Distribution and Source Analysis of Elevated Chloride

Concentration in the Wilcox Aquifer of Northwestern Louisiana

Thomas P. Van Biersel and Douglas A. Carlson

Louisiana Geological Survey – Louisiana State University,

3079 Energy, Coast and Environment Bldg., Baton Rouge, Louisiana  70803

ABSTRACT

From November 2007 to May 2008 the Louisiana Geological Survey, under a grant from the Louisiana Department of Transportation and Development, sampled domestic water supply wells to determine groundwater chemistry variability in the Wilcox Aquifer of Southern Caddo Parish.  This information, as well as pre-existing data, was used to delineate sub-areas in Caddo Parish where chloride concentrations in groundwater exceed the U.S. Environmental Protection Agency’s National Secondary Drinking Water Standard.  This dataset identifies at least a dozen sub-areas where elevated chloride concentrations exist and potential chloride sources need to be investigated.  The distribution of elevated concentrations in the locally shallow Wilcox Aquifer was compared to the distribution of potential surficial and at-depth sources.  The sources under consideration are the saline Red River, several saline lakes, oil wells/fields/brine, faulting, and naturally occurring saline groundwater.  The wells sampled were drilled to a depth of 100 to 290 feet below the surface representing up to three sand layers interbedded with clay layers.  Locally, the next deeper sand layer is saline, and unusable for drinking water purposes.  This and deeper sands are separated from the freshwater sands by approximately 600 to 700 feet of clay (Midway Formation).

The results of the investigation indicate that most sub-areas where excessive groundwater salinity has been observed are locations at, or near, surface water bodies, which are known to have elevated salinities themselves.  These locations have little or no surficial clays protecting the aquifer.  Similarly, most locations are located near areas where the piezometric surface is depressed.  In addition, elevated saline concentrations near Hosston, Louisiana, and Black Bayou and Caddo Lakes, may be further affected by the presence of known faults.  Where data availability allows, a comparison of salinity tracers indicates that connate saltwater from within the aquifer, or from units below the aquifer, is the likely source of saline water.

Van Biersel, T. P., and D. A. Carlson, 2009, Distribution and source analysis of elevated chloride concentration in the Wilcox Aquifer of northwestern Louisiana:  Gulf Coast Association of Geological Societies Transactions, v. 59, p. 753-767.

AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana