Environmental controls of the seasonal variation in oxygen uptake in sulfidic tailings deposited in a permafrost-affected area

Oxygen consumption, sulfide oxidation, and acid mine drainage (AMD) of pyri tic mine tailings were investigated at Nanisivik Mine, which is located in an area with continuous permafrost on Baffin Island in northern Canada. Tai lings of varying age and water content have been deposited under alkaline c onditions. One area consisting of. tailings deposited on land in 1992. was selected for detailed measurements of in situ oxygen uptake rates at the ta iling surface in the summers 1998 and 1999 and periodically during autumn a nd winter in 1998, Measurements included oxygen gas, water content, and tem perature in profiles, as well as chemical analyses of pore solution and sol ids. Additional oxygen consumption rates were measured under controlled tem perature conditions on columns filled with partly oxidized tailings. On the basis of temperature dependency of pyrite oxidation observed in the labora tory, an Arrhenius diffusion equation with soil temperature as input was us ed to simulate the observed temporal variation in oxygen uptake. Field data reveal that the ongoing sulfide oxidation of well-drained tailings primari ly takes place in the upper 30 cm and thar oxidation has resulted in a depl etion of pyrite, carbonates, and metals from this reaction zone. The model provides a reasonable fit to the observed trend in oxygen consumption and d ocuments that oxidation of sulfide minerals in tailings is not reduced to n eglectable levels at 0 degreesC. The AMD generation rate has been quantifie d based on the changes in concentration of oxidation products in the pore w ater and oxidation rates based on in situ measurements of oxygen consumptio n, The two rate descriptions provide comparable estimates of seasonal AMD g eneration and provide detailed information on weather-related controls of A MD generation, i.e,, ground temperature, freezing, water content, and snow cover. These environmental controls are crucial for the design of frozen co ver schemes in permafrost regions, where the aim is to force the frost tabl e to the top of the tailings, maintaining tailings at subzero temperatures year-round and thereby controlling the AMD generation.