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ABSTRACT: Groundwater Sources and Flow Patterns Derived from Stable Isotopes and Elemental Chemistry in the Southeast Louisiana Freshwater Aquifers

Gerard J. Gonthier, Paul Aharon

The freshwater aquifers of southeast Louisiana constitute the core of the giant "Southern Hills" regional aquifer system that is confined to the north and east by outcropping Miocene beds and the groundwater divide, respectively, and to the south and west by the respective flushing limits. The aquifer system above -900 m consists of interbedded sands and confining clays that are Miocene and younger in age and are dipping south at about 2.8 m/km. The freshwaters contained in the stratified sand aquifers have a gravity-driven fluid flow and a dominant cation-anion combination of NaHCO3. These aquifers provide the main source of drinking water for the Baton Rouge metropolitan area and its industrial corridor. Intensive pumping of groundwater from the deep aquifers since the early 1900s has resulted in a pronounced cone of depression centered north of Baton Rouge, and a significant decrease in hydraulic heads, which affected substantially the velocities and directions of groundwater flow.

In order to evaluate the groundwater origins and flow patterns in the stratified aquifers, and predict possible migration path of contaminants from hazardous waste disposal sites into the aquifers, we analyzed oxygen and hydrogen isotopes and the sodium and chloride contents of 73 water wells above -900 m (<3000 ft) depth. Precipitation and Mississippi River water samples were also analyzed for isotopes and elemental chemistry for the duration of the study in order to establish the hydrological background of possible recharge sources to the aquifers.

The mean ^dgr18O value for precipitation in the study area is -3.8 ^pmil (SMOW) whereas the Mississippi River at St. Francisville has an annual weighted mean ^dgr18O value of -6.9 ^pmil. ^dgr18O values of freshwater wells range from -3.5 to -4.7 ^pmil and their ^dgr18O/^dgrD relationship obeys Craig's meteoric water line equation. The maximum age of the groundwater in the aquifer system is estimated to be between 4000 and 8000 YBP on the basis of Darcy's law. Stable isotopes and hydraulic head distributions suggest that rainfall, and not the Mississippi River, is the primary source of groundwater in the aquifer system. On a north-south cross section through the aquifers, ^dgr18O values are highest at shallow depths and a tongue of ^dgr18O values greater than -4.0 ^pmil extends into the deep aquifers coincidental with the cone of depression. Spatial distribution of ^dgr18O, Na, and Cl suggests that groundwater less than 100 years old is downwelling at a fast rate of about 5 m/year as a result of the excessive pumping producing the cone of depression in the Baton Rouge area. Vertical hydraulic head gradients are sufficiently high to produce downwelling from the shallow aquifers to the major "1500 ft" sand aquifer within 20 years. The latter estimate should be of concern in case of a catastrophic leakage of contaminants into the shallow aquifers.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990