Ground-Water Protection and Monitoring Program

The strategy for protecting ground water at the Hanford Site is presented in the Hanford Site Ground-Water Protection Management Plan (DOE 1994h). Two of the key elements of this strategy are to 1) protect the unconfined aquifer from further contamination, and 2) conduct a monitoring program to provide an early warning when contamination of ground water does occur. These elements are reaffirmed by the recommendations of the Hanford Future Site Uses Working Group to "protect the Columbia River from contamination" and to "deal realistically and forcefully with ground-water contamination" (Drummond et al. 1992). The ground-water monitoring program at Hanford monitors and documents ground-water quality to effectively meet the needs of these elements. The monitoring program at Hanford is designed to document the distribution and movement of existing ground-water contamination. This information is used to assess the movement of contamination into previously uncontaminated areas. The monitoring provides the historical baseline for evaluating current and future risk from exposure to the contamination and for deciding on remedial options. The geology and hydrology of the Hanford Site are the major controls on the movement of contaminants in ground water so hydrogeologic studies are integrated into the monitoring program.

Geology

Columbia River basalts erupted from volcanic fissures, starting 17 million years ago, to ultimately cover () of Washington, Oregon, and Idaho. The basalt flows consist of generally dense, impermeable basalt that have more permeable top and bottom portions. At first, there was little time between eruptions for the development of soils or accumulation of sediments between flows. However, the frequency of eruptions eventually slowed, and the regional river system eroded the basalt, depositing sediments across the basalt surfaces between eruptions. These sediments form the Ellensburg Formation of sedimentary interbeds that are found between the basalt flows. Zones between the basalt flows and the sedimentary interbeds are frequently water-bearing zones that are used as water sources in areas around the Hanford Site. Flow between the basalt aquifers and the surficial aquifer generally occurs along faults that bring a water-bearing interbed in contact with other sediments or where the overlying basalt has been eroded to reveal an interbed (Graham et al. 1984, Newcomb et al. 1972, Reidel et al. 1992).

During the period of basalt deposition, tectonic pressure was very slowly deforming the basalt flows into the generally east-west trending ridges that border the Pasco Basin today. Basins also developed at this time. These basin ridges gradually began to affect the distribution of the river beds, moving them toward the Pasco Basin. Ringold Formation deposition began after the last major eruption 8.5 million years ago with the ancestral Columbia River meandering across the relatively flat basalt surface and depositing sand and gravel in the central portion of the Pasco Basin. This pattern continued for the next 5 million years, with two major interruptions occurring when the Columbia River was blocked downstream, which caused a lake to develop in the Pasco Basin. Relatively thick mud layers accumulated in the lake each time. The mud layers are much less permeable than the sand and gravel layers, and act as partial barriers to vertical ground-water flow within the Ringold Formation.

About 3.4 million years ago, the Columbia River began to erode, rather than deposit, sediments in the Pasco Basin. The uppermost lacustrine mud was eroded from much of the Pasco Basin, and in places an impermeable caliche layer, part of the Plio-Pleistocene unit, developed on the eroded Ringold surface. The Ringold Formation sediments have undergone varying degrees of consolidation and cementation, which has decreased their permeability.

The Hanford formation sediments in the Pasco Basin are represented primarily by sand and gravel deposited by catastrophic ice age floods during the past 700,000 years. These floods were caused by collapse of glacier ice dams blocking an immense lake in Montana. The floodwater eroded some of the sediments in the Pasco Basin and deposited large gravel bars in the main channels and sand in the turbulent areas. The Hanford formation sediments are unconsolidated and generally much more permeable than similar Ringold Formation sediments. In places, these sediments are covered by up to a few meters of recent alluvial or windblown deposits.

More detailed information on the geology of the Pasco Basin can be found in Connelly et al. (1992a and b), DOE (1988), Hartman and Lindsey (1993), Lindberg (1993a and b), Lindsey and Jaeger (1993), and Swanson (1992).

Ground-Water Hydrology

The saturated thickness of the unconfined aquifer is greater than 61 m (200 ft) in some areas of the Hanford Site and thins out along the flanks of the basalt ridges (Figure 5.8.3). Depth from the ground surface to the water table ranges from less than 0.3 m (1 ft) near the Columbia River to more than 106 m (348 ft) in the center of the Site. The unconfined aquifer is bounded below by either the basalt surface or, in places, the relatively impervious clays and silts of the Ringold Formation. The water table defines the upper boundary of the unconfined aquifer. Laterally, the unconfined aquifer is bounded by the basalt ridges that surround the Pasco Basin and by the Yakima and Columbia Rivers. The basalt ridges have a low permeability and act as a barrier to lateral flow of ground water where they rise above the water table (Gephart et al. 1979). The elevation of the water table in meters above mean sea level for the Hanford Site and adjacent portions of Franklin and Grant Counties is shown in Figure 5.8.4.

The water-table elevation contours shown in Figure 5.8.4 indicate the magnitude of hydraulic gradient in the unconfined aquifer. Ground-water flow is generally perpendicular to the water-table contours from areas of higher elevation or head to areas of lower head. Areas where the contours are closer together are high-gradient areas where the "driving force" for ground-water flow is greater. However, sediments with low permeabilities inhibit ground-water flow and produce steeper gradients, therefore high gradient does not necessarily mean high ground-water velocity. The permeability of the Ringold sediments is generally lower than that of the Hanford sediments, so lower transmissivity and steeper gradients are often associated with areas where the water table is below the Hanford formation. Figure 5.8.5 shows the distribution of transmissivity used in current ground-water flow models.

Recharge to the unconfined aquifer originates from several sources (Graham et al. 1981). Natural recharge occurs from infiltration of precipitation along the mountain fronts, runoff from intermittent streams such as Cold Creek and Dry Creek on the western margin of the Site, and limited infiltration of precipitation on areas of the Hanford Site that have loose soil. The unconfined aquifer is recharged by the Yakima River where it flows along the southern boundary of the Hanford Site. The Columbia River recharges the unconfined aquifer for short periods during high stages when river water is transferred to the aquifer along the riverbank. For most of the year, the Columbia River is the primary discharge area for the unconfined aquifer. Recharge from infiltration of precipitation is highly variable on the Hanford Site and depends on soil texture, vegetation, and climate (Gee et al. 1992). The recharge rate from precipitation ranges from near zero, where fine-grained soils and deep-rooted vegetation are present, to more than 10 cm/yr (4 in./yr) in areas where soils are coarse-textured and bare of vegetation.

Large-scale artificial recharge to the unconfined aquifer occurs from liquid-waste disposal in the operating areas and offsite agricultural irrigation. The operational discharge of water has created two major ground-water mounds in the 200 Areas. The first of these mounds was created by past disposal at U Pond in the 200-West Area. The water table beneath U Pond rose 18 m (59 ft) from 1950 to 1980 (Newcomer 1990). This mound is slowly dissipating because the pond was decommissioned in 1984. The second mound was created by discharge to B Pond, east of the 200-East Area. The water-table elevation near B Pond increased by a maximum of about 9 m (29 ft) before 1990 (Newcomer 1990) and has decreased slightly over the last 5 years because of reduced discharge. These mounds have altered the unconfined aquifer's natural flow pattern, which is generally from the recharge areas in the west to the discharge areas (primarily the Columbia River) in the east and north. Water levels in the unconfined aquifer have changed continually during Site operations because of variations in the volume and location of waste water discharge. Consequently, the movement of ground water and its associated constituents has also changed with time. Ground-water mounding has also occurred in some of the 100 Areas and the 300 Area. Ground-water mounding in these areas is not as great as in the 200 Areas because of lower discharge volumes, high permeability and proximity to the Columbia River.

Water-table elevations are currently declining in response to the decrease in liquid-waste discharges from Hanford operations. One result of decreasing water levels is that a number of monitoring wells are becoming difficult to sample or are going dry. A ground-water flow model based on predicted changes to discharge indicated that this trend will continue, and many more wells will become impossible to sample during the next 10 years (Wurstner and Freshley 1994).

In the 100 and 300 Areas, water levels are greatly influenced by river stage. Water levels in the Columbia River fluctuate greatly on annual and even daily cycles. The river level is controlled by the operation of Priest Rapids Dam upstream of the Hanford Site. As the river stage rises, the increased water pressure is transmitted inland, increasing water levels in wells near the river. Very near the river, water will flow from the river into the aquifer as the river stage rises and will flow in the opposite direction as the river stage falls. This produces some dilution of contaminants near the river and makes it difficult to define the exact extent of contamination.

Recharge from irrigation in the Cold Creek Valley enters the Hanford Site as ground-water flow across the western boundary. Recharge to ground water across the Columbia River from the Hanford Site is primarily from irrigation and irrigation canal leakage. As indicated in Figure 5.8.4, the water-table elevation to the east of the Columbia River is from 100 to 150 m (328 to 492 ft) higher than the water-table elevation on the Hanford Site.

Figure 5.8.1

Figure 5.8.2

Figure 5.8.3

Figure 5.8.4

Figure 5.8.5

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