SIZING UP MERCURY IN NEW IDRIA – USING CONCEPTS FROM THE CRITICAL ZONE TO UNDERSTAND THE FATE OF MERCURY IN CONTAMINATED SETTINGS

Compared to the natural geologic time span, human activity over the past 100 years has released mercury at unprecedented rates within the New Idria mining region of San Benito County, California—a rate ~4 orders of magnitude faster than the natural background crustal processing (Hazen, 2012; Studemeister, 1984). Today, river sediments and soils surrounding the site have been reported to contain mercury in excess of 20,000ppb (Gehrke, 2011; Weiderhold, 2013). Left as is since its closure in 1972, exposed waste piles of tailings with elevated levels of mercury raises questions about long it will take for such areas to naturally attenuate back to background, pre-mining, and safe conditions (~40ppb, GERM Database; 20-625ppb, ATSDR; 10 to 700 ppb, Jonasson and Boyle, 1971). There are also questions about how abatement naturally happens—chemically and/or physically?

In this study, we use concepts from the critical zone to understand the fate of mercury in this contaminated setting. We performed geochemical analyses on river sediments draining the area, as well as the coarser sediments taken from along the channel banks, to determine mercury transport downriver from New Idria. Using a novel application of tau--which is a mass transfer coefficient typically used critical zone studies and work on soil production and weathering—the downstream weathering, accumulation, and transport of mercury was determined. Our initial geochemical data showed higher tau values of mercury within the banks of the contaminated streambed, as well as a higher accumulation of mercury near the pollution source (i.e., the mine tailings, (τ ~ 10³). Tau results also show elevated mercurial levels also existing downstream, with accumulations in mid- (τ ~ 10²) and down-stream (τ ~ 10) reaches. Combining tau results with more traditional indices of chemical weathering (CIA), mercury at the New Idria site is consistent with low levels of chemical weathering and higher dominance of coupled physical-anthropogenic weathering, with important implications for contaminant fate and remediation.