Legacy contamination by PAHs, pesticides, and metals in an urban mangrove estuary (Florida, USA), and the importance of sediment-core analyses to assessing future risks.

Clam Bayou is a mangrove-dominant urban estuary in Boca Ciega Bay (Florida, USA). Concerns about potential contaminants in the estuary prompted Florida Department of Environmental Protection to implement a study to document contemporary and historical potential contamination. This study examined polycyclic aromatic hydrocarbon, organic-pesticide, metal and metalloid analytes in 5 sediment cores and 42 surface-sediment samples from various locations in Clam Bayou. Twenty-six out of 81 analytes exceeded potential or probable sediment-quality toxicity guidelines for benthic aquatic organisms. Because multiple contaminants often exceeded TEL or PEL guidelines in the same samples, toxicity risks are likely to be higher than expected for individual contaminants. We detected localized hotspots of potential toxicity in mangrove surface sediments, and legacy contamination up to 60 cm deep in sediment cores that resulted from historical contaminant deposition. 210Pb dating of one sediment core revealed that accumulation rates of many of these analytes have decreased in recent years, but potential contaminant concentrations have continued to increase. The present study highlights the importance of site-selection criteria and careful monitoring of contaminant concentrations in urban estuaries, and a need for more routine analysis of deeper sediments in risk assessments. This is particularly important in estuaries that are vulnerable to increased risks of erosion and redistribution of potentially contaminated sediments caused by increased flooding from rises in tropical storm and hurricane activity.