CASE STUDY: 20 YEARS OF ACID ROCK DRAINAGE CHEMISTRY IMPROVEMENTS AFTER A BACTERICIDE APPLICATION 1

The Fisher site is a backfilled and reclaimed (in 1984) surface coal mine in western Pennsylvania, USA. A post-closure toe seep at the site discharged acid rock drainage generated in pyritic rock zones that were identified using geophysical techniques. In 1995, sodium hydroxide and bactericide solutions were injected through cased boreholes into the pyritic zones in a two-step process: sodium hydroxide followed by bactericide. Prior to the event, the toe seepage had been treated with the addition of sodium hydroxide followed by a series of settling ponds and wetland zones. Post- injection, the seepage exhibited net-alkaline chemistry and the sodium hydroxide amendment was discontinued. Based on the prevailing wisdom at the time, the effects of the injection event were expected to be temporary. Two decades later, the beneficial effects of the two-step injection event appear to persist and bond release for the site is pending. The seep chemistry has been monitored for over 25 years and the data trends suggest that the steady-state condition of net alkalinity in the seep water entering the ponds and wetland may be permanent. One current view is that the initial suppression of Acidithiobacillus ferrooxidans bacterial community with the sodium hydroxide and bactericide has been maintained by the seasonal infusion of anti-bactericidal organic acids derived from the robust vegetative cover. The situation appears to be self- sustaining. Others may view the data skeptically; that is, the cause and effect of bactericide application and the sustained benefits of the vegetative cover are not proven conclusively. Certainly, the observations suggest that additional focused study is warranted.