Cold desert evapotranspiration cover system design

Abstract

Round Mountain Gold Corporation (RMGC) mine is located in central Nevada, USA at an elevation varying between 1,770 m and 2,070 m above mean sea level. The climate is characteristic of a cold desert climate, with cold winters and hot summers. Average annual precipitation is 11.8 cm, approximately 13 times less than the calculated average annual potential evapotranspiration (PET) of 159 cm. RMGC constructed evapotranspiration (ET) cover system test plots in 2012 to evaluate the effectiveness of 0.3 m, 0.9 m, and 1.5 m thick monolayer ET cover systems in minimizing net percolation of precipitation into underlying potential acid generating waste rock which may result in poor water quality seepage outflow. Each test plot is approximately 200 m2 and includes three cover system performance monitoring stations consisting of sensors placed at approximately 0.6 m intervals along a vertical profile in the cover and waste rock to a maximum depth of 2.4 m. The sensors measure soil matric potential, temperature, water content, and direct net percolation water flux. Soil water content and matric potential data are used to evaluate the cover systems’ capacity to store infiltrated precipitation and to remove water via ET. Direct net percolation flux measurements are collected below the estimated depth of ET and these measurements provide a point estimate of net percolation flux into the waste rock. The site-specific seed mix applied to the test plots did not establish, thus test plot vegetation is primarily comprised of invasive annuals Russian Thistle (Salsola tragus) and Halogeton (Halogeton glomeratus). Annual precipitation from 2012 through 2021 was representative of long-term average precipitation conditions. Wetting was observed to a maximum depth of 1.2 m at the 0.3 m and 1.5 m ET cover system test plots. The deepest wetting (1.8 m) was observed in the 0.9 m ET cover system test plot due to focused run-on at the location of two of the three monitoring stations. At all test plots, drying of the soil profile occurred in late spring and early summer in response to decreased precipitation and increased ET. The average annual net percolation flux over the monitoring period was zero for the 0.3 and 1.5 m ET cover systems and 0.1% of precipitation for the 0.9 m cover system, most likely due to run-on to the test plot. Results to date indicate no difference in the effectiveness of 0.3 m, 0.9 m, and 1.5 m ET cover system thicknesses in minimizing net percolation of precipitation into underlying waste rock.