Spatial Distribution of Arsenic and Antimony Levels in Manadas Creek, An Urban Tributary of the Rio Grande in Laredo, Texas

High concentrations of antimony and arsenic have been found along the Manadas Creek, an urban tributary of the Rio Grande in Laredo, Texas. This Creek is located in a highly industrial area of Laredo near warehouses, a major railway line and an antimony smelter decommissioned in 1999. To further examine the spatial distribution of arsenic and antimony contamination in this creek, five sites were selected along the creek with one being located in upstream of the antimony smelter and four in downstream. Water and sediment core samples were collected in February and April, 2008, (to represent a dry period). Conductivity, pH, salinity, temperature and flow rate were recorded during each sampling trip. Moisture content, organic carbon content and particle size distribution of sediment samples were determined in the laboratory. The total metal content of both the sediment and water were determined by digesting the samples in acid, followed by analysis using an ICP-MS. Fractions of metal concentrations associated with the sediment were determined using a sequential extraction method and analyzing via ICP-MS. Results show that the concentrations of antimony in the water and sediment samples are considerably high in the two sites located near the decommissioned antimony smelter. Antimony levels decrease in the sites located downstream. At the upstream site, the concentrations of both arsenic and antimony are noticeably low. The vertical distribution of arsenic in sediment remains constant at different depths, while antimony varies significantly. Sequential extraction analysis for the least contaminated site shows that 60 to 80% antimony is in its residual form, up to 20% bound to organic matter and the remaining in the soluble and surface adsorbed fractions. These results provide insight on the fate and transport patterns of arsenic and antimony in Manadas creek and valuable information that can be used in future remediation design.