By Berenice Quirino
Bereniceq@record-bee.com @bquirino15183 on Twitter
Clearlake Oaks >> It starts with small organisms at the bottom of Clear Lake. They become exposed to mercury contaminated sediment and are then eaten by fish, which then get eaten by bigger fish — thus mercury enters the food chain. That’s the problem.
The Environmental Protection Agency (EPA) recently completed its two-year remedial investigation of the Sulphur Bank Mercury Mine Superfund Site through the use of two caps at the bottom of Clear Lake located at the southeastern end of the Oaks Arm. The caps appear to have held up well and the agency is awaiting results of sediment samples.
The caps are 8-inch layers of sand and gravel about 120-square feet in size placed 20 and 40 feet beneath the surface of the lake. Their purpose is to physically block the path where the small organisms come into contact with mercury in the sediment, according to EPA Environmental Engineer and Superfund Project Manager Gary Riley.
“What this is doing and what a future cap would do is it would cut off the path into the food chain from sulphur bank mine,” he said.
Samples of sediment accumulated on top of the caps as well as water and small creatures at the bottom of the lake were monitored and tested every six months, and compared to pre-baseline samples taken before the construction of the caps.
“The primary goals were to evaluate the methods we could use to construct caps in Clear Lake and then evaluate the construction to verify best methods to place caps,” Riley said. “I’m pleased that both caps have remained in place, and it doesn’t look like there’s erosion that would degrade the integrity of these caps.” Mercury, an element toxic for humans to consume, was first discovered by accident in the 1970s by the California Department of Fish and Game testing DDT pesticide levels in fish. This led to a state issued advisory to limit fish consumption from the lake that’s still in place today.
The 150-acre site was mined for sulfur and mercury on and off beginning in the 1860s. The mine, once one of the largest produces of mercury in California, has been inactive since 1957. U.S. Geological Survey studies show that mercury in sediments increased during that time period.
Since then, monitoring by the University of California, Davis in the 1990s and again from 2005 to 2007 have shown that there has been no significant upward trend of mercury contamination in sediment or water, EPA Public Affairs Specialist Suzanne Skadowski stated. The EPA also worked on various cleanup projects during the time period.
The test cap program, which ran from March 2013 to March 2015, was to gather information to evaluate larger cap options and necessary because of characteristics unique to Clear Lake, Riley said.
The relatively shallow water and very soft sediment at the bottom of the lake presented challenges, as well as organic matter in the lake that can cause low visibility at times in water, particular at the end of the Oaks Arm.
“The third challenge was to actually do the construction to place the sand and gravel. Eight inches is a pretty small dimension when compared to scale at the bottom of the lake,” Riley said. Wind was also an obstacle during construction.
However, none of the elements created any significant problems.
It took about two months beginning in January 2013 to complete the caps and involved a crew of 30 personnel from the EPA and various agencies and contractors that included engineers overseeing the project, an underwater construction crew and a dive team to verify activity at the bottom surface of the lake.
“The biggest initial success is the construction methods, we were able to place the material and achieve the design required,” Riley said. “How long it would take for the caps to affect mercury levels in lake is difficult right now to predict.”
It will take another two or three months to receive completed lab results from the samples. Once the EPA receives the data, they will prepare a technical report with recommendations that will be completed at the end of the year. The EPA estimated past and future to add up to about $80 million.
“The technical conclusion will give us a better idea of what the construction of a larger cap would require, and then from there we’ll be taking that information to evaluate the expense that the cleanup needs.”
