STABILITY AND MEROMIXIS IN A WATER-FILLED MINE PIT

A field experiment examined stability and stratification in a disused mine pit filled with 1.75 x 10(7) m(3) of water. Vertical profiles of conductivity and temperature indicated that, due to substantial chemic al stratification, spring and autumn overturn did not penetrate to the base of the pit. Although parameterization indicated that double diff usion should be expected, we found only circumstantial evidence of ass ociated step structure. The available data suggest that groundwater in flow created a warm salty pool of water at the base of the water colum n, giving the appearance of a meromictic structure with a monimolimnio n. However, this pool was not 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 10(-7) and 5 x 10(-5) m(2) s(-1). 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 f or a range of processes, and second, it illustrates how these processe s relate to diffusion parameterization. It is clear that reliable para meterization is vital for long-term modeling required for prediction o f water quality over decadal timescales.