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SPECIAL SECTION: UNDERSTANDING SUBSURFACE FLOW AND TRANSPORT PROCESSES AT THE IDAHO NATIONAL ENGINEERING & ENVIRONMENTAL LABORATORY (INEEL) SITE

Carbon Isotopic Evidence for Biodegradation of Organic Contaminants in the Shallow Vadose Zone of the Radioactive Waste Management Complex

^a Earth Sciences Division, MS 70A-4418, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b Earth Sciences Division, MS 90R-1116, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^* Corresponding author (MSConrad{at}lbl.gov).

Received 20 May 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Waste material buried in drums in the shallow subsurface at the Radioactive Waste Management Facility (RWMC) of the Idaho National Engineering and Environmental Laboratory (INEEL) contained significant amounts of organic compounds, including lubricating oils and chlorinated solvents. Carbon dioxide concentrations in pore gas samples from monitoring wells in the vicinity of the disposal pits are three to five times higher than the concentrations in nearby background wells. The stable C isotope ratios (¹³C values) of CO₂ from the disposal pits averaged 2.4 less than CO₂ from the background wells, indicating that the elevated CO₂ concentrations around the pits were derived from source materials with ¹³C values in the range of –24 to –29. These ¹³C values are typical of lubricating oils, but higher than most solvents. The radiocarbon (¹⁴C) contents of CO₂ across most of the site were significantly elevated above modern concentrations due to reactor blocks buried in a subsurface vault at the site. However, several samples collected from the high-CO₂ zone on the far side of the RWMC from the reactor blocks had very low ¹⁴C contents (<0.13 times modern), confirming production from lubricating oils manufactured from fossil hydrocarbons. The magnitude of the CO₂ anomaly observed at the site is consistent with intrinsic biodegradation rates on the order of 0.5 to 3.0 t C yr^–1.

Abbreviations: CAMS, Center for Accelerator Mass Spectrometry • INEEL, Idaho National Engineering and Environmental Laboratory • RWMC, Radioactive Waste Management Facility • SDA, Subsurface Disposal Area • TSA, Transuranic Storage Area

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
INTRINSIC BIOREMEDIATION—the use of naturally occurring microorganisms to convert organic contaminants to harmless forms—is gaining widespread acceptance as a viable method for remediating sites contaminated with hydrocarbon compounds. In an extensive study of groundwater sites impacted by leaking underground fuel tanks, Rice et al. (1995) demonstrated that natural processes, especially under aerobic conditions, quickly degrade most petroleum hydrocarbons. Wiedemeier et al. (1999) identified a range of conditions that are favorable for biodegradation of chlorinated solvents (e.g., trichloroethene, carbon tetrachloride). For the most part, however, these studies have been focused on groundwater contamination, leaving the potential for intrinsic biodegradation of organic contaminants in the unsaturated zone poorly understood.

One of the primary challenges for determining the efficacy of intrinsic bioremediation as a remediation option is monitoring its progress in the subsurface. One technique is to try to detect changes in the concentrations of metabolic byproducts of the biologic activity. For aerobic metabolism of organic compounds, the primary product is CO₂. Increased CO₂ concentrations in the vicinity of organic contaminants can indicate that biodegradation of the contaminants is occurring. However, this interpretation can be complicated by other potential sources of CO₂, such as plant root respiration, biodegradation of natural soil organic matter, and dissolution of carbonate minerals. To further constrain the origin of subsurface CO₂, the ¹³C value of the CO₂ can be analyzed. Hydrocarbon compounds are generally relatively depleted in ¹³C (low ¹³C values) relative to most other sources of C (Schoell, 1984). Therefore, microbial metabolism of compounds manufactured from hydrocarbons tends to produce soil gas CO₂ with low ¹³C values where significant degradation of hydrocarbons is occurring. A number of field studies of aerobic hydrocarbon bioremediation have noted this effect above groundwater plumes (Suchomel et al., 1990; Ostendorf and Kampbell, 1991; Aggarwal and Hinchee, 1991; Feng et al., 2000).

However, ¹³C analyses alone may lead to ambiguous results because there can be significant overlap between the stable C isotope ratios of the contaminants and background organic matter. In addition, microbial processes such as methanogenesis and methane oxidation can cause the isotopic compositions of CO₂ produced to be enriched rather than depleted in ¹³C (Revesz et al., 1995; Landmeyer et al., 1996; Conrad et al., 1999). One method of resolving most of the uncertainties associated with stable C isotopic measurements is to measure the ¹⁴C content of the metabolic byproducts (Suchomel et al., 1990; Conrad et al., 1997; Aelion et al., 1997). The ¹⁴C content of natural organic matter in near surface environments is usually at or near modern atmospheric levels, whereas fossil fuels have no measurable ¹⁴C. Further, the variations in ¹⁴C/¹²C ratios are sufficiently large that they are not significantly affected by isotopic fractionation attending microbial processes.

In this paper, we present data on the concentrations, ¹³C, and ¹⁴C values of CO₂ in pore gas samples collected from the vadose zone in and around the RWMC at INEEL. These data clearly indicate that extensive biodegradation of organic contaminants in the shallow vadose zone is occurring at the site.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Background
The RWMC is located in the southwest section of the INEEL on the Snake River Plain in eastern Idaho (inset of Fig. 1) . The geology of the upper 200 m at the RWMC site consists of 10 basalt-flow groups with seven major sedimentary interbed units ranging in age from 100000 to 600000 yr. Each basalt group is made up of one to five separate flows erupted from a common source during periods of volcanic activity lasting <200 yr (Anderson and Lewis, 1989). The sedimentary interbeds consist of river and lake sediments, paleosols, and wind-blown dust deposited during periods of volcanic quiescence. These interbeds are up to 10 m thick, constituting approximately 10% of the total stratigraphic section (Anderson and Lewis, 1989). The unsaturated zone is approximately 180 m thick and contains perched water horizons above some of the interbeds.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Map of the Radioactive Waste Management Facility (RWMC) site showing the location of the wells sampled for this study and the Subsurface Disposal Area (SDA) and Transuranic Storage Area (TSA). Also plotted are the locations of the waste disposal pits, the soil vault containing the Be reactor blocks, the vapor extraction units, and the A–A' longitudinal cross section in Fig. 4. The inset shows the location of the RWMC within the INEEL and the state of Idaho.

View larger version (34K): [in this window] [in a new window]   Fig. 4. Longitudinal cross section A–A' (Fig. 1) passing through the high-CO2 zone in the Radioactive Waste Management Facility, with contours of the labeled average CO2 concentrations measured for pore gas samples collected from different sampling port in wells within 100 m of the section. The approximate extent of the surficial sediments and the sedimentary interbeds are shown as shaded areas on the section.

To remediate the solvent plume emanating from the disposal pits in the SDA, a vapor extraction system was installed at the site and began operation shortly before this project began (Fall of 1996). The system originally consisted of three treatment units, but after continuing problems with one of the units (Unit C on Fig. 1), that unit was shut down. The other two units also had significant periods of down time for repairs and maintenance.

During operation, the vapor extraction units pump pore gas out of extraction wells. The chlorinated solvents are stripped from the pore gas using activated charcoal, and the air is released to the atmosphere. The extent of influence of the system on subsurface processes is poorly understood. The contaminant levels in the extracted vapors drop to very low levels when the extraction units are in operation, but rebound quickly when the units are shut down. One of the secondary goals of this project was to determine the extent of influence of the extraction wells.

Methods
To determine the extent of natural biodegradation of organic contaminants at the RWMC, more than 200 gas samples were collected from 57 sampling ports in 24 different monitoring wells during a 2-yr period (October 1996–November 1998). These samples were taken from a combination of shallow soil gas probes and deep gas sampling ports (to almost 180 m below ground surface) installed in monitoring wells in and around the RWMC. The details of the installation and subsurface characteristics for the deep vapor sampling ports are in a report by Parsons Engineering Sciences (1995) and summarized here. The vapor sampling ports were installed on the outside of the well casing. The sampling intervals are filled with size 4-6 sand. Intervals with only vapor ports are typically about 1.5 m long, but intervals where the well casing are screened for vapor extraction are as long as 10 m. The sand-filled intervals are sealed above and below with bentonite layers ranging from 0.6 to 2.0 m thick. The rest of the casing was grouted in place. The vapor ports were constructed of approximately 9.5-mm (3/8'')-o.d. stainless-steel tubing with approximately 3.2-mm (2 1/8'')-diameter perforations every 7.6 cm. The ports were connected to the surface with 1-cm (3/8'') tubing and are kept sealed when not being used. Several of the vapor sampling ports were sealed and could not be used, and at least one port (9V-2) became sealed during this study.

The samples were collected in 1- or 3-L Tedlar bags from selected vapor ports using a portable diaphragm pump after purging the port for a specified time period (approximately equal to the time required to pump three times the volume of the sample tubing). The concentrations of N₂, O₂, and CO₂ in the samples were analyzed by gas chromatograph. These analyses are accurate to ±0.1%. Samples containing low concentrations of CO₂ (<0.2%) were analyzed using an infrared gas analyzer (Li-Cor, Lincoln, NE) and are accurate to within ±1% of the measured values. Carbon dioxide concentrations measured by both techniques are given in Table 1. The CO₂ concentrations measured for duplicate samples collected during the same sampling trip were within ±10% of each other.

[1]

Variations between the ¹³C values of separately prepared aliquots of the NaOH solution were generally within ±0.2. Duplicate samples collected in the field were within ±0.5. The ¹³C values are given in Table 1 (where duplicate analyses were done, only an average value is reported).

The ¹⁴C content of CO₂ aliquots split from the stable isotope samples were analyzed at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory. Carbon dioxide was converted to graphite following the procedures documented by Lloyd et al. (1991). The ¹⁴C content of the graphite was then analyzed using the CAMS accelerator. The results from these analyses are reported as a fraction of modern (pre-1950) C (values >1 represent samples containing radiocarbon produced during aboveground testing of nuclear weapons). The precision of the analyses is 0.01 times modern C.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Table 1 contains a compilation of the data collected for this study. Carbon dioxide concentrations ranged from 66 µL L^–1 (considerably less than the 380 µL L^–1 concentration in the atmosphere) to 2.1% by volume. The measured ¹³C values were between –24.3 and –9.8, and the ¹⁴C concentrations ranged from below the lower detection limit of the CAMS accelerator (<0.01 times modern) to more than five times modern (the upper detection limit of the CAMS accelerator).

Background Carbon Dioxide
Samples were collected from three sites to determine the background CO₂ signal in the vicinity of the RWMC. Boreholes VVE-6 and M6S are located approximately 400 m southeast of the TSA and more than 1 km from the SDA on a knoll overlooking the site (20 m in elevation above the level of the SDA). Twenty-nine samples from five depths were collected from these wells during the project. The WWW boreholes are located about 300 m west of the SDA and 700 m from the nearest burial pit. Eleven samples from four different depths were collected from this site during the second year of the study. In addition, a single sample was collected from Port 6 in Borehole 77-1 during the early part of the project. This borehole is located about 250 m from the SDA and 500 m from the nearest burial pit.

The CO₂ concentrations and isotopic compositions measured for the samples from the background wells are plotted with the data for samples from the SDA on Fig. 2 . The CO₂ concentrations were variable, but generally decreased from approximately 0.3% in the shallow subsurface (<5 m) to <0.1% below 100 m. This is consistent with higher levels of biologic activity (e.g., root respiration, microbial degradation of organic matter) in the shallow soils. These data establish that background CO₂ concentrations between 10 and 70 m deep are between 0.1 and 0.4% and average about 0.2%.

View larger version (19K): [in this window] [in a new window]   Fig. 2. Carbon dioxide concentrations and 13C vs. versus depth for samples from the Subsurface Disposal Area (SDA) and from background wells. The main depth interval of elevated concentrations is between 10 and 30 m (shown shaded). Slightly elevated values occur down to the 75-m depth. Soils from the TEM1 locality and from farther to the west of that locality have particularly high CO2 values within a few meters of the surface. These values require very high local production rates of CO2 (from root respiration).

The ¹⁴C contents of four samples from the VVE-6/M6S site were measured. Two of the samples had ¹⁴C contents close to or slightly less than atmospheric CO₂ (currently at about 1.12 times modern due to production of ¹⁴C from aboveground nuclear testing). Two of the samples, however, had ¹⁴C contents above atmospheric values. The Be-reflector blocks buried in the southeast corner of the SDA, more than 1 km from the VVE-6 and M6S, are the most likely source of the ¹⁴CO₂ (Fig. 1). Transuranics are also stored in the TSA, but these are all in aboveground containers. This clearly indicates that VVE-6 and M6S have been influenced by the activities at the site. However, concentrations of radiocarbon in soil gas CO₂ from adjacent to the vault containing the Be-reflector blocks are up to 4000 times modern (Ritter and McElroy, 1999). Therefore, the total input of CO₂ from the SDA required to produce the ¹⁴C content measured in VVE-6 is small (0.1%).

Carbon Dioxide in the Subsurface Disposal Area
The CO₂ concentrations measured for pore gas samples from the vicinity of the SDA were generally significantly higher than the CO₂ concentrations in the background wells. The highest CO₂ concentration measured in any of the background samples was 0.4%. Within the SDA, nearly one-half of the samples (74 of 151) contained more than 0.4% CO₂, and a significant number (25) were 1.0% or higher (Fig. 2). There were some very high concentration samples (to >2% CO₂) in the shallow subsurface (<10 m depth). Samples from the same sample locations were highly variable, with a weak seasonal correlation related to the growing season of surface plants. Concentrations tended to be higher during the spring and early summer and lower during the late summer and fall. More significantly, there was a zone of highly elevated CO₂ concentrations between the 15- and 35-m depths. The relationship between these samples and the waste material in the disposal pits is discussed below.

Location of Elevated Carbon Dioxide Zone
The highest CO₂ concentrations in the deeper vadose zone of the SDA were concentrated in the vicinity of the burial pits. Figure 3 contains a series of maps with the average CO₂ concentrations for samples from between depths from 10 to 25, 25 to 40, and 40 to 70 m. There is an area of elevated CO₂ just to the north of Pit 4 and 6, especially at the 10- to 25-m depth. Figure 4 is an east–west longitudinal section passing through the center of the high CO₂ zone (all data from sampling points within 100 m of the section have been projected onto the section). Also shown on this figure are the extent of surficial alluvium and the sedimentary interbeds. Most of the higher CO₂ concentrations (>0.3%) were measured for samples taken from the vicinity of the burial pits, above the B–C interbed (one of the more continuous interbed units in the RWMC subsurface). Within this area, however, there were some distinctive fluctuations in the depth of the CO₂ contours. Most of this variability is probably the result of the vapor extraction units that began operating at the site shortly before the beginning of this study.

View larger version (17K): [in this window] [in a new window]   Fig. 3. Average CO2 concentrations (in volume %) in pore gas samples collected from different depth intervals in and around the Radioactive Waste Management Facility (RWMC) site.

View larger version (18K): [in this window] [in a new window]   Fig. 5. Carbon dioxide concentrations measured over time in three vapor sampling ports in Borehole 8801, showing the effect of vapor extraction from Borehole 8901 (located approximately 20 m from 8801). Dashed lines indicate intervals where a sampling period was missed.

Subambient Carbon Dioxide Concentrations
There were also several samples collected from Ports 3 and 4 in Borehole M10S that had CO₂ concentrations below atmospheric levels (380 µL L^–1). This is most likely caused by exchange with pore water dissolved inorganic C. At higher pHs, the equilibrium concentration of dissolved inorganic C compounds in water shifts from dissolved CO₂ to carbonate and bicarbonate ions, resulting in lower CO₂ pressures in the gas phase. The existence of such low CO₂ concentrations in the samples from M10S suggests that chemical trapping of CO₂ must be occurring in the vicinity of those sampling ports.

Source of Elevated CO₂ in the Subsurface Disposal Area
The concentration data clearly indicate that the elevated CO₂ in the SDA is associated with the disposal trenches. There are a variety of potential sources for the additional CO₂, such as degradation of the lubricating oils and/or chlorinated solvents in the waste, degradation of other organic matter buried with the waste, and differences in plant cover at the surface. The origin of the CO₂ cannot be determined from the concentration data alone; however, the C isotope data help place additional constraints on the source of the CO₂.

The average ¹³C value of CO₂ in samples from between the 10- and 70-m depths in the high-CO₂ zone of the SDA (defined by the area within the 0.3% contour on Fig. 3a) is –20.6. This is 2.4 lower than the average ¹³C value of CO₂ in samples from the same depth range in the background wells (–18.2). This relationship is further demonstrated on Fig. 6 , which is a plot of the inverse CO₂ concentration vs. the ¹³C values of CO₂ in samples from the high-CO₂ zone. Allowing for C isotopic fractionation of 4.4 resulting from diffusive loss of CO₂ to the atmosphere (Cerling, 1984), this indicates that the source of the CO₂ has a ¹³C value between –24 and –29. Carbon dioxide produced from aerobic biodegradation of organic compounds is generally 1 to 3 lower than the substrate, suggesting that the source of the CO₂ in the SDA was between –21 to –28. This is within the range of ¹³C values that would be expected for petroleum hydrocarbons (e.g., the lubricating oils). However, plant respiration and/or microbial degradation of natural organic matter could also produce CO₂ with ¹³C values in this range. Although there is no obvious increase in the amount of natural organic matter in the vicinity of the disposal pits, it is possible that the activities at the site (e.g., wetting down the ground for dust abatement) might have caused enhanced biological activity in the near surface.

View larger version (11K): [in this window] [in a new window]   Fig. 6. Inverse concentrations of CO2 in pore gas samples from between 10 and 70 m depth in the high-CO2 zone in the Subsurface Disposal Area (SDA). Allowing for a shift of 4.4 for diffusive fractionation (Cerling, 1984), this data suggests that the source for the increased CO2 in the vicinity of the disposal pits of has a 13C value of between –24 and –29.

View larger version (19K): [in this window] [in a new window]   Fig. 7. Radiocarbon contents (in fraction of modern atmospheric C) in CO2 samples collected for this study. Values in parentheses indicate the depth at which the sample was collected.

Of the organic contaminants at the site, the lubricating oils represent a more likely source for the CO₂ than the chlorinated solvents. The ¹³C values of the CO₂ are in the appropriate range for oil (–24 to –32; Schoell, 1984) but are on the high side for chlorinated solvents. ¹³C values reported in the literature for the chlorinated solvents range from –33 to –47 for carbon tetrachloride, –23 to –37 for perchloroethene, –28 to –43 for trichloroethene, and –26 to –32 for 1,1,1-trichloroethane (van Warmerdam et al., 1995; Holt et al., 1997; Beneteau et al., 1999; Hunkeler and Aravena, 2000; Jendrzejewski et al., 2001). Furthermore, the stable C isotope ratios of two samples of bulk chlorinated solvents in gas samples collected from the RWMC were trapped onto resin columns and analyzed, giving ¹³C values of –32 and –38. It is possible that there was preferential degradation of higher ¹³C solvents (e.g., perchloroethene), but it is unlikely that a significant amount of the CO₂ in the SDA was derived from breakdown of the solvents.

Estimated Rate of Biodegradation of Oils
The rate of degradation of the lubricating oils translates to a rate of production of CO₂ in the subsurface, which can be estimated roughly from the elevated CO₂ concentrations in the pore gas. We derive this estimate from a simplified model of the CO₂ production and transport in the region under the 0.7% contour shown in Fig. 3a. We assume that CO₂ is being produced in a volume of rock that has the areal dimensions of the 0.7% contour (area 75000 m²) and a vertical dimension corresponding to the depth interval 10 to 30 m below the ground surface. The total volume of producing rock is then 1.5 x 10⁶ m³. The CO₂ produced in this volume of rock must diffuse out through the pore space, and with time the CO₂ concentration may reach a steady-state concentration that reflects the balance between production and loss by diffusion into the surrounding low-CO₂ pore air. If the producing volume of rock is modeled as a one-dimensional system, which is reasonable insofar as the lateral dimensions are much greater than the vertical, then the steady-state maximum CO₂ concentration is given by

[2]

where the brackets represent concentrations of CO₂ in pore air, G_CO2 is the production rate of CO₂ per unit volume of soil, a is the half-thickness of the CO₂ producing layer in the subsurface, d is the depth to the middle of the producing layer, D_CO2 is the diffusivity of CO₂ in air, and and are porosity and tortuosity. Rearranging this equation, we can express the total production of CO₂ as

[3]

This equation applies to a layer of thickness 2a at depth d, with CO₂ concentration held at the ambient value at the top and bottom boundaries, uniform CO₂ production within the layer, and transport only by diffusion through the pore space (porosity, 0.1) in the vertical direction. Substituting the values D_CO2 = 756 m² yr^–1, = 0.1, = 0.5, a = 10 m, d = 20 m, maximum and ambient CO₂ concentrations (m³ m^–3 air) of 0.011 and 0.002, and the estimated area (A_prod = 75000 m²), the resultant value is about 3400 m³ CO₂ yr^–1. This corresponds to the conversion of 1.7 t of C to CO₂ per year. The concentrations predicted by the model are plotted vs. depth with the data on Fig. 8 . These calculations are also consistent with the lower ¹³C values for CO₂ observed within the SDA.

View larger version (31K): [in this window] [in a new window]   Fig. 8. Comparison of model results for a = 10 m and d = 20 m and measured CO2 concentrations vs. depth. This model uses a specific CO2 production rate of 0.0023 m3 CO2 m–3 rock yr–1 in the producing layer between the 10- and 30-m depths (shown shaded) and gives a total CO2 production for a 150 by 500 m area equivalent to the degradation of approximately 1.7 t C yr–1.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The primary goal of this project was to evaluate the levels of intrinsic bioremediation of organic contaminants in the burial pits at the RWMC. The CO₂ concentrations in the subsurface around the burial pits were elevated by three- to fivefold relative to background. The stable C isotope ratios of the CO₂ indicate a source with ¹³C values between –21 and –28, and the low ¹⁴C contents of several samples confirm that the CO₂ is mainly derived from aerobic biodegradation of fossil hydrocarbons, with the primary source being the lubricating oils and not the chlorinated solvents. The high O₂ content of the pore gas (>18%) and the existence of a subsurface bacterial community at the RWMC dominated by strict aerobes (Colwell, 1989) supports this conclusion.

The CO₂ anomaly corresponds to a biodegradation rate for the lubricating oils of 1.7 t C yr^–1. The total amount of lubricating oil in the subsurface at the SDA is estimated at 125 t (Anderson and Lewis, 1989). Assuming that the oils are 80% C by weight, approximately 100 t of C in the form of lubricating oils was buried at the site. If all of the oil has leaked out of the containers and is available to be degraded (which is highly unlikely), this suggests an approximate intrinsic biodegradation rate for the oils of 1 to 2% yr^–1.

The estimated biodegradation rate for the lubricating oils is low relative to rates estimated at other sites. For a production volume of 1.5 x 10⁶ m³ of rock, the degradation rate is 1.15 g C cm^–3 yr^–1. By comparison, Baker et al. (2000) measured biodegradation rates for BTEX compounds and gasoline in soil of hundreds to thousands of grams per cubic meter per year. However, lubricating oils tend to have very low vapor pressures, and it is probable that the biological activity within the production zone is limited to areas within close proximity to the oils. Given that the RWMC production zone is hosted in fractured basalts, the distribution of the oils within this zone is undoubtedly highly heterogeneous. Therefore, the actual volume of the rock in which active biodegradation of the oils is occurring could easily be <1% of our production zone.

The results of this study also have significant implications for gas transport in the vadose zone at the RWMC. Elevated ¹⁴C-CO₂ was detected in samples at distances of >1 km from the suspected source. However, the impact of the vapor extraction units on CO₂ concentrations appears to be limited to <100 m. This suggests that advective gas transport is limited to local fracture networks and large-scale transport through the rock matrix is diffusion limited.

	ACKNOWLEDGMENTS

 
Eric Miller of the INEEL was instrumental in arranging the field-sampling program for this project. Assistance with sample collection by Larry Lazzarotto and Donald Song is greatly appreciated. Help with sample analyses by Donald Song and Alexis Templeton is also gratefully acknowledged. Michael Singleton provided helpful suggestions to improve this manuscript. Support for this work was provided by the Assistant Secretary for Environmental Management, Office of Science and Technology, under the Environmental Management Science Program of the U.S. Department of Energy under Contract no. DE-AC03-76SF00098.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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