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4.15 The influence of redox chemistry on groundwater vulnerability

4.15.1 Background

Reduction and oxidation processes exert an important control on the distribution of dissolved substances under natural conditions in groundwater. They also play a major role in aquifer pollution problems such as nitrate from fertilizers. Indeed, problems in aquifers often concern the addition of an oxidant, like oxygen or nitrate, to an aquifer system containing a reductant, like a sulphide mineral or organic carbon (Appelo & Postma 1999). In assessing the vulnerability of an aquifer to nitrate contamination, it may be quite useful to consider the redox chemistry of the groundwater. The same is true of data on the ability of the subsurface sediments to chemically reduce nitrate. 4.15.2 Sediment chemistry

The thickness and distribution of suitable clay cover layers above an aquifer plays a most important part in protecting the aquifer, as they form a physical barrier which to some degree prevents polluted groundwater from reaching the aquifer. Clay cover layers, and indeed most other Pleistocene and Miocene unconsolidated sediments likely to be encountered above aquifers in Northwest Europe, also provide a varying degree of chemical protection of aquifers. The reason for this is that they, with the exception of coarse-grained or monomineralical deposits such as gravel or quartz sand, contain various amounts of minerals and organic matter – reductants – capable of reacting chemically with nitrate in the groundwater and, in effect, break it down. Minerals such as pyrite (iron sulphide – FeS2) as well as organic matter occur in varying but appreciable amounts in the subsurface. For example, a pyrite content of 0.1–1 % is quite common in many types of Pleistocene and Miocene sediments in north-western Europe.

When groundwater with dissolved nitrate is moving through subsurface sediments containing pyrite, bacterially assisted chemical reactions break down the nitrate and pyrite into free nitrogen + iron + sulphate + water. With organic matter as reductant in stead of pyrite, a similar type of reaction produces carbon dioxide and bicarbonate ions in addition to free nitrogen and water. The nitrate reduction capacity of a given sediment volume can be calculated from chemical analyses of their pyrite, organic carbon and ferrous iron content. When this is combined with data on nitrate-supply via the infiltrating groundwater, the number of years it takes to use up the reduction capacity of one metre of sediment in a given area may then be calculated. The finer grained sediments – the clays – appear to offer the highest nitrate reduction capacity. As water passes very slowly through clay layers, there is ample time for the chemical reactions to use up the available reagents. Some clays have nitrate reduction capacities running into hundreds or even a thousand years per metre the redoxcline is pushed (Ringkjøbing Amt 2006), at a constant agricultural-level nitrate infiltration rate. The redoxcline is the depth where conditions change from oxidising to reducing, see Section 4.15.3. However, the major part of aquifer recharge will happen where a more coarse-grained lithology dominates the subsoil, favouring the infiltration of groundwater to deeper levels. The nitrate reduction capacities of silts and fine- to middle-grained sands appear to be significantly lower than for clays, but even these coarser sediments can have effective reduction capacities up to a hundred or two-hundred years per metre the redoxcline is pushed (Ringkjøbing Amt 2002, 2006). 4.15.3 Water chemistry

Depending on its source and transport through the atmosphere, rainwater will contain a number of ions in solution prior to hitting the surface of the earth. Typical components of rainwater will be chloride and sodium from natural seawater and also anthropogenic nitrates and sulphates

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from industrialised areas, ultimately responsible for the acid rain phenomenon. The chemical composition of groundwater starts out with the rainwater chemistry. Its final composition results from the chemical action the water has been exposed to on its way from the surface to the aquifer. The water interacts with the sediments through which it passes during the infiltration from surface to aquifer. Various elements can be taken up by the water from sources such as organic matter and minerals in the subsurface sediments. The agricultural application of fertilizers and manure can add high concentrations of nitrate to the infiltrating groundwater. This may reach deeper-seated aquifers, if these aquifers are vulnerable towards nitrate, i.e. are open to the introduction of an oxidised environment where nitrate is stable. A redox-based chemical classification system for groundwater is useful for assessing the vulnerability and age of groundwater. The classification set up below is a simplified version of the one used by the Danish EPA (Danish EPA 2000). It is based on the water’s content of redox-sensitive elements like oxygen, nitrate, iron, sulphate and methane. The system has four basic water classes ranging from the most oxidised to the most reduced: A. The Oxygen zone. Recently formed

groundwater, only a few years old at the most. It contains considerable amounts oxygen as well as nitrate and sulphate. Vulnerable in relation to nitrate.

B. The Nitrate zone. Characterised by its nitrate content but has little or no oxygen. The development of this zone is controlled by the presence of nitrate reducing substances like sulphide minerals, iron and organic matter in the subsurface. The depth/limit to which the nitrate-zone water has progressed is called the “nitrate front” or Redoxcline. In the subsurface sediments the redoxcline is usually accompanied by a change in colour from grey and green (reduced) colours to yellow and red (oxidised) colours.

A continuing supply of nitrate-containing groundwater will push the redoxcline along the direction of the groundwater flow, thus making nitrate stable in larger and larger groundwater volumes as the nitrate reduction capacity of the subsurface sediments is used up. Eventually the oxidised chemical conditions may reach the aquifer used for drinking water supply. High sulphate content in the nitrate zone water is a sign of possible nitrate reduction with pyrite in the sediment, as this reaction among other things produces sulphate. The reaction may also lead to an increase in the content of nickel or arsenic in the water, because these unwanted metals can be part of the pyrite mineral structure. As nitrate and pyrite reacts with each other, this breakdown process will release eventual metals to the groundwater. In some areas, including buried valleys, serious problems with high concentrations of nickel or arsenic in the groundwater may arise. Nitrate zone groundwater is vulnerable with respect to nitrate and relatively young, usually less than 50 years.

C. The Iron-sulphate zone. Moderately reduced conditions with little or no nitrate, oxygen and methane. High content of dissolved iron. The sulphate content may be as high as in the oxygen and nitrate zones. The groundwater is relatively old, typically more than 50 years. As a rule, the groundwater is not vulnerable and pollution is rare. Increasing sulphate contents accompanied by a decrease of the bicarbonate ( 3HCO ) ion in the water indicates pressure on the zone in the form of a sliding redoxcline.

D. The Methane zone. A strongly reduced chemical environment where methane occurs. There is no free oxygen or nitrate. The sulphate content is low, less than 20 mg/l.

“Brown water” containing dissolved organic material or elevated chloride concentrations may occur. The water is not very vulnerable, and the risk of pollution is low. More than fifty and up to hundreds of years old or more.

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Fig. 4.15.1: Depth of redoxcline, example of Tarm, Ringkjøbing County.

Common to the zones B, C and D, an inappropriately large water extraction can “pull” an overlying zone or elements of this downwards and initiate a slide in the water chemistry to a more oxidised and thereby more vulnerable water type. Even in the short term this can cause problems with nitrate and perhaps nickel or pesticide metabolites like BAM in an aquifer. This process may be accelerated if the aquifer in question is situated near-surface or in a sand-dominated buried valley, without suitable clay cover layers or any significant nitrate reduction capacity in the subsurface. Here, the redoxcline will likely be pulled down in a matter of years, causing a waterwork extracting groundwater from this particular aquifer to experience nitrate and perhaps nickel problems.

Figure 4.15.1 shows a typical example. It is a contoured map of the depth to the redoxcline in an area near the small town of Tarm, Ringkjøbing County, Denmark. The map is based on sediment colour-changes recorded in well descriptions. A complex buried valley system runs through the area. The waterworks of the nearby town has its wells situated within this valley system, the infill of which is strongly sand-dominated in this area. Outside the limits of the buried valley system Miocene clays provide excellent cover layers, and they, as well as the sands of similar age found here, contain appreciable amounts of pyrite and organic material. These Miocene sediments are found very close to the surface, and here the redoxcline lies only a short distance below the topsoil.

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Inside the valley the redoxcline is clearly pulled down around the waterwork well field, which contains several wells extracting about 800,000 cubic metres of groundwater per year. Indeed, nitrate and the pesticide metabolite BAM is found in wells at depths up to 100 m below surface. Another pulldown in the redox-cline surface is seen to the southeast, situated across the border of the buried valley system. This is due to pumping at a large landfill site situated here. 4.15.4 Conclusion

In addition to the physical protection of aquifers accorded by clay layers, the level of chemical protection may also be important. Groundwater and sediment chemistry investigations can be used to assess susceptibility of aquifers to nitrate pollution, and form an important part of the vulnerability mapping process. 4.15.5 References

Appelo CAJ, Postma D (1999): Geochemistry, groundwater and pollution: 239–290. Balkema, Rotterdam.

Danish EPA (2000): Groundwater zoning. Guide nr. 3, 2000. Danish EPA , Ministry of Environment, Copenhagen. In Danish.

Ringkjøbing County (2002): Hydrogeological mapping at Brande. Ringkjøbing County, Department of Environment & Infrastructure. In Danish.

Ringkjøbing Amt (2006): Action planning in Ringkjøbing County: Hydrogeological mapping in Skjern and Egvad municipalities, September 2006. Ringkjøbing County Department of Environment & Infrastructure. In Danish.