Geochemical Modeling of Ground Water in the Vicinity of the Pantex Plant, Southern High Plains, Texas

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1995

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The Ogallala (High Plains) aquifer is the largest aquifer in the United States (Zwingle, 1993). Beneath the Southern High Plains of Texas, pumpage for irrigation and, to a lesser extent, for municipal, industrial, and domestic uses since the 1930s has caused water levels in the aquifer to decline locally more than 30 m (Knowles and others, 1982; Dugan and others, 1994). These declines have motivated numerous physically based studies of rates and sources of recharge to the Ogallala aquifer, as reviewed by Mullican and others (1994). However, hydrochemical studies have been fewer. Hydrochemical studies can complement physically based studies by delineating sources of recharge and rates of subsurface water movement as well as by elucidating controls on water quality. In particular, studies of compositional evolution during recharge to and flow within the Ogallala aquifer, such as have been conducted for other regional aquifers in North America (for example, Hendry and Schwartz, 1990; Plummer and others, 1990; Murphy and others, 1992), have been limited. In this report, we discuss the results of solute-speciation modeling involving data from wells sampled by the Bureau of Economic Geology (BEG) in the vicinity of the Pantex Plant (fig. 1). For a subset of these wells, solute data and data on the isotopic compositions of waters, soil gases, and sediments have been entered into a reaction-path (net mass-transfer) model. We use speciation and reaction-path modeling together to (1) corroborate inferred sources of recharge, (2) constrain estimates of travel times through the unsaturated zones above and below the perched aquifers, and (3) identify plausible reactions controlling the composition of groundwater.

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