on Stratigraphy, Spatial Distribution, and Proximity to Lignite
in Southern Caddo Parish, Louisiana
Douglas Carlson and Thomas Van Biersel
Louisiana Geological Survey – Louisiana State University,
3079 Energy, Coast and Environment Bldg., Baton Rouge, Louisiana 70803
The Wilcox Aquifer is the principal source of groundwater for Bossier, Caddo, De Soto and Sabine parishes in northwestern Louisiana. It appears, from examination of approximately 140 wells, that water chemistry within the Wilcox Aquifer is a function of sand horizon, spatial distribution and proximity to lignite seams.
Water samples were collected from two sand layers located within the base of the Wilcox Aquifer south of Shreveport, Louisiana. The upper sand (15 samples), as determined from the well screen centers, is located typically 50 to 100 ft below the surface. The deeper sand (108 samples) is located typically 160 to 200 ft below the surface. The lower sand has significantly difference concentrations of bromide, calcium, chloride, magnesium, manganese, pH, silica and sodium than the upper sand.
Water chemistry within the lower sand, as a function of spatial distribution, was considered for a select set of quantifiable parameters (>100 values above detection limit). In general, for most species, the areas of higher concentration lied southeast and southwest of Shreveport. Within some of these areas, a few concentrations of chloride, iron, and total dissolved solids (TDS) exceed the Environmental Protection Agency (EPA) national secondary drinking water standards.
The samples were also categorized by the proximity to lignite into three categories: wells without lignite; wells with
lignite within borehole but not within screen interval; and wells with lignite within screen interval. There were at least 30 wells sampled within
in each category. Lignite presence or absences was determined from review of the well completion reports submitted to the Water Resources Division
of the Louisiana Department of Transportation and Development. The presence of lignite within the well boring has a significant impact on concentrations
of ammonia-nitrogen, boron, chromium, fluoride, phosphate, sodium, and strontium.