Mining for Scientific Insight
By Heather Rock Woods
Chris Kim collects a mine waste sample from the Oat Hill mercury mine in Northern California. The majority of mercury mine wastes at these sites are present as loose, unconsolidated piles, facilitating the transport of mercury-bearing material downstream into local watersheds. (Photo courtesy of Christopher Kim, Chapman University)
Chris Kim (Chapman University) has struck a scientific gold mine of information—and has applied it to real mercury mines to determine which mines are most toxic and need to be cleaned up first.
Kim and his colleagues James Rytuba (U.S. Geological Survey) and Gordon Brown (Stanford) have developed a sensitive x-ray spectroscopy technique at SSRL which uses extended x-ray absorption fine structure spectroscopy (EXAFS). The technique identifies the types of mercury compounds present and quantifies the proportions of the different compounds in samples collected from mercury mines and gold mining regions of California and Nevada. The type, or species, of mercury compound dictates its toxicity and solubility in water.
The silvery substance that once filled every thermometer is one of the most strictly regulated pollutants in the country. California has the second largest mercury deposits in the world. During the Gold Rush,miners extracted mercury from new mines, many in the San Jose vicinity, in order to use the metal to separate gold from gold ore. As a result, California watersheds got a double dose of mercury contamination: around the mercury mine sites and in the Sierra Nevada mountains where prospectors introduced mercury into the surrounding environment to get to the gold.
Mercury continues to leach from the mine sites into waterways that flow to the Bay and the ocean. Mercury can cause terrible health problems; it is a neurotoxin that accumulates and concentrates in animal tissues as it moves up the food chain. That is why large fish can contain levels of mercury dangerous to people.
“I wanted to go right to the source, to the mercury mines,” Kim said.
The researchers collected hundreds of samples from some 25 mines. They dug into mining waste piles, fished sediments out of steam beds, and shoveled crushed mercury-containing ore, a strawberry red color, into a giant sieving machine that sorts the rocks by size.
Their technique, borrowed from methods used for other metals, allows researchers to just drop their samples, intact and unaltered, into the x-ray beam. They don’t need to chemically treat, heat or crystallize their mercury compounds, allowing a more accurate analysis of the samples in conditions similar to field conditions.
Their findings showed that to find the most dangerous sources of mercury, it is important to consider the geologic origin of the rock, the size of the mercury-containing rocks, and the ‘roasting’ effect.
The bright red ore, called cinnabar, contains mercury sulfide, which is less soluble in water, and thus less likely to contaminate watersheds and living creatures. Miners crushed the ore and roasted it at 600 degrees to extract the mercury. The leftover ore, called metacinnabar, is slightly more soluble than the unroasted ore.
A significant difference in toxicity stems from the type of rock the mercury comes in, the geological source. Deposits formed in near-surface hot springs have a larger proportion of mercury chloride compounds—very soluble and thus a higher potential for contamination—than those found in silica carbonate alteration rocks, which are located at greater depths in the earth.
“Hot spring deposits of mercury are less common in number but more important to clean up,” said Kim. “If you have two sites with the same mercury concentration, we now know you would go for the one with hot spring deposits to clean up first.”
Another important factor in contamination is the size of the mercury particles. The researchers found that smaller particles have higher concentrations of mercury. Smaller particles have higher surface area and travel farther from their source, which increases the distribution of mercury, but release may be slow because the smaller particles also tend to contain the less-soluble mercury sulfide.
“Our goal is to explain to agencies responsible for regulation and cleanup that the speciation of the mercury—what types and proportions—is a pretty important factor to take into account to get a sophisticated sense of what the problem is at each site and how serious it is.”