GEOLOGIC CONTROL OF MERCURY TRANSPORT IN BIG CYPRESS BAYOU AND CADDO LAKE WATERSHEDS HARRISON COUNTY, TEXAS

Big Cypress Bayou flows along the border of Harrison and Marion counties in East Texas, and is one of the primary tributaries of the Caddo Lake watershed. The watershed covers approximately 164km²; the study area includes a 12 km segment of Big Cypress Bayou from the town of Jefferson, Texas to where it empties into the western portion of Caddo Lake. The stratigraphy in the area consists of the Eocene-aged Wilcox formation, overlain by formations from the Claiborne Group, namely the Reklaw and Queen City Sand. The Wilcox Formation is of special interest because it contains lignite coal, which has a direct link to mercury. Caddo Lake has been listed on the Texas Commission on Environmental Quality 303(d) list for impairment due to mercury content since 1995; previous research has focused on fossil fuel combustion as the primary source of dry deposition mercury, but has not addressed the potential watershed contribution to mercury concentrations in Caddo Lake.

Three sets of water and sediment samples were collected from ten sites along Big Cypress Bayou, selected based on physicochemical parameters (temperature, pH, conductivity, dissolved oxygen, and turbidity). Sample locations were determined by deploying a YSI 6920 multi-parameter sonde along a 12 km section of the waterway. Ten 30cm cores were extracted to determine particle size distribution and sedimentation rates. Geochemical sampling and analyses of water and sediment has been conducted since October 2016 to determine mercury concentration and potential environmental conditions that may contribute the liberation and transport of mercury through the Caddo Lake watershed. Geochemical analyses of water and sediments indicate waters with cation and anion concentrations reflective of the lithologies over which and through which water is transmitted, with negligible concentrations of mercury present.