A coupled hydrodynamic-biogeochemical model for the prediction of mine pit lake water quality

Abstract

Pit lakes are a common feature of the post-closure landscape at mine sites where mining voids are allowed to fill with surface runoff, direct precipitation, and groundwater. Pit lakes also commonly serve as focal points for post-closure water management, often serving as a receptacle for various mining-related drainages and the final point of discharge for mine effluents. Therefore, an accurate and acceptable numerical model capable of predicting pit lake water balance, mixing characteristics, and water quality is needed to support post-closure management. In this paper, a coupled hydrodynamic-biogeochemical model (PitMod) for water quality prediction is described. The model, described in Crusius et al. (2002) and Dunbar (2013), simulates the physical and geochemical evolution of pit lakes over pit filling times ranging from monthly to century time scales. Within PitMod, the pit lake is approximated based on a one-dimensional, horizontally averaged vertical layer scheme. PitMod calculates the time-dependent vertical distribution of physical and geochemical pit lake properties, including temperature, salinity, conductivity, and dissolved oxygen. In this regard, the physical component of PitMod considers the effects of pit morphology, climate data, multiple surface and sub-surface (groundwater) inflows/outflows, precipitation/evaporation, surface ice formation/melting, vertical mixing due to surface wind stress, convective circulation, and turbulent mixing. The geochemical portion of PitMod utilizes a customized version of PHREEQC, capable of a wide variety of aqueous geochemical calculations, including speciation, saturation index calculations, mineral equilibria, surface complexation (adsorption) reactions, ion exchange, and redox processes. The model can also incorporate predictions of dissolved metal scavenging by biogenic particles in response to lake primary production.PitMod, which has been applied at over 50 mine projects since 2002, incorporates physical processes like those found in other lake models such as DYRESM (Imerito, 2007), and has been validated against field observations. However, unlike DYRESM, PitMod offers the advantage of being able to incorporate various non-conservative geochemical and biological processes that are relevant to predictions of long-term water quality.