Unleashing a dioxin legacy

For decades, paper mills, municipal waste incinerators, and petrochemical industries lining the banks of the Houston Ship Channel (HSC) have filled the waters with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its equivalents. The sediments lying underneath still bear that toxic legacy. A new study published today on ES&T’s website (DOI: 10.1021/es062917p) suggests that soil erosion, hurricanes, tides, shipping, and dredging may be churning up contaminants, transporting them to different parts of the channel, and reintroducing them into the aquatic environment. The findings may help reveal why mysteriously high dioxin levels are in fish and shellfish in the channel despite tight regulations on emissions.

The HSC is a 52-mile-long, very narrow channel and is one of the busiest ports in the U.S. It receives freshwater from the San Jacinto and Trinity rivers and connects to the Gulf of Mexico via Galveston Bay. High concentrations of dioxins have been found in blue crab and catfish in the HSC since Texas first started testing for the chemicals in the early 1990s. In other places in the state, tissue levels of dioxins plummeted within 1–2 years of reductions in emissions, but in the HSC, levels have remained almost constant, even after a decade of tightened regulations on industries and years of cleaning up. This suggests that “something different was happening [there]”, says Larry Koenig, manager of the Texas Commission on Environmental Quality’s (TCEQ’s) HSC Total Maximum Daily Load (TMDL) project, which is determining acceptable amounts of daily dioxin input into the channel.

“We’re finding alarming amounts of it, and what we want to know is: is this dioxin derived from continuing inputs, or is it legacy dioxin,” says Kevin Yeager of the University of Southern Mississippi’s department of marine sciences, who teamed up with Hanadi Rifai of the University of Houston’s civil and environmental engineering department and other colleagues for the current study.

Yeager and his team collected 55 sediment cores from the upper part of the channel. Most of the cores were so heavily mixed that they were unsuitable for analysis. Continued dredging, shipping, and hurricanes have caused heavy mixing of the channel’s sediments and the dioxins in them. The team settled on eight sediment cores that showed the least amount of mixing and analyzed them for the rate of sedimentation and the rate of deposition of 17 of the most toxic dioxins and other airborne pollutants. They also tested for dioxin content and changes in dioxin concentrations within each core. They compared these findings with a background atmospheric dioxin deposition rate obtained from one sediment core from a wetland farther northwest.
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