AN ESTIMATE OF LONGEVITY OF POLLUTION FROM THE RICHMOND MINE AT IRON MOUNTAIN, CALIFORNIA

The Pollutant Loadings Above average Pyrite Influenced Geochemistry Pollutant Sources and Sinks in Underground Mines (PLAYING POSSUM) model, developed at Newcastle University utilizes object-oriented programming techniques and geochemical algorithms to simulate hydrogeochemical changes in mine water. PLAYING POSSUM may be applied to abandoned coal and metal mines in order to confirm the governing geochemical processes in the system and to predict the longevity of polluting drainages. The model solves for hydrogeochemical changes based on mineral weathering of a suite of 28 minerals, mineral precipitation, reversible sorption, dissolution and precipitation of acid generating salts and addition of pollutants from dispersed inflows. PLAYING POSSUM has been used to simulate the discharge from Richmond Mine at Iron Mountian, Shasta County, California, in an attempt create a process-based estimate of the longevity of the polluting drainage. In contrast to the U.S. Geological Survey steady-state estimate of 3200 years until the ore body is exhausted, the simulation solution created by PLAYING POSSUM estimates that the mine water chemistry will decrease to asymptotic levels after approximately 3500 years. Although these results seem compatible, the asymptotic pollution levels are only approximately 10% of the initial levels and, therefore, the discharge is still highly polluting. During the 5000 year simulation period, only 40% of the remaining ore body weathers. The estimate of pollution longevity produced by PLAYING POSSUM shows the need for non steady-state estimates based on geochemical controls acting in abandoned mine systems.