Formatted Title:

Evaluation of Multiple Sediment Amendments at a Mercury-Contaminated Reservoir Using Bench-Top Microcosm Treatability Testing

Background/Objectives:

The Black Butte Mercury Mine Superfund site in Lane County, Oregon released mercury to the downstream Cottage Grove Reservoir. Reservoir water level is lowered each year during the wet weather winter season and kept low from November through February to provide water storage and reduce downstream flooding. Investigations of mercury methylation dynamics have shown that the seasonally inundated sediments within the reservoir experience elevated methylmercury within porewater relative to the permanently inundated sediments. EPA Region 10 and CDM Smith are conducting a bench-scale treatability study on these impacted sediments to investigate the efficacy of surface sediment amendment treatments for reducing methylmercury formation and enrichment within porewater, with the ultimate goal of minimizing its release into surface water and entry into the food web.

Approach/Activities:

Seven amendments were selected for evaluation in a bench top microcosm study. The amendments were selected based on published studies demonstrating their performance in sequestering total mercury and consist of a variety of engineered, activated carbon, and biochar amendments. However, not all amendments had been tested for effectiveness in reducing the formation or sorption of methylmercury. Triplicate microcosms of amended seasonally inundated sediment and surface water collected from the site were incubated in jars as microcosms for 4 weeks in the dark and shaken every three days. Three different doses were applied for each amendment, using literature or vendor recommended dosing on a weight basis. In parallel, microcosms of unamended sediment and amended inert sand were incubated as controls under the same conditions. The microcosm incubations were terminated by extracting decanted water, filtered to represent surrogate porewater, and sediment from the jars. Samples were analyzed for total mercury and methylmercury, TOC (sediment), and DOC (porewater). Water quality parameters (ORP, conductivity, pH, and DO) were measured and recorded in the surrogate porewater.

Results/Lessons Learned:

Amendments were evaluated based on their ability to maintain low concentrations of methylmercury and total mercury (the non-methylmercury fraction of total mercury) in porewater, with a secondary objective to maintain low methylmercury concentrations in the solid phase. This talk will describe the performance of the seven amendments on the seasonally inundated sediments from Cottage Grove Reservoir. The top three performing amendments (which include an engineered, activated carbon, and biochar based treatments) will be used to design additional microcosm and mesocosm studies and will potentially be used to design future field-scale pilot studies and screen remedial options for these reservoir sediments.

Chris Eckley

Affiliation

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Evaluation of Multiple Sediment Amendments at a Mercury-Contaminated Reservoir Using Bench-Top Microcosm Treatability Testing