Upper Basalt-Confined Aquifer

Transcription

1 2.14 Upper Basalt-Confined Aquifer D. R. Newcomer and J. P. McDonald Upper basalt-confined aquifer groundwater quality is monitored because of the potential for downward migration of contaminants from the overlying unconfined aquifer. Contaminants that reach the upper basalt-confined aquifer have the potential to migrate through this aquifer and deeper confined aquifers to areas off the Hanford Site. The upper basalt-confined aquifer is also monitored to assess the potential migration of contaminants onto the Hanford Site from offsite sources. The upper basalt-confined aquifer is affected much less from contamination than the overlying unconfined aquifer system. Contamination found in the upper basalt-confined aquifer is most likely attributed to areas where confining units of basalt have been eroded away or were never deposited and where past disposal of large amounts of wastewater resulted in downward hydraulic gradients. In some areas, wells penetrating the upper basalt-confined aquifer system provided a downward pathway for contaminant migration. Because of these factors, intercommunication between the aquifers permitted groundwater flow from the unconfined aquifer to the underlying confined aquifer, thereby increasing the potential to spread contamination. The quality of groundwater in the upper basalt-confined aquifer is monitored because of the potential for downward migration of contaminants from the overlying unconfined aquifer. An area of intercommunication between the unconfined and upper basaltconfined aquifer systems was first identified in the north part of the 200 East Area (RHO-BWI-ST-5; RHO-RE-ST-12 P). Several confined aquifer wells north and east of the 200 East Area have shown evidence of intercommunication with the overlying unconfined aquifer (PNL-10817). Intercommunication between the cn In the area of Gable Mountain, shown above, erosion into the basalt or lack of deposition has resulted in isolated communication between the unconfined aquifer and the upper basalt-confined aquifer. Upper Basalt-Confined Aquifer

2 unconfined and confined aquifers in this region has been attributed to erosion of the upper Saddle Mountains Basalt and a downward hydraulic gradient that resulted from groundwater mounding associated with past wastewater disposal to the ground. However, the groundwater mound has diminished in recent years (see Section ). Contaminants have the potential to migrate to areas off the Hanford Site after reaching the upper basalt-confined aquifer system. Several potential offsite migration pathways for contaminants within the upper basalt-confined aquifer have been identified (PNL-10817). These pathways include migration across the south boundary of the Hanford Site migration across the east-southeast boundary of the Hanford Site migration to deeper confined aquifers in the Gable Mountain-Gable Butte structural area The potential exists for contaminants to migrate off the Hanford Site. However, groundwater monitoring results indicate that contaminants have not migrated off the Hanford Site to the south and southeast, toward urban development. The potential exists for contaminants to migrate through the upper basaltconfined aquifer across the south boundary of the Hanford Site to offsite areas. The potential for contaminants to migrate across this boundary is highly uncertain because of a lack of hydrogeologic and hydrochemical data for the upper basaltconfined aquifer in this area. However, urban development and agricultural activity continues to increase in this region. As a result, groundwater use from sources within the upper basalt-confined aquifer system continues to increase in response to the demand to meet agricultural, municipal, and domestic needs. A limited investigation on the hydrogeology and hydrochemistry of this region was reported in PNNL (see Section ). The potential for offsite migration of contaminants across the east-southeast boundary of the Hanford Site is low for several reasons. Lateral groundwater flow velocities along this flow path are low in the upper basalt-confined aquifer. The Columbia River forms a discharge area for the upper basalt-confined aquifer in this region, and high hydraulic head conditions east of the river inhibit eastward flow across this boundary. Contaminants that reach the upper-basalt confined aquifer in the Gable Mountain-Gable Butte structural area have the potential to migrate downward to transmissive Wanapum Basalt aquifers underlying the Saddle Mountains Basalt. A potential for offsite migration through the deep Wanapum Basalt aquifers exists because of highly transmissive interflow aquifers within the Wanapum Basalt, their deep hydrogeologic setting, and offsite regional uses for agricultural and domestic water supply. However, the potential for offsite migration through this pathway is uncertain Groundwater Flow Confined groundwater generally flows from west to east beneath the Hanford Site, eventually discharging to the Columbia River through the unconfined aquifer. Within the upper basalt-confined aquifer system, groundwater occurs within basalt fractures and joints, interflow contacts, and sedimentary interbeds within the upper Saddle Mountains Basalt (see Section of PNNL-13080). The thickest and most widespread sedimentary unit in this system is the Rattlesnake Ridge Interbed, which is present beneath much of the Hanford Site. Groundwater also occurs within the Levey Interbed, which is present only in the south portion of the site. An interflow zone occurs within the Elephant Mountain member of the upper Saddle Mountains Basalt, and also may be significant to the lateral transmission of water. This system is confined by the dense, low-permeability, interior portions of basalt flows and in some places by Ringold Formation silt and clay units overlying the basalt. Groundwater flow rates within the Rattlesnake Ridge Interbed Hanford Site Groundwater Monitoring 2002

3 have been estimated to be between 0.7 and 2.9 meters per year (PNL-10817). This flow rate is considerably slower than most estimates for the overlying unconfined aquifer system. Recharge to the upper basalt-confined aquifer system is believed to occur along the margins of the Pasco Basin and results from the infiltration of precipitation and surface water where the basalt and interbeds are exposed at ground surface. Recharge also may occur through the Hanford/Ringold aquifer system in areas where the hydraulic gradient is downward. Recharge may also occur from deeper basalt aquifers where an upward gradient is present. The Columbia River represents a regional discharge area for this aquifer system, at least in the south portion of the site. Discharge also occurs to the overlying Hanford/Ringold aquifer system in areas where the hydraulic gradient is upward. Discharge to overlying or underlying aquifers in the vicinity of the Gable Butte-Gable Mountain structural area may occur through erosional windows in the basalt. The groundwater project measures hydraulic heads annually in the upper basaltconfined aquifer system. Figure , constructed by manual contouring, presents an approximation of the March 2002 potentiometric surface for this aquifer system south of Gable Butte and Gable Mountain. Measurements in the Rattlesnake Ridge Interbed (17 wells), the Levey Interbed (2 wells), and the Elephant Mountain Interflow zone (1 well) were primarily used to construct this map. Additional measurements in the upper Saddle Mountains Basalt (10 wells) were used for general contouring. The monitoring well network used for hydraulic head monitoring is presented in PNNL The region to the north of Gable Butte and Gable Mountain was not contoured because of insufficient well control. See PNL-8869 for a generalized potentiometric surface map of this area. South of the Umtanum Ridge/Gable Mountain area, groundwater in the upper basalt-confined aquifer system generally flows from west to east across the Hanford Site toward the Columbia River. The elevated regions to the west and southwest of the site are believed to be recharge areas for this aquifer system, and the Columbia River represents a discharge area. The Yakima River may also be a source of recharge. In the vicinity of the 200 East Area, the potentiometric surface in Figure is similar to the potentiometric surface for the Ringold Formation confined aquifer (compare with Figure ). The basalt in this area was significantly eroded by late Pleistocene catastrophic flooding (RHO-BWI-LD-5), which facilitates aquifer intercommunication. A diagnostic water level-barometric response analysis conducted at wells A (unconfined aquifer) and B (Rattlesnake Ridge Interbed) indicate that the Rattlesnake Ridge Interbed behaves as an unconfined aquifer in this area. In the vicinity of the 200 East Area and to the immediate north, the vertical hydraulic gradient between the upper basalt-confined aquifer system and the overlying Hanford/Ringold aquifer system is upward. Therefore, it is likely the upper basalt-confined aquifer system currently discharges to the overlying Hanford/Ringold aquifer system in this region. Water-table and potentiometric surface maps of the upper basalt-confined aquifer system (see Figure ), the unconfined aquifer (see Figure 2.1-1), and the Ringold Formation confined aquifer (see Figure ) indicate that a downward hydraulic gradient from the Hanford/Ringold aquifer system to the upper basalt-confined aquifer occurs in the west portion of the Hanford Site, in the vicinity of the B Pond recharge mound, as well as in the regions north and east of the Columbia River. In the vicinity of B Pond, the vertical head gradient between the unconfined aquifer system and the upper basalt-confined aquifer system has diminished in recent years, but remains downward. In other areas of the Hanford Site, the hydraulic gradient is upward from the upper basalt-confined aquifer to the Hanford/Ringold aquifer system. A downward hydraulic gradient from the Hanford/Ringold aquifer system to the upper basaltconfined aquifer occurs in the west portion of the Hanford Site, near the B Pond recharge mound, and in the regions north and east of the Columbia River. In other areas of the Hanford Site, the hydraulic gradient is upward from the upper basalt-confined aquifer to the Hanford/Ringold aquifer system. Upper Basalt-Confined Aquifer

4 Water levels in the upper basalt-confined aquifer beneath the Hanford Site declined over the period from March 2001 to March The decline in the 200 East Area and to the immediate north and east (near B Pond) ranged from to meter over the 12-month period (well B had an increase of 0.18 meter). Water level declines near the 200 West Area ranged from to meter. These declines are in response to curtailed effluent disposal activities in the 200 Areas and are consistent with water-level declines in the overlying Hanford/Ringold aquifer system. The maximum hydraulic head associated with the B Pond groundwater mound has shifted to the northeast since periodic monitoring of the upper basalt-confined aquifer system began in Figure shows a trend plot of hydraulic head at well C (located at B Pond and formerly the approximate center of the groundwater mound) and well B (the current approximate center of the groundwater mound). This figure shows that water levels beneath B Pond peaked higher and earlier and are now declining more rapidly than the water level at well B. This more rapid decline beneath B Pond causes the apparent shift in the groundwater mound. The May Junction Fault, located east of B Pond and trending north-south, acts as a barrier to groundwater flow in Hanford/Ringold aquifer system (PNNL-12261). It may also impede the movement of water in the upper basalt-confined aquifer system, which would explain the shifting of the groundwater mound toward the east Groundwater Quality Figure shows the locations of the upper basalt-confined aquifer monitoring wells on the Hanford Site. Most of the wells are completed in the Rattlesnake Ridge Interbed near the 200 East Area in the central part of the Hanford Site. A few wells are completed in the Elephant Mountain interflow zone, the Levey Interbed, or a composite of one or more interbeds and /or interflow zones within the upper Saddle Mountains Basalt. Approximately 20 wells completed in the upper basalt-confined aquifer system are routinely sampled on the Hanford Site, according to the current sampling schedule (PNNL-14111). Sixteen of these wells are sampled every three years, and three are sampled annually. (a) During fiscal years 2000 through 2002, 23 samples were collected from these wells and analyzed for chemical and radiological constituents. Many of the samples were analyzed for tritium, iodine-129, and nitrate because these constituents (1) are the most widespread in the overlying unconfined aquifer, (2) are some of the most mobile constituents in groundwater, and (3) provide an early warning for potential contamination in the upper basalt-confined aquifer system. Groundwater samples from the upper basalt-confined aquifer were also analyzed for anions (besides nitrate), cations, cyanide, gross alpha, gross beta, gamma-emitters, strontium-90, technetium-99, and uranium isotopes. Data for the primary constituents of interest are listed in Table A full data set is included in the data files that accompany this report. Distribution of sample results for selected constituents and wells across the Hanford Site for fiscal years 2000 through 2002 is shown in Figure Elevated tritium in the upper basalt-confined aquifer is limited to one well near the Gable Mountain/200 East Area. This well ( C) contained 5,770 pci/l tritium in 2000, but concentrations have been decreasing since 1996 (Figure ). Nearby wells completed in the overlying Ringold Formation contain tritium at levels exceeding the 20,000-pCi/L drinking water standard (see Section 2.9.3) Hanford Site Groundwater Monitoring 2002 (a) One well was dropped from the sampling schedule after fiscal year 2000.

5 However, tritium trends in these wells have been declining, as represented by well E in Figure Tritium over most of the Hanford Site (excluding the Gable Mountain/200 East Area) ranged from less than the detection limits near the discharge area in the east-southeast portion of the Hanford Site to 43.3 pci/l at a recharge area near the Yakima River. Near the burial ground, where a source of tritium has contaminated the unconfined aquifer at high levels, tritium was detected at a concentration of 25.3 pci/l in the upper basalt-confined aquifer in fiscal year In the north part of the 200 East Area, technetium-99 is elevated in the upper basalt-confined aquifer in one well (see Figure ). The technetium-99 concentration was 1,120 pci/l in this well (299-E33-12) in However this level, which exceeds the 900-pCi/L drinking water standard, is slightly lower than concentrations in the last ~10 years (Figure ). Contamination in this well is attributed to migration of high-salt waste down the borehole during construction when it was open to both the unconfined and confined aquifers (RHO-RE-ST-12 P). This well is located in the vicinity of a technetium-99 plume in the overlying unconfined aquifer (see Section ). Technetium-99 levels have generally been higher and more variable in the unconfined aquifer, as represented by the trend plot for well 299-E33-13 (see Figure ). Most of the samples collected from upper basalt-confined aquifer wells near the Gable Mountain/200 East Area were analyzed for iodine-129. These wells are located beneath or near the iodine-129 plume contained within the overlying unconfined aquifer. Iodine-129 was not detected in the upper basalt-confined aquifer during fiscal years 2000 through 2002 (see Table ). Cyanide and nitrate are elevated in an upper basalt-confined aquifer well (299-E33-12) in the north part of the 200 East Area (see Figure ). However, these co-contaminants are at levels that do not exceed their respective drinking water standards. Concentrations of cyanide and nitrate have not changed significantly at this well in the last ~10 years (Figure ). Like technetium-99, this contamination is associated with migration of high-salt waste down the borehole during well construction when it was open to both the unconfined and confined aquifers (RHO-RE-ST-12 P). Cyanide and nitrate are co-contaminants with much higher concentrations in the unconfined aquifer in the north part of the 200 East Area. Table indicates that the majority of wells showing elevated nitrate in the upper basalt-confined aquifer occur near Gable Mountain and the 200 East Area. Elevated nitrate in the upper basalt-confined aquifer is a hydrochemical indicator of intercommunication with the overlying contaminated unconfined aquifer (RHO-BWI-ST-5; RHO-RE-ST-12 P; PNL-10817). Across the rest of the Hanford Site, nitrate levels in the upper basalt-confined aquifer ranged from less than detectable to ~1 mg/l in fiscal years 2000 through Strontium-90 from four upper basalt-confined aquifer wells showed concentrations below the minimum detection limits during fiscal years 2000 and 2001 (see Table ). Two of these wells are located near the former Gable Mountain Pond in the central part of the Hanford Site (see Figure ), where strontium-90 contamination occurs in the overlying unconfined aquifer. Moderately mobile in groundwater, strontium-90 contamination in the unconfined aquifer is not expected to migrate to and reach detectable levels in the upper basalt-confined aquifer. A few samples collected from upper basalt-confined aquifer wells were analyzed for gamma-emitting and uranium isotopes. Gamma-emitting isotopes were not detected in the upper basalt-confined aquifer on the Hanford Site, including the Gable Mountain/200 East Area. Uranium isotopes were not detected in this aquifer in the east part of the Hanford Site during fiscal years 2000 through 2002 (see Figure ). Elevated concentrations of cyanide, nitrate, and technetium-99 present in the overlying unconfined aquifer, were detected in one basalt-confined well in the northwest 200 East Area. Another basalt-confined well near the former B Pond has slightly elevated tritium. Other basalt-confined wells are uncontaminated. Upper Basalt-Confined Aquifer

6 Hanford Site Groundwater Monitoring 2002 In summary, elevated tritium in the upper basalt-confined is limited to one location near the former B Pond in the Gable Mountain/200 East Area, but concentrations have been decreasing. Cyanide, nitrate, and technetium-99 are elevated in an upper basalt-confined aquifer well in the north part of the 200 East Area. Migration of high-salt waste via the well during its construction caused this contamination. Contaminants on the Hanford Site have not migrated through the upper basalt-confined aquifer system to several offsite sample locations south and southeast of the Hanford Site (PNNL-14107).

7 Table Potential Contaminants in Upper Basalt-Confined Aquifer, Fiscal Years 2000 Through 2002 Upper Basalt-Confined Aquifer Specific Sample Cesium-137 Cobalt-60 Cyanide Gross alpha Gross beta Iodine-129 Nitrate Conductance Strontium-90 Technetium-99 Tritium Well Date (pci/l) (µg/l) (mg/l) (pci/l) (pci/l) (pci/l) (µg/l) (µs/cm) (pci/l) (pci/l) (pci/l) 199-H4-2 07/20/00 NA NA NA -0.2 U NA B U NA -1.2 U 299-E /03/00 NA NA NA NA NA 0.09 U NA NA G 299-E /12/ U 4.37 U U NA 1, /14/01 NA NA NA 0.25 U 9.84 NA U 351 NA NA 7.55 (a) C 02/08/ U 2.19 U NA 0.56 U U U -5.7 UP 110 U C 06/28/01 NA NA NA 0.83 U 6.82 NA U 336 NA NA /15/00 NA NA NA NA NA NA 9.3 D 377 NA NA 3.32 U P 11/01/01 NA NA NA NA U 378 NA NA B 05/15/00 NA NA NA NA NA 0.05 U NA NA 7.13 J C 05/18/00 NA NA NA NA NA 0.32 U NA NA 5, E9B 05/16/ U U NA 0.23 U U B 424 NA NA 9 J E9B 05/15/ U 1.45 U NA 0.87 U U U 420 NA NA 4.61 U E9B 08/09/ U U NA 0.42 U U U 425 NA NA 1.2 U B 06/13/01 NA NA NA U NA U B 04/26/00 NA NA 1.60 U NA U NA -6.5 UP 84 U A 06/13/01 NA NA NA NA U NA /13/01 NA NA NA 0.75 U 7.05 NA 16.4 DH 315 NA NA /14/01 NA NA NA NA 4.87 D 321 NA NA /01/ U 1.07 U 1.60 U NA U U P U 699-S2-34B 08/30/01 NA NA NA NA NA U UH 593 NA NA 37.5 U 699-S11-E12AP 05/16/00 NA NA NA U 6.18 NA B 365 NA NA 7.98 J 699-S11-E12AP 05/29/02 NA NA NA NA NA NA NA NA NA NA S24-19P 07/24/01 NA NA NA NA NA NA NA NA 43.3 Q Wells Located South and Southeast of the Hanford Site 699-ORV-1 11/06/00 NA NA NA 0.42 U 5.15 NA NA NA N28E04G01 11/06/00 NA NA NA 0.07 U 8.87 NA 49 U 377 NA NA N28E06C02 11/06/00 NA NA NA 0.82 U NA 2, NA NA N27E14F03 11/06/00 NA NA NA 0.90 U 7.07 NA 49 U 282 NA NA N27E14F03 11/06/00 NA NA NA U 8.07 NA 49 U 282 NA NA N28E24R03 10/27/00 NA NA NA 0.57 U NA 49 U 382 NA NA N29E02Q01 10/27/00 NA NA NA U NA 49 U 456 NA NA N29E19E01 10/27/00 NA NA NA U NA 49 U 384 NA NA (a) Sample was re-analyzed because batch blank result exceeded criteria. B = Greater than minimum detection limit but below the analytical laboratory s required detection limit. D = Analyzed at a secondary dilution factor. G = Result is valid according to further review. H = Analyzed after recommended holding time. J = Estimated value. NA = Not analyzed. P = Potential problem; collection/analysis circumstances make value questionable. Q = Associated quality control sample is out of limits. U = Below detection limit.

8 Figure Potentiometric Map of Upper Basalt-Confined Aquifer System, March Hanford Site Groundwater Monitoring 2002

9 C B Suspect data not shown Water-Level Elevation (NAVD88), m Jan-89 Jan-91 Jan-93 Jan-95 Jan-97 Jan-99 Jan-01 Jan-03 Date mac02194 Figure Water Levels in Upper Basalt-Confined Aquifer in Well C Near B Pond and Well B North of B Pond Upper Basalt-Confined Aquifer

10 Figure Groundwater Monitoring Wells Sampled in the Upper Basalt-Confined Aquifer on the Hanford Site, Fiscal Years 2000 Through Hanford Site Groundwater Monitoring 2002

11 Figure Distribution of Chemical and Radiological Constituents in the Upper Basalt-Confined Aquifer, Fiscal Years 2000 Through 2002 Upper Basalt-Confined Aquifer

12 120, , C E DWS Open symbols used for non-detect values, suspect data not shown 80,000 Tritium, pci/l 60,000 40,000 20,000 0 Jan-82 Jan-86 Jan-90 Jan-94 Jan-98 Jan-02 Collection Date Figure Tritium Concentrations in Wells C (Upper Basalt-Confined Aquifer) and E (Unconfined Aquifer) mac ,000 3, E E33-13 DWS 3,000 Technetium-99, pci/l 2,500 2,000 1,500 1, Jan-88 Jan-91 Jan-94 Jan-97 Jan-00 Jan-03 Collection Date mac02196 Figure Technetium-99 Concentrations in Wells 299-E33-12 (Upper Basalt-Confined Aquifer) and 299-E33-13 (Unconfined Aquifer) Hanford Site Groundwater Monitoring 2002

13 80 70 Cyanide Nitrate Open symbols used for non-detect values 299-E Cyanide, ug/l Nitrate, mg/l Jan-86 Jan-88 Jan-90 Jan-92 Jan-94 Jan-96 Jan-98 Jan-00 Jan-02 Collection Date 0 Figure Cyanide and Nitrate Concentrations in the Upper Basalt-Confined Aquifer mac02197 Upper Basalt-Confined Aquifer