Abstract Two distinct groundwater systems in the southern Laramie Basin, Wyoming, are inferred from water composition diagrams developed using physico-chemical data collected at springs and wells. The two groundwater systems are: (1) the Casper comprised of, in ascending order, Precambrian igneous and metamorphic rocks, Pennsylvanian Fountain Fm. (arkosic sandstone), and Permo-Pennsylvanian Casper Fm. (interbedded sandstone, limestone, dolomite(?)), and (2) Permian age Satanka shale and Forelle limestone followed by the Triassic age Chugwater shale and mudstone, collectively named the Redbed groundwater system. Sub-aquifers are existent in both groundwater systems as a result of interbedded impermeable material. A series of faults deform the Casper system where it outcrops along the western flank of the Laramie Range. The Redbed system confines the Casper system from just east of the city of Laramie continuing west into the basin. Casper wells west of the confining contact are free-flowing. Both aquifers receive recharge through local precipitation, and ground-water flow direction is generally east to west.
Water in the Casper system has a uniform chemical composition over most of the study area, dominated by Ca (13-85 mg/L) and HCO3 (129 - 251 mg/L). Chemical speciation calculations conducted using WATEQ2 (Ball et al., 1980) demonstrate that equilibrium with respect to calcite (limestone) is attained almost immediately upon infiltration and recharge. Low concentrations of Na, K, and Cl reflect precipitation input modified by a slight evapotranspiration signature, and no effect of equilibration with shales, K-feldspar, or halite.
Higher solute concentrations are observed in the Redbed system with the following ranges: Ca 37 - 480 mg/L; Mg 20 - 126 mg/L; HCO3 181 - 392 mg/L; and SO4 12 - 1580 mg/L. The large range in concentrations for the major elements are the result of wells sampling from sub-aquifers within the Redbed system. Concentrations of Na, K, and Cl are also low, reflecting the effect of evapotranspiration enhancement.
Mixing between the two groundwater systems is inferred from water composition diagrams in a small cluster of Casper wells (C15, C16, C22, C41, C48, C49, C65, C93) located in the middle of the basin. Two competing hypotheses exist for the mixing mechanism: (1) instantaneous mixing within the well during extraction, caused by either well failure or a screened interval spanning the contact between the Casper and Redbed groundwater systems, or (2) mixing of waters at the contact between the two groundwater systems. In the Redbed system, mixing occurs via two mechanisms, (1) along faults, and (2) in wells completed over more than one sub-aquifer.
Major element chemistry obtained for this study indicate no evidence of anthropogenic contamination in either aquifer. However, no chemical data currently exist for either pesticides or human waste by-products. Local sources of contamination from industry (petroleum, railroad ties) are known to exist, but specific analyses are required for detection.
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