UC Davis Studies
The tests of lake sediments and water for mercury from 1992 through 1998 show increasingly higher levels of mercury from samples taken closest to the mine site. How the mercury is getting into the lake and its food chain is still unclear. The UC Davis researchers' theory is that acid mine drainage is seeping through the contaminated waste rock and then entering the lake from underground in addition to surface runoff, leaching mercury into the lake. The source of the acidity may be the flooded Herman pit which has a tested pH level of 3.2.
This underground system is difficult to study as, "the miners ripped the relatively compact natural sulfide deposit to pieces, jumbled uneconomic minerals together with miscellaneous overburden rock, and piled the resulting poisonous mélange 30 or 40 feet deep over the tunnel-laced rock of the former underground mine. Finding out how water and air flow through the resulting mess is a very difficult task."
This difficulty is shown by tracer experiments conducted by UC Davis in 1997 and 1998 to find the subsurface water flow from the Herman Pit to the lake. The tracers used were Rhodamine-WT, sulfur hexafluoride, and a mixture of sulfur hexafluoride and neon-22.
"The three sets of experimental results presented all support a through-flow rate in Herman Pit. Even though it is known that water from Herman Pit is flowing into Clear Lake, it cannot be estimated from these calculations how much is going into the lake or the precise path the flow is taking" the study noted. "Of the approximate 630 liters per second flowing into and out of Herman Pit, fluid may leave the pit and flow directly into the lake through the waste rock piles, through the native sediment that underlies the waste rock piles, or simply flow elsewhere." wrote S. Geoffrey Schlado, Jordan F. Clark, in the study, Use of Tracers To Quantify Subsurface Flow Through A Mining Pit.
Another theory is of geothermal springs being the source for inorganic mercury in Clear Lake, as there are abundant springs emanating from the lake bed. Deep core samples of sediments taken in the 1980s show peaks of mercury during prehistoric times that "likely originated from natural processes such as volcanic and/or tectonic activity within the Clear Lake Basin" A report published in Geology, November 1987 theorized, "that mercury-rich geothermal fluids rose along the activated fractures and faults and were discharged into the lake, causing the anomalously high Hg content of the sediments and leading to deposition of the Sulphur Bank Hg deposit. The total amount of Hg discharged into Clear Lake over the past 15,000 years is estimated to be at least 2,400 metric tons."
The EPA-funded research by UC Davis found that total levels of mercury (TotHg) in the lake sediments has "not declined significantly a decade after" the remediation work in 1992 at Sulphur Bank Mine.
Read more about this topic: Sulphur Bank Mine, Present Day
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