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 <0.24 and 3,975 pg/g in sludge. Based on the historical average reported TEQ concentration, the annual load of dioxins to the HSC via wastewater (assuming all the potential sources in the area discharge dioxins) was estimated to be about
2.9 g TEQ/yr.
Data will be presented on the extensive sampling effort of water, sediment, and tissues, as well as tributaries, treatment plants and air deposition that has been conducted in the HSC. Overall, more than 1270 samples have been collected in all media for dioxins/furans. This
includes over 280 samples using the high-volume water sampling techniques for both particulate and dissolved dioxins/furans in ambient water, municipal and industrial effluent. Results from five sampling events from 2002 – 2005 showed that, in general, dioxin levels in the main channel
continue to be as high or 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 for saltwater (0.093 pg/L) more than 80% of the time. In addition, 83% of the sediment
samples exhibited dioxin levels higher than the TOC-normalized sediment concentration target of 117 ng/kg. Finally, the fish tissue health-based standard of 0.47 ng TEQ/kg 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 1001 showed the highest concentrations in water and sediment, while fish tissue was highest in 1006 and crabs collected from segment 1005 exhibited the highest dioxin levels. A QUAL-TX model was used in a preliminary effort to simulate
the point and non-point sources of dioxin and the resulting ambient dioxin water concentrations in the Channel. A more refined model using WASP has now been established to use for the TMDL. A summary of municipal and industrial loads as well as nonpoint source runoff (utilizing HSPF) and atmospheric
deposition will also be presented with the modeling results.
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