Scientists have found new ways to reduce mercury in wetlands, providing hope that Sacramento-area waterways can be decontaminated of the potentially toxic element that dates back to Gold Rush-era mining activities.
The new research, published recently in Environmental Science and Technology, found that dosing wetlands with two chemicals, iron or aluminum salts, was successful in removing mercury from wetlands.
Researchers spent two years on the project and built nine wetlands on public land in the Sacramento-San Joaquin Delta at Twitchell Island. They applied water with aluminum salts to three wetlands, water with iron to another three and regular water to the last set of three, according to Tamara Kraus, a research soil scientist with the U.S. Geological Survey and a co-author of the study.
To determine the effects, researchers introduced mosquitofish. They found mercury levels in the wetland water decreased by 62 percent in wetlands dosed with aluminum salts and by 76 percent in wetlands where iron was used.
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The mercury lingering in area rivers, creeks and wetlands dates back to the 19th century, when miners used the element to help extract gold in the Sierra Nevada foothills.
Controlling mercury levels in area rivers, creeks and wetlands is a key health issue because many people continue to eat fish they catch from mercury-contaminated waters, despite advisories.
“The mercury levels are of concern to us because people are still fishing regularly out of the Delta,” said Sonney Chong, chairman of Capital, an umbrella organization representing the Asian American and Pacific Islander community.
A 2013 assessment by the California Department of Public Health found that Hmong, Laotian and Cambodian residents are avid fishermen in the Delta, but that some have low awareness of the mercury issue.
“They’re feeding their family. It’s an outright source of food, so they’ve ignored the advisories,” Chong said.
Ingestion of mercury can lead to problems that include mental impairment and other developmental abnormalities, especially in fetuses and young children.
Mercury bioaccumulates in fish tissue and is passed up the food chain in greater quantities as larger fish eat small fish.
In 2011, a survey of 16 species of sport fish from 63 locations done by the State Water Resources Control Board found that fish caught in the Sacramento-San Joaquin Delta watershed had higher concentrations of mercury than anywhere else in the state. The highest contamination was in fish high up the food chain – smallmouth and largemouth bass, striped bass and the Sacramento pikeminnow.
Contaminated river sites included the American River at Discovery Park and the south fork of the American River at Coloma. Fish tested from the San Joaquin River pier at Point Antioch and at Louis Park in Stockton also showed high mercury levels.
Scientists are focused on methyl mercury, a more toxic form that occurs when mercury binds with organic molecules in the environment. It is the only form that accumulates appreciably in fish, birds and humans.
“Wetlands are really good environments for fish and wildlife. They are also very good at methylating mercury,” said Joshua Ackerman, lead author of the study and a research wildlife biologist at USGS.
Besides lingering mercury from the Gold Rush, mercury seeps into wetlands and waterways from the air. Coal-fired power plants are one of the largest sources of mercury in the nation, according to the U.S. Environmental Protection Agency.
In the new study, researchers found that iron and aluminum salts acted as coagulants, changing mercury so it can settle to the bottom, according to Kraus. That takes mercury out of the water column, so fish are less likely to consume it, but leaves it in the sediment.
“The smaller particles clump together to form larger clusters,” Kraus said.
Leaving mercury in the sediment has been an issue for state agencies, including the CALFED Bay-Delta Program, which was an original sponsor of the research, said Keith Coolidge, spokesman with the Delta Stewardship Council.
“The CALFED agency did express concern,” Coolidge said. “The agency felt that disturbing wetlands could allow inert mercury to convert into methyl mercury which could then be absorbed into the food chain.”
Although promising, the coagulant approach is also challenging due to high costs, said William Horwath biogeochemist at UC Davis. He said there may not be enough political support to pay for a plan that still leaves mercury in the riverbed.
In some cases, such as large rivers, flow patterns would make dosing difficult. Reservoirs would require a large amount of dosing and engineering compared with wetlands, Horwath said.
“It would cost millions,” Horwath said.
The coagulant approach at Twitchell Island is an offshoot of earlier research that showed water quality could be improved by using coagulants. Chemicals were used as coagulants in Florida’s Everglades and at Lake Tahoe to remove organic carbon from drainage water, Horwath said.
The coagulant approach is also being studied at the Cache Creek settling basin near Woodland. That basin is affected by the nearby Reed, Harrison and Manhattan mercury mines. The Cache Creek watershed is responsible for a large amount of the mercury that enters the Delta estuary.
In that planned research effort, coagulated material could possibly be collected in basins and moved off site to a wastewater facility.
Edward Ortiz: (916) 321-1071 firstname.lastname@example.org