Whole Sediment TIEs: A TMDL Case Study

Patrick Bayou, a shallow, tidal tributary of the Houston Ship Channel located in Harris County, Texas, receives treated industrial and municipal wastewater and stormwater discharges, as well as non-point source stormwater runoff from nearby suburban areas. Based on an assessment of water quality, the Texas Commission on Environmental Quality (TCEQ) placed the bayou on the State of Texas 303(d) list of impaired waters due to chronic sediment toxicity. The Patrick Bayou Lead Organization, comprising NPDES-permitted dischargers to the bayou, was formed to assess impairment in the Bayou and develop a TMDL where appropriate.The technical assessment study plan for Patrick Bayou sediments was based on use of the Sediment Quality Triad (SQT) approach. The SQT seeks to integrate sediment physical-chemical characteristics data, sediment toxicity data, and benthic community structure information in a weight-of-evidence framework to support impairment evaluations. Therefore, sediment chemistry profiles, whole sediment toxicity test results, and benthic community structure analyses were obtained at multiple sampling stations in the bayou during August 2001-October 2002.Chemical analysis identified many contaminants in the Patrick Bayou sediments. Whole sediment bioassays conducted with Leptocheirus (an amphipod) and Neanthes (a polychaete), showed that all of the sediments were toxic, to one or the other test organism, at least once during the study period. In contrast, pore water toxicity was not observed in any of the samples that exhibited whole sediment toxicity. The lack of pore water toxicity showed that the causative toxicants were tightly bound to the sediment.Since pore water did not exhibit toxicity, whole sediment toxicity identification evaluation (TIE) was attempted. Whole sediment TIE is an emerging science and standardized protocols have not been established; however, a variety of experimental methods, including use of synthetic resins, ion-exchange media, and activated charcoal, were identified and incorporated into the sediment TIEs.Although isolation of toxic fractions and identification of individual constituents was not possible, organic compounds (rather than metals, or other chemical constituents) were identified as a potential toxicant(s). Results of the sediment TIEs will be presented and the capabilities of the current methods to resolve sediment toxicity in samples from complex, industrialized settings will be discussed.