# Mercury studies at Sulphur Bank Mercury Mine and Clear Lake, California

## Science Center Objects

The abandoned Sulphur Bank Mercury Mine on the shores of Clear Lake in Northern California has been designated as a "Superfund Site" by the U.S. Environmental Protection Agency (EPA). This means that the EPA has determined that the area is contaminated by hazardous waste and requires cleanup because it poses a risk to human health and/or the environment. One of the chief contaminants at the site is mercury (Hg). Methylmercury (MeHg) is a highly toxic form of mercury that becomes magnified as it works its way up the food chain. To help the EPA address the problem, the USGS has been asked to study the presence and spread of methylmercury at the Clear Lake site.

Sulfur Bank Mine on the eastern shore of Clear Lake, is one of the most notable mercury mines in the state of California.

### Background

The Sulphur Bank Mercury Mine is an inactive mercury mine located on a 200-acre property on the southeast shores of Clear Lake. The mine site includes a flooded open pit mine and approximately 3,000,000 cubic yards of contaminated mine waste. Contaminants from the Sulphur Bank Mercury Mine have been detected in lake sediments in nearby areas. Previous work at Clear Lake has shown that primary production (algae growth) driven by an excessive richness of nutrients is a key driver to methylation of mercury and its introduction into the food web. Getting a better understanding of mercury cycling in Clear Lake will depend on understanding nutrient cycling and its effects on the depletion of dissolved oxygen in the water column and surface sediment, which increases mercury methylation.

In addition, a research effort on nutrient cycling in Clear Lake by the University of California, Davis (UCD)  began in 2018 in association with the Blue Ribbon Committee for the Rehabilitation of Clear Lake, affording the USGS the opportunity to integrate their work with the ongoing UCD study.

### Objectives

The overarching goal of the planned research is to improve understanding of the relationships between contaminants derived from Sulphur Bank Mercury Mine with concentrations of mercury in sediment, water, and fish tissue in Clear Lake. A related goal is to determine the proportion of methylmercury in the food web of Clear Lake that is derived from the Sulphur Bank Mercury Mine using geochemical and isotopic methods. A third goal is to further examine and develop the relationships between various dissolved and particulate mercury fractions and optical properties in the water column, such that these relationships can be leveraged to develop monitoring platforms that will generate data that can inform future efforts. A fourth goal is to develop a model for mercury cycling in Clear Lake that is a useful management tool for the EPA and other stakeholders for testing the potential benefits of reducing levels of MeHg bioaccumulation in the Clear Lake food web.

Location map showing Sulphur Bank Mercury Mine, Clear Lake

### Science Plan

The planned work is divided into eight tasks:

1. Technical assistance (USGS scientists with expertise in several areas to assist the EPA)
2. Prepare Quality Assurance Project Plan and Field Sampling Plans
3. Water sampling and analysis
4. Solids sampling and analysis
5. Biological sampling and analysis
6. Mercury isotope analysis
7. Evaluating water column mercury methylation rates
8. Modeling of mercury cycling in Clear Lake

### Budget, Timeline, and Deliverables

A five-year project is planned with a budget of about $2M. Funding will be provided by EPA on a year-to-year basis. The initial Interagency Agreement has$319,500 for work mostly in Fiscal Year 2020, with data releases planned in Fiscal Year 2021. Interpretive reports are planned for Fiscal Year 2024.

Related Data Release

Geochemical Data for Mercury and Other Constituents in Redox-Manipulated Sediment Cores from Clear Lake, Lake County, California