Th. Suchanek et al., Monitoring inter-annual variability reveals sources of mercury contamination in Clear Lake, California, ENV MON ASS, 64(1), 2000, pp. 299-310
Mercury (Hg) in the aquatic ecosystem of Clear Lake has been documented sin
ce the 1970s when fishes were found to have elevated levels of toxic methyl
mercury (meHg). Mining practices at the Sulphur Bank Mercury Mine (active
intermittently from 1872-1957) along the shoreline of Clear Lake included t
he bulldozing of waste rock and overburden ore into the shallow nearshore r
egions of the lake and the creation of steeply sloped piles of waste rock a
t the water's edge. This process, plus erosion of the waste rock piles, res
ulted in the accumulation of an estimated 100 metric tons of Mg in Clear La
ke. A monitoring program to assess Mg in Clear Lake was established in 1992
, and conducted continuously from 1994. Drought conditions in California ha
d persisted for fa. 6 yrs prior to 1992, when the U.S. Environmental Protec
tion Agency (USEPA) remediated the steeply sloped eroding waste rock piles,
which appeared to reduce sediment Hg concentrations significantly. In Apri
l 1995, a white flocculent material was observed in Clear Lake adjacent to
the mine and has been observed every year since, leading to the discovery o
f ongoing acid mine drainage (AMD), low pH fluids high in Mg and extremely
high in sulfate. AMD is now believed to be the most likely cause of elevate
d meHg in Clear Lake. The discovery of this source of meHg production in Cl
ear Lake, which will significantly influence remedial options, was only mad
e possible by implementation of a diligent monitoring program.