Spatial Variations in Groundwater Chemistry, Southern Hills Aquifer System – A Three-Dimensional View

Jeffrey S. Hanor, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803: hanor@lsu.edu

Potable Na-HCO3 groundwaters are characteristic of siliciclastic aquifers in the Gulf of Mexico and Atlantic coastal plains. The origin of these waters has been variously attributed to cation (base) exchange and to silicate hydrolysis. The Southern Hills regional aquifer system in southwest Mississippi and southeast Louisiana provides an excellent natural laboratory for the study of the origin these waters in part because of the extensive areal coverage of available groundwater analyses. In addition, the U.S. Geological Survey defined a vertically-stacked series of five ‘regional permeable zones’, A (shallowest) through E (deepest), in a hydrogeologic modeling study of the Southern Hills system. Zones A through D have sufficient well control that areal variations in groundwater chemistry can be established for each. A vertical comparison of the zones thus provides a framework for a three-dimensional examination of spatial variations in groundwater chemistry.

Each of the four zones is characterized by a progressive evolution in groundwater chemistry from their respective recharge areas down-gradient to their pre-development discharge areas. There is initially a progressive increase down-gradient in the concentrations of dissolved silica, Na, Ca, and HCO3 as plagioclase feldspar is dissolved by microbial acids to form kaolinite. When the waters become saturated with smectite, dissolved silica concentrations now decrease down dip. Eventual saturation with respect to calcite and chalcedony appear to buffer dissolved Ca and silica at low concentrations, and the waters become truly dominated by Na and HCO3. The geographic locations of the transitions between these compositional domains vary by zone and appear to reflect differences in pre-development groundwater flow paths and groundwater residence times.

Key words: Na-HCO3 groundwater, Southern Hills aquifer, silicate hydrolysis, cation exchange
Preference: oral presentation

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