Evaluating coagulation techniques to reduce the transport of Hg from mine-affected and active geothermal Hg-source watersheds

Science Center Objects

Our objective is to determine the effectiveness of coagulation and adsorption techniques in removing mercury from contaminated surface waters of the Cache Creek watershed.

Maps showing Cache Creek watershed

Maps showing the Cache Creek watershed.

(Public domain.)

map showing mercury sources in the target source area

Maps showing primary Hg sources in the target source area (from Domagalski and others 2004).

(Public domain.)

Mercury (Hg) is targeted as a priority contaminant because it is a potent neurotoxin that affects the nervous systems of a wide variety of life forms affecting reproductive success, neurological development and survival rates. In California, the extensive contamination of sediment from active and historic mining activities is believed to affect a diverse range of habitats from high elevation mines to coastal habitats including the Sacramento-San Joaquin Bay/Delta Estuary (Bay Delta). There is a pressing need to control the migration of Hg from the mining-affected foothills of the Coast Ranges and Sierra Nevada to the sensitive, low gradient environments, and/or sequester the Hg at or near the source within a complex that is less available for methylation, the process through which Hg becomes most bioavailable. The use of in situ coagulation techniques to remove mercury (Hg) from surface waters has gained attention as a potentially viable control measure for Delta ecosystems, but little is known about the effectiveness of coagulation for the removal of Hg from waters with the characteristics encountered in mining-related particulate or mine seep sources (high turbidity, complex geochemistry, variable salinity). There is a strong regional, national, and global need to determine whether in situ treatment in combination with source control is an effective strategy to reduce contamination in the environment to acceptable levels, and thereby advance remediation science, restoration of sensitive habitats, and reduce the risk of human exposure.

Our objective is to determine the effectiveness of coagulation and adsorption techniques in removing mercury from contaminated surface waters of the Cache Creek watershed.

Funding from the RARE program will enable EPA and USGS to evaluate the use of the coagulation and sorbent techniques in removing Hg from suspension in important Hg source areas of the Cache Creek watershed, thus preventing the formation and bioaccumulation of MeHg in sensitive downstream habitats identified in the TMDLs and Superfund actions. Application of coagulation and adsorption techniques to the diverse Cache Creek sources will identify the relative efficacy of removal for the different sources. A review of literature will prioritize coagulants, coagulant aids/enhancers and adsorbents based on applicability to the sources, dosing rates, cost, potential toxicity, etc. Batch jar tests and exchange column experiments will evaluate adsorption isotherms, kinetics, initiation and production of the flocculants of the top three to five identified coagulants and sorbents. The Hg removal efficiency will be quantified for each source’s top two performing treatment materials for turbidity removal or floc formation in the coagulation tests and a subset of samples selected from the exchange column tests. Results will be used to inform managers and regulators about the engineering requirements for Hg removal using the top treatment options and the potential costs of implementation for the different Hg sources while minimizing deleterious effects in the environment.