Over a century of iron mining in NE Minnesota has created numerous open pits and stockpiles that line the northern fringe of the St. Louis River watershed. Sulfate (SO₄) released by sulfide oxidation in mine wastes crosses the landscape and enters the St. Louis River through a series of small tributaries. Sampling of waters from these and other tributaries from 2007 to 2009 revealed little correlation between dissolved SO₄ and methylmercury (MeHg) [1] concentration. The watershed conditions during these sampling events ranged from relatively dry to moderately wet. Many of the same tributaries were sampled following a major rain event in August 2010. These waters were found to have elevated MeHg levels compared to those from the earlier study, but total dissolved mercury (THg) was linearly correlated to dissolved organic carbon (DOC) for the entire data set (2007-2010). We argue that these chemical trends are a natural consequence of stream recharge processes associated with precipitation events. Most stream recharge occurs through the oxygen-depleted, organic rich materials that surround and line small streams throughout the watershed. MeHg/DOC and THg/DOC ratios in the water are defined initially as waters pass through reduced pore fluid environments where competing oxidation and reduction mechanisms buffer methyl and inorganic mercury concentrations and mobilize DOC. In-stream demethylation decreases MeHg/DOC ratio from the initial values, largely defined by pore fluid chemistry, but preserves the THg/DOC ratio. Departures from the consistent THg/DOC and MeHg/DOC relationships observed in the region may provide a means to evaluate where and when SO₄is impacting MeHg levels in streams.

^{1. Berndt, M.E. and T.K. Bavin, Methylmercury and dissolved organic carbon relationships in a wetland-rich watershed impacted by elevated sulfate from mining. Environmental Pollution, 2012. 161: p 321-327.}