Evaluation of the Heterogeneity of Constructed Landforms for Rehabilitation Using Lysimeters

Rainfall ingress into sulfidic rocks or tailings from metalliferous mining operations can result in acid mine drainage. Waste rock cover systems in semiarid areas are commonly designed to retain all precipitated water within benign material, from where it is removed by evapotranspiration. The long-term effectiveness of covers is often predicted from models that are based on data obtained from single trial plots assuming that the trials are homogeneous and representative of large areas. Two cover designs were tested in semiarid monsoonal northwest Queensland: (i) 1.5 m of unconsolidated waste rock overlying 0.5 m of consolidated waste rock; and (ii) 2.0 m of unconsolidated benign waste rock. Three identical plots comprised each treatment in which water balance equation parameters were estimated from meteorological measurements and vertical arrays of soil suction and moisture sensors and seepage collection using a 3-m-deep lysimeter inserted in the waste rock. In a wet season with 900 mm of precipitation, water movement through the covers was followed through changes in moisture content, suction, and seepage. Greater differences in these parameters occurred within than between cover treatments. Water retention and water movement varied substantially and seepage ranged from 2 to 80% of total rainfall. The internal heterogeneity of hydraulic properties had more effect on cover performance than did the initial cover design. Therefore, it is important to include internal heterogeneity in mine waste cover water balance models to improve their applicability.