Valley Water working to reduce methylmercury (toxins) in local waterways

For more than 15 years, Valley Water has measured mercury levels in reservoirs and creeks in the Guadalupe River Watershed and studied ways to reduce the metal's harmful impacts.

Parts of the Guadalupe River watershed, which covers about 171 square miles, are contaminated with mercury from the former New Almaden Mining District. The mining and processing of mercury occurred in the area from 1845 through 1971. These operations released large amounts of mercury into parts of the Guadalupe River watershed, which flows into South San Francisco Bay.

Mercury-enriched sediment from mining waste made its way into creeks and reservoirs within the watershed. Creeks flowing in the watershed carry that sediment down the Guadalupe River to San Francisco Bay, especially during wet years. Under the right conditions, sediment mercury can be converted by bacteria and other microbes to methylmercury, the most toxic form of mercury. Methylmercury accumulates up the food chain until it is highly concentrated in top predator fish like largemouth bass in reservoirs, and sharks and other species in the Bay. It's important to note that reservoir and creek water itself does not pose a significant mercury risk to people. The primary risk to people comes from eating fish with high mercury levels.

Valley Water is working to identify, test, and implement actions to help reduce mercury and methylmercury in reservoirs and in fish. Our work is consistent with the San Francisco Bay Regional Water Quality Control Board's regulations and plans for mercury.

Under our agency's Impaired Water Bodies Improvement project, Valley Water studies and implements methods to reduce methylmercury formation in reservoirs. We also help create and carry out realistic plans to reduce contaminants such as nutrients, bacteria, pesticides and other toxic chemicals in local creeks and reservoirs. The Impaired Water Bodies Improvement project is funded through the voter-approved Safe, Clean Water and Natural Flood Protection Program.

Oxygen systems in reservoirs

One goal we're trying to achieve is to reduce the conversion of mercury into methylmercury in reservoirs. For the past 12 years, Valley Water has used machines to pump oxygen into the bottom waters of reservoirs to reduce methylmercury production.

These systems pull oxygen from the air and distribute it through lines which bubble that oxygen into the bottom waters. This creates unfavorable conditions for methylmercury production. However, these oxygen systems can also increase water temperatures, stimulate surface algae growth, and make water cloudier, particularly when reservoir levels are low.

Monitoring Plan

Valley Water scientists measure water quality every month in Almaden, Calero, Guadalupe, and Stevens Creek reservoirs and quarterly at Almaden Lake. Teams drop a water quality measurement device to various depths to check temperature and other parameters and collect water at various depths to bring back for laboratory analysis.

To monitor mercury levels in fish, Valley Water biologists collect tissue samples from bass and other non-native fish every spring and summer in those same reservoirs. The samples are sent to a laboratory for analysis to determine the total amount of mercury in the fish tissue. Valley Water is currently conducting its spring fish sampling. Although we have achieved some success reducing methylmercury in bottom water, most recent data show that fish in these reservoirs are still unhealthful to eat.

Collaboration with Northern California universities

Valley Water continued its collaboration with the University of California, Merced to study the use of adsorbent materials for mercury remediation in reservoirs. Adsorbent materials (or sorbents) are materials that stick to other materials. In this case, successful sorbents will bind to mercury or methylmercury or otherwise make it unavailable to microbes. The research team is currently carrying out preliminary experiments to evaluate and rank different sorbents based on effectiveness and suitability for application.

Following this evaluation, priority sorbents will be used to experimentally evaluate several application methods. In September 2023, Valley Water and UC Merced researchers collected water samples from Lexington Reservoir to extract dissolved organic carbon for use in their bench top experiments. Results will inform potential implementation actions in Guadalupe Reservoir.

Valley Water is also working with UC Santa Cruz on a study using lichens to estimate local atmospheric mercury deposition near the New Almaden Mining District. This will help identify additional sources of mercury to reservoirs.

A third study with researchers at UC Davis aims to reveal the finer scale locations of methylmercury production within Guadalupe Reservoir. Understanding the locations of methylmercury production within the reservoir will help guide the application of remedial actions such as those being evaluated through the UC Merced sorbent studies.

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