Inputs and outputs of mercury from terrestrial watersheds: a review

Abstract

This review focuses on mercury (Hg) inputs and outputs in temperate and boreal terrestrial systems. It covers deposition via throughfall and litterfall, whose sum (ca. 38 µg m–2 a–1) is greater than that via precipitation (ca. 10 µg m–2 a–1). Outputs considered include volatilization, soil sequestration, and streamflow. The former is highly uncertain, but the mean rate (11 ng m–2 h–1) over a growing season is equivalent to about 32 µg m–2 a–1. Modern rates of soil sequestration (ca. 5 µg m–2 a–1) and streamflow fluxes (ca. 2 µg m–2 a–1) balance the annual budget. The majority of the uncertainty in the budget is related to volatilization. Nonetheless, a large fraction of atmospheric Hg is likely a product of continuing deposition and volatilization. Watershed characteristics related to streamflow fluxes of both Hg and methylmercury (MeHg) are discussed. Both runoff concentration and flux of Hg are weakly and inversely related to watershed size. Dissolved organic carbon (DOC) and particulates are important carriers of Hg; watershed activities that affect either affect Hg flux. Runoff flux of MeHg is skewed with about 80% of observations less than 0.15 µg m–2 a–1. Although there is no pattern of MeHg flux with watershed size, there is a strong positive relationship between flux and wetland area. Wetlands are a site of MeHg production and their presence increases water residence time; both increase MeHg flux. Concentrations of MeHg in streamflow from watersheds with wetlands are near the current water quality criterion, and effective control measures in those watersheds appear problematic.Key words: deposition, non-point pollution, methylmercury, wetlands, dissolved organic carbon (DOC).