Drew presents CalTrout findings on mercury contamination
CalTrout received a grant from the Lahontan Regional Water Quality Control Board in 2012 to monitor water quality in Mammoth Creek. The study revealed elevated levels of mercury, with the highest concentrations at the historic Mill City Stamp Mill, a former gold ore processing facility about one mile east of Twin Lakes.
The Stamp Mill was one of Mammoth’s three major mining camps, operating from 1878-1880.
According to Dr. Drew’s presentation, an estimated 52 million pounds of mercury were used for gold mining efforts across California; 3-8 million pounds were lost to the environment.
Unsurprisingly, mercury levels rose above the Lahontan Regional Water Quality Objective of 50 parts per million (ppm) in June and July at the Stamp Mill and Highway 395 Gauge.
Dr. Mark Drew.
Dr. Drew noted that these months have high snowmelt runoff. “More water flow means more dilution, so the higher levels of mercury were perplexing,” he said. It was this contradiction that led CalTrout to hypothesize “that mercury was sourced from somewhere and throughout time was moving down the system, settling in areas with lower gradients,” he said.
CalTrout subsequently intensified sampling between Sherwin Creek Road and Hayden Cabin, finding mercury levels of 120-130 ppm during June and July.
Based on these findings, the U.S. Forest Service (USFS) conducted a preliminary soil sample analysis in 2013.
USFS sampling revealed five out of eight sites near the Stamp Mill with mercury levels above the Environmental Protection Agency (EPA) Human Risk Management Criteria (HRMC) for mercury of 40 ppm. The five sites ranged from 40.83 – 210.49 ppm, according to the USFS Preliminary Assessment.
Dr. Drew stressed that mercury contamination would not affect Mammoth’s drinking water supply, which the Water District diverts at Lake Mary, several miles above the Stamp Mill.
However, the presence of mercury may indicate that methylmercury exists in Mammoth Creek. Methylmercury is an organic form of mercury that can bioaccumulate into the aquatic food chain, from bacteria to fish, and may be harmful to humans at elevated concentrations.
Dr. Drew said that there could be several environments conducive to the methylization process, including Hidden Lake.
But, “We do not now understand whether the methylization process is occurring, and where,” he said. “We do not understand whether our critters [and] our fish are accumulating that.
“We need to be thinking about investigating that further … There very well may be a part two to this study.”
Dr. Drew noted that because both CalTrout and USFS studies located mercury on Forest Service lands, the USFS will be performing an in-depth Site Investigation soon. “[Mammoth Creek] has become their number two priority for the region as a whole,” he said.
The end result of a USFS investigation could be something as simple as signage to warn residents and recreators, or, Dr. Drew said, “It could be dump trucks hauling away dirt … It’s a popular area, and we do want to be careful and make sure people aren’t exposed to a contaminant that could be harmful to them.”
