Environ. Sci. Technol. 2006, 40, 2895-2902
Measurement of Helium Isotopes in Soil Gas as an Indicator of Tritium Groundwater Contamination KHRIS B. OLSEN,* P. EVAN DRESEL, JOHN C. EVANS, WILLIAM J. MCMAHON,† AND ROBERT POREDA‡ Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352
species remaining within the lithium aluminate target material after processing. Tritium in this form exchanges freely with meteoric water from precipitation as it migrates toward and then mixes with the groundwater. The use of 3He/4He ratios relative to air (RA) in soil gas as a tool to determine subsurface tritium contamination is a modification of a successful technique developed in the late 1960s for age-dating deep ocean water as part of the GEOSECS (4) ocean monitoring program. The technique was also applied to shallow aquifers in the late 1980s and 1990s by Poreda et al. (5), Schlosser et al. (6), Schlosser (7), and Salomon (8-10). These studies were based on the presence of tritium, which decays to a stable, inert isotope, 3He: 3
The focus of this study was to define the shape and extent of tritium groundwater contamination emanating from a legacy burial ground and to identify vadose zone sources of tritium using helium isotopes (3He and 4He) in soil gas. Helium isotopes were measured in soil-gas samples collected from 70 sampling points around the perimeter and downgradient of a burial ground that contains buried radioactive solid waste. The soil-gas samples were analyzed for helium isotopes using rare gas mass spectrometry. 3He/4He ratios, reported as normalized to the air ratio (RA), were used to locate the tritium groundwater plume emanating from the burial ground. The 3He (excess) suggested that the general location of the tritium source is within the burial ground. This study clearly demonstrated the efficacy of the 3He method for application to similar sites elsewhere within the DOE weapons complex.
Introduction This study was undertaken at the U. S. Department of Energy’s (DOE’s) Hanford Site in southeastern Washington State approximately 200 miles southeast of Seattle and 120 miles southwest of Spokane. In this work, the 618-11 burial ground was studied in an attempt to identify the source of a groundwater tritium plume emanating from that location. The 618-11 burial ground consists of 3 trenches, 2-5 largediameter caissons, and 50 vertical pipe storage units. The site covers an area of 3.5 hectares (8.6 acres) (1). The burial ground received low- to high-activity dry waste, fission products, plutonium, and other transuranic constituents in a variety of waste forms from DOE’s research operations on the Hanford Site during the period 1962-1967 (2). The operation of this burial ground coincided with the development of the lithium aluminate tritium target project on the Hanford Site. Circumstantial evidence suggests that tritium targets, as reactor irradiated lithium aluminate, had been disposed to the burial ground during its operation. Production-sized pellets were used in a series of experiments designed to determine the characteristics of the tritium extraction (3). Results of that study estimated that each target contained approximately 51 curies of tritium, and residual tritium remaining with the lithium aluminate target after processing ranged from 0.204 to 2.55 curies (3). The condensable form, primarily HTO, is believed to be the main * Corresponding author phone: 509-376-4114; fax 509-372-1704; e-mail:
[email protected]. † CH2M Hill Hanford Group, Richland, WA. ‡ University of Rochester, Rochester, New York. 10.1021/es0518575 CCC: $33.50 Published on Web 04/01/2006
2006 American Chemical Society
H (tritium) f 3He + β-t1/2 ) 12.32 yr
When soil moisture reacts with tritium-bearing waste, by exchanging a tritium atom for a proton in water, the result is tritiated moisture in the vadose zone surrounding the source material within the burial ground. The tritiated soil moisture mixes with meteoritic moisture from precipitation and migrates downward to the water table. Concurrent with tritium’s release to the vadose zone, its daughter product, 3He, begins to increase in the vadose zone and groundwater due to tritium decay. The 3He then diffuses away from its vadose or groundwater source and migrates toward the surface. Because helium is nonreactive, it is a good surrogate tracer to identify tritium in the vadose zone and groundwater. A previous study on the Hanford Site (11) indicated that a 3He signature could be detected in soil gas above a tritium groundwater plume, whereas enriched (relative to tritium level in meteoric water) tritium in soil moisture was not detectable at those locations. The 3He/4He ratio at a given location above a tritium groundwater plume increased with depth, suggesting that the source of the 3He was the tritium in the groundwater and that exchange of the 3He into the soil gas and subsequent diffusion/migration from the source (groundwater) created the observed depth profile in the soil column. Tritium was detected in a monitoring well immediately downgradient of the burial ground at an initial level of 66,000 Bq/L (1,800,000 pCi/L) in January 1999 (12). Because of environmental and human health concerns from this level of tritium in groundwater, an investigation was initiated. This investigation was designed to use 3He/4He ratios (as RA) to identify the source of tritium in the groundwater, identify the boundaries of the tritium groundwater plume, determine the farthest extent of the plume from the source, and identify locations for additional groundwater monitoring wells to verify the soil-gas results.
Hydrogeology The DOE’s Hanford Site is located in the arid shrub-steppe in the rain-shadow of the Cascade mountains. Normal annual precipitation is 177 mm (13). The burial ground has been revegetated with bunch grass; surrounding undisturbed areas are covered with a mixture of bunch grass, cheat grass, rabbit brush, and sage brush; and the industrial areas are graveled or paved. Infiltration and recharge is highly dependent on the soil type and vegetation coverage. Recharge for sandy soil estimated as 8.6 mm/year for shrub, 11.3 mm/year for bunch grass, and 55.4 mm/year for no plants provides a reasonable estimate of the range for the study area (14). The Hanford Site is in the Columbia Basin, formed by a thick sequence of Miocene-Age tholeiitic basalt flows of the Columbia River Basalt Group, overlain by Pliocene-Age VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2895
Ringold Formation fluvial sediments. These sediments consist of variably cemented and compacted gravel, sand, silt, and clay. Pleistocene-Age cataclysmic Missoula flood deposits, locally known as the Hanford formation, consisting of highly permeable gravel and coarse- to fine-grained sands overlay and are incised into the Ringold Formation (15, 16). In the immediate vicinity of the 618-11 burial ground, the Ringold Formation sediments are described generally as sand, sandy gravel, and gravelly sand. The water table occurs within the Ringold Formation near the Hanford formation contact in the study area. East of the 618-11 burial ground, the vadose zone consists of fill containing excavated Hanford formation sediments with concrete and miscellaneous debris from the construction of a nuclear reactor. This fill may extend down to the water table, which occurs at a depth of approximately 18 m below ground surface (bgs). The Ringold Formation sediments have a hydraulic conductivity between 49 and 150 m/day. The hydraulic gradient is approximately 0.002 resulting in a groundwater velocity estimated to range from 7.3 to 19.5 m/day. The regional hydraulic gradient on this portion of the Hanford Site is generally toward the east with a probable northeast component. Details of the flow direction were not well known prior to this study due to potential anisotropy in the fluvial sediments and sparse well coverage (17).
Experimental Section Seventy soil-gas sampling points were installed and sampled during the course of this study. Forty-three sampling points were initially installed around the 618-11 burial ground and within 122 m to the east (hydraulically downgradient) of the burial ground. Twenty-seven sampling points were then installed in four transects to the east of the burial ground during the second phase of the investigation. Sampling points ranged from 305 m north, 518.5 m south, and 945.5 m east of groundwater monitoring well 699-13-3A. Sample Point Installation. Sampling points for soil-gas sampling were installed using a truck-mounted Geoprobe model 5400 system equipped with a probe 3.2 cm in diameter with a detachable steel tip. Target depth of installation of the screen interval was 6.1 m bgs, but actually ranged from 4.4 to 6.3 m bgs. Depth to groundwater ranges from approximately 18.6 m near the burial ground to 10.7 m at the eastern side of the study area. When the tip achieved its desired depth, a 20.3-cm-long, fine-mesh, stainless steel sampling point connected to the surface with a polyethylene tube (0.64-cm o.d. by 0.24-cm i.d.) was strung down the center of the push rod. The rod assembly was withdrawn 15.2 cm to release the steel tip and allow the sampling point to extend into the void space just below the push rod. Approximately 250 mL of 20-40 mesh washed silica sand was added around the sampling point through the center of the push rod. The push rod was slowly withdrawn and bentonite pellets were added through the center of the push rod. The bentonite was hydrated with 250 mL of water several meters above the screened interval. Additional bentonite pellets were added to the surface of the hole. To complete the sampling points, a cement cap was poured around the sampling tubes at the ground surface. Each sampling location was allowed to equilibrate for at least 24 h and up to several weeks before soil-gas samples were collected. Soil-Gas Sample Collection. All samples were collected with the aid of a Thomas model 107CA14 flexible diaphragm pump. Power to the pumps was supplied with a portable generator. Pressurized samples (∼2 atm Ptotal) were collected for helium isotope analysis from each sampling location. The sampling vessels were 50-mL stainless steel cylinders with one end sealed with a pipe plug and the other end consisting of a high vacuum needle valve with a 0.64-cm Swagelok fitting. Each vessel was evacuated to less than 5 2896
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 9, 2006
Torr before sampling. After a short equilibration period (minutes), a Kurz Instruments mass flow meter was placed in the flow stream between the polyethylene raiser tube and the bottom of the rotometer. The initial flow was adjusted to 1 L/min. The soil-gas sampling point was allowed to purge at 1 L/min for a minimum of 15 min. At the end of the purging period, a hose was connected to the pressure side of the pump, and the sampling cylinder was pressurized to the maximum pressure of the pump, allowed to vent to atmospheric pressure twice without removing the sampling tube from the sample cylinder, then allowed to pressurize to the maximum pressure of the pump. For every 20 samples, 1 random duplicate sample was collected, and an ambient air sample was included for each sampling day. To minimize temporal effects on the 3He/4He ratios when sampling numerous sites (e.g., around the burial ground), extraordinary attempts were made to sample all locations in a single day. If this was not possible, several of the previously samples sites were resampled, and the 3He/4He ratio results were compared. This comparison provided an estimate of any temporal variability between the two sampling events. Soil-gas sampling sites downgradient of the burial ground and within the same transect were sampled within several hours of each other, and the resulting 3He/4He ratios were directly compared. Groundwater Sampling. Groundwater tritium samples were taken at six locations suggested by the 3He/4He ratio results. Six boreholes to groundwater were drilled using a cable-tool drill rig. Once groundwater was reached, drilling continued to about 0.9 m into the aquifer and then the drill casing was back-pulled to allow water to enter the hole for sampling. Water was not added during drilling, so the water entering the borehole was considered representative of groundwater at that location. All the groundwater samples were collected with a bailer; because tritium is not significantly affected by sorption or volatilization, the bailer samples are considered representative to the concentration of tritium in the aquifer at the sampling point. Four of these six boreholes were later converted to permanent monitoring wells. Sample Analysis: Helium Isotopes in Soil Gas. After collection, soil-gas samples were sent to the University of Rochester for helium isotopic (3He and 4He) analysis by rare gas mass spectrometry. Experimental details can be found in Poreda et al. (5). Sample Analysis: Tritium in Groundwater. Tritium in groundwater was measured using liquid scintillation beta counting (18). An aliquot of groundwater was made alkaline with sodium hydroxide and distilled. After distillation, the distillate and a scintillation cocktail were mixed into a scintillation vial. The solution was placed into a refrigerated liquid-scintillation counter and counted. Detection limits range from 11 Bq/L (300 pCi/L) to 14.7 Bq/L (400 pCi/L) tritium.
Results and Discussion Burial Ground Helium Results. Figure 1 displays the results from soil-gas samples collected from the perimeter and in the immediate vicinity of the burial ground downgradient of well 699-13-3A during the first phase of the investigation. Normalized 3He/4He ratios ranged from 1.00 to 62.5 RA in this area. The highest 3He/4He ratios were located on the north side of the burial ground where two distinct maxima were observed. One maximum, 62.5 RA, was located midway along the north side fence line, near the location where disposal caissons are located within the burial ground. The second maximum, 10.93 RA, was located at the northeast corner of the burial ground. 3He/4He ratios decline to atmospheric levels (1.0 RA) at the northwest corner and along the south side of the burial ground.
FIGURE 1. 3He/4He ratio results (RA) from soil-gas samples from around 618-11 burial ground and immediately downgradient of well 699-13-3A. The maximum 3He/4He ratio along the north side of the burial ground strongly indicates a nearby tritium source. However, a groundwater sample collected from a boring at this location contained 239 Bq/L (6510 pCi/L) of tritium, which is consistent with regional concentrations. The probable explanation for the greatly elevated 3He/4He ratio is diffusion of 3He from a vadose zone source within the burial ground south of the sampling points. A second 3He/4He maximum on the north side of the burial ground is located at the northeast corner. Since no boring to groundwater was drilled at that location, the source of the elevated 3He/4He ratios could be from a vadose zone source of tritium or a tritium groundwater plume. However, it is believed that the source of the elevated 3He/4He ratios at this location is the result of tritium groundwater plume that affects well 699-13-3A. The groundwater flow direction is generally toward the east, but local variation may occur due to minor differences in gradient or anisotropy. It appears that the highest groundwater tritium concentration is slightly to the north of well 699-13-3A, corresponding to the highest 3He/4He ratios at the northeast corner of the burial ground, though this has not been confirmed by groundwater sampling. To estimate the concentration of tritium in groundwater a response factor can be calculated based on the relationship between the 3He/4He ratio, 3.73 RA nearest the well to the tritium concentration at the well (307,267 Bq/L; 8.38 million pCi/L). Therefore, the tritium concentration
in groundwater below the point with the 3He/4He ratio of 10.93 RA is estimated to be 1,118,333 Bq/L (30.5 million pCi/L). The 3He/4He ratios on the east side of the burial ground are highest at the northeast corner, adjacent to well 699-13-3A, and decrease to the atmospheric ratio of 1.0 RA at the southeast corner of the burial ground (Figure 2). The maximum 3He/4He ratio, 3.73 RA, is from the tritium groundwater plume affecting well 699-13-3A and follows the discussion presented above for the north side of the burial ground. There appears to be a second smaller maximum, 1.63 RA, located 45.7 m north of the southeast corner of the burial ground. The second maximum may be a vadose zone source of tritium or a groundwater tritium source. The tritium source was not confirmed in this area. The remaining points along the south and west sides of the burial ground approach the atmospheric 3He/4He ratio. The 3He/4He ratios at soil-gas locations within 122 m of the downgradient side of the burial ground ranged from 1.12 to 1.38 RA. A groundwater sample was collected from a boring east of the burial ground adjacent to a cluster of three soilgas sampling points whose 3He/4He ratios averaged 1.37 RA (Well A). Tritium in the groundwater at that location was 55,000 Bq/L (1.50 million pCi/L). Based on the 3He/4He results from the perimeter of the burial ground and within a radius of 122 m of well 699-13-3A in the hydraulically downgradient direction and the two VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2897
FIGURE 2. 3He/4He ratio results (RA) from soil-gas samples collected from the east side perimeter of the 618-11 burial ground.
FIGURE 3. 3He/4He ratio results (RA) and isopleths from soil-gas samples collected downgradient of the 618-11 burial ground. groundwater grab samples, a tritium groundwater plume appears to be heading east-northeast from the burial ground. Downgradient 3He/4He Results. Based on the results from the perimeter and near-field sampling, a second phase of the sampling focused downgradient from the burial ground. Downgradient sampling points were located in four transects crossing the hypothesized plume axis trending to the eastnortheast. Twenty-seven additional soil-gas sampling points were installed in the four transects approximately parallel to the east side of the burial ground ranging from 305 m north, 518.5 m south, and 945.5 m east of groundwater monitoring well 699-13-3A (Figure 3). To ensure that there were no additional tritium sources in the area of the 618-11 burial ground, Transects 1 and 3 spanned 793 and 732 m, respectively. Given the extended lengths of the transects, the 3He/4He ratios are expected to return to the atmospheric ratio (1.00) if no other sources of tritium are present in the area. Transect 2 consisted of two soil-gas sample points. Transect 2 was limited because of industrial facilities and 2898
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 9, 2006
construction activities in the immediate area. Transect 4 was located 945.5 m from well 699-13-3A and spanned 213.5 m across the potential path of the tritium plume. The fourteen 3He/4He ratio results from Transect 1, located about 122 m from the burial ground, ranged from 0.99 to 1.68 RA. Spatially, the higher helium ratios are located off the northeast corner of the burial ground and slightly elevated 3He/4He ratios off the east side. Background 3He/4He ratios were obtained at the ends of this transect, and the maximum 3He/4He ratios were near the plume axis generated from the burial ground and near-field data collected during phase 1 characterization. The plume width at Transect 1 is estimated to be 366 m. The graph of data from Transect 1 shows that the 3He/4He profile across the transect has an asymmetrical shape with elevated ratios extending farther south of the maximum point (Figure 3). This asymmetry appears to be an extension of the secondary maximum seen along the east boundary of the burial ground (Figure 2). The 3He/4He ratios at Transect 2, approximately 259 m from the burial ground,
FIGURE 4. Plot of tritium concentrations measured in well 699-13-3A from January 1999 through January 2002. ranged from 1.06 to 1.27 RA. The two readings at Transect 2 continued to track the hypothesized centerline of the tritium groundwater plume from the burial ground. The nine readings at Transect 3, approximately 518 m from the burial ground, ranged from 0.987 to 1.049 RA. The 3He/4He ratios suggest the plume has narrowed to 204 m because only two points indicate the presence of tritium in the groundwater. Background 3He/4He ratios were obtained at the ends of each transect, and the maximum 3He/4He ratios were near the hypothesized centerline of the tritium plume. The four readings from Transect 4, located approximately 945 m from the burial ground, ranged from 0.987 to 1.104 RA. Background 3He/4He ratios were obtained at the ends of this transect. Only one sample point indicated tritium in the groundwater. Based on the 3He/4He ratio results, the width of the plume at Transect 4 is estimated to be approximately 88 m. Although the 3He/4He ratio (1.104 RA) from the soil-gas point at Transect 4 looks relatively high compared to the Transect 3 values, the depth to groundwater along Transect 4 is approximately 6 m less than the distance at the other transects. This would result in higher 3He/4He ratio per unit concentration of tritium in the groundwater. The highest 3He/4He ratio values across each transect was used to determine the hypothetical downgradient groundwater center line of the plume in Figure 3. The edge of the plume was estimated by taking half the distance between the point approaching the atmospheric ratio and the next highest point in toward the center of the plume. Groundwater Tritium Results. Seven groundwater sampling locations for installation of temporary or permanent groundwater monitoring wells were identified based on the 3 He/4He analysis results. This was in addition to the preexisting groundwater monitoring well 699-13-3A that is directly downgradient near the northeast corner of the burial ground. One site was located directly adjacent to the north side of the burial ground, and the remaining six sites were downgradient or cross gradient to the hydraulic groundwater flow direction outward from the burial ground. The highest 3He/4He ratio, 62.5 RA, along the north side of the burial ground was believed to be either from a tritium source in the vadose zone or tritium contaminated groundwater. To determine the likely source of 3He, a groundwater sample was collected from a boring at this location. The
groundwater sample contained 239 Bq/L (6510 pCi/L) of tritium. Given the high 3He/4He ratio at that location, a groundwater concentration of 6,893,333 Bq/L (188 million pCi/L) of tritium would be expected using the 3He/4He ratio at that location and the 3He/4He ratio directly adjacent to well 699-13-3A to tritium concentration in that same well. Failure to find significant levels of tritium in groundwater beneath the highest 3He/4He ratio in the study strongly suggested that the highest 3He concentrations midway along the north side of the burial ground boundary were from a vadose source of tritium located within the burial ground. Thus, the likely source of the 3He in that area is from tritiumcontaining waste residing in a series of caissons located along the north side of the burial ground. Tritium results of groundwater sampling from well 69913-3A can be seen in Figure 4. Mean tritium concentrations of multiple samples ranged from 30,767 to 66,000 Bq/L (1.80 million to 8.38 million pCi/L). The 3He/4He results from the soil-gas survey along the north and east sides of the burial ground suggest the main body of the tritium plume to be just north of this well. If the main body of the tritium plume moves farther north, a decrease in the tritium level would be expected. Conversely, if the plume moves south toward the well, the tritium concentration would increase. This behavior may help explain the large temporal variations in the tritium concentrations at well 699-13-3A. A groundwater sample was collected from a boring near a cluster of soil-gas sampling points approximately 45.7 m east of well 699-13-3A (Figure 3). The three 3He/4He ratios at this location ranged from 1.34 to 1.39 RA. The tritium concentration measured in a sample of groundwater was 55,000 Bq/L (1.50 million pCi/L) (Well A, Figure 5). This concentration is significantly less than the 307,267 Bq/L (8.38 million pCi/L) observed at well 699-13-3A and supported the 3He/4He results that suggested the main body of the tritium plume was to the north of this location. Two groundwater monitoring locations were installed within Transect 1 (Figure 3). One location was at the fringe of the plume, as indicated by the 3He/4He results. The 3He/4He ratio at two soil-gas sampling points adjacent to the monitoring well ranged from 1.19 to 1.23 RA. The tritium concentration in groundwater at that location measured 7187 Bq/L (196,000 pCi/L) (Figure 5, Well B). A second sampling location was installed farther to the north at the highest VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2899
FIGURE 5. Tritium groundwater isopleths (pCi/L) generated from samples collected in the vicinity of 618-11 burial ground.
TABLE 1. 3He Response Factors for Groundwater Sampling Points Downgradient of 618-11 Burial Ground well
location
699-13-3A A
downgradient well
B
Transect 1 (SE)a
C
Transect 1 (HP)b
D
Transect 3 (HP)
E
Transect 3 (NE)c
F
Transect 3 (SE)
G
Transect 4 (HP)
a SE ) south end of transect. enrichment unit.
b
tritium GW concn Bq/L (pCi/L)
3He/4He ratio, RA
307,267 (8,370,000) 55,000 1,500,000 7,187 (196,000) 25,300 (690,000) 4,217 (115,000) 88 (2,400) 47 (1,200) 931 (25,400)
ratio, 1.68 RA, in Transect 1. The tritium concentration at this location was measured at 25,300 Bq/L (690,000 pCi/L) (Figure 5, Well C). Three groundwater monitoring locations were installed within or directly adjacent to Transect 3. The tritium groundwater concentrations at the perimeter of the plume, as indicated by the 3He/4He ratio in the overlying soil gas, ranged from 88 Bq/L (2400 pCi/L) (Figure 5, Well E) on the north and 44 Bq/L (1200 pCi/L) (Figure 5, Well F) on the south boundary of the plume. These levels are considered background concentrations for this location since there is some contribution from upgradient tritium sources. The tritium groundwater concentration at the monitoring well located adjacent to the highest 3He/4He ratio in Transect 3 measured 4217 Bq/L (115,000 pCi/L) (Figure 5, Well D). A single groundwater monitoring location was installed at the highest 3He/4He location within Transect 4. The tritium groundwater concentration at the location was measured at 931 Bq/L (25,400 pCi/L) (Figure 5, Well G). All the remaining three points in Transect 4 had 3He/4He ratios at or below the 9
enrichment unit (3He/4He ratio - 1)
response factor
3.74
2.74
3.82
1.37
0.37
2.88
1.21
0.21
12.49
1.68
0.68
11.49
1.049
0.049
4.97
1.017
0.017
82.57
0.987
-0.013
NDd
1.104
0.104
47.73
HP ) highest 3He/4He ratio in transect. c NE ) north end of transect.
3He/4He
2900
3He
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 9, 2006
d
ND ) not determined; negative 3He
atmospheric ratio. Based on the groundwater tritium result, the tritium plume originating from within the 618-11 burial ground has migrated 945 m downgradient of the burial ground boundary. Based on the comparison of the 3He/4He results and the tritium groundwater concentrations measured at the six downgradient wells, the 3He/4He contours were remarkably accurate in identifying the boundaries and center point of the tritium plume (Figure 5). Furthermore, the 3He/4He results also suggest the location of the tritium groundwater source is within the burial ground. Relationship between 3He/4He Ratio and Tritium Groundwater Concentration. During the course of this investigation several attempts were made to estimate the tritium groundwater concentrations using the 3He/4He ratio measured in soil gas directly adjacent to a groundwater sampling location. The concentration of 3He in air equals 7.252 pptv determined by eq 1. Table 1 compares the excess 3He* in soil-gas samples to tritium groundwater concentrations on an atom per mole basis at several
locations according to eqs 2-4. 3
He in air ) 1.384 × 10-6‚[4He]
(1)
where [4He] ) 5.24 ppmv and the abundance factor for 3He in air is 1.384 × 10-6. 3
He* (atom/mole air) ) (R - RA)‚(7.278 × 10-12 atoms 3He/atoms air)‚(6.02 × 1023 atoms air/mole air) (2)
where R ) the measured 3He/4He ratio in a sample, RA ) 1, 7.278 × 10-12 is the 3He abundance factor on an atom/atom ratio in air and 6.02 × 1023 is the number of atoms in a mole of air. 3
H (atom/mole H2O) ) [3H]‚2.08 × 107 atom/pCi 55.5 moles/L H2O
(3)
where [3H] ) the measured tritium concentration in groundwater pCi/L, 2.08 × 107 is the number of atoms of tritium per pCi/L in water, and 55.5 equals mol/L H2O. 3
Response Factor )
He* (atom/mole air)
3
H (atom/mol H2O)
(4)
Data from well 699-13-3A (Figure 1) was selected as the initial test case because it had well characterized groundwater tritium concentrations and a soil-gas sampling point within approximately 7.6 m of the well. The measured tritium groundwater concentration, 307,267 Bq/L (8.38 million pCi/ L), was from a sample collected within a month of the soilgas sampling. A 3He/4He ratio of 3.74 was measured at the soil-gas sampling point directly adjacent to the well. Therefore, the 3He enrichment factor of 2.74 is calculated (3.74 minus 1 ) 2.74). The response factor was calculated using eqs 2-4. The response factor equaled 3.79. The response factors for several other locations were calculated in a similar manner and ranged from 2.86 to 82.05. Because there was such a large spread of response factors under the most favorable conditions it became apparent that the use of 3He/4He ratios is only a qualitative measure of tritium concentration in the groundwater. Reasons for this variability may be due to one or more of the following factors: (a) Impermeable or less permeable layers in the subsurface may affect the rate of diffusion of 3He from its source to the sampling point. (b) Residence time of the tritium plume in an area will affect the 3He/4He ratio (e.g., allows the additional ingrowth of 3He into the soil gas). (c) Varying depth to groundwater from the soil-gas sampling point will affect 3He/4He ratio. (d) 3He/4He ratios are affected by the total mass of tritium in groundwater near the sampling point. The thickness of the groundwater tritium plume may have a profound effect on the 3He flux at a specific location. (e) Pathways for outgassing of 3He from the soil gas and incursion of atmospheric air into the vadose zone will affect 3He/4He ratio (e.g., wells screened across the water table which allow exchange of air through atmospheric pumping). (f) Temporal differences resulting from the influence of atmospheric pumping and ingrowth of 3He from tritium decay. Further research into the variables that influence the response factors would be needed to provide quantitative
predictions of groundwater tritium concentrations from soilgas helium isotopic measurements. Multi-level sampling points at a single site would assist in determining the flux of 3He from the tritiated groundwater and may prove to be a better indicator of the tritium source than a single measurement per site. Additional characterization of aquifer properties and barometric effects would likely be needed to form more detailed predictions. In the absence of such information, careful control of temporal variables is needed when applying qualitative methods.
Acknowledgments Funding for this project was provided by the U.S. Department of Energy Office of Environmental Management Richland Operations Office through Bechtel Hanford, Inc. The authors would like to acknowledge the staff of CH2M HILL for their assistance in installing the soil-gas sampling points. Special thanks go to Jane Borghese, Roger Ovink, and James (Mike) Faurote of CH2M HILL Hanford, Inc, Launa Morasch of PNNL for her editing assistance, and Chris Newbill for graphic support.
Literature Cited (1) Demiter, J. A.; Greenhalgh, W. O. Characterization of the 618-11 Solid Waste Burial Ground, Disposed Waste, and Description of the Waste-Generating Facilities; HNF-EP-0649; Waste Management Federal Services, Inc.: Richland, WA, 1997. (2) Johnson, A. B.; Kabele, T. J.; Gurwell, W. E. Tritium Production from Ceramic Targets: A Summary of the Hanford Coproduct Program; BNWL-2097; Battelle Northwest Laboratories: Richland, WA, 1976. (3) Yunker, W., Ed. Extraction of Tritium from Lithium Aluminate; HEDL-TME 76-81; Hanford Engineering Development Laboratory, Westinghouse Hanford Company: Richland, WA, 1976. (4) Clark, W. B.; Beg, M. A.; Carey, H. Excess 3He in Sea: Evidence for Terrestrial Primordial Helium. Earth Planet. Lett. 1969, 6, 213-220. (5) Poreda, R. J.; Cerling, T. E.; Salomon, D. K. Tritium and Helium Isotopes as Hydrologic Tracers in a Shallow Unconfined Aquifer. J. Hydrol. 1988, 103, 1-9. (6) Schlosser, P.; Stute, M.; Dorr, H.; Sonntag, C.; Munnich, K. O. Tritium/3He Dating of Shallow Groundwater. Earth Planet. Sci. Lett. 1988, 89, 353. (7) Schlosser, P. Tritium/3He Dating of Waters in Natural System. In Isotopes of Noble Gases as Tracers in Environmental Studies; Loosli, H. H., Mazon, E., Eds.; International Atomic Energy Agency: Vienna, Austria, 1992; pp 123-145. (8) Salomon, D. K.; Poreda, R. J.; Schiff, S. L.; Cherry, J. A. Tritium and Helium-3 as Groundwater Age Tracers in the Borden Aquifer. Water Resour. Res. 1992, 28 (3), 741-755. (9) Salomon, D. K.; Schiff, S. L.; Poreda, R. J.; Clarke, W. B. A Validation of the 3H/3He Method for Determining Groundwater Recharge. Water Resour. Res. 1993, 29, 2951-2962. (10) Salomon, D. K.; Poreda, R. J.; Cook, P. G.; Hunt, A. Site Characterization using 3H/3He Groundwater Ages, Cape Cod, MA. Ground Water 1995, 33 (6), 988-996. (11) Olsen, K. B.; Patton, G. W.; Dresel, P. E.; Evans, J. C.; Poreda, R. Measurement of Tritium in Gas-Phase Soil Moisture and 3He in Soil Gas at the Hanford Townsite and 100-K Area; PNNL13217; Pacific Northwest National Laboratory: Richland, WA, 2000. (12) Hartman, M. J., Morasch, L. F., Webber, W. D., Eds. Hanford Site Groundwater Monitoring for Fiscal Year 2000; PNNL13404; Pacific Northwest National Laboratory: Richland, WA, 2001. (13) Hoitink, D. J.; Ramsdell, J. V.; Burk, K. W.; Shaw, W. J. Hanford Site Climatological Summary 2004 with Historical Data; PNNL15160; Pacific Northwest National Laboratory: Richland, WA, 2004. (14) Fayer, M. J.; Walters, T. B. Estimated Recharge Rates at the Hanford Site; PNL-10285, UC-702; Pacific Northwest Laboratory, Richland, WA, 1995. (15) Hartman, M. J., Ed. Hanford Site Groundwater Monitoring: Setting Sources and Methods; PNNL-13080; Pacific Northwest National Laboratory, Richland, WA, 2000. VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2901
(16) DOE. Standardized Stratigraphic Nomenclature for Post-RingoldFormation Sediments Within the Central Pasco Basin; DOE/ RL-2002-39, Rev 0; U.S. Department of Energy, Richland Operations Office: Richland, WA, 2002. (17) Faurote, J. M. Borehole Summary Report for the 618-11 Burial Ground Tritium Investigation; BHI-01567; CH2M Hill Hanford, Inc.: Richland, WA, 2001.
2902
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 9, 2006
(18) American Society for Testing and Materials. D-2476, Standard Test Method for Tritium in Water. Volume 11.2 in Annual Book of ASTM Standards; West Conshohocken, PA, 2005.
Received for review September 20, 2005. Revised manuscript received December 19, 2005. Accepted February 1, 2006. ES0518575
