INTEGRATING PREDICTIVE HYDROGEOLOGIC AND GEOCHEMICAL ASSESSMENTS IN PIT-LAKE REGULATION AND RECLAMATION IN NEVADA

Pit lakes form as groundwater recovers following the closure of open pit mines, which are common throughout the State of Nevada. Management of both existing and predicted future pit lakes is overseen by the Nevada Division of Environmental Protection (NDEP) - Bureau of Mining Regulation & Reclamation (BMRR). Water quality of existing pit-lakes varies from meeting drinking water standards to representing significant threats to water resources. Formation of some pit lakes preceded current regulations; these pit lakes lack a responsible party and have become abandoned mine lands. Reclamation of pit lakes may be required depending on current or predicted characteristics including groundwater outflow rates and water quality. Currently the BMRR manages 42 existing pit lakes, with additional pit lakes predicted to form in the future. This presentation focuses on case studies and strategies related to pit lake prediction and regulation.

Standard scientific assessment of the cumulative impacts of pit lakes includes predictive hydrogeologic and geochemical models. These models are built using site-specific constraints including hydrogeologic parameters, meteorological conditions, and the geochemistry of deposit host rocks and background groundwater. Models typically contain a large number of unknown or difficult to estimate quantities that introduce inherent uncertainties into model predictions.

Several existing pit lakes with poor water-quality are examples of the risk of relying heavily on predictive models for regulatory decisions, as these predictive geochemical models have varied in their accuracy and applicability. Reclamation strategies for pit lakes with poor water-quality vary depending on geologic characteristics, but strategies should be included in predictive models.

In addition to water-quality issues, the quantity of water that will eventually be stored in pit lakes, and lost to evaporation from pit-lake surfaces, may be of concern. Application of reclamation strategies may alleviate water-quantity concerns related to pit lake formation.

The myriad of technical, environmental, and policy issues related to pit lakes mean the critical assessment of pit lake predictive studies and reclamation strategies are crucial in properly managing these controversial mining legacies.