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Mercury's Lasting Legacy: Measuring Contamination in Cache Creek

USGS scientists collect samples to test for a range of water-quality constituents, especially mercury levels, in the Cache Creek Settling Basin.

On a cold, rainy morning in late January, a group of scientists from the U.S. Geological Survey's California Water Science Center gather on a foggy levee road just east of the City of Woodland. Shivering as the wind howls around them, the crew pulls on thick yellow raincoats, rubber waders, and life vests. They're preparing to collect water-quality and sediment samples from the murky, brown water below – an area known as the Cache Creek Settling Basin. The scientists will test the samples for a range of water-quality constituents, but their focus is set on something specific to this site: mercury.

A picture of a levee of Cache Creek Settling Basin
Kevin Brown, an engineer with the California Department of Water Resources, stands on a levee road that divides the Cache Creek Settling Basin (left) and the Yolo Bypass floodplain that leads to the Sacramento River. Photo: Bonnie Dickson, USGS (Public domain.)

Used primarily for agriculture and rangelands today, the Cache Creek Watershed was once a hotbed for mining activity. Mercury deposits discovered in the mid-1800s were mined through the 1950s. Mercury production peaked during 1860s through the 1880s when one of its main uses was mercury amalgamation to recover gold from hard-rock gold ores and unconsolidated sediment known as placer deposits. Liquid elemental mercury – sometimes referred to as quicksilver – can still be found in sediments and streams in gold-mining areas where mercury was used. At least 40 abandoned mine sites have been identified in the Watershed. As trusted experts on the environmental impacts of historical mining, the USGS was enlisted by the California Department of Water Resources (DWR) to study the lingering presence of mercury contamination in the area's water and sediments.

Charlie Alpers, a research chemist with the USGS, has been studying the environmental legacy of mining in California for nearly 30 years. He is the project chief here at the Cache Creek Settling Basin and leads several other mercury studies around the state.

"Our studies have identified several mine sites around the state that are considered mercury ‘hot spots' that either have been remediated or are in need of remediation to protect public health and the environment," Charlie said. "The environmental legacy of mining is an aspect of California history that potentially affects the health of California's human residents as well as its biological resources."

A picture of a spillway of Cache Creek into Yolo Bypass
An engineered spillway, operated by the California Department of Water Resources, controls the flow of water and sediment from the Cache Creek Settling Basin into the Yolo Bypass. Photo: Bonnie Dickson, USGS. (Public domain.)

Charlie walks over to a nearby truck where his crew prepares equipment for sampling. Physical science technician Shanna Rose is prepping water-quality testing equipment. Shanna is in charge of calibrating the equipment on-site at the beginning of each sampling trip, a responsibility that fits naturally with her focused and deliberate disposition. She carefully pours a sequence of liquids from different bottles into a large plastic cup. The cup connects to a cylinder with several protruding electrodes and sensors. The whole device looks something like a Jedi lightsaber.

"That's a multi-parameter water quality sonde," exclaimed Charlie, his eyes wide with excitement.

Shanna briefly breaks her focus and smiles as Charlie continues to explain the nuances of water-quality equipment and the resulting technical data produced from each sample.

"Charlie is crazy about data," she said. "He always wants more. More data, more pieces of the puzzle to put together."

Kevin Brown, an engineer with DWR, nods in agreement. He has worked with Charlie for almost a decade.

"The USGS does the science, and we do the building and remediation," he said of the symbiotic USGS-DWR partnership. "Charlie is one of those people that, when you're around him, you hope some of what he knows rubs off on you – he's just so smart."

A picture of water quality equiptment
USGS research chemist, Charlie Alpers, explains the nuances of water quality equipment to physical science technician, Shanna Rose. Photo: Bonnie Dickson, USGS. (Public domain.)

Mercury studies in California's water are essential to ensuring the health of the state's human, wildlife, and ecological populations. When exposed to bacteria in water, soil, and plants, mercury is converted into methylmercury by bacteria that thrive in areas with low oxygen conditions. Methylmercury is a dangerous neurotoxin that impairs the human nervous system and disrupts reproduction in birds and other wildlife. It also happens to be the form of mercury most readily incorporated into the tissues of fish and other animals – a process known as bioaccumulation. Methylmercury is highly toxic to humans, especially fetuses and young children.

The USGS and DWR have been monitoring water quality at this site since 2010, after it was discovered that Cache Creek is a significant contributor of mercury and methylmercury to the Sacramento-San Joaquin Delta through the Yolo Bypass. The California Regional Water Quality Control Board has estimated that the Cache Creek supplies 10-30 percent of the mercury loads into the Sacramento River system, though it only represents only about two percent of the water flow.

The risk of mercury-loaded sediment moving into the Yolo Bypass from the Basin is highest during and after storms that flood the Bypass and increase flows in the Delta. Sampling during these times is crucial for data-collection purposes.

Picture of some of the sampling crew at Cache Creek
Kevin Brown, of CA DWR, left, and Charlie Alpers, right, oversee crews during storm sampling of the Cache Creek Settling Basin. Photo: Bonnie Dickson, USGS. (Public domain.)

"There are a lot of moving parts in California's water system," Charlie explained. "It's impossible to collect samples 24/7, but we try to collect at specific times during high flows to capture the dynamics of the system."

The first months of 2017 have brought back-to-back storms and high flows aplenty, which means USGS scientists are busy chasing critical high-flow measurements in dynamic systems like Cache Creek. At the end of a long day on site, crews will carefully pack up water samples that will be processed at the USGS laboratory on the Sacramento State University campus. The processing includes filtration and additional laboratory measurements, resulting in dozens of sample splits that will be shipped to water-quality laboratories at other USGS locations that will turn the samples into data. Tomorrow – and for the next 10 days – CAWSC crews will repeat their sampling routine at different sites throughout the Basin. The long hours, hard work, and wet weather don't deter them because they know the value their work contributes not only to science, but to the improved health and wellness of California's environment and population.

picture of a USGS crew
USGS crews wrap another day of field work in the Cache Creek Settling Basin. Photo: Bonnie Dickson, USGS.​​​​​​​ (Public domain.)

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