For the September Delta Lead Scientist report, Lead Scientist Dr Laurel Larsen spotlighted the study, Effects of vegetation on methylmercury concentrations and loads in a mercury-contaminated floodplain, which looked at the role that mercury-contaminated floodplains play in methylmercury contamination in the Delta.
Mercury is a potent environmental neurotoxin; chronic exposure can cause tremors, speech problems, emotional instability, and hallucinations. Mercury contamination is a major issue for California’s waterways due to its extensive use during the Gold Rush era. Mercury was used to enhance gold recovery in all types of mining operations, but especially in placer mining, which used jets of water to recover gold from the sand and gravels along the waterways.
Mercury came from mines throughout California’s coastal range. It was added to the sluices with the sediments, where it would amalgamate with the gold; since the mercury is dense, it helps the gold settle out from the rest of the sediment. But the water flowing through the sluices was quite turbulent, and much of that mercury was lost to the environment.
“I want to stress that this was a lot of mercury,” said Dr. Larsen. “Historical reports state that globs of quicksilver, another term for mercury, could often be seen in the rivers near these placer mining operations. The USGS estimates that the total amount of mercury lost to the environment from the placer mining operations alone was about 10 million pounds, about 80 to 90% of which was in the Sierra Nevada.”
Mercury in the form that comes directly from the mines isn’t necessarily a problem for human health because it isn’t easily taken up into living tissue. However, microbial processes that happen in sediments, particularly wetlands where sulfur and organic matter are available, convert it to methylmercury, a form that can be readily taken up by organisms.
“My late graduate school advisor at the USGS, George Aiken, used to refer to this carbon-sulfur-mercury trifecta as the mercury Axis of Evil,” said Dr. Larsen.
Once an organism takes up methylmercury, it is not metabolized, so as it moves up through the food chain, that mercury becomes more magnified. With the fish at the top of the food chain that humans like to consume, it could be a severe health threat to humans.
Under the Clean Water Act, the Water Boards are required to regulate methylmercury. Regional water boards establish regulations through their basin plans. To regulate methylmercury in the Delta, the Central Valley Regional Water Quality Control Board must adopt the total maximum daily load for methylmercury as part of its Basin Plan Amendment. Their strategy for regulation is to allocate required reductions and methylmercury load to the major sources of its production in a phased adaptive management approach that allows regulated entities to conduct studies to determine the feasibility of load reductions.
The scientific analysis that underlies the Central Valley Regional Board’s Delta Mercury Control Program revealed that wetlands in the Delta and flooded agricultural soils in the Yolo Bypass are major sources of methylmercury to the Delta, which would make the Department of Water Resources one of the key responsible parties for load reduction.
“Now, this is altogether a troublesome finding because there are many benefits for zooplankton food webs and Chinook salmon that come from managed floodplain inundation in the Yolo bypass,” said Dr. Larsen. “But methylmercury contributions that come from these actions are a distinct drawback.”
In 2021, the Delta Science Program convened a peer review panel to evaluate the Delta Mercury Control Program, and one of the findings in that report is that there are many gaps in scientific understanding of how flooded agricultural soils in the Yolo Bypass contribute to the methylmercury load, and what, if anything, can be done.
The study, Effects of vegetation on methylmercury concentrations and loads in a mercury-contaminated floodplain, aimed to address that uncertainty. The study was led by a researcher at San Jose State with Close involvement of DWR. The team tested the hypothesis that decaying vegetation might play a key role in the mercury ‘axis of evil’ that allows for the methylation of large quantities of mercury in the Yolo Bypass.
To test that hypothesis, they conducted both field and laboratory studies. For the field study, they harvested chunks of soil and whatever vegetation it contained from different locations throughout the Yolo Bypass; the samples were from grazed lands, ungrazed lands, and grazed land that had been disced (meaning farm equipment was used to turn the soil and bury the vegetation. With minimal disturbance, they put those chunks into a large tub called a mesocosm. They set them on a West Sacramento levee where they would experience identical conditions, such as temperature or precipitation.
“The researchers found that the mesocosms with vegetation from both grazed and ungrazed fields produced significantly more methylmercury than the ones that had been disced where the vegetation was effectively buried,” said Dr. Larsen.
The laboratory study involved collecting different amounts of soil and vegetation from the Yolo Bypass and incubating them under conditions similar to those experienced when the bypass is inundated.
“In that experiment, the investigators found that you need both the sediment, which is the source of the microbes, and the mercury and vegetation to produce the greatest amounts of mercury methylation,” she said. “As you increase the amount of vegetation in the experiment, the amount of mercury methylation increases concomitantly.”
“Together, these two experiments affirm that inundating large swaths of vegetated floodplains does indeed have the potential to contribute in major ways to the Delta’s methylmercury problem,” she continued. “And in planning for multi-benefit wetland and floodplain inundation projects, this potentially adverse effect must be accounted for and weighed against the foodweb, recreational, cultural, and ecological benefits of these projects as called for in the Science Action Agenda. In the case of the Yolo bypass, it may be the case that some reduction of vegetation load could be considered to balance those effects a little more.”
Dr. Larsen also suggested it might be worth investigating if there were hotspots with the potential for mercury methylation and focusing vegetation controls on those areas.
During the discussion period, Dr. Larsen was asked if the mercury could be removed. She said it’s embedded in the sediments and is a legacy problem. “The best we can hope for really is reducing the extent to which it gets methylated and taken up into those food webs.”
Dr. Larsen also noted that a lot more research investment is needed regarding water quality in the Delta, as mercury is a persistent problem, as well as contaminants of emerging concern. Next month’s science spotlight will feature recent studies on contaminants, such as pharmaceuticals and suncreens, that are released from wastewater treatment plants, and how those are likely distributed once released into the environment.
“When you talk to the USGS, which is looking at pesticides and other emerging contaminants, in terms of what is being put into the Delta is changing so rapidly, it’s really hard for scientists to stay on top of those impacts,” she said. “So one of the things we will see in the upcoming Data Science Conference will be a focus on water quality.”
SCIENCE NOTES
Upcoming research proposals: The Delta Science Program’s upcoming call for research proposals emphasizes co-production and participatory research, in which information is co-produced by both the researchers and the communities affected by the research. This requires that the communities are engaged early in the research process. To facilitate connections between researchers, Tribes, and community-based organizations (CBOs), the Delta Science Program asks researchers to respond to this survey by November 1, 2023. The solicitation will be released in fall 2023. For the latest updates, folks can subscribe to receive the Council’s email announcements.
Peer review of Reclamation’s Water Temperature Modeling Platform: In September, the Delta Science Program hosted a three-day meeting for the second part of the peer review of the Bureau of Reclamation’s water temperature modeling platform. Temperature management is a key parameter for protecting species with specific cold water needs, such as Chinook salmon, and is one of the most complex considerations in the operation of the Central Valley Project. The platform can provide suggestions for operations and monitoring to assist research resource managers of major Central Valley Project reservoirs with balancing water resources for downstream uses as well as instream temperature needs. The peer review panel is comprised of five hydrological and engineering experts from across the US.
“This panel has been a huge success story for us and Reclamation,” said Dr. Larsen. “It’s been extremely engaged and has provided helpful feedback. The dialogue between panelists and Reclamation was hugely productive. The panel members will provide comments to improve the process of developing and implementing water temperature modeling tools, and a framework for the Central Valley Project. The final report of the panel review will then be available in early November.”
