Predicting mixing trends in mine-receiving waters using water quality modeling

Abstract

Complete mixing of the water column of lakes is important to ensure that nutrients and dissolved oxygen are replenished and equalized throughout the lake. This ensures that organisms within the lake at all trophic levels can perform necessarily metabolism and have a maximized habitat range. Lakes that receive effluent high in total dissolved solids from mine operations can be prevented from performing complete water column turnover. When this occurs the lake is termed meromictic. Two lakes, one naturally remediating and the other actively receiving effluent discharge, have both exhibited historical meromictic conditions (meromixis). Over a three year period, the water quality in both lakes was analyzed and related to their current meromictic state. The CE-QUAL-W2 model was calibrated to predict total dissolved solids concentrations within the active receiver lake and one-year predictive simulations were run with total dissolved solids reductions of 10%, 25%, 50% and 75% at the lake inflow. The lake undergoing natural remediation exhibited substantial breakdown of meromictic stability during the study and was deemed monomictic while the active receiver demonstrated meromictic stability and variability in mixolimnion turnover depth. Calibration of CE-QUAL-W2 revealed that fluctuations in surface total dissolved solid concentrations and dissolved organic carbon from natural and effluent inputs had substantial influences on heat distribution in the effluent receiver. Reducing total dissolved solids inputs into the active receiver lake only elicited change in mixolimnion total dissolved solids concentrations but little change in monimolimnion concentrations. The 50% and 75% reduction simulations resulted in a much shallower mixolimnion depth and stronger meromictic stability in the lake.