AN INTEGRATED PROCEDURE TO PREDICT PIT LAKE GEOCHEMISTRY: PART II. PHYSICAL LIMNOLOGY, MARTHA MINE, WAIHI, NEW ZEALAND

The physical limnology of a pit lake will influence the vertical distribution of dissolved oxygen in lake water and affect the potential for subaqueous pyrite oxidation and acid generation. This paper demonstrates the application of the one-dimensional hydrodynamic model DYRESM to a future pit lake that will develop in the Martha mine, North Island, New Zealand, by 2012. DYRESM predicts vertical temperature and density structure for lakes based on a set of forcing functions applied to lake morphology, meteorological data, and other parameters. To assess the accuracy of these predictions for the Martha pit lake, DYRESM was also applied to the existing Blowout pit lake in Utah, USA which exhibits similar characteristics to the future Martha lake, and for which limnologic data is already available. The primary similarity between the lakes is morphologic proportionality, expressed in terms of relative depth (maximum depth as a percentage of mean diameter). Martha lake will have a surface area of approximately 28.7 ha, a maximum depth of 192 m, and a relative depth of 32%. Blowout lake has a surface area of 3.4 ha, a maximum depth of 70 m, and a relative depth of 34%. The lakes are equidistant from the equator and so receive similar solar radiation, with Martha lake at 37° S, and Blowout lake at 37° N. However, major differences include altitude, precipitation/evaporation rates, and annual temperatures. Martha lake will exist 100 m above sea level, will receive 3 m of precipitation annually, and will respond to air temperatures that remain above freezing throughout the year. Blowout lake exists 1900 m above sea level, responds to evaporation rates that exceed precipitation rates by a factor of 10, and responds to air temperatures that drop below freezing in winter producing surface ice. Previous study has show that Blowout lake circulates annually and contains oxygen-rich water at depth. DYRESM modeling predicts only the density stratification in the future Martha lake, from which dissolved oxygen concentrations are inferred. This must be combined with quantitative mineralogy defined in Part I, to accurately model water-rock reactions within the pit lake, and the effects of these reactions on pit lake water quality, over short and long time periods.