Total and methyl mercury in wetland soils and sediments of Louisiana's Pontchartrain Basin (USA)

Abstract

Accumulation of methylmercury (MeHg) in aquatic biota is a primary toxicological concern associated with Hg contamination in the environment. This study reports total mercury (THg) and MeHg measurements in 11 swamp and 24 marsh soils/sediments in wetlands surrounding Lake Pontchartrain and Lake Maurepas located in Louisiana's Pontchartrain Basin. The salinity level ranged from fresh, brackish to salt water. Average THg content in the swamp soils/sediments (112.3 microg kg(-1), n = 10) was significantly higher (P = 0.04) than in the marsh soils/sediments (56.5 microg kg(-1), n = 24). The THg content in the marsh soils/sediments tended to decrease with salinity increase, probably due to geographical locations of the sampling sites with less Hg input in more saline regions. Average MeHg content in the soils/sediments was 1.3 microg kg(-1) (n = 34), higher than reported values in the bottom sediments of Lake Maurepas (0.8 microg kg(-1), n = 27) and Lake Pontchartrain (0.6 microg kg(-1), n = 147). Average MeHg/THg ratio in the marsh soils/sediments (0.022) was considerably higher than in the swamp soils/sediments (0.012). Analysis of MeHg/THg ratio along the salinity gradient at the marsh soils/sediments show that the highest MeHg/THg ratio (up to 0.040, n = 5) was found at the fresh/brackish water sites, and the lowest (0.002, n = 1) at the salt water site. Results suggest that there was a greater potential for MeHg formation in wetland soils/sediments than in bottom sediments of adjacent lakes. Results suggest that wetland surrounding the lakes may be a potential source of MeHg to the aquatic food chain and significance is governed by area of the adjacent wetland.