ABSTRACT
A test plot study has been initiated to study the performance of a store-and-release cover consisting of alluvial soil over tailings proposed for final closure of a large tailings facility in New Mexico. The test plot study consists of two closed lysimeters covered with 0.23m (9”) and 0.60m (24”) of alluvial soil (silty gravel) over backfilled tailings and an instrumented deep in-situ tailings profile with a 0.28m (11”) alluvial cover with existing grass/shrub vegetation. All test plots were instrumented with temperature/suction sensors and moisture content sensors to monitor moisture movement in the cover/tailings profile. The lysimeters are free-draining and outflow is collected and monitored continuously using a tipping bucket. The lowest rate of net infiltration (20mm or 6% of total precipitation) during the first year of monitoring was observed in the deep in-situ cover/tailings profile. Net infiltration into the unvegetated lysimeter test plots were 55.9mm (17.3%) for the 0.23m alluvial cover and 117mm (36%) for the 0.60m alluvial cover. Initial calibration of a soil atmosphere model (SoilCover) to the test plot data suggested that the lower rates of net infiltration into the in-situ profile are a result of (i) higher evapotranspiration due to the presence of vegetation and (ii) lower vertical hydraulic conductivity of the in-situ tailings (relative to the backfilled tailings in the closed lysimeters). The observed rates of net infiltration for the unvegetated, backfilled lysimeters may therefore significantly overestimate cover fluxes for long-term (post-closure) conditions. The calibrated model should be used to predict cover performance for a range of climate conditions (e.g. wet year vs dry year) and cover design parameters (e.g. cover thickness).
