Abstract. Post-reclamation acid discharge has haunted abandoned mine lands reclamation programs and the mining industry reclamation efforts. Many mine sites have been reclaimed, as mandated by regulations, but continue to generate acid. Such sites represent perpetual treatment problems for operators and, in the case of bond forfeitures or abandoned mine reclamation projects, they represent sources of perpetual surface and groundwater degradation. Post-reclamation discharge occurs when the Thiobacillus ferrooxidans bacteria, not destroyed prior to soil cover in the reclamation process, continue to thrive, even in a supposedly anaerobic environment. A post-reclamation remediation system utilizing subsurface injection of sodium hydroxide and an anionic surfactant offers a two phase integrated technologies approach. In Phase 1, the site is characterized by geophysical mapping with (1) electromagnetic terrain conductivity meters to determine locations of subsurface aquifers and (2) proton processing magnetometers to delineate zones of pyritic oxidation in the subsurface environment. Infiltrometers are also used to determine permeability which influences pressure requirements and distance between wells. Site conditions and water analyses help quantify requirements for injection of sodium hydroxide and bactericide. Phase 2 involves drilling two sets of injection wells; the first into the acidified water table for injection of20% sodium hydroxide solution to neutralize existing acid water, and the second into the acid producing material for injection of a 20% sodium hydroxide solution to neutralize existing acid salts, followed by a 2% solution of bactericide. The sodium hydroxide is injected to neutralize existing acid which prevents future acid generation. Results from a site in Pennsylvania, USA, which was treated in this manner are reported.

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.