978-1-4244-4605-6/09/$25.00 ©2009 IEEE

Monitoring of Sequential Gasoline-Ethanol Releases using High Frequency Ground

Penetrating Radar

Cameron H. McNaughton, John D. Mosquera, Anthony L. Endres and Juliana G. Freitas Department of Earth and Environmental Sciences

University of Waterloo Waterloo, Ontario, Canada N2L 3G1

alendres@sciborg.uwaterloo.ca (A. L. Endres)

Abstract—Our hydrogeophysical field experiment evaluated the ability of high frequency (450 & 900 MHz) ground penetrating radar (GPR) to monitor sequential releases of gasoline followed by ethanol. The initial GPR images of the gasoline release zone reveal the development of shallow (i.e., above 10 ns) high reflectivity events and a significant velocity pull-up of an underlying stratigraphic reflection. Temporal evolution of the impacted zone reflectivity and the velocity pull-up indicate a progressive redistribution of gasoline in the impacted zone. GPR profiles obtained during frozen soil conditions clearly show that the presence of gasoline affects the freezing process. Preliminary analysis of the sequential ethanol release GPR data indicates an significant expansion of the impacted zone following this release.

Keywords-GPR, ethanol, gasoline, immiscible contaminants, LNAPL

I. INTRODUCTION Contamination by immiscible organic liquids is widely

recognized as a serious groundwater quality issue. These liquids are divided into two categories on the basis of their densities. Liquids with specific gravities greater than water, such as chlorinated solvents, are referred to as DNAPLs (dense non-aqueous phase liquids). These fluids migrate downwards through the saturated zone, forming immiscible pools at boundaries with subtle contrast in permeability.

Liquids with specific gravities less than water, such as gasoline and jet fuel, are referred to as LNAPLs (light non-aqueous phase liquids). These immiscible contaminants are largely confined to above the water table due to their buoyancy. These liquids displace water in tension saturation zone (i.e., capillary fringe), reducing the fringe thickness, and may depress the water table depending on LNAPL mass. After their initial emplacement, fluctuations in the water table vertically redistribute the immiscible contaminants and increase the thickness of the LNAPL impacted zone. The increasing use of ethanol as a fuel component raises a number of issues regarding gasoline contaminated sites. In particular, the introduction of ethanol into an existing gasoline impacted zone is expected to increase the mobility of the LNAPL phase, causing its redistribution.

The large contrast in relative dielectric permittivity between water ( ~ 80wκ ) and immiscible organic liquids ( 2 4oκ = − ) makes ground penetrating radar (GPR) an effective geophysical

technique for the detection and monitoring of these contaminants. While previous studies (e.g., [1]-[5]) have demonstrated the ability of GPR to characterize controlled LNAPL releases in repacked test cells, only accidental LNAPL releases have been investigated in natural aquifers (e.g. [6]-[9]). Further, there has been no geophysical monitoring of the effects of ethanol releases on a gasoline contamination site. In this paper, we present data from a recent field experiment using GPR to monitor sequential releases of gasoline followed by ethanol into a natural aquifer and subjected to a fluctuating water table, as well as winter freeze-thaw conditions. Our preliminary results clearly demonstrate the ability of high frequency (450 and 900 MHz) GPR to image the temporal evolution of these releases in the shallow subsurface.

II. EXPERIMENT DESCRIPTION The experiment site was located in the unconfined aquifer

at Canadian Forces Base Borden which is situated approximately 90 km northwest of Toronto, Ontario, Canada. The Borden test site is consists of fine to medium sand aquifer underlined by a clayey aquitard. The water table elevation at this test site location seasonally fluctuates from above the ground surface in the early spring to a maximum 2 m depth in late summer-early fall. Overlying the water table is a 30-40 cm thick capillary fringe and a transition zone of similar thickness.

Our experiment was conducted in a 7.35 m width open end cell that was hydraulically connected to the surrounding aquifer; this design permitted naturally occurring horizontal flow through the cell. Both the gasoline and ethanol were released in a 1.5 m (width) × 0.8 m (length) × 0.2 m (depth) trench excavated along the center line of the cell. After the gasoline release, the trench was repacked with the original material. Field testing found that this repacking had a very minor impact on the GPR imaging of the subsurface. After the ethanol release, the trench was repacked with clean fill to avoid the presence of a gasoline phase overlying the ethanol release zone. While this procedure had some limited impact on the GPR monitoring, subsurface changes due to the ethanol release were clearly observed.

A mixture of 180 liters of gasoline and 20 liters of ethanol was released on August 21, 2008 and monitored using GPR at progressively increasing time intervals over the next year. The subsequent release of 175 liters of ethanol and 9.2 liters of gasoline occurred on September 2, 2009 and was again

monitored at progressively increasing time intervals. The GPR data was acquired using a Sensor & Software pulseEKKO 1000 unit with 450 and 900 MHz antennae. For the gasoline release, constant offset reflection profiling was done along six survey lines; to improve characterization of the initial gasoline impacted zone, two additional survey lines were acquired to monitor the effects of the ethanol release. A number of common midpoint (CMP) surveys were also performed on each monitoring data to obtain electromagnetic wave velocity profiles. In addition, the 900 MHz CMP surveys were acquired using two antenna polarizations (TE and TM) to investigate difference in reflectivity of the impacted zone. This presentation will focus on the preliminary results of the profile imaging.

III. TEMPORAL CHANGES IN GPR RESPONSE A time series of 900 MHz GPR reflection profiles acquired

along Line 3 for the initial gasoline release are given in Figure 1. The comparison of the pre-release background data (Figure 1a) with the post-release profile obtained the day of the release (Figures 1b) clearly shows the development of shallow (i.e., above 10 ns) high reflectivity in the vicinity of the trench (i.e., 2.80-4.40 meters). The profiling on Day 20 (Figure 1c) coincided with the seasonal water table low stand. At this point, the region of high reflectivity has undergone some additional lateral expansion (i.e., 2.60-4.95 meters); this represents the widest lateral extent of the observed high reflectivity zone. Throughout the remainder of the gasoline monitoring, the GPR profiling observed a long-term dimming of reflectivity in the gasoline impacted zone at its periphery. This trend is clearly seen in a profile acquired the following summer (Day 369) shown in Figure 1d.

Figure 1 900 MHz GPR profiles along Line 3 showing the initial gasoline release.

An analogous time series of 450 MHz GPR reflection profiles are shown in Figure 2. Due to the shallow depth of the gasoline impacted zone, it is not as well imaged using this antenna frequency due to its superposition with the direct air-ground wave arrival. A more diagnostic attribute of the gasoline impacted zone for this frequency is the velocity “pull-up” due to the displacement of low velocity water by high velocity gasoline. The velocity “pull-up” is clearly seen by comparing the pre-release profile (Figure 2a) with the post-release profile obtained on Day 26 after the release (Figures 2b). In particular, a continuous stratigraphic reflection can be seen between 45-50 ns on the pre-release profile; this event is distorted by the velocity “pull-up” between 2.70-5.00 meters on the post-release profile. The magnitude of the “pull-up” was qualitatively observed to gradually decrease over the next year (Figures 2c-d).

We have quantified the velocity “pull-up” by determining the difference in arrival times of the direct air wave and the

stratigraphic event for each profiling date; Line 3 results for selected dates are shown in Figure 3. To compensate for variations due to water table variations, we have used the differencing results from Line 4 that were unaffected by the gasoline release to adjust the differences obtained along Line 3 for varying near-surface soil moisture conditions due to the vertical movement of the water table. The maximum pull-up was observed on Day 20 (Sept. 11, 2008) during the water table low stand. Prior to Day 26 (Sept. 17, 2008), a rapid water table rise resulted in gasoline reaching the surface; the decrease in the pull-up magnitude suggests that considerable redistribution of the gasoline occurred in the subsurface during this event. The progressive reduction in the pull-up magnitude during the remainder of the observation period indicates continued redistribution of the immiscible gasoline phase.

Figure 2 450 MHz GPR profiles along Line 3 showing the initial gasoline release.

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Figure 3 Traveltime variation relative to pre-spill conditions for the stratigraphic event along Line 3

Figure 4 900 MHz GPR profiles along Line 3 showing frozen soil conditions after the initial gasoline release.

Because of the shallow depth of the gasoline impacted zone, we had the opportunity to observe the effects of freezing on its GPR signature during the winter period. Figures 4a and 4b display the 900 MHz profiling along Line 3 on Day 153 (January 22, 2009) and Day 187 (February 25, 2009); the base of the frozen soil is clearly imaged on both days as a high amplitude reflection event at 5-10 ns. It can be seen that the presence of the gasoline impacted zone affects the depth of freezing, causing a depression of the frozen soil base. Further, the dimming of the direct air-ground wave complex indicates that the contaminant phase brought to the surface by the water table fluctuations have impacted the nature of the near-surface freezing.

Figures 5a-d show a time series of 900 MHz GPR reflection profiles acquired along Line 3 for the subsequent ethanol release. Initially after the release, high reflectivity events redevelop beneath the trench (Figure 5b). As time progresses, these events diminish in amplitude while the development of high reflectivity event are observed adjacent to the trench (Figure 5c). These events expanded laterally until they reached their maximum extent (i.e., 1.90-5.55 meters) on October 7 (Figure 5d), indicating a significant expansion of the impacted zone after the introduction of the ethanol. This lateral expansion also occurred on the other survey lines where the initial gasoline release was imaged by GPR profiling. Further, the effects of ethanol release were observed in the GPR profiling along Line A; the initial gasoline release was not detected along this survey line.

Figure 5 900 MHz GPR profiles along Line 3 showing the subsequent ethanol release.

IV. CONCLUSIONS Our results clearly show the ability of high frequency GPR

to detect and monitor very shallow (i.e., less than 0.5 meter depth) LNAPL impacted zones. Because of the presence of the direct air and ground wave arrivals, the high resolution capacity of the 900 MHz antennas was better for directly image the impacted zone. While the 450 MHz antennas produced less well-defined direct imaging of the impacted zone, its better depth of penetration allowed us to evaluate the effects of gasoline induced velocity changes on the imaging of an underlying stratigraphic reflecting boundary (i.e., a velocity pull-up feature). Quantification of the velocity pull-up over time suggest that there is an ongoing redistribution of the immiscible gasoline phase that is most like due to fluctuating water table.

GPR profiling during the winter period of frozen soil conditions clear show that presence of the gasoline impacted zone affects the depth of freezing. Further, changes in the

direct air-ground wave amplitude suggest that the contaminant phase brought to the surface has affected the near-surface freezing process.

The ethanol release initially caused high reflectivity events to redevelop beneath the trench. Subsequently, the GPR profiling indicted a significant expansion of the impacted zone.

ACKNOWLEDGMENT This work was supported by Collaborative Research &

Development and Individual Discovery Grants to A. L. Endres from the Natural Sciences and Engineering Research Council of Canada (NSERC).

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