Development of a Dioxin TMDL for the Houston Ship Channel in Texas

Polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxin) are extremely persistent in the environment, and can affect human health at low concentrations for many years. As a result of dioxin found in fish and crab tissue, a seafood consumption advisory was issued by the Texas Department of Health in September 1990 for the upper portion of Galveston Bay and the Houston Ship Channel System (HSC) in Texas. As a result of the advisory, the HSC system was placed on the 303(d) list and a Total Maximum Daily Load (TMDL) study was initiated. This paper presents the current results of the most comprehensive, aquatic multimedia dioxin study performed in the US and summarizes loads from significant sources such as municipal and industrial WWTPs, nonpoint source, and atmospheric deposition.Historical data compiled and analyzed in this study confirm that dioxins are present at elevated concentrations in sediment and tissue. Sediment total Toxicity Equivalency (TEQ's) concentrations found in the main channel were between 2 and 43 times the average background TEQ concentration observed in North America (5.31 ppt). In addition, analysis of tissue data indicated that the segment with the highest average tissue TEQ concentration (Segment 1006- Houston Ship Channel Tidal) had an average TEQ-WHO98 of 7.1,15.2, and 8.2 ppt in catfish, crab, and oyster tissue, respectively. These values were 4, 25, and 14 times the concentrations observed in non-impacted areas in catfish, crabs, and oysters, respectively.Major dioxin sources were believed to have been historical inputs from bleached kraft paper mills. Effluent data gathered from permit files and permit applications for these industries show that TEQ concentrations varied between 4.85 and 17.46 pg/L in wastewater and between