Scientific Investigations Report 2008–5089
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5089
Wastewater-disposal sites at INL facilities are the primary sources of radiochemical and chemical constituents in ground water at the INL. These sites included infiltration ponds and ditches, lined evaporation ponds, drain fields, pits, and disposal wells. During 2002–05, wastewater was discharged to infiltration and lined-evaporation ponds. Liquid and solid waste materials buried at the RWMC (fig. 1) also are sources of some constituents in ground water. Davis (2006b) provides detailed information on waste disposal amounts and types of constituents discharged at each facility.
Radiochemical and chemical constituents in wastewater migrate to the Snake River Plain aquifer through perched ground water beneath wastewater infiltration ponds at the RTC and INTEC. Perched ground water beneath the RWMC formed from infiltration of snowmelt and rain and recharge from the Big Lost River and INL spreading areas. This perched water contains constituents leached from buried radioactive and organic-chemical wastes. The extent of this perched ground water is affected by the waste-disposal practices.
Bodies of shallow and deep perched ground water formed at the RTC in response to wastewater disposal to radioactive-, chemical-, cold-, and sanitary-waste ponds (fig. 4). Selected radiochemical and inorganic chemical constituents in wastewater have been monitored in the shallow and deep perched ground water since the early 1960s.
Water samples from five wells (CWP 1, 3, 8, TRA A 13, and TRA A77) (fig. 21) completed in shallow perched ground water near the RTC routinely were collected and analyzed for selected radiochemical and chemical constituents during 2002–05. Wells TRA A 13 and TRA A 77 were dry during this period, and no samples could be collected. Water samples also were collected from 18 wells (PW,8, 9, USGS 53 through 56, 60 through 63, 66, 68 through 73, 78) (fig. 21) completed in deep perched ground water beneath the RTC. Sampling was discontinued at many perched water wells during the 2002–05 reporting period because of lack of water in the wells (fig. 21). Selection of radiochemical and chemical constituents for analyses was based on waste-disposal history at the RTC. Selected radiochemical constituents were tritium, strontium-90, and gamma analyses (which may yield detections of cesium-137, cobalt-60, or chromium-51); chemical constituents were dissolved chromium, sodium, chloride, and sulfate.
Well TRA A 77 (fig. 21) is completed in shallow perched ground water in alluvium near the RTC retention basin (fig. 22), where radioactive wastewater flowed before it was discharged to the radioactive-waste infiltration ponds (fig. 21). Some wastewater reportedly leaked to the subsurface through cracks in the retention basin (U.S. Department of Energy, 1991, p. 29). To prevent discharge of radioactive wastewater to the retention basin, the retention basin was isolated in 1993 when discharge to the lined evaporation ponds began (Bartholomay and Tucker, 2000). The largest tritium concentration in water from well TRA A 77 during 1989–91 was 3,790±50 pCi/mL (Tucker and Orr, 1998, p. 15). In 1992, the largest tritium concentration increased slightly to 3,940±60 pCi/mL (Bartholomay, 1998, p. 41). By October 1995, the tritium concentration in water from well TRA A 77 had decreased to 22.4±0.9 pCi/mL (Bartholomay, 1998, p. 41) and by April 1997 to 1.0±0.15 pCi/mL (Bartholomay, 1998, table 2). Historically, tritium concentrations were variable in water from well TRA A 77. Because of the shallow depth of well TRA A 77 and its proximity to the leaking retention basin, the variability in tritium concentrations in this well could have resulted from changes in tritium disposal rates (Bartholomay, 1998, p. 10). No samples were collected from well TRA A 77 since April 1997 because no water was present in the well during scheduled sampling dates. The lack of water in this well may be the result of discontinued use of the retention basin for transfer of radioactive wastewater.
Maximum concentrations of tritium in water from well TRA A 13 decreased from 158±2 pCi/mL during 1982–85 to 1.1±0.3 pCi/mL during 1986–88 (Cecil and others, 1991, p. 33); during 1989–2001, tritium concentrations in water from this well were less than the reporting level. No samples were collected from this well after April 2001 because of equipment problems. The decrease in tritium concentrations in well TRA A 13, between the radioactive-waste infiltration ponds and the cold-waste ponds (fig. 21), likely is attributed to the large quantity of nonradioactive water discharged to the cold-waste ponds (Bartholomay and Tucker, 2000).
Wells CWP 1 through CWP 9 monitor shallow perched ground water around the cold-waste ponds at the RTC. Discharge of cooling-tower wastewater to the TRA disposal well ceased in 1982 and this water subsequently was discharged to the cold-waste ponds. During 1982–2005, tritium was less than the reporting level in water from wells CWP 1 through CWP 7. A tritium concentration of 0.8±0.2 pCi/mL was measured in water from well CWP 8 in November 1988, and since then, concentrations have been less than the reporting level. Tritium concentrations in water from well CWP 9 (fig. 21) decreased from 6.3±0.2 pCi/mL during 1982–85 to 1.1±0.2 pCi/mL during 1986–88 (Cecil and others, 1991, p. 35). No samples have been collected at CWP 9 since 1988. The absence of detectable tritium concentrations in most CWP wells was attributed to the large quantity of nonradioactive wastewater discharged to the cold-waste ponds since 1982, which has diluted any residual radioactive-waste infiltration pond water (Bartholomay and Tucker, 2000). Lack of available perched water to sample, and the history of non-reportable values of tritium in most of these wells resulted in the decision to remove wells CWP 2, 4, 5, 6, 7, and 9 from the sampling schedule at various times during 2002–05 (fig. 21).
Tritium concentrations in water from nine wells completed in deep perched ground water (PW 9, USGS 53, 55, 56, 61, 66, 70, 71, and 73) generally were greater than the reporting levels during at least one sampling event during 2002–05. Concentrations decreased in some wells and varied randomly in other wells (table 10). Tritium concentrations in water from six wells (USGS 60, 62, 63, 69, 72, and 78) were less than the reporting level during 2002–05 (table 10). Tritium concentrations varied between reportable and nonreportable concentrations in water from three wells, PW 8, USGS 54 and 68, during 2002–05 (table 10).
During April–October 2005, the most recent tritium concentrations in water from eight wells completed in deep perched ground water at the RTC exceeded the reporting levels (fig. 22; table 10). Tritium concentrations ranged from 0.19±0.06 pCi/mL (well USGS 68) to 46.2±2.0 pCi/mL (well PW 9). During April–October 2005, reportable tritium concentrations in water from wells completed in deep perched ground water (fig. 22) were less than concentrations measured during July–October 2001, with the exception of well PW 9, which was slightly higher (Davis, 2006b, fig. 5).
Water samples collected in October 2005 from wells USGS 73 and PW 9 contained tritium concentrations of 8.9±0.5 and 46.2±2.0 pCi/mL, respectively. These concentrations represent a decreasing trend since 1993 when the radioactive-waste infiltration ponds were taken out of service. Water in well USGS 74 (not shown in figures) contained 93.1±1.7 pCi/mL in April 1992; however, no samples have been collected since 1992 because the well was dry and the well was removed from the sampling schedule in October 2001. These three wells are more than 1,500 ft west of the radioactive-waste infiltration ponds (fig. 21). Historically large tritium concentrations in water from these wells indicate that the chemistry of perched ground water west of the RTC was affected by radioactive-waste infiltration pond disposals. Discontinuation of wastewater discharge to the radioactive-waste infiltration ponds and subsequent use of lined evaporation ponds, together with the radioactive decay process, may account for the decreased tritium concentrations in this area and could indicate an eastward migration of the extent of deep perched water relative to well USGS 74.
Water-level hydrographs for wells USGS 60 and 73 (fig. 23) indicate that wastewater disposal to the cold-waste ponds since 1982 has hydraulically affected perched ground-water flow to the west and east. Disposal to the cold-waste ponds affected water levels much less in wells USGS 54 and 70, to the north and northeast of the cold-waste ponds (fig. 23). Water levels in all four wells decreased significantly in 1992 (fig. 23), when wastewater discharge to the cold-waste ponds was much less than during other years (Bartholomay and Tucker, 2000). Because of the effect of disposal to the cold-waste ponds on water levels and the removal of the radioactive-waste infiltration ponds as a water source, tritium concentrations in perched ground water at the RTC likely decreased as nonradioactive wastewater from the cold-waste ponds mixed with water derived from earlier radioactive-waste infiltration pond disposal (Bartholomay and Tucker, 2000).
Bartholomay (1998) determined that increases in tritium concentrations in water from wells USGS 53, 56, and 70 corresponded partly to tritium disposal rates. The hydraulic connection between the radioactive-waste infiltration ponds and wells USGS 53 and 56 also can be demonstrated because well USGS 53 dried up and the water level in well USGS 56 declined below the pump intake subsequent to cessation of wastewater disposal to the ponds. Water was present and samples were collected from well USGS 53 in October 2003; the concentration of tritium was above the reporting level at 3.1±0.2 pCi/mL. However, this was a significant decrease in concentration from the sample analyzed in October 1995, which had a concentration of 126±4 pCi/mL. Samples also were collected from well USGS 56 in October 2004 and 2005, with concentrations above the reporting level at 23.3±0.9 and 23.3±1.1 pCi/mL, respectively. These concentrations were a significant decrease from the last sample analyzed in April 1997, which had a concentration of 148±5 pCi/mL.
Bartholomay (1998) noted that increases and decreases in tritium concentrations in water from well USGS 73 lagged from 3 to 13 months behind increases and decreases in well USGS 56. This time lag indicated that tritium in ground water moved from the radioactive-waste infiltration ponds to well USGS 73 during that period. Bartholomay (1998) also determined that changes in tritium concentrations in water from well USGS 54 did not correspond directly to monthly changes in tritium disposal. The lack of correspondence indicated that other factors, including hydraulic effects and dilution from the cold-waste ponds, affected tritium concentrations in water from that well.
Several factors affected the distribution of tritium in perched ground water at the RTC, including proximity of wells to the radioactive-waste infiltration ponds, depth of water below the ponds, variations in tritium disposal rate, and radioactive decay. Since 1982, tritium concentrations also have been affected by dilution from the cold-waste ponds. Replacement of the radioactive-waste infiltration ponds with the lined evaporation ponds in 1993 contributed to decreases in tritium concentrations in perched ground water and decreases in perched water in some wells. Infiltration from the Big Lost River during 1999, early 2000, and 2005 may have contributed to diluted tritium concentrations in perched ground water southeast of the RTC . Tritium concentrations in wells USGS 54, 61, 62, 66, and 71 decreased slightly during 2002–05 (fig. 22, this report; Davis, 2006b, fig. 5).
During 1996–98, strontium-90 concentrations in water from wells TRA A 77 and TRA A 13, completed in shallow perched ground water, were above the reporting levels (Bartholomay and Tucker, 2000, table 2). Concentrations in water from well TRA A 77 ranged from 4,710±140 pCi/L in October 1996 to 6,800±200 pCi/L in April 1997. Well TRA A 77 was not sampled during 1999–2005 because of well access problems or lack of water in the well. In October 1998, the concentration of strontium-90 in water from well TRA A 13 was 23.5±1.2 pCi/L. Water from well TRA A 13 exceeded the reporting level during 1999–2001 and in April 2001 the concentration was 22.1±1.1 pCi/L, consistent with the October 1998 concentration. The well was not sampled in October 2001–05 because of an obstruction in the well, or the well was dry. Well CWP 1, also completed in shallow perched ground water, exceeded the reporting level in June 1999 with a concentration of 3.1±0.7 pCi/L and in October 2004 with a concentration of 2.0±0.6. However, the concentrations of strontium in this well varied between reportable and non-reportable concentrations during 2000–05 (table 10, this report; Davis, 2006b, table 2).
During April through October 2005, concentrations of strontium-90 in water from wells PW 8, USGS 54, 55, 56, 63, and 70, completed in deep perched ground water at the RTC were greater than reporting levels (table 10 and fig. 24); concentrations ranged from 5.4±0.9 pCi/L in well USGS 63 to 71.7±1.8 pCi/L in well USGS 54. The concentration was slightly higher than the October 2001 concentration in water from well USGS 63 and slightly lower in water from well USGS 54. The distribution of strontium-90 concentrations in water from these wells during 2002–05 is attributed to exchange reactions between strontium-90 in solution and sediments beneath the radioactive-waste infiltration ponds. No strontium‑90 concentrations were detected in water from the Snake River Plain aquifer beneath the RTC (Bartholomay and others, 1997, p. 30). The absence of detectable concentrations indicates that strontium‑90 in solution is removed possibly by sorption and (or) exchange reactions in the unsaturated zone. Study of strontium distribution coefficients for samples of surficial sediment, sedimentary interbeds, and sediment-filled fractures in basalts (Liszewski and others, 1997, 1998; Pace and others, 1999) at the INL support this theory.
Water in wells USGS 60, 61, 62, 63, 66, 69, 71, 73, and 78 contained strontium‑90 at concentrations less than reporting levels in some samples collected during 2002–05, but greater than reporting levels in other samples (table 10). Because 2002–05 strontium-90 disposal data were not available, fluctuations could not be correlated with disposal during 2002–05. In addition, lined evaporation ponds were in use, which probably prevented contaminated water from percolating into the ground.
During 1999–2005, no reportable concentrations of cesium-137 were detected in water from any wells completed in either shallow or deep perched ground water. The general absence of reportable concentrations of cesium-137 in perched ground water at the RTC probably is due to decreasing cesium-137 disposal rates, change from using the radioactive-waste infiltration ponds to lined evaporation ponds, and sorption and (or) exchange of cesium-137 to minerals in sediments. During 1996–97, cesium-137 concentrations in water from shallow well TRA A 77 exceeded the reporting level and ranged from 42,300±1,800 pCi/L in April 1996 to 1,200±110 pCi/L in April 1997. No samples were collected from well TRA A 77 during 1998–2005 because the well was dry. The intermittent presence of cesium-137 in water from well TRA A 77 may have been due to the proximity of the well to the retention basin and the amount of suspended sediment in water samples collected onto which cesium-137 may have sorbed.
Chromium-51 has a half-life of 27.7 days (Walker and others, 1989, p. 24). About 2,390 Ci of chromium-51 was in wastewater discharged to the radioactive-waste infiltration and lined evaporation ponds during 1979–98. Data were not available for the amount of chromium-51 discharged during 1999–2005. The average disposal rate of chromium-51 during 1979–81 was 766 Ci/yr (Pittman and others, 1988, p. 35). A total of 25.7 Ci of chromium-51 was discharged during 1986‑88, an average of 8.6 Ci/yr (Cecil and others, 1991, p. 35). During 1989–91,11.6 Ci was discharged for an average of 3.9 Ci/yr (Tucker and Orr, 1998, p. 17). During 1992–95, 10 Ci was discharged, an average of 2.5 Ci/yr (Bartholomay, 1998, p. 16). During 1996–98, 6.2 Ci was discharged, an average of 2.1 Ci/yr (Bartholomay and Tucker, 2000).
Because of the decreased amount of chromium-51 discharged through time, and the relatively short half-life, this radionuclide was not detected in water from wells completed in deep perched ground water during 1986–88 (Cecil and others, 1991, p. 35). Chromium-51 was not detected in shallow perched ground water from wells TRA A 13 and CWP 1 through CWP 9 during 1982–88. During 1989–91, chromium-51 was detected in water from wells TRA A 77, USGS 53, and USGS 56 (Tucker and Orr, 1998, p. 17). During 1992–95, chromium‑51 was detected only in shallow well TRA A 77; concentrations ranged from 24,500±1,300 pCi/L in October 1992 to 2,700±500 in April 1995 (Bartholomay, 1998, p. 16). Chromium-51 was not detected in any wells during 1996–2005.
During 1996–98, cobalt-60 concentrations in water from wells TRA A 77 and USGS 56 exceeded the reporting level. Concentrations of cobalt-60 in water from well TRA A 77 ranged from 7,700±260 to 44,000±1,400 pCi/L. The concentration in water from well USGS 56 was 220±30 pCi/L (Bartholomay and Tucker, 2000). The presence of cobalt-60 in these wells probably is due to their proximity to the ponds and retention basin. During 1999–2005, no samples were collected from well TRA A 77 because the well was dry. Cobalt-60 was not detected in any water samples analyzed during 1999–2005.
During 1996–98, dissolved chromium concentrations in shallow perched ground water ranged from less than 5 μg/L in several wells to 26 μg/L in well TRA A 77 (Bartholomay and Tucker, 2000). During 1999–2001, wells TRA A 77, CWP 6, and CWP 7 could not be sampled because the wells were dry. During 1999–2001, dissolved chromium was not detected in shallow perched ground water (Davis, 2006b). During 2002–05, dissolved chromium was detected in shallow perched ground water from wells CWP 1 and 3. The concentrations ranged from 2 to 6 μg/L (table 11). The LRL for dissolved chromium varied from 2 to 10 μg/L during 2002–05; consequently, concentrations within that range were designated according to those LRLs as detections or nondetections during 2002-05. Estimated concentrations (table 11) less than the LRLs are treated as nondetected concentrations for consistency with treatment in previous publications, and because an estimated concentration is considered a “qualitatively detected analyte” (Childress and others, 1999, p. 7).
Dissolved chromium was detected in water from 17 wells (PW 8, 9, USGS 53-56, 60-63, 66, 68-71, 73, and 78) completed in deep perched ground water at the RTC during 2002–05 (table 11). Chromium was not detected in well USGS 72 during 2002–03; the well was not sampled for chromium during 2004–05. During 1996–98, the maximum concentration of dissolved chromium was 200 μg/L in well USGS 56 in April 1996; this well was not sampled during 1999–2001 because the water level was below the pump intake. In 2004 and 2005, water samples collected from well USGS 56 contained concentrations of chromium of 114 and 86 μg/L, respectively. The 2004 value of 114 μg/L also was the maximum concentration of chromium in deep perched water at the RTC during 2002–05. During April–October 2005, the most recent concentrations of dissolved chromium in wells completed in deep perched ground water near the RTC ranged from 3 μg/L in well USGS 69 to 86 μg/L in well USGS 56 (table 11 and fig. 25). The largest concentrations were in water from wells north and west of the radioactive-waste infiltration ponds (PW 9 and USGS 55, 68, and 73). The presence of dissolved chromium in water from wells completed in perched water indicates that water from these wells contains chromium and other constituents that were discharged to the radioactive-waste infiltration ponds before 1965, when disposal practices changed to injection of cooling-tower blowdown water to the disposal well.
During 2002–05, no analyses were made for dissolved sodium concentrations in shallow perched ground water at the RTC. Concentrations of dissolved sodium wells completed in shallow perched ground water were not available because (1) wells were dry during 2002–05, (2) wells could not be sampled because of equipment problems, or (3) analyses for dissolved sodium were not requested from the laboratory (table 11). Dissolved sodium concentrations in water from 16 wells completed in deep perched ground water were determined. During April–October 2005, dissolved sodium concentrations ranged from 6 to 27 mg/L in all wells except well USGS 68 (370 mg/L) (table 11), a decrease from October 2001 when the concentration was 413 mg/L (Davis, 2006b, table 3). However, sodium concentrations in this well varied during 2002–05 and ranged from 370 to 737 mg/L. Reasons for the variability of these concentrations are unknown, but may be due to movement of remnant water through the unsaturated zone from the chemical waste pond which was closed in 1999.
During April–October 2005, dissolved chloride concentrations in shallow perched ground water ranged from 10 mg/L in well CWP 3 to 32 mg/L in well CWP 1. Dissolved chloride concentrations in deep perched ground water ranged from 3 mg/L in well USGS 78 to 35 mg/L in well USGS 68. Concentrations of sodium in most wells remained fairly constant or decreased slightly compared to the 1999–2001 reporting period with the exception of well USGS 68, which increased from 23 mg/L in October 2001 to 35 mg/L in April 2005. This may be a result of movement of remnant water through the unsaturated zone from the chemical waste pond which was closed in 1999.
The maximum dissolved sulfate concentration in shallow perched ground water was 396 mg/L in well CWP 1 in October 2005. Concentrations of dissolved sulfate in this well vary greatly. During 2002–05, the concentrations ranged from 26 to 396 mg/L. The higher concentrations are attributed to sulfate disposal to nearby cold-waste ponds. Concentrations of dissolved sulfate ranged from 66 to 276 mg/L during April–October 2005 in water from wells USGS 54, 60, 63, 69, and PW 8, completed in deep perched ground water near the cold-waste ponds (fig. 7). These large concentrations indicate that water in the wells also was affected by discharge into the cold-waste ponds. During April–October 2005, the maximum concentration of dissolved sulfate in deep perched ground water was 951 mg/L in well USGS 68 (table 11), west of the chemical-waste pond (fig. 7). The dissolved sulfate concentration in this well varied during 2002-05, however there was an overall decrease from 1,409 mg/L in October 2001 (Davis, 2006b, table 3), and from 2,278 mg/L in December 1998 (Bartholomay and Tucker, 2000, table 3), which partly may be the result of a decrease in disposal rates or movement of remnant water through the unsaturated zone from the chemical waste pond which was closed in 1999.
Two wastewater-infiltration ponds were constructed south of the INTEC in 1984 and 1985 to replace the INTEC disposal well (fig. 4). Wastewater infiltrating from these ponds formed perched ground water in the basalt and sedimentary interbeds above the eastern Snake River Plain aquifer. The volumes of wastewater discharged to the well and infiltration ponds during 1962–2005 are shown in figure 9.
Many auger holes were drilled in 1983 to obtain geohydrologic and engineering data at the site of the planned INTEC infiltration ponds. Two holes (SWP 8 and 13 [fig. 21]) subsequently were used as monitoring wells to sample and measure water levels in shallow perched ground water in surficial sediment at the ponds. Attempts were made to sample wells SWP 8 and 13 annually during 1999–2001; however, the wells were dry in some years. Only one sample was obtained from SWP 8 during 2002–05. Wells PW 1, 2, 3, 4, 5, and 6 were completed in 1986 to monitor deep perched ground-water levels and water-quality changes under the INTEC percolation ponds (fig. 21). Well USGS 50 was used to monitor deep perched ground water near the INTEC disposal well. Lack of available perched water to sample, and the history of non-reportable values of tritium in most of these wells resulted in the decision to remove wells SWP 8 and 13, and PW 2, 3, and 5 from the sampling schedule at various times during 2002–05 (fig. 21).
In July 2002, the tritium concentration in well SWP 8, completed in shallow perched water was below the reporting level with a concentration of 0.16±0.14 pCi/mL. This well was dry during 2003 and sampling was discontinued due to lack of water in the well. During 2002–05, tritium concentrations in water from wells completed in deep perched ground water beneath the infiltration ponds ranged from less than the reporting level in wells PW 1 and PW 5 to 1.8±0.2 pCi/mL in well PW 4 (table 12), an increase in concentration from 2001 when all concentrations were less than the reporting level. Tritium concentrations in water from wells near the percolation ponds decreased significantly from concentrations during 1986–88, when disposal of tritium was about 185 Ci/yr (Orr and Cecil, 1991). During 2002–05, tritium concentrations in perched ground water in the wells closest to the ponds (PW 1 through 5, and SWP 8) increased slightly or remained fairly constant compared to the 1999–2001 reporting period (table 12, this report; Davis, 2006b, table 4). During 2002–04, tritium concentrations in water from well USGS 50 (fig. 7), near the disposal well, decreased slightly from 29.3±0.7 pCi/mL in April 2002 to 22.0±0.7 pCi/mL in November 2004 (table 12). Well maintenance problems prevented sample collection at this well during 2005. The large tritium concentrations in water from well USGS 50 may be due to leakage of wastewater from ruptures in the upper part of the disposal well casing or to leakage from wastewater lines at the INTEC (Tucker and Orr, 1998). The slight decrease in tritium concentrations can be attributed mostly to radioactive decay or dilution of well water from a nonradioactive source such as landscape irrigation. Figure 26 shows concentrations of tritium in wells near the INTEC during April–October 2005. Many wells were dry, were not sampled because of equipment problems, or sampling was discontinued prior to 2005.
During 2002, the concentration of strontium-90 in water from well SWP 8, completed in shallow perched ground water had a concentration of 0.7 ± 0.7 pCi/L. This well was dry during 2003, and sampling was subsequently discontinued.
During 2002–05, concentrations of strontium-90 varied in water from all wells completed in deep perched ground water beneath the INTEC percolation ponds. In April–October 2005, strontium-90 concentrations in deep perched ground water in wells closest to the ponds were not sampled because of access problems, dry wells, or sampling was discontinued prior to 2005 (table 12, fig. 27).
The largest concentrations of strontium-90 in perched ground water at the INTEC were in well USGS 50 near the INTEC disposal well. During 2002–04, strontium-90 concentrations in water from well USGS 50 decreased from 150±3 pCi/L in April 2002 to 105±2 pCi/L in November 2003 and then increased to 145±3 in November 2004 (table 12). These concentrations represent an overall decrease in strontium-90 concentrations since the 1980s when concentrations were as high as 620±30 in October 1982. Well maintenance problems prevented sample collection in 2005. Strontium‑90 concentrations in water from well USGS 50 may be due to the 1972 leak of 18,100 Ci of strontium-90 in soils at the INTEC Tank Farm (Cahn and others, 2006), leakage of wastewater from ruptures in the disposal well casing, or leakage from wastewater pipelines at the INTEC.
During 2002–05, concentrations of cesium-137 did not exceed the reporting level in shallow or deep perched ground water in wells closest to the infiltration ponds or in well USGS 50 (table 12). The absence of reportable concentrations of cesium-137 in perched ground water at the INTEC probably is due to decreased disposal and to sorption and (or) exchange of cesium-137 to minerals in sediments.
Water from well SWP 8, completed in shallow perched ground water, contained a concentration of 120 mg/L of dissolved sodium in July 2002 (table 13). This concentration was slightly higher than the July 2001 concentration of 102 mg/L (Davis, 2006b, table 5). During 2002–05, dissolved sodium concentrations in deep perched ground water in wells closest to the infiltration ponds (PW 2 and 4) ranged from 106 mg/L in well PW 2 in October 2002 to 83 mg/L in well PW 4 in October 2003 (table 13). Some wells could not be sampled due to lack of water in the well, well maintenance problems or sampling was discontinued during 2002–05.
Dissolved sodium concentrations in three water samples from well USGS 50 varied during 2002–05. The maximum concentration was 60 mg/L in November 2003 (table 13), the same concentration reported by Davis (2006b, table 5) for 1999–2001. These dissolved sodium concentrations may be due to leakage of wastewater from pipelines or infiltration of landscape irrigation at the INTEC.
The dissolved chloride concentration in well SWP 8 (fig. 21) was 145 mg/L in July 2002, similar to the July 2001 concentration of 153 mg/L. No samples were collected from well SWP 8 during 2003 because the well was dry, and sampling was subsequently discontinued. During 2002–05, dissolved chloride concentrations in deep perched ground water in wells closest to the infiltration ponds (PW 1 through 5) ranged from 118 to 322 mg/L in well PW 4 (table 13). The variability and values of concentrations of dissolved chloride in this well are similar to the 1999–2001 reporting period. Wells PW 1–3 and PW 5 were not sampled after 2002 because the wells were dry or sampling was discontinued during 2002-05. Dissolved chloride concentrations in water from wells PW 1, 2, and 5 were 169, 205, and 160 mg/L, respectively during April–May 2002 (table 13).
During 2002–04, dissolved chloride concentrations in water from well USGS 50 ranged from 44 mg/L in October 2002 to 56 mg/L in April 2002 (table 13), similar to concentrations during the 1999–2001 reporting period. The dissolved chloride concentrations may be due to leakage of wastewater from ruptures in the disposal well casing or leakage from wastewater pipelines at the INTEC. Dissolved chloride concentrations in water from this well steadily decreased since sampling began in 1959.
The dissolved sulfate concentration in shallow perched ground water from well SWP 8 was 49 mg/L in July 2002 (table 13). Dissolved sulfate concentrations in water from wells completed in the deep perched ground water closest to the INTEC infiltration ponds (PW 2 and 4) were 35 mg/L in October 2002 (table 13). After 2002, no samples were collected from wells PW1 through 3 and PW5 because of access problems, dry wells, or sample collection discontinued during 2002–05. Historically, dissolved sulfate concentrations in these wells fluctuated between about 22 and 41 mg/L.
Concentrations of dissolved sulfate in samples from well USGS 50 (table 13) ranged from 26 to 40 mg/L during 2002–04. Historically, dissolved sulfate concentrations in water from well USGS 50 have fluctuated around these values. The dissolved sulfate concentrations in water from this well are attributed to leakage from wastewater pipelines at the INTEC.
Water from well USGS 50 was analyzed for dissolved nitrite plus nitrate (as nitrogen) during 2002–04. Nitrite analyses indicated that almost all dissolved nitrite plus nitrate concentration is from nitrate. During 2002–04, dissolved nitrite plus nitrate (as nitrogen) concentrations in water from well USGS 50 ranged from 3.6 mg/L in October 2002 to 33.3 mg/L in November 2004 (table 13), an increase from the 1999–2001 reporting period, but maintaining an overall decreasing trend since sampling began in 1988. The nitrate concentrations may be due to leakage from wastewater pipelines at the INTEC.
Perched ground water beneath the RWMC is in sedimentary interbeds in basalts and can be attributed primarily to local snowmelt and rain infiltration and recharge from the Big Lost River and the INL spreading areas.
Well USGS 92 (fig. 4) is in the SDA at the RWMC and is completed in a sedimentary interbed (Anderson and Lewis, 1989, p. 29) 214 ft below land surface. Perched water in this well has moved through overlying sediments and basalt and may contain waste constituents leached from radiochemical and organic chemical wastes buried in the SDA. Small amounts of water in well USGS 92 frequently preclude collection of an adequate sample for all requested analyses. Adequate samples for requested analyses were collected during spring 2002–03. During 2002–03, radiochemical constituent concentrations in all water samples from well USGS 92 (table 14) were less than the reporting level. Tritium concentrations in water from well USGS 92 have varied through time.
Historically, the concentration of americium-241 was above the reporting level in October 1992, and the concentration of plutonium-238 was above the reporting level in November 1994 (Bartholomay, 1998).
Dissolved chloride concentrations in water from one sample collected from well USGS 92 was 87 mg/L in April 2002 (table 14). This dissolved chloride concentration is consistent with concentrations measured historically.
In 1987, 9 VOCs were detected in water from well USGS 92 (Mann and Knobel, 1987, p. 16–17); in January 1990, 6 VOCs were detected (Tucker and Orr, 1998); and in April 1992, 18 VOCs were detected (Bartholomay, 1998, p. 28; Greene and Tucker, 1998). During 1996–98, 14 VOCs were detected (Bartholomay and Tucker, 2000). During 1999–2001, water from well USGS 92 was analyzed for the same VOCs as in previous years. During 2002–05, attempts were made each year to sample well USGS 92, completed in perched water at the RWMC; however, lack of water in the well precluded obtaining an adequate sample during most sampling events. Most of the same VOCs except chloroethane that were detected during 1999–2001 were detected during 2002–03; additionally, bromodichloromethane was detected. Table 15 lists the concentrations of 16 VOCs detected in 2002–03. Most VOCs fluctuated through time and show no distinct trend. The sample collected on April 11, 2002 was foamy or contained high levels of contaminants when received by the laboratory, so the sample was diluted 1:10. Because the sample was not analyzed full strength, the laboratory raised the reporting level to less than 2 μg/L for this sample (L. Murtagh, C. Adamson, National Water Quality Laboratory, written commun., 2002). The MRL for some VOCs varied between 0.1 to 0.2 μg/L during 2002–05, a change that could result in detections of smaller concentrations and (or) different VOCs than detected in previous years.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5089
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