Regional geochemical study of the Western Interior Plains Aquifer System and the Great Plains Aquifer System in the mid-continent, United States
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A wide range of chemical and isotopic variability characterizes the Great Plains Aquifer System (GP) and the Western Interior Plains Aquifer System (WIP), reflecting a range of fluid mixing processes and water:rock interaction. The study area extends 1200 km from eastern Colorado to central Missouri and from Nebraska to the Oklahoma panhandle. The GP comprises Cretaceous sandstones that are both overlain and underlain by shales. The WIP comprises carbonates and sandstones of Cambrian through Mississippian age. This aquifer system underlies Pennsylvanian through Jurassic shales and overlies the Precambrian basement. The WIP groundwater has been classified into three distinct groups; Group 1 is a fresh water originating in the Ozark Plateau, Group 2 is a saline groundwater possibly originating in the Colorado Front Range, and Group 3, originating in the Anadarko Basin, is the most saline groundwater of the three. Results from fluid mixing models suggest that the groundwater in the WIP can be explained by the mixing between these three groundwater groups. The salinity of the WIP could be the result of two different mechanisms, halite dissolution and evaporation of seawater. The GP groundwater can be divided into two groups; GP1 is a fresh groundwater present in the unconfined portion of the aquifer while GP2 is a saline groundwater present in the confined portion of the aquifer. GP2 groundwater acquired its salinity from interaction with groundwaters upwelling from the Permian shales and evaporites. The strontium isotopic composition of GP2 also implies that these groundwaters have mixed with groundwater migrating upwards from the Pennsylvanian shales. The light isotopic signature of both GP1 and GP2 suggests that this groundwater recharged at higher elevations and colder climatic conditions in the Colorado Front Range. Fluid-mixing modeling results between the two groundwater types suggest that the groundwater from the confined system and the groundwater from the unconfined system are not mixing.