Total mercury and methylmercury in freshwater and salt marsh soils of the Mississippi river deltaic plain

Mercury entering wetland environments can be microbially methylated to methylmercury. The purpose of this study was to investigate the historical rate of mercury accumulation and distribution of total and methylmercury in soil profile of Louisiana coastal marshes. Two sediment cores each were taken from Louisiana freshwater marsh and salt marsh. Vertical accretion was determined using the 137Cs dating technique. Total and methylmercury were determined with depth in the soil profiles. The fresh marsh soil on a dry weight basis contained more total and methylmercury than the salt marsh. Average vertical accretion rates in freshwater marsh and salt marsh were 0.90 and 0.75 cm year−1, respectively. Average total and methylmercury content (to a depth of 30 cm) was 140 and 4.19 μg kg−1 and 80 and 1.34 μg kg−1 for the fresh and salt marsh, respectively. Due to greater sediment input resulting in a higher bulk density the salt marsh contained more total mercury per m2 (to 30 cm depth) than the fresh water marsh (5340 μg m−2 as compared to 2929 μg m−2). The amount of methylmercury per m2 to depth of 30 cm was approximately the same for each marsh.