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Harpur Hill, Buxton, Derbyshire, SK17 9JN Telephone: +44 (0)1298 218000 Facsimile: +44 (0)1298 218590

Report Title

Report Number

Project Leader: Penny Simpson Author(s): Vincent Crook, Penny Simpson, Beth Rawson and Derrick Wake

Science Group: Environmental Sciences

© Crown Copyright 2006

Report Title

Report Number

Investigation of PXRF procedures for measuring contaminated land

HSL/2006/102

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ACKNOWLEDGEMENTS

Beth Rawson and Penny Simpson carried out the bulk of the research and experimental work for the project. Beth now works at the HSE as an inspector. Thanks to Derrick Wake of HSL for his help with the GPS system and the field walk through. Craig Taylor of HSL carried out the ICP-AES analyses and several members of HSL were involved in feasibility discussions. These included Dave Mark, James Wheeler, Owen Butler and John Groves, all of HSL.

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CONTENTS

1 Background of project........................................................................................................... 1 2 Introduction ........................................................................................................................... 2 3 Introduction to the PXRF device........................................................................................... 4

3.1 PXRF Instrumentation details ....................................................................................... 4 3.2 Field use ........................................................................................................................ 5 3.3 Laboratory use............................................................................................................... 5 3.4 Relation to this work ..................................................................................................... 6

4 Aims of this work.................................................................................................................. 7 4.1 Introduction ................................................................................................................... 7 4.2 This study ...................................................................................................................... 7

5 Experimental and disscussion ............................................................................................... 8 5.1 Limits of detection (LOD)............................................................................................. 8 5.2 Sample preparation: Soil Samples................................................................................. 9 5.3 Data quality ................................................................................................................... 9 5.4 Particle size effect ....................................................................................................... 10 5.5 Moisture content effect................................................................................................ 11 5.6 Sampling strategies ..................................................................................................... 13

6 Final Conclusions................................................................................................................ 18 7 Further work........................................................................................................................ 19 8 References ........................................................................................................................... 20

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EXECUTIVE SUMMARY

OBJECTIVES Portable x-ray fluorescence (PXRF) analysers have been developed to meet a variety of applications, one of which is the analysis of soil to assess contaminated land for metals. This research aims to investigate the effects of particle size and moisture content on the level of contaminant in soils detected by the Niton 700 PXRF analyser. Findings will be used to relate results from measurements taken in the field, to results from the same samples after drying and sieving. The outcome from the research will show whether HSE field scientists could effectively assess the level of metals in contaminated land using HSL’s Niton 700 PXRF. In addition, the ability of HSL to facilitate a viable contaminated land survey will be investigated. MAIN FINDINGS The concentration of Pb that was measured in soil samples reduced in an approximate direct relation to moisture content. This relationship affords prediction of ‘dry’ result from in-situ measurements when the moisture content is known. Particle size has a less predictable effect. Investigation has suggested that sieving to < 125 µm is beneficiary. Sieving to < 125 µm is also a requirement according to the BSI standard BS7755. This work has confirmed that the PXRF takes measurements that are comparable in accuracy to that measured with ICP-AES analysis for fully dried and ground samples. However, the limits of detection (LOD) of the PXRF are above some of the soil guidance value (SGV) levels and it likely that this deficiency would limit the scope for use of the instrument. A walk through of a transect sampling task was conducted in a field at HSL using HSL’s GPS device. Given that there is an average positional uncertainty in the location of 4.6 m it was concluded that the GPS system that HSL presently owns is not suitable for sampling at points that are a short distance apart (< 20 m). The wet chemistry skills and knowledge that are required to provide a full land contamination assessment survey for HSE field scientists already exist within HSL. Therefore given the limitations of the Niton 700 PXRF device an assessment survey could be accomplished without the use of the PXRF device. Currently, HSL’s Niton PXRF could only be used as a part of a soil assessment survey as a qualitative analysis tool. The PXRF could find use in the initial soil investigation process before a main investigation is started –this initial part of an investigation frequently includes some analysis to help support the conceptual model from which the main investigation is run. If a mobile laboratory is to be used as a base to dry and prepare samples for analysis it could be possible to use the PXRF as an analysis tool as part of a more complete quantitative investigation. However, owing to the length of time that is required to dry out soil samples it would be difficult to report on the same day.

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RECOMMENDATIONS PXRF has other strengths and capabilities, for which there may be an external market, and which field scientists could make greater use of. There is clearly scope for development. The PXRF is very good at the evaluation of surface contamination. It is uniquely suited for looking at metals in the surface of immovable and hard objects such as floors, work surfaces, walls and ceilings. The device is also able to accurately measure contamination on filters, wipes and suit materials in a fast and inexpensive manner. In addition, the PXRF device can uniquely be used as a demonstrative tool during factory assessments and education programmes.

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1 BACKGROUND OF PROJECT

This project was begun in February 2001 as a response to a suggestion that it may be possible for HSL to provide a viable soil contamination analysis service for HSE using Exposure Control Section’s PXRF as the lead instrument. The project was intended to strengthen HSL’s capacity to support the HSE in the enforcement of HSG 66, which is entitled: ‘Protection of workers and the general public during the development of contaminated land’. HSL have considerable expertise in quantitative analysis of samples with an array of instrumentation for the purpose including an inductively coupled plasma atomic emission spectrometer device (ICP-AES). The laboratory is ISO 9001:2000 quality accredited. However, since 2002 it has also become necessary for laboratories to attain MCERTS accreditation for soil testing. The Environment Agency has established a ‘Monitoring Certification Scheme’ (MCERTS) for soil sampling services.[1] The certification is to ensure that laboratories deliver high quality chemical testing of soil. MCERTS sets out a performance standard for laboratories that are undertaking soil analysis that ensures that they are able to demonstrate the validity of their analytical methods and it includes the need for laboratories to have ISO 17025:2005 accreditation.[2] In addition, since the beginning of this project, HSL recruited staff devoted to sample booking and recording. HSL is also aiming to become ISO 140000 accredited.[3] This standard encompasses some previous BSI standards relating to environmental monitoring and expands the standard into the area of management. It functions as a ‘guide to environmental management principles, systems and supporting techniques’. The idea was that an investigating officer with a PXRF device would provide the initial assessment of contaminants and the initial delineation of areas of high concentrations of contaminants on the site. The officer would use a GPS system as a real-time location identifier and provide a report on the day of investigation. In order to provide a high level service, the PXRF should be capable of achieving this onsite, without sacrificing accuracy. Based on this initial investigation the customer would then be able to make an informed decision on the level of further investigation that was necessary, which HSL would be able to supply by transporting samples back to the laboratory for wet analysis. To achieve this service standard operating procedures (SOPs) for all of these steps are required. In the early stages of the work HSL’s ability to supply a competitive price for the service was questioned. In light of this revelation this report serves the purpose of presenting the findings of the laboratory investigation work into the capabilities of the PXRF and the paper exercise that went alongside it. It does not present new SOPs for a PXRF led service, or for the use of a GPS device, as this would be an unnecessary effort. HSL already has SOPs in place that cover the sample booking procedure and the ICP-AES analysis of soil samples. The ability of HSL to handle the work is not questioned in this report. This work solely investigates the reliability and feasibility of using the PXRF device and makes some general comments on its market competitiveness.

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2 INTRODUCTION

As the use of Brownfield sites increases so does the demand for the assessment of contamination on such sites. Not all of the sites classed as Brownfield have been previously used for industrial purposes, but high percentages have. The assessment of contaminated land involves a lot more than taking samples and assessing them for contaminants, however this is a major part of the assessment. Full details of what is required in the UK for a survey of suspected contaminated land is available from the BSI code of practice.[4] Contaminated land is defined in the UK’s Environmental Protection Act as “any land which appears to the relevant local authority to be in a condition, by reason of substances in, on or under it, which is causing significant harm or there is ‘significant possibility’ that it could cause such harm, or pollution of controlled waters”.[5] Reducing the time taken to analyse the samples from Brownfield and other contaminated areas could considerably cut the time required for an assessment of the land. The traditional method for soil samples analysis has involved collection of samples which are then taken to a laboratory and analysed using a number of methods, dependant on the contaminants being looked for. Typical preparation includes full digestion of samples using acid and then analysis by relevant wet methods or drying and grinding of the sample in to a homogenous dust followed by analysis by x-ray fluorescence. All these methods are costly in both time and materials. A portable x-ray fluorimeter (PXRF) has the advantage of being portable (no more collecting samples for return to the laboratory), and quick (giving results in a matter of minutes). The US Environmental Protection Agency has a published standard method for the use of a field portable XRF.[6] Currently, the UK does not have similar guidelines. The PXRF can be used to take readings directly from the soil without taking samples or by taking a small amount in a sample cup the readings can be taken on site. The important question is, when it comes to using the PXRF in this manner are the readings of the same accuracy and precision as those found from laboratory analysis? The main features of soil that may affect the results of PXRF analysis are the moisture content, particle size and the non-homogeneity of the soil being tested. If these were major factors is it possible to reduce the cost of analysis by preparing the soil but testing using the PXRF? This could still be done on site with a mobile laboratory. Another area to be examined in this work is the use of global positioning systems (GPS) to map the areas where the samples or readings are taken from. Finally, the BSI standard [4] states that “in most investigations, samples collected from the site should be sent to a laboratory for detailed examination“. This work can be seen as an attempt to refute this statement and prove that it is possible to use the PXRF device to give detailed investigation on site. The document also refers to six occasions where onsite investigation is appropriate. Of these six, five are most certainly within the capabilities of the PXRF device and HSL as an organisation. These occasions, are listed below:

a) the detection and initial assessment of contaminants; b) the rapid analysis of soils, [and] fill materials…during site clearance, development or

remediation, in order to inform decisions on disposal or remediation; c) the initial delineation of possible localized areas of high concentrations of

contaminants; d) screening of large numbers of samples to reduce laboratory costs; e) helping to determine the positions of further sampling points.

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Currently, Forest Research, an agency of the forestry commission, use a PXRF for these very purposes.[7] This helps to minimise costs by reducing the number of samples being unnecessarily sent for ‘wet’ chemical analysis. The standard also refers to the use of a field XRF device, stating a need for laboratory space onsite “since the soil moisture content requires control”. Put more straightforwardly this means that the soil needs drying before it is analysed for chemical content.

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3 INTRODUCTION TO THE PXRF DEVICE

3.1 PXRF INSTRUMENTATION DETAILS

The Niton XL-700 Series spectrum analyser is an energy dispersive x-ray fluorescence (EDXRF) spectrometer that uses one of three 3 radioisotope sources (109Cd; 55Fe; 241Am) allowing the detection of a total of 76 chemical elements in test samples. The device carries out a non-destructive test (NDT) in which the radioisotope source excites a test sample's constituent elements. These elements subsequently emit characteristic electromagnetic radiation in the X-ray region. The XRF device is capable of detecting this radiation in a continuous manner. The wavelength of each x-ray detected identifies the particular element that is present in the sample. The rate at which each x-ray of a given wavelength is detected providing a determination of the quantity of that element that is present in the sample. Sample test results are displayed in ppm for bulk samples. The device is capable of self-calibration on start-up. This process is fully automated and includes Compton-normalisation, Fundamental Parameters analysis and automatic correction for source aging. A particular feature of this device is that it weighs only 1.13 kg. In addition, the radioactive source is closed thereby allowing no operator access and making the device portable and an extremely useful tool for a field scientist to have available. Figure 1 The Niton PXRF 700 instrument

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3.2 FIELD USE

The PXRF devices find use in inspections of factories in diverse industries such as aerospace, medical device manufacturing, petrochemical & pharmaceutical processing, regulatory compliance and R&D. In testing of metal alloys, polymers, scrap metal recycling, electronic components, soils and sediments and Pb in paint assessment. Such analysis is useful in quality analysis and quality control, environmental monitoring, site remediation and geo-chemical exploration. The device is shown being used in two different investigations in Figure 11.

Figure 2 Examples of PXRF use by HSL scientists 3.3 LABORATORY USE

Wheeler et al. [8-10] have studied the device in three papers on behalf of the HSE and currently the device is in regular use by HSL’s field scientists and exposure control research scientists. Examples of uses of the PXRF device in the laboratory include analysis of contamination on and in:

• Overalls • Dirichlet tessellation (Figure 12) • Gloves • Filters • Tape-lifts • Wipes • Soil samples

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Figure 3 A Dirichlet tessellation plot representing levels of contamination on a Tyvek suit 3.4 RELATION TO THIS WORK

The Niton 700 Portable PXRF was developed to directly measure environmental contaminants in soils on contaminated land. On-site measurements are simple, fast, reliable and comprehensive. The PXRF gives an instant direct reading of the concentration of any element from sulphur to uranium, within 2 to 6 minutes. The PXRF should be the perfect solution for a survey of contaminated land, particularly brown field construction sites. With the PXRF it is possible to do all the measurements while on site. Three types of soil measurements can be made. These are:

• Samples collected from the soil surface • Samples collected from bore holes • Direct measurement of surface soil in-situ

XRF is often a preferable method of analysis over destructive type wet chemistry methods. For example Si, Hg, Se, As and possibly Pb and Cd are better determined by XRF. These elements can be are lost to the atmosphere during acid digestion procedures. Another advantage of using the PXRF is that it does not rely on the solubility of the contaminant –some minerals will not completely dissolve using acid digestion procedures.

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4 AIMS OF THIS WORK

4.1 INTRODUCTION

Prior to this work HSL did not know whether the moisture in the soil affects the accuracy of the PXRF. The manufacturer indicates that the PXRF cannot be used to measure soil contamination if the moisture content of the soil is more than 25 %. It is known from a previous project (JS20.00524 Contaminated land: Survey and soil analysis toxic metals) that the PXRF appears to underestimate the amount of contaminant in the soil compared to soil that had been dried and crushed. It is however, not clear whether the underestimation was due solely to the moisture content of the soil or to the particle size of the soil. Therefore, it is necessary to know to what extent the moisture content and the particle size of the soil effects the measurements and whether we can compensate for them in the calibration. 4.2 THIS STUDY

The study is divided into four parts:

1. In the first part of the study the moisture content of contaminated soil will be varied. The moisture content of the soil will be measured and these readings will be related to the concentration of contamination measured using the PXRF.

2. In the second part of the study, the effect particle size has on the PXRF readings will be investigated.

3. Conventionally soil is dried and crushed into a fine powder or digested to release the contaminant. This part of the study will determine the minimum amount of sample preparation that is required in order to get reproducible and accurate results.

4. The fourth part of the study will look at sampling protocols. The advantage of using the PXRF is that it allows a more comprehensive sampling strategy of contaminated land to be made. The speed of analysis means that more measurements can be made on one site within a short time scale and in a cost effective way.

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5 EXPERIMENTAL AND DISSCUSSION

5.1 LIMITS OF DETECTION (LOD)

For the purposes of investigating the instrument’s limits of detection this work concerns itself only with the elements for which there are soil guidance values (SGV). Assessment of contaminated land relies on values above which the land can be said to pose a risk to health. In the past the UK has used so-called ICRCL (interdepartmental committee on the redevelopment of contaminated land) values. In more recent times Soil Guideline Values (SGV) have superseded these. The current SGVs for common elements are displayed in Table 5. An SGV has been defined for four different land uses. These are residential use where there is plant uptake, residential use where there is no plant uptake, allotment use and commercial/industrial uses. Table 1 Soil Guideline Values. (*assumes all Cr is Cr(VI). **based on total mercury –do not apply to elemental or organic mercury compounds. $ above LOD for PXRF)

SGV according to Land Use

Element

Residential with plant uptake

(ppm) Residential without plant uptake (ppm)

Allotment (ppm)

Commercial/ Industrial (ppm)

LOD of Niton PXRF

Cr * 130 200$ 130$ 5000$ 141.9 Se 35$ 260$ 35$ 8000$ 8.8 Ni 50 75 50 5000$ 80.6 Pb 50$ 450$ 450$ 750$ 15.0 Hg ** 8 8$ 15 480$ 11.5 As 20 20$ 20 500$ 80.6

pH 6 1 30$ 1 1400$ 58.8 pH 7 2 30$ 2 1400$ 58.8 Cd pH 8 8 30$ 8 1400$ 58.8

If the PXRF is to be used in the field to measure the elements shown in Table 5, it is important to know the limit of detection (LOD) of the instrument. In addition, it is vital for providing a soil analysis service that the LOD of the instrument is below the SGV of the element being measured. LOD is defined as 3 times the standard deviation of fluctuation in the background. An estimate of the LOD for each element was found by measuring a blank sample. In this work measurements (n=30) were taken of a sample of SiO2

and the average was taken to be the LOD. The average LOD values are displayed in Table 5, and where those values fall below the SGV, the SGV is marked with a $. From the point of view of using the PXRF as a tool for measuring contaminated soil samples at contamination levels more than or equal to the SGV the outlook is good. The LOD of the PXRF is above the SGV levels for Commercial/Industrial use for all of the metals tested and for all but one of the SGV levels for residential use without plant uptake. However, the LOD of the instrument is not low enough to detect most of the elements tested at the SGV for residential use where there is plant uptake and allotment use. It must be decided whether these deficiencies would limit the commercial use of the instrument. Indeed, the British Standards Institute document BS 10175:2001 [4], raises particular concerns about the detection limit of field XRF devices for some metals, specifically cadmium.

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5.2 SAMPLE PREPARATION: SOIL SAMPLES

Samples were prepared for analysis first by drying them to constant weight in an oven at 40 oC. The soil was subsequently ground and sieved to be less than 125 µm. The resultant powder was sealed in a sample cup, consisting of five components –a sealing ring, a main tube, cotton wool bung, a base and a cellophane cover– which all fit together to house the sample (see Figure 13) and analysed on a specially designed platform (available from Niton) (see Figure 14)

Figure 4 The four sample cup components and a completed sample cup containing a soil sample within it (right) Figure 5 The Niton PXRF device sitting on the soil test platform. The sample cup sits at the head of the platform, positioned directly below the window of the instrument’s source 5.3 DATA QUALITY

The quality of the data produced by the PXRF was tested by comparing results from the PXRF with that obtained by ICP-AES analysis of the same samples. The results are displayed in Figure 15.

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y = 1.0994x - 212.36R2 = 0.9999

0

5000

10000

15000

20000

25000

0 5000 10000 15000 20000 25000

ppm determined by ICP-AES

ppm

det

erm

ined

by

PX

RF

ppm Pb1 to 1Linear (ppm Pb)

Figure 6 Comparison of concentration of Pb in three soil samples as determined by both ICP-AES and PXRF. The data is fitted to a linear trend line, the equation for which is displayed on the chart. Comparison of the results reveals that as the concentration of Pb in the soil samples increased the results deviate further from the ‘ideal’ 1:1 relationship. The ICP-AES is calibrated with standard solutions. The samples were completely digested as opposed to leached. The resultant solution is diluted in order to reduce the effect of the matrix and inter element interference. Therefore, we must assume for the purpose of this work that the deviation from parity is owing to deficiencies in the PXRF device. However, the deviation is appreciably small, and it is only at high levels (> 10,000 ppm, (way above the SGV for Pb) that it becomes significant. According to the Niton 700 manual; “Niton XRF’s are calibrated to give accurate values…in concentrations of 10,000 ppm or less”. At levels above 10,000 the linearity of the Compton Normalisation method of internal calibration is lost. The manual states that “beyond 20,000 ppm…readings may exhibit even greater deviation” –above 2 %. All SGV fall below 10,000 pm. Therefore, in terms of providing a service for contaminated land assessment, <10,000 ppm is also the region where accurate readings are essential and can be provided by the PXRF device. 5.4 PARTICLE SIZE EFFECT

5.4.1 Aims and methods

The aim of this part of the work was to investigate the effect of soil particle size on Pb measurements taken using the Niton PXRF. Other research has also established a concern with these matters.[11] The author demonstrated that sieving to <0.5 mm had the effect of reducing the apparent contamination in soil in comparison to that observed for soil fractions of >2mm and 2 to 0.5 mm and that sieving improved the reproducibility of the readings. Two soil samples, each having 4,600 ppm and 4,800 ppm of Pb contamination were sieved to give 2 mm, 250 µm and 125 µm fractions. Each fraction was analysed separately and the results are displayed in Table 6. Unpredictable changes in the apparent Pb concentration were observed. This, perhaps, reveals a need for a standard sieving procedure. What is clear is that sieving to smaller sizes reduces the SD of multiple readings, this is an indication of the improved reproducibility of the readings. Following analysis of the sieved samples the samples

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were each ground up and reanalysed. This produced an apparent average increase in Pb in the samples of 10.5 %. The greater increases were for the larger fraction (< 2 mm). Those samples that had been ground to < 125 µm each showed no increase in apparent Pb concentration following subsequent grinding. Table 2 Particle size effect upon PXRF measurement of two soil samples

Sample Sieved fraction

Pb (ppm) after

sieving SD

Pb after grinding

(ppm) SD

Increase after

grinding (ppm)

Increase after

grinding (%)

A >2mm 4658 125.1 6192 213.7 1534 32.9 A 250µm - 2mm 4801 98.4 5516 81.3 715 14.9 A 125µm - 250µm 4401 90.8 5089 43.7 688 15.6 A <125µm 4942 59.4 4942 59.4 0 0 B >2mm 5072 320.6 5393 132.7 321 6.3 B 250µm - 2mm 5035 38.2 5756 114.1 721 14.3 B 125µm - 250µm 4978 62.1 4959 115.6 -19.2 -0.4 B <125µm 4354 41.8 4354 41.8 0 0 AVE 494.9 10.5

5.4.2 Particle Size Conclusions

This series of experiments has shown that particle size differences can cause variations in data. Given that particles of soil will always have vacant space separating them, it is necessary to grind a sample until it approximates one that does not. It is recommended therefore, that all samples be sieved to below 125 µm diameter. The BSI code of practice recommends the same.[4, 12] By way of a comparison, the particle size problem is not experienced in ICP analysis of soil because it involves total digestion of a known mass of soil. Unfortunately, PXRF measurement is only capable of giving an apparent Pb concentration in soil because assumes that the concentration of soil in a volume of sample is 100 %. 5.5 MOISTURE CONTENT EFFECT

5.5.1 Introduction, aims and methods

Presently probes are available for soil moisture determination. However, these are prohibitively expensive. Therefore, throughout this work moisture content was determined by gravimetric methods. The soil is simply dried in a warm oven until it is a consistent weight. Soil was dried at a temperature high enough to evaporate water but not to oxidise organic mater in accordance with the methodology detailed in the British Standard code of practice; BS7755-3.1: 1994.[12, 13] The equation below was used for calculating soil moisture content.

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Gravimetric soil moisture (%) = (Mass of wet soil – Mass of dry soil) * 100 Mass of dry soil The aim of this part of the work was to investigate the effect of soil moisture content on Pb measurements taken using the Niton PXRF. Three contaminated soil samples were selected each having approximately 20,000, 7,000 and 1,500 ppm of Pb respectively. These were dried at 40 oC, sieved and subsequently analysed using the PXRF. Each sample was divided into five 10 g portions and subsequently analysed to ensure homogeneity throughout the portions. Deionised water was subsequently added to each portion in order to obtain moisture contents of 5, 10, 20, 30 and 40 %. 5.5.2 Results and discussion of moisture effect

The effect that moisture content had upon PXRF readings of the soil samples is displayed in Table 7 and in Figure 16. An approximate direct relationship between moisture content and the reduction in the PXRF reading is seen. This relationship is less strong at moisture concentrations above 20 %, the reason for this is unclear. Table 3 Effect of moisture content upon PXRF readings of soil

A 20,377 ppm B 7,273 ppm C 1,649 ppm

Moisture (%) Reduction (%) Reduction (%) Reduction (%) Average (%)

0 0.0 0.0 0.0 0.0

5 7.1 1.7 5.8 4.9

10 12.9 10.4 10.8 11.4

20 22.4 19.3 19.0 20.2

30 29.0 25.7 25.2 26.8

40 35.2 31.9 31.7 32.9

13

0

10

20

30

40

0 10 20 30 40

Moisture content (%)

Red

uctio

n in

Pb

mea

sure

d (%

)20,000 ppm of Pb

7,000 ppm of Pb

1500 ppm of Pb

1:1

Figure 7 Effect of moisture content upon PXRF readings of soil. A 1:1 equivalent line is shown for comparison 5.5.3 Moisture content effect conclusions

In conclusion the concentration of Pb measured reduces in an approximate direct relation to moisture content at moisture content less than ~20 %. This relationship affords prediction of ‘dry’ result from in-situ measurements when the moisture content is known. With this knowledge HSL may be capable of providing an initial assessment of contaminated land sites without removing samples from the ground or with minimal sample preparation. However, HSL would be unable to guarantee the accuracy of the results on the day of measurement without drying the samples. Drying the soil is a task that takes several hours. It is usually carried out overnight. This would make it impossible to report on the same day. 5.6 SAMPLING STRATEGIES

5.6.1 Introduction to sampling strategies

Various strategies are available for measuring contaminated land from simple straight grids to the common herringbone pattern design to the random wander. Each type of design has its advantages and disadvantages. Pattern designs are rigid and usually not flexible, whereas the random patterns can be hit or miss, but are flexible. The depth of sampling can be from 1 cm, 30 cm to 3 m, or more, depending on the use and purpose of the land. By adopting a flexible approach to the choice of technique and by using reliable sampling regimes an effective land

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survey can be implemented. It is proposed that central to this must be the use of a global positioning satellite (GPS) device. The best use of this technology would be to utilise GPS technology for positioning the measurement sites. In this work it was important to assess whether HSL’s GPS equipment was accurate enough for samples to be taken from the same spot at different times. However, given that the BSI standard [4] states that “the use of Global Positioning Systems (GPS) should not preclude the inclusion of permanent marks”, the need for precise GPS readings is not critical. This statement implies that sampling points could be marked with a stake or even paint. A simple program could be designed for running on a portable computer to aid the sampling and reporting. This program could mark the position of the area to be surveyed on a map and produce a standard sampling pattern such as a herringbone pattern with coordinates for each sample point. The principle is easily demonstrated: Figure 17 displays a screenshot of a simple ExcelTM spreadsheet that generates a standard sampling grid from a single starting point co-ordinate. In addition, the size of each grid square can be specified.

Figure 8 ExcelTM spreadsheet to generate standard sampling pattern from a single starting point

co-ordinate 5.6.2 HSL’s GPS device

The Global Positioning System (GPS) used for this work was an Ashtech (Formally Magellan Corporation) Reliance Workabout, SCA-12 Satellite Receiver used in conjunction with an Ashtech BR2 Beacon Receiver and Survey Antenna. The system is mounted in a backpack and controlled from a Psion Workabout hand held computer. The combination of GPS receiver and beacon receiver, which is called a Differential Global Positioning System (DGPS) is capable of

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sub-metre accuracy with post processing of the data gathered. In use in real-time, accuracies of around two metres are possible. The USA has a system of 24 satellites constantly orbiting the earth forming the basis of a geographic positioning and navigation tool. This is called Navstar (the Russians also have a similar system). The reception of any four or more of these satellites allows a GPS receiver to compute it’s own three-dimensional coordinates relative to the World Geodetic System, 1984 (WGS-84). The American system is in two forms each operating at a particular frequency. These are the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). The latter is reserved for use by the US military (and others), whereas the former less accurate service is provided free of charge on a worldwide basis, for civilian use. Basic GPS is capable of extremely high accuracy, of the order of several millimetres. Therefore, to reduce the level of accuracy for the SPS, in the interests of preventing terrorism, the Americans broadcast a Selective Availability (SA) precision degrading signal so that the accuracy of most civilian GPS systems is only about 100 metres. To get round this problem when more precise navigation is an issue, maritime navigation for example, it is possible to use a beacon receiver in addition to the GPS receiver, in other words a DGPS. The Ashtech BR2 Receiver contains an extremely sensitive 300 kHz minimum shift keying engine (MSK) that demodulates differential GPS corrections broadcast by a system of navigational radio beacons maintained by the International Association of Lighthouse Authorities (IALA). In the UK there are 14 such stations, each with a range of between 185 and 370 miles so that all of the UK is covered. 5.6.3 Experimental work with the GPS

A walk through of a transect sampling task was conducted in a field at HSL near the explosion control section. Five points (A-E) were selected along an approximately 100 m long stretch of grass that was known to have been contaminated with strontium carbonate in previous work by the exposure control section. The PXRF was used in thin film mode and readings were taken at each point. Each reading was taken over a period of 60 nomsecs using the cadmium source. The GPS device was used to obtain a map reference at each measurement site. Following each reading a marker was left at the site. This marker was subsequently returned to and a repeat set of readings with the PXRF and the GPS system were taken for comparison. The GPS readings that were taken at each visit to each sample point are plotted in Figure 18 and are displayed in Table 8.

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53.2265

53.2266

53.2267

53.2268

53.2269

53.2270

53.2271

53.2272

1.9141

1.9142

1.9142

1.9143

1.9143

1.9144

1.9144

1.9145

1.9145

1.9146

1.9146

1.9147

deg W

deg

N 1st Visit

2nd Visit

Figure 9 GPS readings that were taken at each visit to each sample point. Values are in

decimal degrees in the form 12.3456 (deg deg . min min sec sec) Table 4 Table combining GPS readings that were taken at each visit to each sample point (A-E)

along a transect and the Strontium PXRF readings that were obtained Visit Site Latitude (deg) Longitude (deg) Positional

uncertainty (m)

PXRF reading of Sr (µg/cm2)

1st visit A 53.22711749 1.91416667 - 1 B 53.2270055 1.9143005 - 2.1 C 53.2268634 1.9144016 - 1.3 D 53.2267514 1.9144563 - 0.7 E 53.2265820 1.9145984 - -

2nd visit A 53.2271339 1.9141913 3.29 1.8 B 53.2270328 1.9142951 3.09 0.9 C 53.2268689 1.9143552 5.20 1.4

D 53.2268115 1.9144754 7.01 LOD E 53.2265437 1.9146120 4.52 0.7

The second visit to the five markers produced appreciably different GPS readings, this throws the accuracy of both readings into question. Calculating the distance between the each pair of GPS reading is a rather complex matter. It involves so-called ‘great circle calculations’. Fortunately a freeware program called ‘Perpendicular distance calculator’ created by Peter Ersts can carry out this calculation rather quickly.[14] The distances between the pairs of GPS readings were calculated using the ‘great circle distance’ function of this program and are displayed in Table 8. The mean distance between each pair of readings is 4.6 m. The length of the transect (point A to E) was found to be 76.5 m, based on the first visit.

17

In conclusion, the GPS system that HSL presently owns may not be suitable for sampling points that are a short distance apart (< 20 m) owing to the relatively large positional uncertainty inherent in using the device.

18

6 FINAL CONCLUSIONS

The main conclusion from this investigation is that it would be difficult to provide a full land contamination assessment service at HSL using the PXRF as the lead instrument. The wet chemistry skills and knowledge that are required to provide a full land contamination assessment survey for HSE field scientists already exists within HSL. Although there are some requirements that are additional to the actual soil analysis that require a certain amount of knowledge and expertise, this could be developed within HSL. However, in light of the existence of highly efficient, high through put analytical laboratories it is unlikely that HSL could price the service competitively. The LOD of the PXRF is below the SGV levels for Commercial/Industrial use for all of the metals tested and for all but one of the SGV levels for residential use without plant uptake. However, the LOD of the instrument is not low enough to detect most of the elements tested at the SGV for residential use where there is plant uptake and allotment use. It must be decided whether these deficiencies would limit the commercial use of the instrument. Certainly some brown field sites have houses with garden built upon them because of land shortages in built up areas. The PXRF would be unsuitable for assessing this type of land. The concentration of Pb that was measured in soil samples reduced in an approximate direct relation to moisture content. This relationship affords prediction of ‘dry’ result from in-situ measurements when the moisture content is known. Particle size has a less predictable effect. Investigation has suggested that sieving to <125 µm is beneficiary owing to the improvement in the reproducibility of the readings. Sieving to <125 µm is also a requirement according to the BSI standard BS7755 [12]. This work has displayed that the PXRF takes measurements that are comparable in accuracy to that measured with ICP-AES analysis for fully dried and ground samples. A walk through of a transect sampling task was conducted in a field at HSL using HSL’s GPS device. Given that there is an average positional uncertainty in the location of 4.6 metre it was concluded that the GPS system that HSL presently owns is not suitable for sampling at points that are a short distance apart (< 20 m). In conclusion, the PXRF could be used as a qualitative analysis tool in the initial investigation process before a main investigation into the soil is started, this part of an investigation frequently includes some analysis to help support the conceptual model from which the main investigation is run. If a mobile laboratory is to be used as a base to dry and prepare samples for analysis it could be possible to use the PXRF as an analysis tool for a more complete quantitative investigation. However, owing to the length of time that is required to dry out soil samples it would be difficult to report on the same day.

19

7 FURTHER WORK It is the opinion of this author that the PXRF has other strengths, expertise and capabilities, other than land contamination, for which there may be an external market for, and which field scientists could make greater use of. Currently the PXRF is in use for only about 30 % of the time. Therefore, basing a service on its use 100 % of the time is probably unwise. However, there is clearly scope for development. The PXRF is very good at the evaluation of surface contamination. It is uniquely suited for looking at metals in the surface of immovable and hard objects such as floors, work surfaces, walls and ceilings. Specifically, the device has found use in HSE factory lead investigations.[15, 16] The device is also able to accurately measure contamination on filters, wipes and suit materials. Currently at HSL such samples are more commonly measured by ICP-AES which is more time consuming and costly. One of the bonuses of the PXRF device is that it displays a measurement very quickly on its screen, therefore it is uniquely able to be used as a demonstrative tool by HSE during factory assessments and education programs. The ability to say, “look there is contamination on this bench” and to prove it in-situ immediately, is valuable.

20

8 REFERENCES

1. Chemical testing of soil. 2006, Environment Agency Website. 2. ISO/IEC 17025:2005 General requirements for the competence of testing and

calibration laboratories. 2005: International Standards Organisation. 3. ISO 14000: Guide to Environmental Management Principles, Systems and Supporting

Techniques. 2004: International Standards Organisation. 4. BS 10175:2001 Investigation of potentially contaminated sites - code of practice.

British Standards Institute, ed. EH/4. Vol. BS 10175:2001. 2001, Milton Keynes: British Standards Institute. 85.

5. Environmental Protection Act (Part 11a). 1990. 6. Method 6200: Field Portable X-Ray Fluorescence spectrometry for the determination of

elemental concentrations in soil and sediment. 1998: Environmental Protection Agency (US).

7. Reclaiming our future, in Office of Science and Innovation: Infinite Monkeys. 2006. p. 4-8.

8. Wheeler, J., C. Sams, and P. Baldwin, Portable X-ray Fluorescence Spectrometer (PXRF) to measure Air Filters and Forensic Tape. HSL Internal Report, 1999. IR/ECO/99/06.

9. Wheeler, J. and F.N. Medler, Calibration and Some Performance Characteristics of the PXRF. HSL Internal Report, 1996. IR/A/96/6.

10. Wheeler, J., Operating procedures for the PXRF (Compilation report). 1998, HSL. 11. Laiho, J.V.-P. Development of portable X-ray fluorescence (PXRF)

instruments reliability and quality control procedures. in Geologian tutkimuskeskuksen ja Suomen ympäristökeskuksen tutkimusseminaaris. 2004. Helsinki.

12. Part 3: Chemical methods. Section 3.5 Pre-treatment of samples for physio-chemical analyses, in BS 7755. Soil Quality. 1995, British Standards Institute: Milton Keynes.

13. Part 3. Section 3.1: Determination of dry matter and water content on a mass basis by a gravimetric method, in BS 7755 Soil quality. 1994., British Standard Institution: Milton Keynes.

14. Ersts, P.J., Perpendicular Distance Calculator. 2006, Center for Biodiversity and Conservation at American Museum of Natural History.

15. Rawson, B., Lead Sampling Visit to F Murphy Alloys (July 2004). HSL Internal Report, 2004. IR/ECO/04/10.

16. Rawson, B., Lead Sampling Visit to F Murphy Alloys. HSL Internal Report, 2002. IR/ECO/02/20.

21

ACKNOWLEDGEMENTS

Beth Rawson and Penny Simpson carried out the bulk of the research and experimental work for the project. Beth now works at the HSE as an inspector. Thanks to Derrick Wake of HSL for his help with the GPS system and the field walk through. Craig Taylor of HSL carried out the ICP-AES analyses and several members of HSL were involved in feasibility discussions. These included Dave Mark, James Wheeler, Owen Butler and John Groves, all of HSL.

22

CONTENTS

1 Background of project........................................................................................................... 1 2 Introduction ........................................................................................................................... 2 3 Introduction to the PXRF device........................................................................................... 4

3.1 PXRF Instrumentation details ....................................................................................... 4 3.2 Field use ........................................................................................................................ 5 3.3 Laboratory use............................................................................................................... 5 3.4 Relation to this work ..................................................................................................... 6

4 Aims of this work.................................................................................................................. 7 4.1 Introduction ................................................................................................................... 7 4.2 This study ...................................................................................................................... 7

5 Experimental and disscussion ............................................................................................... 8 5.1 Limits of detection (LOD)............................................................................................. 8 5.2 Sample preparation: Soil Samples................................................................................. 9 5.3 Data quality ................................................................................................................... 9 5.4 Particle size effect ....................................................................................................... 10 5.5 Moisture content effect................................................................................................ 11 5.6 Sampling strategies ..................................................................................................... 13

6 Final Conclusions................................................................................................................ 18 7 Further work........................................................................................................................ 19 8 References ........................................................................................................................... 20

23

EXECUTIVE SUMMARY

OBJECTIVES Portable x-ray fluorescence (PXRF) analysers have been developed to meet a variety of applications, one of which is the analysis of soil to assess contaminated land for metals. This research aims to investigate the effects of particle size and moisture content on the level of contaminant in soils detected by the Niton 700 PXRF analyser. Findings will be used to relate results from measurements taken in the field, to results from the same samples after drying and sieving. The outcome from the research will show whether HSE field scientists could effectively assess the level of metals in contaminated land using HSL’s Niton 700 PXRF. In addition, the ability of HSL to facilitate a viable contaminated land survey will be investigated. MAIN FINDINGS The concentration of Pb that was measured in soil samples reduced in an approximate direct relation to moisture content. This relationship affords prediction of ‘dry’ result from in-situ measurements when the moisture content is known. Particle size has a less predictable effect. Investigation has suggested that sieving to < 125 µm is beneficiary. Sieving to < 125 µm is also a requirement according to the BSI standard BS7755. This work has confirmed that the PXRF takes measurements that are comparable in accuracy to that measured with ICP-AES analysis for fully dried and ground samples. However, the limits of detection (LOD) of the PXRF are above some of the soil guidance value (SGV) levels and it likely that this deficiency would limit the scope for use of the instrument. A walk through of a transect sampling task was conducted in a field at HSL using HSL’s GPS device. Given that there is an average positional uncertainty in the location of 4.6 m it was concluded that the GPS system that HSL presently owns is not suitable for sampling at points that are a short distance apart (< 20 m). The wet chemistry skills and knowledge that are required to provide a full land contamination assessment survey for HSE field scientists already exist within HSL. Therefore given the limitations of the Niton 700 PXRF device an assessment survey could be accomplished without the use of the PXRF device. Currently, HSL’s Niton PXRF could only be used as a part of a soil assessment survey as a qualitative analysis tool. The PXRF could find use in the initial soil investigation process before a main investigation is started –this initial part of an investigation frequently includes some analysis to help support the conceptual model from which the main investigation is run. If a mobile laboratory is to be used as a base to dry and prepare samples for analysis it could be possible to use the PXRF as an analysis tool as part of a more complete quantitative investigation. However, owing to the length of time that is required to dry out soil samples it would be difficult to report on the same day.

24

RECOMMENDATIONS PXRF has other strengths and capabilities, for which there may be an external market, and which field scientists could make greater use of. There is clearly scope for development. The PXRF is very good at the evaluation of surface contamination. It is uniquely suited for looking at metals in the surface of immovable and hard objects such as floors, work surfaces, walls and ceilings. The device is also able to accurately measure contamination on filters, wipes and suit materials in a fast and inexpensive manner. In addition, the PXRF device can uniquely be used as a demonstrative tool during factory assessments and education programmes.

1

9 BACKGROUND OF PROJECT

This project was begun in February 2001 as a response to a suggestion that it may be possible for HSL to provide a viable soil contamination analysis service for HSE using Exposure Control Section’s PXRF as the lead instrument. The project was intended to strengthen HSL’s capacity to support the HSE in the enforcement of HSG 66, which is entitled: ‘Protection of workers and the general public during the development of contaminated land’. HSL have considerable expertise in quantitative analysis of samples with an array of instrumentation for the purpose including an inductively coupled plasma atomic emission spectrometer device (ICP-AES). The laboratory is ISO 9001:2000 quality accredited. However, since 2002 it has also become necessary for laboratories to attain MCERTS accreditation for soil testing. The Environment Agency has established a ‘Monitoring Certification Scheme’ (MCERTS) for soil sampling services.[1] The certification is to ensure that laboratories deliver high quality chemical testing of soil. MCERTS sets out a performance standard for laboratories that are undertaking soil analysis that ensures that they are able to demonstrate the validity of their analytical methods and it includes the need for laboratories to have ISO 17025:2005 accreditation.[2] In addition, since the beginning of this project, HSL recruited staff devoted to sample booking and recording. HSL is also aiming to become ISO 140000 accredited.[3] This standard encompasses some previous BSI standards relating to environmental monitoring and expands the standard into the area of management. It functions as a ‘guide to environmental management principles, systems and supporting techniques’. The idea was that an investigating officer with a PXRF device would provide the initial assessment of contaminants and the initial delineation of areas of high concentrations of contaminants on the site. The officer would use a GPS system as a real-time location identifier and provide a report on the day of investigation. In order to provide a high level service, the PXRF should be capable of achieving this onsite, without sacrificing accuracy. Based on this initial investigation the customer would then be able to make an informed decision on the level of further investigation that was necessary, which HSL would be able to supply by transporting samples back to the laboratory for wet analysis. To achieve this service standard operating procedures (SOPs) for all of these steps are required. In the early stages of the work HSL’s ability to supply a competitive price for the service was questioned. In light of this revelation this report serves the purpose of presenting the findings of the laboratory investigation work into the capabilities of the PXRF and the paper exercise that went alongside it. It does not present new SOPs for a PXRF led service, or for the use of a GPS device, as this would be an unnecessary effort. HSL already has SOPs in place that cover the sample booking procedure and the ICP-AES analysis of soil samples. The ability of HSL to handle the work is not questioned in this report. This work solely investigates the reliability and feasibility of using the PXRF device and makes some general comments on its market competitiveness.

2

10 INTRODUCTION

As the use of Brownfield sites increases so does the demand for the assessment of contamination on such sites. Not all of the sites classed as Brownfield have been previously used for industrial purposes, but high percentages have. The assessment of contaminated land involves a lot more than taking samples and assessing them for contaminants, however this is a major part of the assessment. Full details of what is required in the UK for a survey of suspected contaminated land is available from the BSI code of practice.[4] Contaminated land is defined in the UK’s Environmental Protection Act as “any land which appears to the relevant local authority to be in a condition, by reason of substances in, on or under it, which is causing significant harm or there is ‘significant possibility’ that it could cause such harm, or pollution of controlled waters”.[5] Reducing the time taken to analyse the samples from Brownfield and other contaminated areas could considerably cut the time required for an assessment of the land. The traditional method for soil samples analysis has involved collection of samples which are then taken to a laboratory and analysed using a number of methods, dependant on the contaminants being looked for. Typical preparation includes full digestion of samples using acid and then analysis by relevant wet methods or drying and grinding of the sample in to a homogenous dust followed by analysis by x-ray fluorescence. All these methods are costly in both time and materials. A portable x-ray fluorimeter (PXRF) has the advantage of being portable (no more collecting samples for return to the laboratory), and quick (giving results in a matter of minutes). The US Environmental Protection Agency has a published standard method for the use of a field portable XRF.[6] Currently, the UK does not have similar guidelines. The PXRF can be used to take readings directly from the soil without taking samples or by taking a small amount in a sample cup the readings can be taken on site. The important question is, when it comes to using the PXRF in this manner are the readings of the same accuracy and precision as those found from laboratory analysis? The main features of soil that may affect the results of PXRF analysis are the moisture content, particle size and the non-homogeneity of the soil being tested. If these were major factors is it possible to reduce the cost of analysis by preparing the soil but testing using the PXRF? This could still be done on site with a mobile laboratory. Another area to be examined in this work is the use of global positioning systems (GPS) to map the areas where the samples or readings are taken from. Finally, the BSI standard [4] states that “in most investigations, samples collected from the site should be sent to a laboratory for detailed examination“. This work can be seen as an attempt to refute this statement and prove that it is possible to use the PXRF device to give detailed investigation on site. The document also refers to six occasions where onsite investigation is appropriate. Of these six, five are most certainly within the capabilities of the PXRF device and HSL as an organisation. These occasions, are listed below:

f) the detection and initial assessment of contaminants; g) the rapid analysis of soils, [and] fill materials…during site clearance, development or

remediation, in order to inform decisions on disposal or remediation; h) the initial delineation of possible localized areas of high concentrations of

contaminants; i) screening of large numbers of samples to reduce laboratory costs; j) helping to determine the positions of further sampling points.

3

Currently, Forest Research, an agency of the forestry commission, use a PXRF for these very purposes.[7] This helps to minimise costs by reducing the number of samples being unnecessarily sent for ‘wet’ chemical analysis. The standard also refers to the use of a field XRF device, stating a need for laboratory space onsite “since the soil moisture content requires control”. Put more straightforwardly this means that the soil needs drying before it is analysed for chemical content.

4

11 INTRODUCTION TO THE PXRF DEVICE

11.1 PXRF INSTRUMENTATION DETAILS

The Niton XL-700 Series spectrum analyser is an energy dispersive x-ray fluorescence (EDXRF) spectrometer that uses one of three 3 radioisotope sources (109Cd; 55Fe; 241Am) allowing the detection of a total of 76 chemical elements in test samples. The device carries out a non-destructive test (NDT) in which the radioisotope source excites a test sample's constituent elements. These elements subsequently emit characteristic electromagnetic radiation in the X-ray region. The XRF device is capable of detecting this radiation in a continuous manner. The wavelength of each x-ray detected identifies the particular element that is present in the sample. The rate at which each x-ray of a given wavelength is detected providing a determination of the quantity of that element that is present in the sample. Sample test results are displayed in ppm for bulk samples. The device is capable of self-calibration on start-up. This process is fully automated and includes Compton-normalisation, Fundamental Parameters analysis and automatic correction for source aging. A particular feature of this device is that it weighs only 1.13 kg. In addition, the radioactive source is closed thereby allowing no operator access and making the device portable and an extremely useful tool for a field scientist to have available. Figure 10 The Niton PXRF 700 instrument

5

11.2 FIELD USE

The PXRF devices find use in inspections of factories in diverse industries such as aerospace, medical device manufacturing, petrochemical & pharmaceutical processing, regulatory compliance and R&D. In testing of metal alloys, polymers, scrap metal recycling, electronic components, soils and sediments and Pb in paint assessment. Such analysis is useful in quality analysis and quality control, environmental monitoring, site remediation and geo-chemical exploration. The device is shown being used in two different investigations in Figure 11.

Figure 11 Examples of PXRF use by HSL scientists 11.3 LABORATORY USE

Wheeler et al. [8-10] have studied the device in three papers on behalf of the HSE and currently the device is in regular use by HSL’s field scientists and exposure control research scientists. Examples of uses of the PXRF device in the laboratory include analysis of contamination on and in:

• Overalls • Dirichlet tessellation (Figure 12) • Gloves • Filters • Tape-lifts • Wipes • Soil samples

6

Figure 12 A Dirichlet tessellation plot representing levels of contamination on a Tyvek suit 11.4 RELATION TO THIS WORK

The Niton 700 Portable PXRF was developed to directly measure environmental contaminants in soils on contaminated land. On-site measurements are simple, fast, reliable and comprehensive. The PXRF gives an instant direct reading of the concentration of any element from sulphur to uranium, within 2 to 6 minutes. The PXRF should be the perfect solution for a survey of contaminated land, particularly brown field construction sites. With the PXRF it is possible to do all the measurements while on site. Three types of soil measurements can be made. These are:

• Samples collected from the soil surface • Samples collected from bore holes • Direct measurement of surface soil in-situ

XRF is often a preferable method of analysis over destructive type wet chemistry methods. For example Si, Hg, Se, As and possibly Pb and Cd are better determined by XRF. These elements can be are lost to the atmosphere during acid digestion procedures. Another advantage of using the PXRF is that it does not rely on the solubility of the contaminant –some minerals will not completely dissolve using acid digestion procedures.

7

12 AIMS OF THIS WORK

12.1 INTRODUCTION

Prior to this work HSL did not know whether the moisture in the soil affects the accuracy of the PXRF. The manufacturer indicates that the PXRF cannot be used to measure soil contamination if the moisture content of the soil is more than 25 %. It is known from a previous project (JS20.00524 Contaminated land: Survey and soil analysis toxic metals) that the PXRF appears to underestimate the amount of contaminant in the soil compared to soil that had been dried and crushed. It is however, not clear whether the underestimation was due solely to the moisture content of the soil or to the particle size of the soil. Therefore, it is necessary to know to what extent the moisture content and the particle size of the soil effects the measurements and whether we can compensate for them in the calibration. 12.2 THIS STUDY

The study is divided into four parts:

5. In the first part of the study the moisture content of contaminated soil will be varied. The moisture content of the soil will be measured and these readings will be related to the concentration of contamination measured using the PXRF.

6. In the second part of the study, the effect particle size has on the PXRF readings will be investigated.

7. Conventionally soil is dried and crushed into a fine powder or digested to release the contaminant. This part of the study will determine the minimum amount of sample preparation that is required in order to get reproducible and accurate results.

8. The fourth part of the study will look at sampling protocols. The advantage of using the PXRF is that it allows a more comprehensive sampling strategy of contaminated land to be made. The speed of analysis means that more measurements can be made on one site within a short time scale and in a cost effective way.

8

13 EXPERIMENTAL AND DISSCUSSION

13.1 LIMITS OF DETECTION (LOD)

For the purposes of investigating the instrument’s limits of detection this work concerns itself only with the elements for which there are soil guidance values (SGV). Assessment of contaminated land relies on values above which the land can be said to pose a risk to health. In the past the UK has used so-called ICRCL (interdepartmental committee on the redevelopment of contaminated land) values. In more recent times Soil Guideline Values (SGV) have superseded these. The current SGVs for common elements are displayed in Table 5. An SGV has been defined for four different land uses. These are residential use where there is plant uptake, residential use where there is no plant uptake, allotment use and commercial/industrial uses. Table 5 Soil Guideline Values. (*assumes all Cr is Cr(VI). **based on total mercury –do not apply to elemental or organic mercury compounds. $ above LOD for PXRF)

SGV according to Land Use

Element

Residential with plant uptake

(ppm) Residential without plant uptake (ppm)

Allotment (ppm)

Commercial/ Industrial (ppm)

LOD of Niton PXRF

Cr * 130 200$ 130$ 5000$ 141.9 Se 35$ 260$ 35$ 8000$ 8.8 Ni 50 75 50 5000$ 80.6 Pb 50$ 450$ 450$ 750$ 15.0 Hg ** 8 8$ 15 480$ 11.5 As 20 20$ 20 500$ 80.6

pH 6 1 30$ 1 1400$ 58.8 pH 7 2 30$ 2 1400$ 58.8 Cd pH 8 8 30$ 8 1400$ 58.8

If the PXRF is to be used in the field to measure the elements shown in Table 5, it is important to know the limit of detection (LOD) of the instrument. In addition, it is vital for providing a soil analysis service that the LOD of the instrument is below the SGV of the element being measured. LOD is defined as 3 times the standard deviation of fluctuation in the background. An estimate of the LOD for each element was found by measuring a blank sample. In this work measurements (n=30) were taken of a sample of SiO2

and the average was taken to be the LOD. The average LOD values are displayed in Table 5, and where those values fall below the SGV, the SGV is marked with a $. From the point of view of using the PXRF as a tool for measuring contaminated soil samples at contamination levels more than or equal to the SGV the outlook is good. The LOD of the PXRF is above the SGV levels for Commercial/Industrial use for all of the metals tested and for all but one of the SGV levels for residential use without plant uptake. However, the LOD of the instrument is not low enough to detect most of the elements tested at the SGV for residential use where there is plant uptake and allotment use. It must be decided whether these deficiencies would limit the commercial use of the instrument. Indeed, the British Standards Institute document BS 10175:2001 [4], raises particular concerns about the detection limit of field XRF devices for some metals, specifically cadmium.

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13.2 SAMPLE PREPARATION: SOIL SAMPLES

Samples were prepared for analysis first by drying them to constant weight in an oven at 40 oC. The soil was subsequently ground and sieved to be less than 125 µm. The resultant powder was sealed in a sample cup, consisting of five components –a sealing ring, a main tube, cotton wool bung, a base and a cellophane cover– which all fit together to house the sample (see Figure 13) and analysed on a specially designed platform (available from Niton) (see Figure 14)

Figure 13 The four sample cup components and a completed sample cup containing a soil sample within it (right) Figure 14 The Niton PXRF device sitting on the soil test platform. The sample cup sits at the head of the platform, positioned directly below the window of the instrument’s source 13.3 DATA QUALITY

The quality of the data produced by the PXRF was tested by comparing results from the PXRF with that obtained by ICP-AES analysis of the same samples. The results are displayed in Figure 15.

10

y = 1.0994x - 212.36R2 = 0.9999

0

5000

10000

15000

20000

25000

0 5000 10000 15000 20000 25000

ppm determined by ICP-AES

ppm

det

erm

ined

by

PX

RF

ppm Pb1 to 1Linear (ppm Pb)

Figure 15 Comparison of concentration of Pb in three soil samples as determined by both ICP-AES and PXRF. The data is fitted to a linear trend line, the equation for which is displayed on the chart. Comparison of the results reveals that as the concentration of Pb in the soil samples increased the results deviate further from the ‘ideal’ 1:1 relationship. The ICP-AES is calibrated with standard solutions. The samples were completely digested as opposed to leached. The resultant solution is diluted in order to reduce the effect of the matrix and inter element interference. Therefore, we must assume for the purpose of this work that the deviation from parity is owing to deficiencies in the PXRF device. However, the deviation is appreciably small, and it is only at high levels (> 10,000 ppm, (way above the SGV for Pb) that it becomes significant. According to the Niton 700 manual; “Niton XRF’s are calibrated to give accurate values…in concentrations of 10,000 ppm or less”. At levels above 10,000 the linearity of the Compton Normalisation method of internal calibration is lost. The manual states that “beyond 20,000 ppm…readings may exhibit even greater deviation” –above 2 %. All SGV fall below 10,000 pm. Therefore, in terms of providing a service for contaminated land assessment, <10,000 ppm is also the region where accurate readings are essential and can be provided by the PXRF device. 13.4 PARTICLE SIZE EFFECT

13.4.1 Aims and methods

The aim of this part of the work was to investigate the effect of soil particle size on Pb measurements taken using the Niton PXRF. Other research has also established a concern with these matters.[11] The author demonstrated that sieving to <0.5 mm had the effect of reducing the apparent contamination in soil in comparison to that observed for soil fractions of >2mm and 2 to 0.5 mm and that sieving improved the reproducibility of the readings. Two soil samples, each having 4,600 ppm and 4,800 ppm of Pb contamination were sieved to give 2 mm, 250 µm and 125 µm fractions. Each fraction was analysed separately and the results are displayed in Table 6. Unpredictable changes in the apparent Pb concentration were observed. This, perhaps, reveals a need for a standard sieving procedure. What is clear is that sieving to smaller sizes reduces the SD of multiple readings, this is an indication of the improved reproducibility of the readings. Following analysis of the sieved samples the samples

11

were each ground up and reanalysed. This produced an apparent average increase in Pb in the samples of 10.5 %. The greater increases were for the larger fraction (< 2 mm). Those samples that had been ground to < 125 µm each showed no increase in apparent Pb concentration following subsequent grinding. Table 6 Particle size effect upon PXRF measurement of two soil samples

Sample Sieved fraction

Pb (ppm) after

sieving SD

Pb after grinding

(ppm) SD

Increase after

grinding (ppm)

Increase after

grinding (%)

A >2mm 4658 125.1 6192 213.7 1534 32.9 A 250µm - 2mm 4801 98.4 5516 81.3 715 14.9 A 125µm - 250µm 4401 90.8 5089 43.7 688 15.6 A <125µm 4942 59.4 4942 59.4 0 0 B >2mm 5072 320.6 5393 132.7 321 6.3 B 250µm - 2mm 5035 38.2 5756 114.1 721 14.3 B 125µm - 250µm 4978 62.1 4959 115.6 -19.2 -0.4 B <125µm 4354 41.8 4354 41.8 0 0 AVE 494.9 10.5

13.4.2 Particle Size Conclusions

This series of experiments has shown that particle size differences can cause variations in data. Given that particles of soil will always have vacant space separating them, it is necessary to grind a sample until it approximates one that does not. It is recommended therefore, that all samples be sieved to below 125 µm diameter. The BSI code of practice recommends the same.[4, 12] By way of a comparison, the particle size problem is not experienced in ICP analysis of soil because it involves total digestion of a known mass of soil. Unfortunately, PXRF measurement is only capable of giving an apparent Pb concentration in soil because assumes that the concentration of soil in a volume of sample is 100 %. 13.5 MOISTURE CONTENT EFFECT

13.5.1 Introduction, aims and methods

Presently probes are available for soil moisture determination. However, these are prohibitively expensive. Therefore, throughout this work moisture content was determined by gravimetric methods. The soil is simply dried in a warm oven until it is a consistent weight. Soil was dried at a temperature high enough to evaporate water but not to oxidise organic mater in accordance with the methodology detailed in the British Standard code of practice; BS7755-3.1: 1994.[12, 13] The equation below was used for calculating soil moisture content.

12

Gravimetric soil moisture (%) = (Mass of wet soil – Mass of dry soil) * 100 Mass of dry soil The aim of this part of the work was to investigate the effect of soil moisture content on Pb measurements taken using the Niton PXRF. Three contaminated soil samples were selected each having approximately 20,000, 7,000 and 1,500 ppm of Pb respectively. These were dried at 40 oC, sieved and subsequently analysed using the PXRF. Each sample was divided into five 10 g portions and subsequently analysed to ensure homogeneity throughout the portions. Deionised water was subsequently added to each portion in order to obtain moisture contents of 5, 10, 20, 30 and 40 %. 13.5.2 Results and discussion of moisture effect

The effect that moisture content had upon PXRF readings of the soil samples is displayed in Table 7 and in Figure 16. An approximate direct relationship between moisture content and the reduction in the PXRF reading is seen. This relationship is less strong at moisture concentrations above 20 %, the reason for this is unclear. Table 7 Effect of moisture content upon PXRF readings of soil

A 20,377 ppm B 7,273 ppm C 1,649 ppm

Moisture (%) Reduction (%) Reduction (%) Reduction (%) Average (%)

0 0.0 0.0 0.0 0.0

5 7.1 1.7 5.8 4.9

10 12.9 10.4 10.8 11.4

20 22.4 19.3 19.0 20.2

30 29.0 25.7 25.2 26.8

40 35.2 31.9 31.7 32.9

13

0

10

20

30

40

0 10 20 30 40

Moisture content (%)

Red

uctio

n in

Pb

mea

sure

d (%

)20,000 ppm of Pb

7,000 ppm of Pb

1500 ppm of Pb

1:1

Figure 16 Effect of moisture content upon PXRF readings of soil. A 1:1 equivalent line is shown for comparison 13.5.3 Moisture content effect conclusions

In conclusion the concentration of Pb measured reduces in an approximate direct relation to moisture content at moisture content less than ~20 %. This relationship affords prediction of ‘dry’ result from in-situ measurements when the moisture content is known. With this knowledge HSL may be capable of providing an initial assessment of contaminated land sites without removing samples from the ground or with minimal sample preparation. However, HSL would be unable to guarantee the accuracy of the results on the day of measurement without drying the samples. Drying the soil is a task that takes several hours. It is usually carried out overnight. This would make it impossible to report on the same day. 13.6 SAMPLING STRATEGIES

13.6.1 Introduction to sampling strategies

Various strategies are available for measuring contaminated land from simple straight grids to the common herringbone pattern design to the random wander. Each type of design has its advantages and disadvantages. Pattern designs are rigid and usually not flexible, whereas the random patterns can be hit or miss, but are flexible. The depth of sampling can be from 1 cm, 30 cm to 3 m, or more, depending on the use and purpose of the land. By adopting a flexible approach to the choice of technique and by using reliable sampling regimes an effective land

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survey can be implemented. It is proposed that central to this must be the use of a global positioning satellite (GPS) device. The best use of this technology would be to utilise GPS technology for positioning the measurement sites. In this work it was important to assess whether HSL’s GPS equipment was accurate enough for samples to be taken from the same spot at different times. However, given that the BSI standard [4] states that “the use of Global Positioning Systems (GPS) should not preclude the inclusion of permanent marks”, the need for precise GPS readings is not critical. This statement implies that sampling points could be marked with a stake or even paint. A simple program could be designed for running on a portable computer to aid the sampling and reporting. This program could mark the position of the area to be surveyed on a map and produce a standard sampling pattern such as a herringbone pattern with coordinates for each sample point. The principle is easily demonstrated: Figure 17 displays a screenshot of a simple ExcelTM spreadsheet that generates a standard sampling grid from a single starting point co-ordinate. In addition, the size of each grid square can be specified.

Figure 17 ExcelTM spreadsheet to generate standard sampling pattern from a single starting

point co-ordinate 13.6.2 HSL’s GPS device

The Global Positioning System (GPS) used for this work was an Ashtech (Formally Magellan Corporation) Reliance Workabout, SCA-12 Satellite Receiver used in conjunction with an Ashtech BR2 Beacon Receiver and Survey Antenna. The system is mounted in a backpack and controlled from a Psion Workabout hand held computer. The combination of GPS receiver and beacon receiver, which is called a Differential Global Positioning System (DGPS) is capable of

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sub-metre accuracy with post processing of the data gathered. In use in real-time, accuracies of around two metres are possible. The USA has a system of 24 satellites constantly orbiting the earth forming the basis of a geographic positioning and navigation tool. This is called Navstar (the Russians also have a similar system). The reception of any four or more of these satellites allows a GPS receiver to compute it’s own three-dimensional coordinates relative to the World Geodetic System, 1984 (WGS-84). The American system is in two forms each operating at a particular frequency. These are the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). The latter is reserved for use by the US military (and others), whereas the former less accurate service is provided free of charge on a worldwide basis, for civilian use. Basic GPS is capable of extremely high accuracy, of the order of several millimetres. Therefore, to reduce the level of accuracy for the SPS, in the interests of preventing terrorism, the Americans broadcast a Selective Availability (SA) precision degrading signal so that the accuracy of most civilian GPS systems is only about 100 metres. To get round this problem when more precise navigation is an issue, maritime navigation for example, it is possible to use a beacon receiver in addition to the GPS receiver, in other words a DGPS. The Ashtech BR2 Receiver contains an extremely sensitive 300 kHz minimum shift keying engine (MSK) that demodulates differential GPS corrections broadcast by a system of navigational radio beacons maintained by the International Association of Lighthouse Authorities (IALA). In the UK there are 14 such stations, each with a range of between 185 and 370 miles so that all of the UK is covered. 13.6.3 Experimental work with the GPS

A walk through of a transect sampling task was conducted in a field at HSL near the explosion control section. Five points (A-E) were selected along an approximately 100 m long stretch of grass that was known to have been contaminated with strontium carbonate in previous work by the exposure control section. The PXRF was used in thin film mode and readings were taken at each point. Each reading was taken over a period of 60 nomsecs using the cadmium source. The GPS device was used to obtain a map reference at each measurement site. Following each reading a marker was left at the site. This marker was subsequently returned to and a repeat set of readings with the PXRF and the GPS system were taken for comparison. The GPS readings that were taken at each visit to each sample point are plotted in Figure 18 and are displayed in Table 8.

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53.2265

53.2266

53.2267

53.2268

53.2269

53.2270

53.2271

53.2272

1.9141

1.9142

1.9142

1.9143

1.9143

1.9144

1.9144

1.9145

1.9145

1.9146

1.9146

1.9147

deg W

deg

N 1st Visit

2nd Visit

Figure 18 GPS readings that were taken at each visit to each sample point. Values are in

decimal degrees in the form 12.3456 (deg deg . min min sec sec) Table 8 Table combining GPS readings that were taken at each visit to each sample point (A-E)

along a transect and the Strontium PXRF readings that were obtained Visit Site Latitude (deg) Longitude (deg) Positional

uncertainty (m)

PXRF reading of Sr (µg/cm2)

1st visit A 53.22711749 1.91416667 - 1 B 53.2270055 1.9143005 - 2.1 C 53.2268634 1.9144016 - 1.3 D 53.2267514 1.9144563 - 0.7 E 53.2265820 1.9145984 - -

2nd visit A 53.2271339 1.9141913 3.29 1.8 B 53.2270328 1.9142951 3.09 0.9 C 53.2268689 1.9143552 5.20 1.4

D 53.2268115 1.9144754 7.01 LOD E 53.2265437 1.9146120 4.52 0.7

The second visit to the five markers produced appreciably different GPS readings, this throws the accuracy of both readings into question. Calculating the distance between the each pair of GPS reading is a rather complex matter. It involves so-called ‘great circle calculations’. Fortunately a freeware program called ‘Perpendicular distance calculator’ created by Peter Ersts can carry out this calculation rather quickly.[14] The distances between the pairs of GPS readings were calculated using the ‘great circle distance’ function of this program and are displayed in Table 8. The mean distance between each pair of readings is 4.6 m. The length of the transect (point A to E) was found to be 76.5 m, based on the first visit.

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In conclusion, the GPS system that HSL presently owns may not be suitable for sampling points that are a short distance apart (< 20 m) owing to the relatively large positional uncertainty inherent in using the device.

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14 FINAL CONCLUSIONS

The main conclusion from this investigation is that it would be difficult to provide a full land contamination assessment service at HSL using the PXRF as the lead instrument. The wet chemistry skills and knowledge that are required to provide a full land contamination assessment survey for HSE field scientists already exists within HSL. Although there are some requirements that are additional to the actual soil analysis that require a certain amount of knowledge and expertise, this could be developed within HSL. However, in light of the existence of highly efficient, high through put analytical laboratories it is unlikely that HSL could price the service competitively. The LOD of the PXRF is below the SGV levels for Commercial/Industrial use for all of the metals tested and for all but one of the SGV levels for residential use without plant uptake. However, the LOD of the instrument is not low enough to detect most of the elements tested at the SGV for residential use where there is plant uptake and allotment use. It must be decided whether these deficiencies would limit the commercial use of the instrument. Certainly some brown field sites have houses with garden built upon them because of land shortages in built up areas. The PXRF would be unsuitable for assessing this type of land. The concentration of Pb that was measured in soil samples reduced in an approximate direct relation to moisture content. This relationship affords prediction of ‘dry’ result from in-situ measurements when the moisture content is known. Particle size has a less predictable effect. Investigation has suggested that sieving to <125 µm is beneficiary owing to the improvement in the reproducibility of the readings. Sieving to <125 µm is also a requirement according to the BSI standard BS7755 [12]. This work has displayed that the PXRF takes measurements that are comparable in accuracy to that measured with ICP-AES analysis for fully dried and ground samples. A walk through of a transect sampling task was conducted in a field at HSL using HSL’s GPS device. Given that there is an average positional uncertainty in the location of 4.6 metre it was concluded that the GPS system that HSL presently owns is not suitable for sampling at points that are a short distance apart (< 20 m). In conclusion, the PXRF could be used as a qualitative analysis tool in the initial investigation process before a main investigation into the soil is started, this part of an investigation frequently includes some analysis to help support the conceptual model from which the main investigation is run. If a mobile laboratory is to be used as a base to dry and prepare samples for analysis it could be possible to use the PXRF as an analysis tool for a more complete quantitative investigation. However, owing to the length of time that is required to dry out soil samples it would be difficult to report on the same day.

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15 FURTHER WORK It is the opinion of this author that the PXRF has other strengths, expertise and capabilities, other than land contamination, for which there may be an external market for, and which field scientists could make greater use of. Currently the PXRF is in use for only about 30 % of the time. Therefore, basing a service on its use 100 % of the time is probably unwise. However, there is clearly scope for development. The PXRF is very good at the evaluation of surface contamination. It is uniquely suited for looking at metals in the surface of immovable and hard objects such as floors, work surfaces, walls and ceilings. Specifically, the device has found use in HSE factory lead investigations.[15, 16] The device is also able to accurately measure contamination on filters, wipes and suit materials. Currently at HSL such samples are more commonly measured by ICP-AES which is more time consuming and costly. One of the bonuses of the PXRF device is that it displays a measurement very quickly on its screen, therefore it is uniquely able to be used as a demonstrative tool by HSE during factory assessments and education programs. The ability to say, “look there is contamination on this bench” and to prove it in-situ immediately, is valuable.

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16 REFERENCES

1. Chemical testing of soil. 2006, Environment Agency Website. 2. ISO/IEC 17025:2005 General requirements for the competence of testing and

calibration laboratories. 2005: International Standards Organisation. 3. ISO 14000: Guide to Environmental Management Principles, Systems and Supporting

Techniques. 2004: International Standards Organisation. 4. BS 10175:2001 Investigation of potentially contaminated sites - code of practice.

British Standards Institute, ed. EH/4. Vol. BS 10175:2001. 2001, Milton Keynes: British Standards Institute. 85.

5. Environmental Protection Act (Part 11a). 1990. 6. Method 6200: Field Portable X-Ray Fluorescence spectrometry for the determination of

elemental concentrations in soil and sediment. 1998: Environmental Protection Agency (US).

7. Reclaiming our future, in Office of Science and Innovation: Infinite Monkeys. 2006. p. 4-8.

8. Wheeler, J., C. Sams, and P. Baldwin, Portable X-ray Fluorescence Spectrometer (PXRF) to measure Air Filters and Forensic Tape. HSL Internal Report, 1999. IR/ECO/99/06.

9. Wheeler, J. and F.N. Medler, Calibration and Some Performance Characteristics of the PXRF. HSL Internal Report, 1996. IR/A/96/6.

10. Wheeler, J., Operating procedures for the PXRF (Compilation report). 1998, HSL. 11. Laiho, J.V.-P. Development of portable X-ray fluorescence (PXRF)

instruments reliability and quality control procedures. in Geologian tutkimuskeskuksen ja Suomen ympäristökeskuksen tutkimusseminaaris. 2004. Helsinki.

12. Part 3: Chemical methods. Section 3.5 Pre-treatment of samples for physio-chemical analyses, in BS 7755. Soil Quality. 1995, British Standards Institute: Milton Keynes.

13. Part 3. Section 3.1: Determination of dry matter and water content on a mass basis by a gravimetric method, in BS 7755 Soil quality. 1994., British Standard Institution: Milton Keynes.

14. Ersts, P.J., Perpendicular Distance Calculator. 2006, Center for Biodiversity and Conservation at American Museum of Natural History.

15. Rawson, B., Lead Sampling Visit to F Murphy Alloys (July 2004). HSL Internal Report, 2004. IR/ECO/04/10.

16. Rawson, B., Lead Sampling Visit to F Murphy Alloys. HSL Internal Report, 2002. IR/ECO/02/20.