The Role of Bank Erosion in Mercury Fux into a Contaminated Stream.

TITLE: Distribution and transport of mercury in a contaminated freshwater stream

JO Dickson, PD Ayres, TL Mehlhorn, LR Rodriguez, EM Pierce, SC Brooks, DB Watson, MJ Peterson, and MA Mayes

Mercury contaminated streams and rivers have been poorly studied compared with lentic aquatic systems. East Fork Poplar Creek (EFPC) in Oak Ridge, Tennessee, which received large quantities of mercury from an industrial facility at its headwaters in the 1950s, provides a useful case study for evaluating mercury distribution and transport in streams. Although mercury export from the upstream point source has substantially declined over the last few decades, recent results suggest that releases from diffuse and historical downstream sources, such as bank soils and stream sediments, may sustain mercury contamination levels in EFPC. Field- and laboratory-scale studies such as bulk chemical analysis and spectroscopic techniques–(SEM, EDS, EMP, XRD)–are underway to better understand the spatial distribution of these downstream sources and the complex physiochemical factors controlling mercury flux.

In this study we evaluated bank mercury flux by estimating erosion using a kayak-based continuous video survey, coupled with bank soil mercury and methylmercury analyses at 145 locations at 15%, 50% and 85% of creek bank heights. Annual bank erosion in EFPC (~1M kg/yr) increases downstream with increasing flow, whereas mercury and methylmercury concentrations in bank soil generally decrease downstream. Overall, the highest bank soil mercury fluxes were from the upper reach of the stream proximal to the point source.  Mercury concentrations of bank soils generally ranged from 7.7 to 24.9 mg/kg (25^th-75^th quartile), with a median of 15.7 mg/kg. Additional sampling revealed localized mercury enrichments exceeding 500 mg/kg. Less than 0.01% of the initial mercury sequestered in bank soils was released into solution, suggesting the recalcitrant nature of the mercury in the system