Abstract:

Pit lakes are commonly characterized by meromictic density structures, whereby permanent water column stratification may occur naturally or through engineered methods. In turn, meromixis typically results in the development of suboxic conditions in the monimolimnion via the bacterial respiration of organic matter. Within suboxic bottom waters, various redox processes have the potential to affect water quality, including oxygen consumption, denitrification, Fe(III)/(Mn(IV) reduction, sulfate reduction, and alkalinity generation. In turn, such redox reactions can influence metal(loid) mobility through changes in redox potential, pH, aqueous speciation, sorption and the solubility of secondary minerals. To support the closure management of pit lakes, accurate prediction of the evolution of suboxia and associated redox processes is relevant to both water quality and aquatic habitat considerations. In this paper, a 1-D hydrodynamic model is described (PitMod) that couples pit lake hydrodynamic process with suboxic redox reactions. Within the geochemical component of PitMod (PHREEQC), oxygen consumption rates are applied to both the water column and sediment surfaces that govern the depletion of dissolved oxygen, allowing for predictions of changing redox conditions over time and with depth. The model mechanics that allow for the coupling of hydrodynamic and redox processes, as well as the model limitations, are discussed.

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Maryam Zarrinderakht