WILCOX AQUIFER, NORTHWEST LOUISIANA: INFLUENCES ON GROUNDWATER CHEMISTRY AS DETERMINED FROM CHLORIDE AND BROMIDE CONCENTRATIONS

Groundwater samples from Wilcox (Paleocene) and Upland Terrace (Pleistocene) aquifers in the vicinity of Shreveport-Bossier City, Louisiana were analyzed to determine the impact on water quality, if any, of human activity, regional weather patterns, and spatial variations in aquifer composition. Samples collected from over 1100 private water wells in southern Caddo, southern Bossier, and northern DeSoto parishes between Aug. 2010 and Dec. 2011 were analyzed for anion concentrations using ion chromatography (IC) and cation concentrations using inductively coupled plasma optical emission (ICP-OES). Concentrations of chloride, [Cl-], and the ratio of chloride to bromide concentrations, [Cl-]/[Br-], are indicators of the impacts of recharge source, aquifer chemistry, climate variations, and anthropogenic activity on groundwater chemistry. For the majority of wells sampled [Cl-] varies mainly between 5 ppm to 500 ppm (mean = 88.2 mg/L, median = 32.9 mg/L) while [Cl-]/[Br-] is confined mostly to the range 100 to 700 (mean = 343.9, median = 253.8) and is, as a cluster, insensitive to variations in [Cl-]. These values are consistent with groundwater chemistry derived from felsic volcanic compositions as documented in published studies. The mineral assemblage of Wilcox sandstone includes a significant percentage (20%) of relatively fresh amphiboles, zoned plagioclase (~An50), and magnetite, as well as doubly terminated euhedral quartz crystals, all interpreted here as volcanigenic. The relative invariance of [Cl-]/[Br-] over a range of [Cl-] values suggests water chemistry influenced primarily by spatial variations in aquifer mineralogy and possibly by fluctuations in recharge rates. A mixture of high [Cl-], low [Br-] brine appears to be present in only a small number of isolated instances.