Upward vertical leakage either throughout the basin, dominantly to the Columbia River, or dominantly to Lake Wallula has been proposed for the discharge of the lower units. Units underlying the Mabton interbed may have a flow scheme similar to those higher units or a flow scheme dominated by interbasin flow. The amount of upward vertical leakage from the Saddle Mountains Formation into the overlying sediments or to the Columbia River is unknown. Gable Butte - Gable Mountain probably is a ground-water sink, although the vertical flow direction in this zone is uncertain. Recharge areas for the Mabton interbed and the Saddle Mountains Formation are the highlands surrounding the basin with the flow for these units toward Gable Butte - Gable Mountain and Lake Wallula. Although a large amount of information is available on the more » hydrology of the unconfined aquifer system, ground-water flow within the basin is, in general, poorly understood. This sequence rests upon a basement of probably diverse rock types that may range in age from precambrian through early Tertiary. The stratigraphic sequence within the basin consists of an undetermined thickness of lower Miocene and younger flood basalts with interbedded and overlying sedimentary units. Pasco basin is a structural and topographic basin of approximately 2000 mi/sup 2/ (5180 km/sup 2/) located within the Yakima Fold Belt Subprovince of the Columbia Plateau. « lessĭiscussion of the regional setting, geology, hydrology, and geochemistry of the Pasco Basin are included in this report. Potentiometric data and hydrochemical evidence collected from recent studies indicate that geologic structures act as areal hydrologic barriers and in some instances, regions of enhanced vertical conductivity. In the Pasco Basin, geologic structures influence groundwater flow patterns within basalt aquifer systems. Results from numerical modeling are used for evaluating potential waste migration as a function of space and time. Data obtained from these studies provide input for numerical modeling of groundwater flow and solute transport. Currently, in excess of 150 test intervals have been tested for hydraulic properties, while in excess of 80 horizons have been analyzed for hydrochemical characteristics at about 30 borehole more » sites within the Pasco Basin. Hydrologic studies within the Pasco Basin involve the areal and vertical characterization of hydraulic head, hydrologic properties, and hydrochemical content for the various basalt groundwater systems. These studies are focused on the Hanford Site, which is located within the Pasco Basin in south-central Washington. = ,Īs part of the Basalt Waste Isolation Project (BWIP), hydrologic studies are being performed to provide an evaluation of groundwater systems within the Columbia River Basalt Group. Some basic modeling considerations of the hydrogeologic systems in the basin with a brief discussion of model input requirements and their relationship to available data are presented. The contaminant transport properties of the rocks in the Pasco Basin are analyzed with emphasis on the dispersion and sorption coefficients and the characteristics more » of the potential reactions between emplaced waste and the surrounding medium. The hydrochemical data obtained are summarized. A summary of the basic properties is tabulated. The hydraulic properties of the various aquifers and non-water-bearing strata are characterized and reported. #SUPERCAL 98G CALCIUM SERIES#This aquifer overlies a series of confined aquifers carrying progressively smaller amounts of groundwater as a function of depth. The subsurface hydrology of the Pasco Basin is characterized by an unconfined aquifer carrying the bulk of the water discharged within the basin. Short-lived ephemeral streams may flow for a short period of time after a heavy rainfall or snowmelt. The surface hydrology of the basin is dominated by the Yakima, Snake, and Columbia rivers. The regional geology of this basin is dominated by tholeiitic flood basalts of the Columbia Plateau. The Pasco Basin itself is a tract of semi-arid land covering about 2,000 square miles in south-central Washington. It also aims at compiling the properties required to evaluate contaminant transport potential within individual subsurface strata in this basin. This report aims at consolidating existing data useful in defining the hydrologic characteristics of the Pasco Basin within south-central Washington.
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