Trace Metal Contamination of Sediments and Organisms from the Swan Lake Area of Galveston Bay

Abstract

Swan Lake is a sub-bay of the Galveston Bay system. The area received runoff from a tin smelter during the 1940s and 1950s via a dredged channel called the Wah Chang Ditch. The ditch was cut off by a hurricane protection levee in 1968. An industrial waste disposal facility borders the area on the north and the surrounding region is highly industrialized. Consequently, there is ample potential for metal contamination. In this study, concentrations of Ag, Al, As, Cd, Cu, Fe, Hg, Mn, Ni, Pb, Se, Sn and Zn were determined in sediments and organisms (oyster, mussel, snail, crab, fish, shrimp, and marsh grass). The geographic trends in trace metal concentrations in sediments strongly suggest that tin smelter residues cause elevations relative to sediment from Galveston Bay and other Texas bays. Especially high concentrations were found along the Wah Chang Ditch, near Highway 197 and in the NE corner of Swan Lake. Sediment in these areas was as high as 8000 ppm in Sn, 3700 ppm in Pb, and 500 ppm in As. Concentrations varied widely over small space scales and with depth in the sediment column due to differential erosion and deposition. Surface sediment was, however, generally much less contaminated than deeper sediment, indicating less input of metal recently than in the past. In contrast to other metals, anthropogenic inputs have not greatly influenced the natural concentrations of Fe, Al and Ni in sediments. Most organisms showed very small spatial variations in metal concentration. However, the oysters in Swan Lake were found to be enriched in most metals relative to those from Galveston Bay and other US Gulf of Mexico areas. The mussels analyzed for this study did not reflect the highly elevated metal concentrations in the sediments from which they were taken. Iron and Pb concentrations in oysters seemed to be directly related to sediment concentrations at each location. For the organisms collected, trace metal concentrations were generally in the order oysters > snail > crab > shrimp > fish.