Pick Our Brain - October 2009

What are the forms of mercury in the Delta and where did they come from? How dangerous is mercury to humans, fish and wildlife and what can be done about it? - Mary McTaggart, Delta resident

The forms of mercury in the Delta are elemental mercury (the silvery metal in thermometers), ionic mercury and methyl mercury - associated with sediments (most) dissolved in water (minor). Mercury is a naturally occurring metal that is now much more widespread in rivers and streams as a legacy of California’s Gold Rush days. When aquatic conditions are right, mercury can convert to a highly toxic form known as methyl mercury that readily accumulates in the food chain. Mercury is transformed into methyl mercury by certain types of bacteria, and that change can be accelerated by low-oxygen environments such as wetland sediments.

High concentrations of methyl mercury can cause neurological damage in humans, particularly children. It also impairs reproductive ability of some fish and wildlife. The primary human concern is consumption of relatively long-lived sport fish that accumulate the highest concentrations of mercury by women of child-bearing age. Many wildlife species and lower food web organisms can have methyl mercury within their bodies without adverse effect. Dissolved mercury is at such low levels that it is not a concern for drinking water.

Factors affecting toxic effects of methyl mercury

Formation of methyl mercury from elemental mercury occurs in areas where conditions promote this bacteriological process. These tend to be wetland and floodplain sediments where mercury is present and where there are periods of wetting and drying.

“In a toxicological sense, the primary problem with mercury in aquatic ecosystems can be defined as biotic exposure to methyl mercury. It follows that an overall challenge for scientists and managers involved with ecological restoration and management in the Bay-Delta ecosystem is to avoid increasing - and to eventually decrease - biotic exposure to methyl mercury.”

From the 2003 CALFED Ecosystem Restoration Program “Mercury Strategy for the Bay-Delta Ecosystem: A Unifying Framework for Science, Adaptive Management, and Ecological Restoration.” (MercuryStrategyFinalReport.pdf)

Exposure of food web organisms to methyl mercury

Methyl mercury is a problem only when food web organisms important to susceptible species are exposed to it for a long enough time period to be taken up by those organisms. Methyl mercury may be produced in a certain location, but could stay within that local area and not reach the food chain important to wildlife that are adversely affected by it or to sport fish (where it is an issue for humans). Methyl mercury also may be transformed back to elemental mercury (“demethylated”) or transported away before higher level food web organisms are exposed to it.

What can be done about it?

For humans:

  • Monitoring to determine methyl mercury levels in the fish people eat most
  • Beefing up outreach to communities that consume these fish, especially subsistence fishing areas
  • Development and distribution of fish consumption advisories by the California Office of Environmental Health Hazard Assessment (http://oehha.ca.gov/fish.html).

Fortunately, humans have an easier time than fish and wildlife in reducing their exposure to methyl mercury through following these advisories.

For fish and wildlife:

  • Mitigation options are on-the-ground activities that reduce methyl mercury methylation* and exposure, including reducing additional mercury source materials from being transported into areas where methylation can occur (e.g., through containment of mercury-contaminated mine wastes) and managing landscapes as well as possible to minimize mercury methylation and exposure. There is much yet to be learned on the landscape management side.

*Methylation is the attachment or substitution of a methyl group on various substrates.

For further information on mercury, please read our Science Action mercury issue here: http://science.calwater.ca.gov/pdf/SIA_mercury_063005.pdf