Predicting the effects of evapoconcentration on water quality in mine pit lakes

Abstract

For mine pit lakes that are hydrologic sinks, evapoconcentration can be an important process affecting long-term water quality. Based on the concept of chemical divides, pit lakes may be categorized according to the ratio of Ca to alkalinity. Lakes with 2 m Ca< m alkalinity can be expected to evolve to alkaline pH, Na–HCO 3–CO 3–(±SO 4)–(±Cl)-dominated solutions. Most predictions of long-term chemistry for pit lakes fall into this first category. Lakes with 2 m Ca> m alkalinity will to evolve to near-neutral pH, Ca–Na–(±SO 4)–(±Cl)-dominated solutions. Most existing pit lakes fall into the second category. This comparison suggests that the models used for predicting long-term pit lake chemistry may overestimate rates of alkalinity-generating processes. The distinction between the two evapoconcentration pathways is important for predicting the concentrations of trace constituents, such as As and Se, that are often chemicals of concern for eco-risk assessments. Pit lakes falling into the first category can reach pH>9, at which anionic As and Se species are not adsorbed, hence accumulate in solution over time. Pit lakes falling into the second category remain at near-neutral pH, under which most of the anionic As and Se species are adsorbed, hence do not accumulate in solution. Observations of solute compositions in existing pit lakes combined with consideration of evapoconcentration pathways in natural lakes can provide useful analogies for understanding the long-term chemical evolution of pit lakes.