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COMPARISON OF DATA FROM SYNTHETIC LEACHATE AND DIRECT SAMPLING OF ACID DRAINAGE FROM MINE WASTES: IMPLICATIONS FOR MERCURY TRANSPORT AND WASTE MANAGEMENT
Citation:
Bates, E R., M. A. Engle, G. J. Reller, D G. Jewett*, D. McMindes, AND E. Manges. COMPARISON OF DATA FROM SYNTHETIC LEACHATE AND DIRECT SAMPLING OF ACID DRAINAGE FROM MINE WASTES: IMPLICATIONS FOR MERCURY TRANSPORT AND WASTE MANAGEMENT. US EPA Conf. Hardrock Mining 2002: Issues Shaping the Industry, Denver, CO, 05/7-9/2002. U.S. Environmental Protection Agency, Washington, DC, 2002.
Description:
The Sulphur Bank Mercury Mine (SBMM) in Lake County, California operated from the 1860s through the 1950's. Mining for sulfur started with surface operations and progressed to shaft, then open pit techniques to obtain mercury. Mining has resulted in deposition of approximately 2.6 million tons of mine wastes (waste rock, ore and tailings) on the ground at the site. Analytical results from synthetic leaching of mine wastes, of acid rock drainage sampling and groundwater sampling were compared to evaluate mercury mobility at SBMM. Groundwater sampling included six monthly sample collection events from four wells completed in mine wastes and containing acid rock drainage (ARD wells) , with water from each ARD well filtered through differing filter sizes to evaluate particulate transport (filtration sampling). Synthetic leaching included the California waste extraction test (WET) at pH of 5, California WET using distilled water, and synthetic precipitation leaching procedure (SPLP) leaching at pH of 5 and 3. Synthetic leachate contained from 0.24 to 1,670 micrograms per liter (ug/L) dissolved mercury. Filtration sampling results show that particulate transport of mercury is insignificant compared to transport of dissolved mercury. ARD well mercury concentrations range from <0.049 to 350 ug/L dissolved mercury. Other workers have conducted synthetic column leaching experiments on mine wastes from SBMM and found that initial leachate from the columns contained significant quantities of mercury on particles (up to 20 ug/L in unfiltered samples), but that as pore volumes of leachate increased the quantity of particles and associated mercury decreased, with final unfiltered leachate mercury concentrations after four weeks of leaching similar to the dissolved mercury concentrations in the filtered leachate. Comparison of ARD well and synthetic leaching results when combined with observations of chemical armoring of preferential infiltration pathways and the presence of oxidation rinds on mine waste piles leads to the following conclusions regarding mine wastes at SBMM:
1. Particulate transport of mercury represents only a small portion of the mercury migrating from in-place mine wastes.
2. The SPLP at a pH of 5 best mimics the leaching of mercury from mine wastes.
3. Water- rock interaction at high Eh (>400 mV) and low pH (<3.5) results in elevated dissolved mercury concentrations (other work indicates that at pH above 8 significant dissolved mercury is present, however site conditions are acidic).
4. Disturbance of mine wastes is likely to release particles containing adsorbed mercury to groundwater.
These conclusions lead to the following implications for management of mine wastes containing mercury minerals at SBMM and other locations:
1. Synthetic leaching methods to evaluate mercury mobility must be selected to mimic site-specific conditions.
2. The effects of sampling disturbance must be accounted for when interpreting synthetic leachate analyses.
3. Disturbance of mine wastes is likely to create an increase in the mass of mercury released until infiltration of sufficient water volumes to chemically armor reactive surfaces and physically trap the particles has occurred.