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a Earth Sciences Division, E.O. Lawrence Berkeley National Lab., One Cyclotron Rd., Berkeley, CA 94720
b Pacific Northwest National Lab., P.O. Box 999, Richland, WA 99352
* Corresponding author (MSConrad{at}lbl.gov).
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Received 12 January 2007.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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Abbreviations: CIG, Center for Isotope Geochemistry • MFD, mole fraction deuterium • VSMOW, Vienna Standard Mean Ocean Water
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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Where the fluid source is localized in time or space, such as an accidental spill or leak from a holding tank or pipe, transient flow fields may occur that differ significantly from normal background conditions. This is of special concern when the leak fluids contain chemical contamination that poses a potential threat to groundwater resources. Tracking fluid and chemical movement under these conditions is a complicated issue that generally requires the use of several complementary monitoring techniques to fully understand what is happening. Geophysical techniques including neutron moderation, electrical resistance, and crosswell radar tomography can accurately detect changes in moisture content in the vadose zone (Hubbard et al., 1997; Binley et al., 2002; Alumbaugh et al., 2003). Tracers can be used to examine transport of chemicals, but processes such as sorption and retardation may make it unclear how they interact with the matrix under unsaturated conditions. Natural variations in the stable hydrogen (
D) and oxygen (18O) isotopic compositions of water can also be used to track water in the vadose zone. These isotope tracers are water with the same properties as those of natural water and are not affected by the chemical and physical processes that affect other tracers, making them ideal tracers for water. Where natural differences in the isotopic compositions of the different fluids are not significant, it is possible to use water labeled with D2O and/or H218O (Swensen, 1997; Anderson et al., 1997).
In this paper, we present data from a field experiment at the Hanford Site in Washington State designed to simulate the movement of water and chemicals released from a point source (e.g., a leaking tank). The primary purpose of this test was to identify the principal mechanisms controlling vadose zone transport processes at the site (Ward and Gee, 2000). D2O was added to the third of five aliquots of water released into the subsurface to track the migration of the water and examine its interaction with the other four aliquots of leak water and the ambient pore water. In addition to D2O, the tracer aliquot also contained bromide and other stable isotopic tracers (e.g., 87Sr) to examine how chemical transport in the vadose zone is affected by interaction with the sediments.
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Materials and Methods |
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During flood events, the water would collect in the Pasco Basin and slowly drain out through the Wallula Gap (Allison, 1933). This led to three basic types of sedimentary deposits: (i) gravel-rich units formed in high-energy channels, (ii) thick sequences of coarse- to fine-grained sands deposited adjacent to the main channels, and (iii) rhythmic graded beds consisting primarily of silt with minor sand and ranging from 0.1 to 1 m thick formed in slack-water areas (Baker et al., 1991). These different units have significantly different hydrologic properties that strongly influence the transport of contaminants, especially in the vadose zone.
In May and June 2000, a mock tank leak test was performed at the 299-E24-111 experimental test well site (also know as the Sisson and Lu site) in the 200 East Area (Fig. 1). This time period was chosen because precipitation in the area is generally minimal during the late spring and summer, and what rain falls at this time of year is rapidly evapotranspired and lost to the atmosphere. The Sisson and Lu site was the location of a series of earlier tracer tests conducted during the early 1980s using nitrate, chloride, barium, rubidium, calcium, and the short-lived radionuclides 134Cs and 85Sr (Sisson and Lu, 1984). Figure 2 is a schematic map of the site showing the locations of a set of steel-cased wells that were installed for those earlier studies. Also shown in Fig. 2 are the locations of the injection well, the sampling (S) boreholes drilled after the injection test, and wells installed for crosswell radar and seismic studies for this test.
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3800 L (1000 gallons) each of water were leaked into the subsurface at the Sisson and Lu site at weekly intervals, following the general protocol used for the earlier sets of tracers studies done at the site during the 1980s (Sisson and Lu, 1984). The water was gravity-fed into the sediments at a depth of 4.5 m through a 15-cm inner-diameter cased borehole (taking approximately 8 h to infiltrate into the sediments). The third aliquot of water contained 2 kg D2O, 3 g NaH13CO3, 25 mg 87Sr, 1 mg 145Nd, 1 mg 179Hf, 3 mg 207Pb, and 1000 mg L–1 bromide. Transport of the tracers with time was monitored by analyzing the isotopic compositions of the tracers and the bromide concentrations in pore water extracted from core samples from the S boreholes. The geology of the Sisson and Lu site consists of coarse- to fine-grained sands and silts from the upper part of the Hanford formation. Last and Caldwell (2001) and Last et al. (2001) identified six distinctive units in the upper 17 m of the sediments (the sequence where the tracer tests were conducted) that could be correlated between boreholes. The stratigraphic column in Fig. 3 summarizes this classification scheme. The key features of the sediments are the two strongly layered units at depths of 6 to 7 m (Unit C) and 10 to 12 m (Unit E).
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2% w/w water). In the two layered units (C and E), the moisture contents of the sediments are higher (to >8% w/w water), especially in the bottom meter of the lower layered unit. No measurements of the hydrogen and oxygen isotope ratios of the pore water in the pretest borehole were done. However, the isotopic compositions of pore water samples from boreholes from several uncontaminated areas in the 200 Areas have been analyzed (DePaolo et al., 2004). In most cases, the oxygen isotopic compositions of the pore waters have been shifted to higher values relative to mean precipitation due to evaporation during infiltration. The magnitude of the isotopic shift varies but is generally 2 to 4
for oxygen isotopes. The 18O values of pore water from the Sisson and Lu site measured on core samples that were not affected by the leak fluids were similar to those from other vadose zone cores (18O = –14 to –16), suggesting that the background 18O values at the site were probably in the same range as other vadose zone pore waters at Hanford.
Core Sampling
The S-2 and S-3 cores were drilled with a 25-cm (10-in) outer-diameter hollow-stem auger. Core samples were collected by removing the bit on the auger and driving a 7.6-cm (3-in) inner-diameter, 0.6-m (2-ft) long split spoon sampler into the undisturbed sediments beneath the auger. The split spoon sampler included four 15-cm (6-in) polycarbonate plastic liners. After the sampler was retrieved, the liners were removed from the sampler and capped at both ends. The auger bit was replaced and advanced to the next interval to be sampled.
The later tracer cores (S-5, S-7, and S-8) were drilled using a cone penetrometer and a wireline sampling tool (Last et al., 2001). For each borehole, the cone penetrometer was initially pushed to 4.5-m depth. From that depth on, the cone was removed and replaced with the sampling unit (2.5-cm diameter and 30-cm length). The cone penetrometer with the sampling unit was then pushed 30 cm at a time and retrieved. The sample was removed from the sampler and immediately placed in a sealed plastic bag, placed on ice, and transported to the laboratory for subsampling.
The core samples were kept sealed and refrigerated until they could be subsampled. For the samples collected in polycarbonate sleeves, the sleeves and caps were cut longitudinally and the core split down the middle. To minimize evaporation, samples for measurements of the isotopic compositions of the pore waters were collected from the central part of the sampled interval immediately after the core was split open. For the samples collected with the cone penetrometer, the bags were opened and immediately sampled. From both sets of samples, approximately 200 g of material for isotopic analyses was placed into a wide-mouthed, plastic sample bottle and sealed.
Analytical Methods
The pore water in the samples collected for isotope analyses was vacuum-distilled from the samples at 100°C at the Center for Isotope Geochemistry (CIG) at the E.O. Lawrence Berkeley National Laboratory. Water contents were determined by weighing the samples before and after the water was extracted and dividing the difference by the weight of the dry sample. The water contents for the samples used for bromide analyses were determined at the Pacific Northwest National Laboratory. For those samples, approximately 100 g of the wet sample was placed in a pan of known weight and dried in an oven at 105°C for 24 h (Last and Caldwell, 2001; Last et al., 2001).
The water contents determined by the vacuum-distillation method were in very good agreement with those determined for the oven-dried samples. Where the water contents of splits of the same sample were measured by both methods, the results were generally within ± 10% of each other. Most of this variability is believed to be the result of the heterogeneity of the samples. The average water contents measured by both methods for samples from S-5 and S-7 (the two cores where splits of the same samples were analyzed) were within 0.1% of each other. This is critical for the isotope analyses, as it has been shown that yields of less than 98% of the total water in a sample can lead to significant shifts in the isotopic composition of the water (Araguás-Araguás et al., 1995).
The stable isotope compositions of the water samples were analyzed at the CIG. The hydrogen isotope ratios (D) of the waters were analyzed using the method of Vennemann and O'Neil (1993). Three-microliter water samples were injected into evacuated borosilicate glass tubes containing approximately 50 mg of zinc metal. The water was reduced to H2 gas by baking the tubes for 20 min at 500°C. D values of the H2 gas were analyzed using the VG Instruments (Manchester, England) Prism Series II isotope ratio mass spectrometer (Prism) at CIG. The oxygen isotope ratios (18O) of the samples were analyzed using a VG Instruments (Manchester, England) Isoprep automated CO2–H2O equilibration system interfaced to the Prism. The isotope ratios are expressed as per mil deviations from the internationally accepted standard Vienna Standard Mean Ocean Water (VSMOW). For hydrogen, duplicate analyses of the D values of the waters were generally within ± 2. For oxygen, the precision of the measurements was ± 0.1.
Strontium isotope compositions (87Sr/86Sr) were measured for deionized water leaches of the dried sediment remaining after the pore water was removed by vacuum distillation method. Thirty grams of dried sediment were combined with 30 mL of 18 M
deionized water, shaken for 90 min, and allowed to stand for 24 h. The strontium contained in the rinse water is assumed to be that which was originally dissolved in the pore fluid, plus any readily exchangeable strontium on the solid phases. The dissolved strontium was extracted from the rinse water using Sr-Spec resin (Eichrom Industries, Darien, IL) in 0.250-mL columns constructed from shrink-fit Teflon tubing (Zeus Industrial Products, Orangeburg, SC). Yields for the chemical separations were about 98%. Total procedural blanks were approximately 5-ng Sr, corresponding to <0.2% of the total amount of Sr extracted. Given the small blank contribution relative to the total amount of Sr analyzed, no blank corrections were applied. Sr isotope ratios were measured on a VG354 multi-collector thermal ionization mass spectrometer in the CIG laboratories on the University of California–Berkeley campus. The average 87Sr/86Sr value for NBS 987 during the analyses was 0.710286 ± 0.00002 (2
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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D values for the samples into D/H ratios using the following formula derived from the definition of delta values:
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D value of –126, the average hydrogen isotope composition we have measured for pore water samples extracted from fine-grained units in the Hanford formation) and (MFD)tracer = 0.0005744 (from an average D value of 2680 determined from measurements of four samples of the tracer solution). Given the uncertainties in the data collection and the initial isotopic compositions of the pore water and the tracer solution, the maximum error in the C/C0 values in Table 1 is estimated to be <0.01.
Boreholes S-2 and S-3
Sampling boreholes S-2 and S-3 were both drilled approximately 2 m downgradient from the injection well, 21 and 25 d, respectively, after the tracer solution was leaked into the subsurface (8 and 12 d after the final aliquot of water was released). The water contents, C/Co for deuterium and bromide in the pore water, and the 18O compositions of the pore water are all plotted versus depth in Fig. 4
. The bromide concentrations and the moisture contents for the bromide samples are from Last and Caldwell (2001). Also shown in Fig. 4 are the approximate depths for the different units identified in Fig. 3.
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The C/C0 values measured for deuterium and bromide are plotted together in the central panels of Fig. 4. High concentrations of both tracers were detected at 9- to 11-m depth in both cores, corresponding to the lower part of Unit D and the upper part of Unit E. There were also smaller peaks in the upper part of Unit C (6-m depth) in both cores. In addition, there was a single sample with a high concentration of deuterium (C/C0 = 0.31) at 13-m depth in borehole S-2. There was a slight enrichment in bromide (3%) in a sample collected just below this sample, but nothing nearly as high, suggesting that this represents an isolated fluid pathway.
The deuterium and bromide data clearly indicate that most of the tracer solution was spreading out laterally between the two layered units (Units C and E). Peak tracer concentrations above the lower layered unit (E) reached greater than 50% of the initial concentration in the tracer solution, with C/C0 values for deuterium generally higher than for bromide. Given that the water content approximately doubled between the two layers, this implies that all of the added water at this depth was from the tracer aliquot. This is notable because the tracer solution represented only 20% of the total water released during the experiment. Furthermore, there were low concentrations of both tracers in the zone between 7- and 9-m depth where increased moisture contents were also observed.
The leak fluids also shifted the 18O values of the pore waters. Although the 18O values of the leak fluids were not intentionally altered, there was a large enough difference between the leak water (Columbia River water) and the pore waters (precipitation shifted by evaporation during infiltration) to detect the leak fluids. Based on the 18O values of pore water samples from similar units in other cores and the 18O values of the pore water from unaffected intervals of these cores (e.g., from Unit B, the lower part of Unit E, and Unit F), the background 18O values of the pore water are estimated to be between –14.5 and –15.5, whereas the leak waters were approximately –17.5. Although this signal is not nearly as pronounced as the deuterium and bromide signals, it is significant enough to identify the presence of the leak fluids in the subsurface and allows us to compare the distribution of the tracers, which were only added to the third leak aliquot, to the distribution of the total amount of leaked water.
The 18O values of the pore water samples from both boreholes are plotted on the right-hand side of Fig. 4. Also shown is an outline of the deuterium data for the same samples. In general the results were similar, but there were some notable differences. Based on the 18O data, all of the pore water between 7- and 11-m depth contained high proportions of the leak water (up to 70%), including areas that contained only minor enrichment of deuterium or bromide. This indicates that there was limited mixing between the different leak aliquots in the subsurface. The water from the tracer aliquot spread laterally mainly near the boundary between stratigraphic units D and E, whereas the water from all of the leaked aliquots is more evenly distributed within units C, D, and E.
The 87Sr/86Sr values measured for pore water samples from S-2 and S-3 were between 0.7139 and 0.7153 (except for one sample from the bottom S-2 that had a value of 0.7118). One sample from Unit D in borehole S-1 had a 87Sr/86Sr of 0.7140, which matches well with strontium isotope ratios reported by Maher et al. (2003) at similar depths in the Hanford formation. The 87Sr/86Sr of the tracer solution was measured at 1.300. For Sr concentrations of 1.6 mg L–1 in the pore water and 100 µg L–1 in the tracer solution and a C/Co of 0.5, the 87Sr/86Sr ratio of the pore water in the peak deuterium samples in S-2 and S-3 should be about 0.753 if strontium was acting conservatively. However, the 87Sr/86Sr ratios in both cores were very close to background, indicating strong retardation of strontium in the sediments.
Borehole S-5
A third borehole, S-5, was drilled at about 2 m from the injection hole on 11 Sept. 2000 (88 d after the tracer solution was released). The data collected for S-5 are plotted in Fig. 5
. The average water content between 7- and 11-m depth in the core dropped to 4.1% w/w versus greater than 5% w/w in the S-2 and S-3 cores but was still significantly higher than before the test (based on changes in gravimetric water contents determined from bulk density estimates and volumetric water contents determined from neutron probe logging). This decrease is greater in the upper part of this zone (corresponding to the upper part of Unit D) than in the lower part of the zone (corresponding to the lower part of Unit D and the upper part of Unit E).
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The 18O data for the S-5 pore waters are plotted on the right-hand side of Fig. 5. As with the S-2 and S-3 cores, the 18O values of the pore water within the high water content zone at the base of Unit D have been affected by the leak waters. In addition, the 18O values of the pore water in the upper part of the core (above the 7-m high water content zone) are also shifted. There is one deep sample (at 16.6 m) with a shifted 18O value and relatively high water content (4.5% w/w). This sample suggests that water from some of the unlabeled leak aliquots may have penetrated to 16-m depth, but water from the tracer aliquot did not.
The strontium isotope ratios in the samples with elevated deuterium and bromide in this core are slightly enriched relative to the earlier samples (averaging 0.7154). This is much lower than would be expected (>0.73) based on the level of deuterium enrichment, but it could be an indication that there was limited transport of strontium during the infiltration experiments.
Boreholes S-7 and S-8
The final two boreholes that were analyzed, S-7 and S-8, were drilled on 23 Mar. 2001, 281 d after injection of the tracer solution. Borehole S-7 was drilled about 3 m from the injection well, and S-8 was drilled approximately 8 m from the injection well (Fig. 2). The data from the S-7 samples are plotted in Fig. 6
(including the bromide moisture contents and concentrations from Last et al., 2001). The water contents between 7- and 11-m depth average 3.4% w/w, which is elevated relative to the pre-injection well (S-1) but significantly less than the concentrations in S-2, S-3, and S-5.
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Pore water samples were extracted from only three samples from the S-8 core. However, the peak concentration of deuterium was at the same depth (11.4 m) as the peak bromide concentration (for which 20 samples were analyzed by Last et al., 2001). C/C0 for deuterium in that sample was 0.06, or 25% of the peak concentration in S-7. C/C0 for bromide in the same sample was 0.08, higher than the value measured for deuterium. Furthermore, despite the greater distance from the injection point for S-8, the bromide concentration was essentially equal to the relative concentration measured in S-7.
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Discussion |
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Stratified Water Flow in the Unsaturated Zone
The tracer data provide strong evidence that the different aliquots of leak fluids did not mix significantly as they moved laterally through the unsaturated sediments. In all four cores drilled at 2 to 3 m from the injection well (S-2, S-3, S-5, S-7), the moisture contents of the samples from between 6- and 11-m depth increased significantly relative to the moisture contents of the samples from the same interval in S-1, the pre-injection core, due to the addition of water from the leak aliquots. This is confirmed by both the geophysical data and the decrease in the 18O values of the pore water samples. However, the zone of increased bromide and deuterium was more limited in extent, with the most highly enriched samples between 9- and 10-m depth. Furthermore, the peak C/C0 values for deuterium in S-2 and S-3 were 0.53 and 0.62, respectively. Given that the moisture content of these samples was approximately double the pretest concentrations in this interval, this indicates that most, if not all, of the added pore water at this depth was derived from the tracer aliquot and not the other four leak aliquots.
The average and peak C/C0 values for deuterium in all four of the boreholes are plotted versus the fraction of water added to the sediments in Fig. 7
. The fraction of "added water," Fadded water, was determined using the following formula:
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peakD is the average moisture content in the interval containing the significantly elevated deuterium and bromide concentrations in each of the tracer cores, and initial is the average moisture content between 9- and 11-m depth in S-1 (2.46%). The upper diagonal line across the figure has a slope of one and represents the position where a sample would plot if all of the added water in the sample were derived from the tracer aliquot. The lower line has a slope of 0.2 and is where the samples would plot if the added water contained 20% tracer solution (the fraction of the total leak water represented by the tracer aliquot). For all cases, at least 50% of the added water in this interval was derived from the tracer aliquot, especially in S-2 and S-3, the two cores drilled within 2 wk after the final release of water. It is likely that the leak fluids displaced some of the pore water originally in the sediments, meaning that there is a greater fraction of added water. However, even if all of the original pore water was displaced, the tracer concentrations were still much higher than 20% in most cases. Conversely, if a similar analysis is done for samples with significant added water from above this interval, the C/C0 values for deuterium fall well below the slope 0.2 line. There was clearly limited mixing between the different aliquots of fluid released during the experiment, leading to strong vertical stratification of the fluids that persisted until 9 mo after the leak test.
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Differential Transport of Deuterium and Bromide
The observed movements of deuterium and bromide during this experiment were significantly different. In almost every case, in the cores drilled within 2 to 3 m of the injection well, C/C0 for deuterium was higher than C/C0 for bromide. Furthermore, this relative difference increased with time. Figure 8
shows a plot of the ratio of the relative concentrations of deuterium to bromide in the same zones of elevated tracer concentrations used for Fig. 7. In S-2 and S-3, the average deuterium concentrations were approximately 1.5 times the average bromide concentrations. In S-5, that ratio of deuterium to bromide increased to greater than 2, and in S-7, drilled 9 mo after the test, the ratio was greater than 3.5. Conversely, in S-8, which was drilled at the same time as S-7, but almost three times as far from the injection well, the ratio of deuterium to bromide was less than 1.
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If anion exclusion is an important process associated with chemical transport in the vadose zone, it could strongly affect the arrival of different contaminants in the groundwater. 99Tc (as pertechnetate), nitrate, and tritium are widespread contaminants and major components of many of the tank fluids. Anion exclusion could cause them to reach the groundwater sooner and/or in higher concentrations than some of the other less mobile contaminant species (e.g., uranium, 90Sr, 137Cs), acting as a precursor to the arrival of high-level groundwater waste plumes.
Strontium Mobility
There was limited evidence that the 87Sr in the tracer solution reached any of the boreholes sampled. The Sr concentration in the tracer solution was approximately 100 µg L–1 with 87Sr/86Sr of 1.3. For the other aliquots of leak water, the concentration is also 100 µg L–1, but the 87Sr/86Sr is about 0.714. The average pore water Sr concentration in vadose zone sediments of the Hanford formation is about 1600 µg L–1 with 87Sr/86Sr of 0.714 (Maher et al., 2003). With these numbers, we can calculate the expected 87Sr/86Sr for the pore waters after the test for a range of distribution coefficient (Kd) values. Those have been plotted in Fig. 9
versus C/C0 for the tracer solution. Also plotted are the strontium isotope ratios for the pore water samples versus C/C0 for deuterium in the pore water from the same sample. Given the uncertainties in the initial 87Sr/86Sr of the background (pretest) pore water (which probably ranged between 0.714 to 0.715), the lack of a discernible breakthrough for the 87Sr in the tracer aliquot is consistent with Kd values greater than 10. Column studies with Hanford sediments conducted by Um and Serne (2005) obtained Kd values for 90Sr of about 18 mL g–1 for coarse-grained Hanford sands. For finer sands from Hanford formation (such as those in this study area), Kd values are estimated to be 40 to 50 mL g–1 (R.J. Serne, personal communication).
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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The implications of these results for contaminant transport at Hanford are significant. It is possible that contaminated water from a point source (e.g., leaking tank, crib) could travel substantial horizontal distances before infiltrating deeper into the sediments. How far is not clear (certainly on the order of tens of meters) and will depend on the types of sediments and the amount and continuity of any layering. In addition, the chemical composition of the waste fluids could be greatly altered by interaction with the sediments, separating the different chemical components from each other. This makes it difficult to predict when and if the different contaminants will reach the groundwater. The results suggest that for layered Hanford sediments, both hydrologic and geochemical aspects of fluid leaks must be known to predict when, or if, the different contaminants will reach groundwater.
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ACKNOWLEDGMENTS |
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