Metals in Sediments of the Upper Laguna Madre VIRENDER K. SHARMA *, KURTIS B. RHUDY , ROBERT KOENING and FELIPE G. VAZQUEZà  Department of Chemistry, Texas A&M University ± Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412, USA àInstituto de Ciencias del Mar y Limnologia, UNAM, Cd., Universitaria, D.F. Apartado Postal 70-305, C.P. 04510, Mexico The Laguna Madre system is the largest hypersaline coastal basin in the United States. Surface sediments from 22 Upper Laguna Madre (ULM) sites were analyzed for grain size and metals (Cd, Cu, Ba, Cr, Mn, Ni, Pb, V, Zn, Fe, and Al) to assess the extent of contamination in the area. Sediments were found to consist mainly of sand texture. Clay and silts were minor constituents (<10%) of the sediments. Anomalies in metal concentrations were found at some sites and were related to probable sources, i.e., recreational and industrial activities. Concentrations of metals were normalized to grain size, Al, and Fe to distinguish natural and anthropogenic sources. Most metals showed positive correlations (p < 0.001) with Fe and Al, suggesting a natural variability of metal concen- trations in sediments. Concentrations of metals, except Cd and Pb, at most sites were found to be below threshold concentrations thought to produce toxic eects in marine and estuarine organisms. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Laguna Madre; metals; sediments; contami- nation. The Laguna Madre of the Texas and Mexico coasts is one of the worldÕs great marine treasures. It is also one of a few known hypersaline estuary systems in the world, the hypersalinity related to limited freshwater in¯ows, high rates of evaporation, and limited exchange with the Gulf of Mexico. The Laguna Madre is eco- nomically important due to its natural and wildlife re- sources. One of the major concerns for the Laguna Madre is the Gulf Intercoastal Waterway (GIWW), a manmade 2092 km (1300 mile) long canal which runs along the northern coast of the Gulf of Mexico (Fig. 1). The GIWW is the third busiest canal in the United States, largely accommodating trac carrying petro- leum products. Other activities such as industrial and agricultural runo, dredging, and algal bloom may also contribute to contamination in the area (Sharma et al., 1997; Rhudy et al., 1999). Laguna Madre has therefore become a part of the Corpus Christi Bay National Es- tuary Program (CCBNEP), an eort to assess the eco- logical health of such systems. A number of human activities, smelting, mining, electroplating, and other industrial processes, have metals in their euents which may result in change in the ecological equilibrium of the area. Some, Mn, Fe, Zn, and Cu, are required by aquatic organisms. Others, Hg, As, and Pb, can be toxic to aquatic organisms, even at trace concentrations (Sunda, 1988, 1989). Therefore, types and amounts of metals entering the environment must be considered in assessing possible toxic eects. Generally, as metals enter a coastal environment, they are transferred to the sediments by various processes, i.e. adsorption onto particle surfaces and coprecipitation with solid phases (Santchi et al., 1990). These sediments become an important reservoir for metals and provide input records of metals to the ecosystem (Zwolsman et al., 1996). Our work encompasses the study of sediment grain size and metal concentrations in the Upper Laguna Madre. Results were normalized to grain size, Al, and Fe, to understand the processes controlling metal con- centrations in the sediments and to assess the extent of metal contamination in the area. Study Area The study area, see Fig. 1, is a coastal embayment extending 200 km (125 miles) southward from Corpus Christi to Port Isabel, Texas, near the Mexican border. The system consists of two shallow basins, an average depth of about 1 m, referred to as the Upper and Lower Laguna Madre. The basins are separated by a 40 km- long wind tidal ¯at known as Saltillo Flats (Brown et al., 1995). The warm and shallow water of the Laguna Madre enhances seagrass beds, a thriving source for ®n®sh and shrimp. Of the 44 ecologically important species of ®sh and invertebrates identi®ed in the Gulf of Marine Pollution Bulletin Vol. 38, No. 12, pp. 1221±1226, 1999 Ó 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/99 $ - see front matter PII: S0025-326X(99)00166-6 *Corresponding author. Present address: Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA. 1221