A review of methods to calculate current and future evaporation rates from pit lakes with high concentrations of total dissolved solids

Abstract

For pit lakes in arid environments, lake evaporation and mechanical evaporation methods (e.g., misters) are sometimes used to manage water levels to avoid discharge to the receiving environment (e.g., aquifers). For terminal lakes, the steady-state water surface elevation remains below the regional water table. This creates a perpetual sink in the local water table such that the pit retains mine impacted water on site. One management strategy for flow-through lakes involves enhancing evaporation using misters to prevent the lake from reaching its steady state water level, thereby producing an artificial terminal pit lake. Evapoconcentration coupled with the deposition of mister-generated aerosols landing within the pit catchment can increase the concentration of total dissolved solids (TDS) in lake water over time. As TDS concentration increases, the activity of water decreases, reducing the vapor pressure and decreasing the evaporation rate. Consequently, the long-term management plan and water balance for artificial terminal lakes must account for TDS concentrations in future projections of evaporation rates. This review evaluates methods to calculate current and future evaporation rates, including methods that consider the impact of TDS concentrations on the activity of water. First, we review methods used to estimate current evaporation rates, including: (1) pan evaporation, (2) water balance, (3) energy balance, (4) combination mass transfer and energy balance method (called 'combination method', e.g. Penman equation), ( 5) pan and combination method (called 'PenPan'), ( 6) water isotope mass balance, ( 7) temperature-only models [e.g. the Hargreaves and Samani (H-S) equation], and ( 8) eddy covariance measurements.Next, using a modified Penman equation paired with an ocean water equation of state, we showed how increased TDS concentrations in a theoretical lake can reduce the activity of water and estimated evaporation rate. Results of this exercise showed that simulated evaporation was greatly impacted above TDS concentrations of 300,000 mg/L. For long term management plans and water balances that utilize predictions of future evaporation rates, it is preferred to use a method like the H-S equation since it requires only downscaled temperature from a climate projection, whereas the modified Penman requires wind speed, relative humidity and other meteorologic variables which are not typically generated by climate projections and have greater uncertainty. To show how a H-S equation can be modified to account for TDS concentrations, we used the modified Penman to establish the coefficients for TDS and site conditions to establish a modified H-S equation.