The Rio Grande is the natural boundary between the United States and Mexico from El Paso, Texas, to Brownsville, Texas. It supports about 12 million people on both sides of the border for municipal, agricultural, industrial, and recreational uses. The rapid population and economic growth along the border region has led to increased pollution in the Rio Grande, which has been linked to several border health issues associated with pesticide contamination. This project was initiated to assess the organochlorine pesticide levels in the water and sediments in Manadas Creek, an urban tributary of the Rio Grande located in north Laredo, Texas. Water and sediment samples were collected monthly during a 6-month period from July to December of 2006 and analyzed using gas chromatography with an electron capture detector after extraction via a solid-phase microextraction technique. Among the water and sediment samples collected, several organochlorine pesticides including alpha-, beta-, and gamma-hexachlorocyclohexane (HCH), heptachlor epoxide, endrin, and 4,4'-DDT were found in either the creek water or sediments. Analysis of variance results indicated that only gamma-HCH had significant variation in the creek water among the sampling periods. Comparison of results with previous findings showed the presence of higher levels of HCH isomers and much lower DDT concentrations in the present study.

Stream–subsurface exchange plays a significant role in the fate and transport of contaminants in streams. It has been modelled explicitly by considering fundamental processes such as hydraulic exchange, colloid filtration, and contaminant interactions with streambed sediments and colloids. The models have been successfully applied to simulate the transport of inorganic metals and nutrients. In this study, laboratory experiments were conducted in a recirculating flume to investigate the exchange of a hydrophobic organic contaminant, p,p′‐dichloro‐diphenyl‐dichloroethane (DDE), between a stream and a quartz sand bed. A previously developed process‐based multiphase exchange model was modified by accounting for the p,p′‐DDE kinetic adsorption to and desorption from the bed sediments/colloids and was applied to interpret the experimental results. Model input parameters were obtained by conducting independent small‐scale batch experiments. Results indicate that the immobilization of p,p′‐DDE in the quartz sand bed can occur under representative natural stream conditions. The observed p,p′‐DDE exchange was successfully simulated by the process‐based model. The model sensitivity analysis results show that the exchange of p,p′‐DDE can be sensitive to either the sediment sorption/desorption parameters or colloidal parameters depending on the experimental conditions tested. For the experimental conditions employed here, the effect of colloids on contaminant transport is expected to be minimal, and the stream–subsurface exchange of p,p′‐DDE is dominated by the interaction of p,p′‐DDE with bed sediment. The work presented here contributes to a better mechanistic understanding of the complex transport process that hydrophobic organic contaminants undergo in natural streams and to the development of reliable, predictive models for the assessment of impacted streams. Copyright © 2015 John Wiley & Sons, Ltd.