Total Hg, methyl Hg and other toxic heavy metals in a northern Gulf of Mexico estuary: Louisiana Pontchartrain basin

Abstract

Concentration of total Hg, methyl Hg, and other heavy metals were determined in sediment collected along a salinity gradient in a Louisiana Gulf Coast estuary. Surface sediment was collected at established coordinates (n = 292) along a salinity gradient covering Lake Maurepas, Lake Pontchartrain, Lake Borgne and the Chandeleur Sound located in the 12,170 km2 Pontchartrain basin estuary southeastern coastal Louisiana. Lake Maurepas sediment with lower salinity contained higher levels of methyl Hg (0.80 μ g/kg) than Lake Pontchartrain (0.55 μ g/kg). Lake Maurepas sediment also had higher levels of total Hg (98.0 μ g/kg) as compared to Lake Pontchartrain (67.0 μ g/kg). Average total Hg content of Lake Borgne and the Chandeleur Sound sediment was 24.0 μg/kg dry sediment and methyl Hg content averaged 0.21μ g/kg dry sediment. Methyl Hg content of sediment was positively correlated with total Hg, organic matter and clay content of sediment. Methyl Hg was inversely correlated with salinity, sediment Eh and sand content. Total Hg and methyl Hg decreased with increase in salinity in the order of Lake Maurepas > Lake Pontchartrain > Lake Borgne/ the Chandeleur Sound. Lake Maurepas containing several times higher amount of methyl Hg in sediment as compared to Lake Pontchartrain and Lake Borgne and the Chandeleur Sound is an area that could serve as potential source of mercury to the aquatic food chain. Methyl Hg content of sediment in the estuary could be predicted by the equation: Methyl Hg = 0.11670–0.0625 × Salinity + 0.05349 × O.M. + 0.00513 × Total Hg - 0.00250 × Clay. Concentrations of other toxic heavy metals (Pb, Cd, Ni, Cu and Zn) in sediment were not elevated and was statistically correlated with sediment texture and iron and aluminum content of sediment.