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

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.