Importance of monsoon rainfall in mass fluxes of filtered and unfiltered mercury in Gwangyang Bay, Korea

Abstract

We investigated the effects of the East Asian Summer Monsoon (EASM), which brings approximately half of Korea's annual rainfall in July, on the concentration and particle–water partitioning, and sources of Hg in coastal waters. Surface seawater samples were collected from eight sites in Gwangyang Bay, Korea, during the monsoon (July, 2009) and non-monsoon dry (April and November, 2009) seasons and the concentrations of suspended particulate matter, chlorophyll-a, and unfiltered and filtered Hg were determined. We found significant (p<0.05) increases in filtered Hg in the monsoon season (1.8±0.019pM) compared to the dry season (0.62±0.047pM). In contrast, the Hg concentrations associated with particles showed no significant differences (p>0.05) between the monsoon (459±141pmolg−1) and the dry season (346±30pmolg−1), which resulted in decreased particle–water partition coefficients of Hg in the monsoon season compared to the values in the dry season: 5.7±0.1 in April, 5.3±0.1 in July, and 5.8±0.1 in November. The annual Hg input to Gwangyang Bay was estimated at 64±6.6molyr−1 and 27±1.9molyr−1 for unfiltered and filtered Hg, respectively. The Hg discharged from rivers was a major source of Hg in Gwangyang Bay: the river input contributed 83±13% of total input of unfiltered and 73±6.0% of filtered Hg. On a monthly basis, unfiltered Hg input was 17±11molmonth−1 in the monsoon season and 3.2±0.70molmonth−1 in the dry season, while filtered Hg input was 7.1±4.1molmonth−1 in the monsoon and 1.3±0.26molmonth−1 in the dry. Consequently, the EASM resulted in an unfiltered Hg input 5.3 times greater than the mean dry month input and a filtered Hg input 5.5 times greater than the mean dry month input, which is mainly attributable to enhanced river water discharge during the monsoon season.