The Importance of Nitrate Dynamics in Mine Pit Lakes of Drier Regions

This study reviewed nitrate (NO3-) dynamics in non-acidic dry-zone pit lakes to indicate likely gains, losses, transformations, and trends over time. Nitrate dynamics are more complex in such lakes as many nitrogen transformation and loss processes are inhibited at low pH. However, NO3- dynamics in drier regions present greater potential hazards due to NO3- evapo-concentration. Mine water inputs from in-pit and ex-pit waters elevated with NO3- from dissolution of blasting agents and contact with mine wastes are likely to be the highest concentration NO3- sources for pit lakes. However, groundwater inflows from aquifers with naturally elevated NO3- concentrations can sometimes contribute markedly to pit lake NO3- budgets. Case studies of pit lakes have found that NO3- appears to undergo net removal over time at rates relative to concentration, through processes that tend to be dominated by denitrification and ammonification of NO3-. As a consequence, although pit lakes often contain elevated NO3- concentrations, these concentrations can diminish over the first few years to decades. Longer term concentrations will depend on the rates of more sustained inputs of NO3- over periods of decades to centuries, including nitrogen fixation and surface and groundwater sources.