Regional geochemical study of the Western Interior Plains Aquifer System and the Great Plains Aquifer System in the mid-continent, United States
Abstract
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.
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