The Importance of Catchments to Mine-pit Lakes: Implications for Closure

Despite the large body of riparian literature for rivers and lakes, there are few studies on the catchments of mine pit lakes. Therefore, the broad objective of this research was to determine if catchment characteristics were related to pit lake nutrient concentrations. We hypothesised that: (1) catchment characteristics would vary among pit lakes, (2) pit lake catchments would differ from co-occurring naturally-forested catchments, and (3) connecting a pit lake (Kepwari) to a naturally-forested catchment via a river flow-through would increase C accumulation in the lake. The research was conducted in pit lakes of the Collie lake district in Western Australia and examined catchment characteristics (soil nutrients, litter biomass, vegetation, and biomass), carbon and sedimentation rates in pelagic and benthic materials, and establishment of a simple nutrient budget for Lake Kepwari. Broadly, results indicated that catchment vegetation differed significantly among pit lakes (although parts were similar to co-occurring natural forest), with differences largely driven by catchment age. None of the pit lake catchments had true riparian species surrounding the pit lakes. The hypothesis that connecting a pit lake to a watercourse would increase carbon concentrations and sedimentation was not supported. Most (87%) of the water that entered the lake was from the river, and river outflow was slightly greater than river inflow. However, the lake acted like a nutrient sink, with more N, P, and TOC calculated in the inflows than the outflow. We found that it took 10–15 years of growth for actively rehabilitated pit lake catchments to approximate natural forest and 60 years for unrehabilitated pit lake catchments. Active rehabilitation that includes riparian-specific planting would likely lead to better outcomes for pit lake catchment vegetation. Connecting a pit lake to a river substantially increased catchment size and large amounts of river-derived carbon were deposited in the lake. However, carbon concentrations were relatively low compared to the size of the lake. Our results suggest that active intervention is required if stakeholders are unwilling to wait decades for the biological development of pit lakes.