Sulfate and Mercury Chemistry of the St. Louis River in Northeastern Minnesota

The St. Louis River and its major tributaries were sampled upstream from Cloquet during periods of high, medium, and low flow between September 2007 and October 2008. Special emphasis was placed on measuring sulfate (SO4) and mercury (Hg) distributions as well as other chemical parameters that might help to determine whether SO4 releases from the Iron Range have an impact on Hg speciation in the St. Louis River. These included, but were not limited to, dissolved organic carbon (DOC), dissolved iron (Fe), and the isotopic ratios for sulfur and oxygen atoms in dissolved SO4 (?34SSO4 and ?18OSO4). Dissolved and particulate fractions of methyl mercury (MeHg), total mercury (THg), and bioavailable mercury (AHg) were additionally determined over a range of hydrologic conditions to identify primary source regions and transport mechanisms for Hg species. Results confirm that the majority of SO4 is derived from the iron mining district, and that SO4 added in the upstream portion of the St. Louis River is generally diluted downstream by waters from larger watersheds containing high percentages of wetlands. SO4, magnesium (Mg), calcium (Ca), sodium (Na), and chloride (Cl) concentrations all increase in the river, especially in the mining region, during periods of low-flow when groundwater inputs dominate chemistry of dissolved components. Variations in the relative concentrations of major elements and in ?34SSO4 and ?18OSO4 among the tributaries provide important clues to specific SO4 sources for each of the individual watersheds under varying flow conditions. Chemical data indicate that most SO4 from the mining region is derived from oxidation of small amounts of iron sulfide minerals present in stock piles, tailings, and pit walls containing Mg-rich carbonate minerals that are common in the Biwabik Iron Formation. Comparison with stream chemistry from 1955 to 1961 indicates SO4 sources were commonly present in the mining region before taconite mining became widespread in the region. Other chemical parameters in these data, particularly Ca and Mg, indicate the primary source for this SO4 was different from today. In contrast to SO4, Hg appears to be derived predominantly from wetlands, and is highest during periods of increased flow in the rivers. THg is well correlated to DOC concentration under most conditions, but quite variable during precipitation events when dissolved AHg and particulate SHg become more abundant in the rivers. MeHg concentrations in the St Louis River and its tributaries are also strongly correlated to DOC. Four sources of DOC are preliminarily inferred to be present in the river depending on the season and watershed characteristics, and it is believed that the relative amounts of DOC from each source may control the MeHg concentrations present in the river. These include: (1) DOC released from surface wetland areas containing low Fe (approximately 0.2 ng/mg Hg and 0.02 ng/mg MeHg in the DOC), (2) DOC containing almost no MeHg that is either produced in-stream or present in small amounts in natural groundwater, (3) DOC released from deep wetland areas following a major summer rain event containing very high MeHg and high Fe, and (4) DOC containing almost no MeHg in waters containing elevated dissolved Fe that seep slowly from deep within wetland areas under dry conditions. MeHg systematics appear to be very similar to those reported in two well-studied low- SO4 tributaries of the Rum River in east-central Minnesota. Additional sampling is planned to verify the above model and to more fully characterize mercury speciation during the warm summer months, particularly during periods when high Fe concentrations are present in the streams.
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