Groundwater Modeling as a Tool for Closure Planning: Prediction ofZinc Transport for Alternative Cover Scenarios

摘要

Groundwater modeling was used to assists in the design of a soil cover for an acid-generating waste rock pile at the Woodcutters lead-zinc mine near Darwin, N.T.,Australia. A three-dimensional groundwater flow (MODFLOW) and solute transportmodel (MT3D) was developed for the site to predict the timing and magnitude of peakzinc concentrations (the primary metal of concern) in shallow groundwater discharging tothe nearest stream for different cover scenarios: (1) removal of the waste rock dump (nocover required); (2) use of a "high quality" cover (allowing net infiltration of 1% ofMAP); (3) use of a "lower quality" cover (10% of MAP); and (4) use of a "low quality"cover (20% of MAP). The predicted times for a "breakthrough" of peak zinc concentrations in a nearby creekranged from ～150 years for Scenario 1, to ～400 years for Scenario 4. While the lowestpeak zinc concentrations were predicted for Scenarios 1 and 2 (～1.2 mg L~(-1)),all scenariosproduced peak zinc concentrations that exceeded the low risk trigger value (0.008 mg L~(-1))for protection of 95% of aquatic biota in the creek. An ecological risk assessment ensuedwhich found that cover scenario 3 (10% of MAP) would be sufficient to protect theecology in the nearby creek. In 2003/2004, the waste rock pile was reshaped and a high quality cover was placed.Detailed groundwater monitoring is ongoing at the site to assess the performance of allsite closure measures, including cover placement. Monitoring over the last five yearsindicates that the groundwater flow system is recovering as predicted whereascontaminant concentrations (e.g. sulphate, zinc) in the local groundwater and theWoodcutters Creek are lower than predicted.