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[1] Determining the controls on dissolved mercury (Hg D ) transport is necessary to improve estimations of export from unmonitored watersheds and to forecast responses to changes in deposition and other environmental forcings. Stream water Hg D and dissolved organic carbon (DOC) were evaluated over a range of discharge conditions in three streams within Shenandoah National Park, VA. Watersheds are distinguished by stream water pH (ranging from neutral to acidic) and soil size fractioning (ranging from clays to sands). At all sites, discharge was a significant but poor predictor of Hg D concentrations (r 2 from 0.13-0.52). Hg D was strongly coupled with DOC at all sites (r 2 from 0.74-0.89). UV absorbance at 254 nm (UV 254 ), a proxy for DOC quantity and quality, slightly improved the predictions of Hg D . Mean DOC quality differed between streams, with less aromatic DOC mobilized from the more acidic watershed. The site with less aromatic DOC and sandy soils mobilized more Hg to the stream for the same quantity and quality of DOC, likely due to the reduced capacity of the larger-grained soils to retain Hg, leaving a greater fraction associated with the organic matter. A similar amount of 0.54 ng Hg D /mg DOC is transported at all sites, suggesting the less aromatic DOC transports less Hg per unit DOC, offsetting the effects of soil type. This research demonstrates that soil composition and DOC quality influence Hg D export. We also provide evidence that soil organic carbon is a primary control on Hg-DOC ratios (0.12-1.4 ng mg À1 ) observed across the U.S. and Sweden.Citation: Riscassi, A. L., and T. M. Scanlon (2011), Controls on stream water dissolved mercury in three mid-Appalachian forested headwater catchments, Water Resour. Res., 47, W12512,

Sediments and floodplain soils in the East Fork Poplar Creek watershed (Oak Ridge, TN, USA) are contaminated with high levels of mercury (Hg) from an industrial source at the headwaters. Although baseflow conditions have been monitored, concentrations of Hg and methylmercury (MeHg) during high-flow storm events, when the stream is more hydrologically connected to the floodplain, have yet to be assessed. The present study evaluated baseflow and event-driven Hg and MeHg dynamics in East Fork Poplar Creek, 5 km upstream of the confluence with Poplar Creek, to determine the importance of hydrology to in-stream concentrations and downstream loads and to ascertain whether the dynamics are comparable to those of systems without an industrial Hg source. Particulate Hg and MeHg were positively correlated with discharge (r(2)  = 0.64 and 0.58, respectively) and total suspended sediment (r(2)  = 0.97 and 0.89, respectively), and dissolved Hg also increased with increasing flow (r(2)  = 0.18) and was associated with increases in dissolved organic carbon (r(2)  = 0.65), similar to the dynamics observed in uncontaminated systems. Dissolved MeHg decreased with increases in discharge (r(2)  = 0.23) and was not related to dissolved organic carbon concentrations (p = 0.56), dynamics comparable to relatively uncontaminated watersheds with a small percentage of wetlands (<10%). Although stormflows exert a dominant control on particulate Hg, particulate MeHg, and dissolved Hg concentrations and loads, baseflows were associated with the highest dissolved MeHg concentration (0.38 ng/L) and represented the majority of the annual dissolved MeHg load. Environ Toxicol Chem 2016;35:1386-1400. Published 2015 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US Government work, and as such, is in the public domain in the United States of America.