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Performance of Mono-Layer Evapotranspirative Covers in Response to High Precipitation and Extended Drought Periods in the Southwestern United States

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  • Properly designed evapotranspirative (ET) cover systems can greatly reduce the amount of infiltration and groundwater recharge (deep percolation) into potential acid generating mine waste. In arid and semi-arid environments, infrequent precipitation, low measurable subsurface fluxes, spatial variability within mine waste and cover material, and evapotranspiration from below the cover system create challenges to monitoring moisture flux in these systems. Sloped ET single-layer (monolayer) cover depths and a variety of reclamation treatments are being studied at a copper tailing impoundment in Arizona, USA. To monitor the performance of different ET cover systems in response to precipitation patterns, four different test plots consisting of two different cover depths and two different vegetation densities were instrumented with heat dissipation sensor nests. The sensors measure the soil water pressure potential and allow hydraulic gradients to be determined within and below the cover systems to depths of 180 cm. Three replicate sensor nests were installed in each test plot to account for variability in materials and test plot treatments. Data collection occurs twice daily and is ongoing. Weather during the 26-month monitoring period was characterised by two months of greater than normal precipitation followed by nine months of normal precipitation and 15 months of abnormally dry conditions. Monitoring data indicate that deep percolation occurs in response to periods of extended precipitation, however, drying was observed to depths of six feet below the surface in all test plots. Using conservative assumptions regarding tailing hydraulic properties, one-dimensional downward flux predictions based on the monitoring data indicate very low (<1.6 mm/year) average annual deep percolation rates. Prior to the abnormally dry period, increased cover depth and dense vegetation reduced the amount of moisture reaching the deeper sensors. During the abnormally dry period, little difference in the soil moisture regime was observed between cover depths and vegetation. Subsequent to the dry period, observed wetting and predicted deep percolation in the dense vegetation test plots was slightly greater than in the sparse vegetation test plots, indicating that the ET cover system performance is dynamic. Additional long-term monitoring and installation of deeper and replicate monitoring sensors into tailings without cover systems are recommended to evaluate the long-term ET cover system performance and develop a better understanding of deep percolation rates at the sites.

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