ABSTRACT

The freshwater aquifers of southeast Louisiana constitute much of the "Southern Hills" regional aquifer system. Freshwaters above -3,000 ft (-900 m) are saturating interbedded sands and confining clays of Miocene or younger age and have a gravity-driven fluid flow. Intensive pumping of groundwater since the early 1900's has resulted in a pronounced cone of depression in Baton Rouge which has affected the velocities and directions of groundwater flow.

To evaluate groundwater origins and flow patterns and predict possible migration path of land surface contaminants, we analyzed oxygen and hydrogen isotopes and the sodium and chloride contents of 73 water wells above -3,000 ft (-900 m) depth. Precipitation and Mississippi River water samples were also analyzed for isotopes and elemental chemistry for the duration of the study.

The mean ¹⁸O values for precipitation and the Mississippi River in the study area in 1988 were -3.8 and -6.9 , respectively. Values of ¹⁸O for freshwater wells range from -3.5 to -4.7 and their ¹⁸O -²H relation obeys Craig's meteoric water line equation. Stable isotopes and hydraulic head distributions suggest that rainfall is the primary water source of the aquifer system. Spatial distribution of ¹⁸O, Na⁺ and Cl^- suggests that groundwater is downwelling as a result of the cone of depression in the Baton Rouge area.