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 dioxin TMDL study for the HSC. The study is focused on identifying the sources of dioxin to the channel and developing load reduction strategies using models.Historical data compiled and analyzed in this study confirm that dioxins are 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 (5.31 ppt) observed in North America. 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 are 4, 25, and 14 times the concentrations observed in non-impacted areas in catfish, crabs, and oysters, respectively.An extensive sampling effort targeting in-stream water, sediment, and tissue is underway. Results from two sampling events in 2002 showed that, in general, dioxin levels in the main channel continue to be as high or they are higher than the concentrations in sediment and tissue observed in 1990, 1993, 1996, and 2001. Dioxin concentrations in water exceed the Texas Surface Water Quality Standard (WQS) for saltwater (0.093 pg/L) more than 80% of the time. In addition, 83% of the sediment samples exhibit dioxin levels higher than the TOCnormalized sediment concentration target of 106 ng/kg. Finally, the fish tissue health-based standard of 0.47 ng TEQ/kg (derived from the Texas WQS) was exceeded in 96% of the samples and in 98% of the crab tissue samples. Overall, high concentrations of dioxins were distributed across the HSC system. Segment 1006 showed the highest concentrations in water, sediment, and fish tissue, while crabs collected from segment 1005 exhibited the highest dioxin levels.An in-depth assessment of possible sources of dioxin contamination to the HSC that includes sampling of sediments in tributaries, industrial and municipal wasterwater treatment plant sludges, effluents, wet/dry deposition and runoff has been conducted over the last two years. Analyses of sludge samples from point sources discharging to the HSC showed concentrations between 0.02 and 2355 ng TEQ/kg-dry weight with an average value of 62.2 ng TEQ/kg-dry weight. Ambient air sampling conducted nine times at five locations yielded dioxin concentrations within the range 2.66–48.60 fg TEQ/m3. Results from stormwater runoff sampling showed total TEQ concentrations between 0.021 and 0.567 pg/L with 8 out of 10 samples exceeding the Texas WQS.