Delineating Controls on the Compostition of Ground Water in the Vicinity of the Pantex Plant, Southern High Plains, Texas

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Chemical, isotopic, mineralogical, and bacteriological analyses indicate that the composition of groundwater in the vicinity of the Pantex Plant reflects distinctive sources of recharge (both natural and anthropogenic) modified by initial evaporation, sediment-water interactions, and microbial processes. As observed regionally by others, the major-ion fades are predominantly Ca-Mg-HCO3, reflecting ion exchange; dissolution of feldspars, calcite, sepiolite, palygorskite, and CO2 gas; precipitation of calcite, Na- and Ca-smectite, sepiolite, and palygorskite; exsolution of CO2 gas; and oxidation of organic carbon. Above-background concentrations of Cl- and NO3- indicate return flow of wastewater, although NO3- exceeds the drinking-water standard in only one well. NO3- concentrations tend to be limited during recharge by microbially mediated denitrification in the vadose zone beneath playas.

Elevated levels of 3H and 14C indicate focused recharge beneath both playas and ditches. Focused recharge is also indicated by values of δD and δ18O that are enriched beneath playas. The coincidence of plots of δD versus δ18O with local seasonal meteoric water lines suggests minimal evaporation, which may reflect episodic recharge or recharge along preferential pathways. Depleted δD and δ18O of H2O and enriched δ13C of dissolved inorganic carbon indicate some diffuse infiltration to perched aquifers in interplaya areas, although the timing of such recharge is uncertain. Dissolved Si and 14C provide evidence of percolation from the perched aquifers to the Ogallala aquifer beneath the Pantex Plant.


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