Sensitivity analyses in pit lake prediction, Martha Mine, New Zealand 1: Relationship between turnover and input water density

Abstract

Turnover events in pit lakes influence the water quality of pit lakes by homogenizing the chemistry of mixing layers and distributing dissolved oxygen, hence limnologic models that predict the depth of annual turnover are required for predictive studies of post-mining pit lake water quality. This study demonstrates a limnologic prediction of a proposed pit lake at the Martha Au–Ag Mine, New Zealand, created with the hydrodynamic program DYRESM. The model considered 5 years of lake-filling conditions where the pit will receive river water, groundwater, pit wall runoff, and direct rainwater followed by 1 year of steady-state conditions where the lake will receive only groundwater, runoff, and rainwater. Sensitivity analyses showed that input groundwater temperatures ≤ 17 °C produced meromictic conditions, whereas groundwater temperatures ≥ 17 °C produced holomictic conditions. Changes in the chemistry of runoff inputs had no effect on turnover because the quantity of runoff is significantly less than the quantity of other inputs. These results suggest that understanding the density of major lake inputs is critical toward predicting the limnologic evolution of a pit lake as small changes in the water chemistry or temperature between inputs may affect input density and the resulting turnover depth. Lakes filled with water from one principle source, such as groundwater, are more likely to be holomictic due to the relatively homogeneous character of the resulting lake water, whereas lakes filled with water from more than one source, such river water and groundwater, may develop meromictic conditions due to slight density differences between input waters. It may be possible for mine managers to engineer holomictic or meromictic conditions in future pit lakes to improve lake water quality.