ABSTRACT
The tendency of pit mine lakes to permanently stratify and become anoxic has tremendous influence on the accumulation of trace metals and thus the environmental liability of mine owners and operators. Most pit mine lakes have a large depth/surface area ratio commonly referred to as the relative depth (zr). This, plus the fact that pit walls tend to shelter the lake from wind stress, means that pit lakes do not mix as easily as natural lakes and therefore have a tendency to stratify. In order to become permanently stratified, the density contrast between the surface and deep waters of a lake must be sufficiently high that wind-induced shear stresses cannot mix the water column. The gradient Richardson number (Ri), defined as the ratio of vertical density gradient to the square of the vertical velocity gradient, is typically used to quantify these two competing forms of energy. A water column is permanently stratified when the Ri > 0.25. This is more likely occur when zr is large because wind energy is less effective at penetrating the water column. Detailed measurements of currents and density at the Yerington, Nevada pit mine lake confirms the generalized application of the Richardson number criteria. Numerical models of lakes also show that intermediate depth lakes (10-30 m) lakes are more likely to become seasonally anoxic while shallow lakes (< 10 m) are neither permanently nor seasonally anoxic.
