By Charles B. Salocks, PhD
Teichert Construction is applying for a Yolo County permit to mine gravel on more than 250 acres of land in lower Cache Creek west of Woodland that is now being used for agriculture.
This proposal is problematic because the Cache Creek watershed naturally contains substantial deposits of mercury ore. It includes a US EPA Superfund site, Sulphur Bank Mercury Mine, located at the east end of Clear Lake.
According to the Environmental Impact Report (EIR), at the end of 30 years the mined property will be reclaimed: approximately two-thirds of the land area will be converted back to agricultural use and one-third will become a permanent water impoundment (or ‘pond’) and turned over to the County. The property will not be restored to its original state, at least not in the foreseeable future.
A toxic compound called methylmercury is produced by certain types of bacteria that live in water and sediment where the concentration of dissolved oxygen is very low. This condition can occur at the bottom of ponds or lakes, such as the reclaimed water impoundments in lower Cache Creek where open pit mining has already occurred.
Methylmercury is taken up by microscopic aquatic organisms much more rapidly than it is eliminated. The process is called bioaccumulation. Because of bioaccumulation, and another process called biomagnification, the concentration of methylmercury in fish can be hundreds of times higher than the concentration in water. Animals that consume these fish -- including wildlife and humans -- are at greatest risk.
In humans, methylmercury is readily absorbed and distributed throughout the body, where it acts as a potent neurotoxin, harming the brain and nervous system. It crosses the placental barrier, passing from mother to unborn child, and is particularly harmful to babies and young children, disrupting brain development.
The project EIR does not include an in-depth examination of this problem. It refers to mercury as a “legacy” contaminant, which of course it is. But mining requires excavation of sand and rock that have remained undisturbed for hundreds of years, and the proposed reclamation plan includes a pond where methylmercury can form. Ultimately, mining appears to accelerate release methylmercury into the ecosystem.
Research studies of methylmercury in several existing mine ponds shows results that are concerning. The fact that that mercury in reclaimed mine ponds is undergoing conversion to methylmercury and is being taken up by fish that live in these ponds is information that should have been presented in the EIR.
In a recent study, the concentrations of methylmercury in fish collected in three existing reclaimed mine water impoundments were significantly greater than those found in fish collected from Cache Creek. Summertime anoxia (lack of oxygen) in water at the bottom of the ponds appeared to be a critical factor leading to increased methylmercury production. This is because the bacteria that produce methylmercury thrive under anaerobic conditions. However, a correlation between bottom-water anoxia and elevated fish mercury was not consistently observed. One pond with elevated fish mercury showed no evidence of oxygen depletion, indicating that factors other than anoxia were influencing methylmercury formation. For example, water clarity may affect the growth of algae, which can oxygenate water and reduce methylmercury production. These factors – and others – compete against one another, some increasing methylmercury formation and others decreasing it.
This complex situation makes it unlikely that we can devise a “one size fits all” remediation strategy for the existing water impoundments. Pumping air into the ponds might reduce deep water anoxia but also might reduce water clarity and limit the growth of algae, leading to less oxygen production. Furthermore, the ponds will probably need to be aerated every season because the conditions that lead to deep water anoxia are re-created each summer.
The latest research shows that we don’t know enough about methylmercury formation in these ponds, and so far we don’t know how to prevent it. One thing is certain – prevention will cost money. How much? We simply don’t know.
Here’s what we do know. We already have multiple reclaimed water impoundments along lower Cache Creek that contain methylmercury, which is being taken up by fish. When these fish are consumed by birds and other animals, the methylmercury concentration increases even more. Methylmercury is highly toxic to humans and wildlife, and we don’t know what to do about it.
We need more research to find solutions. Is it wise to allow more gravel mining and another methylmercury-generating pond along a planned parkway designed to attract families with young children? That’s a decision the Yolo County Board of Supervisors must make on January 11.
The Supervisors should take a step back and require a revision of the EIR based on research findings that were coming to light when the document was being drafted.
Dr. Charles Salocks has a Ph.D. in Environmental Toxicology from UC Davis and extensive experience evaluating human health risk assessments for hazardous waste sites, including the Sulphur Creek Mercury mine at the east end of Clear Lake.