Characterization of mercury concentration, form, and air-surface exchange associated with the area surrounding and within two Nevada gold mines
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This project focused on developing a dataset that would allow for prediction of mercury (Hg) release to the air from an area prior to mining, and the potential for mining releases to impact Hg concentrations and cycling in surrounding soils. The study areas for this project were around the Twin Creeks Mine and Cortez-Pipeline Mine located in north-central Nevada. Hg flux from substrates was measured using dynamic flux chambers in-situ and in a more controlled laboratory setting, in conjunction with environmental parameters. The mean daily flux for the Twin Creeks area (88 ± 282 ng m-2 d-1, n = 15) was greater than that of the Cortez-Pipeline area (14 ± 19.5 ng m-2 d-1, n = 14). Mean substrate Hg concentrations were also higher at Twin Creeks (342 ± 426 ng g-1, n = 73) relative to Cortez-Pipeline (95 ± 78 ng g-1, n = 74). Multiple linear regression analysis revealed that Hg flux was significantly (p < 0.05) correlated with substrate Hg concentration, solar radiation, and soil moisture. Using this data, pre-disturbance Hg emissions from the current active mine areas at Cortez-Pipeline and Twin Creeks were estimated to be 0.1 kg yr-1 and 1.7 kg yr-1, respectively, one to two orders of magnitude less than the estimated current releases occurring from the same mining disturbed areas. Solid- phase thermo-desorption analysis revealed that for mining and undisturbed substrates the majority of the Hg was released at temperatures greater than 450 °C, indicative of a mineral phase such as HgS or HgO. The amount of Hg volatilized at lower temperature (80-125 °C) was generally less than 1%. Hg0 was observed to deposit to mining and undisturbed substrates under elevated air Hg concentrations (> 100 ng m-3) at deposition velocities ranging from 0.13 to 0.46 cm s-1. The results indicate that atmospheric Hg0 deposition is possible under favorable conditions (high air concentration, low light intensity, low atmospheric oxidant concentrations), and may have a potential for net accumulation.