Stability and meromixis in a water-filled mine pit

Abstract

A field experiment examined stability and stratification in a disused mine Fit filled with 1.75 X lo7 m3 of water. Vertical profiles of conductivity and temperature indicated that, due to substantial chemical stratification, spring and autumn overturn did not penetrate to the base of the pit. Although parameterization indicated that double diffusion should be expected, we found only circumstantial evidence of associated step structure. The available data suggest that groundwater inflow created a warm salty pool of water at the base of the water column, giving the appearance of a meromictic structure with a monimolimnion. However, this pool was noi. a consistent feature, suggesting both a variable inflow and significant diffusion rates. From the temperature data, estimates indicated that hypolimnetic vertical eddy diffusivity varied between 1 X 10e7 and 5 X lo-” m* s-l. The observations identify short-term relatively energetic internal wave events that may have a significant impact on this value. The present study is useful in two ways: first, it shows how pit-lakes form a natural laboratory fcr a range of processes, and second, it illustrates how these processes relate to diffusion paranieterization. It is clear that reliable parameterization is vital for long-term modeling required for prediction of water quality over decadal timescales. With the closure of an open-cut mining operation come the dual problems of rehabilitation of the mine pit and containment of deleterious material generated by the mine during its life. In many cases the pit fills with water, either naturally or through mine activities. Thus, an artificial lake of moderate volume is created (Davis and Ashenberg 1989). Characterization of the transport processes within the water column is important for two reasons. First, the pit is likely to contain hazardous levels of dissolved metals, and second, if vertical transport rates are low then the pit may prove suitable as a storage location for hazardous material. Water-filled mine pit lakes (“pit-lakes”) have received consideration in the literature but with a biological or geochemical emphasis (e.g.. Klapper and Schultz 1995). Crater lakes are the natural system most similar to these artificial water bodies (Martin 1985; MacIntyre and Melack 1985). Pit-lakes are unique for several reasons: they are often deep relative to natural lakes of similar surface area; they have high surrounding walls that often generate a microclimate different to that outside the pit; they typically have very thin sediment layers; they may undergo substantial and rapid changes in water level; and they are likely to contain significant concentrations of dissolved material. The final point is important if the dissolved material is environmentally harmful or if it leads to meromixis. With these departures from what might be associated with typical natural lakes,