When Density Matters: Hydraulic and Salinity Evolution in Groundwater-Fed Pit Lakes in Semiarid and Arid Climates.

Although groundwater flow directions are influenced by hydraulic gradients and density gradients, density gradients are often assumed negligible outside coastal zones and saline lake environments. Density gradients are rarely considered in mining studies, but can alter groundwater flow patterns, and cause outflow of water from pit lakes despite inward hydraulic gradients. Open mine pits often intersect regional water tables, requiring dewatering during mining operations. At the end of mine life, groundwater abstraction ceases, frequently leading to the development of pit lakes. Due to prolonged water residence time, and evaporation, pit lake water quality may deteriorate with salinization being a common problem. While salinity differences between pit lake water and groundwater are small initially, they increase with time, inducing density contrasts. Consequently, dense pit lake water may move along density gradient towards less dense groundwater. This changes the flow patterns around the pit lake and impact on surrounding aquifer water quality. In this study, we advance process understanding of how density effects alter flow and salinity patterns in pit lake environments post-mining using numerical modeling. We show the impact of ambient groundwater salinity, regional hydraulic gradients, evaporation rates, and hydraulic conductivities on the interaction between a pit lake and the surrounding aquifer. We demonstrate how density effects can substantially increase lake water outflow and decrease pit lake water salinities. Pit lakes can turn from terminal sinks into throughflow systems purely due to variable-density flow. Understanding the hydraulic and salinity evolution of pit lakes is crucial for planning post mining rehabilitation.