Evapotranspiration cover performance in a high desert environment, north waste rock disposal facility, Rain Mine, USA

Authors: Creek, M; Wickham, M; Gjerapic, G

DOI https://doi.org/10.36487/ACG_rep/1152_33_Creek

Cite As:
Creek, M, Wickham, M & Gjerapic, G 2011, 'Evapotranspiration cover performance in a high desert environment, north waste rock disposal facility, Rain Mine, USA', in AB Fourie, M Tibbett & A Beersing (eds), Mine Closure 2011: Proceedings of the Sixth International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 303-312, https://doi.org/10.36487/ACG_rep/1152_33_Creek

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Abstract:
Newmont Mining Corporation conducted a performance audit of an evapotranspiration (ET) cover constructed in 2002 at an acid generating waste rock disposal facility located at 2,000 m elevation. Based on technology available at that time, the monolithic ET cover design included approximately 0.9 m of local alluvium with native sagebrush steppe vegetation. The site receives an average of 540 mm of precipitation annually, 84% of which falls during October to April. The cover did not significantly reduce infiltration into the waste rock, as measured by a seepage collection system constructed beneath the waste rock facility. Average annual seepage following reclamation has ranged from 38,000 to 193,000 cu. m, or 13 to 44 % of annual precipitation for the 76 ha facility. Peak monthly average rates during the spring have been as much as 10 litres per second (L/s). Seepage chemistry is poor, with pH typically below 3, total dissolved solids concentration ranging from 5 to 30 g/L, and elevated metals concentrations. Newmont proactively conducted studies of the reclamation cover including installation and monitoring of moisture sensor nests, laboratory testing, surface geophysics, monthly snow surveys, geochemical characterisation of the waste rock, and groundwater monitoring. A detailed assessment of the available design and construction data, site climate, and cover monitoring data was conducted in 2010 to assess cover performance. The study included precipitation gauge catch corrections, modelling of potential ET (PET) by aspect and slope, a direct method for computing infiltration from the moisture content sensor data, an analysis of seepage chemistry, and calibrated numerical modelling. The assessment concluded that the compromised cover performance was due to the compounded effects from winter precipitation, snow drifting, slope and aspect influence on PET, available water holding capacity of the cover material, and cover construction unconformities. While the ET cover is able to fully reset the available water holding capacity of the cover each year, the cover profile is fully wetted each spring from snowmelt, resulting in significant infiltration into the waste rock. Newmont is using the results of this study together with an engineering analysis of alternative reclamation designs to identify a final reclamation strategy for the facility.

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