Geophysical Mapping and Subsurface Injection for Treatment of Post-Reclamation Acid Drainage

Abstract

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.

Case Study: 20 Years of Acid Rock Drainage Chemistry Improvements After a Bactericide Application

“…That changed when Plocus and Rastogi (1997) demonstrated that subsurface application of anionic surfactants using injection techniques could successfully reduce ARD at the Fisher coal mine site in Banks Township, Indiana CO, PA. This was accomplished by identifying the acid-generating zones using geophysical techniques and designing a multiple-stage borehole based injection program that targeted the ARD "hot spots.…”

“…Geophysical mapping with electromagnetic terrain conductivity meters and magnetometers were utilized to identify pyritic zones which were responsible for the high acid production on the Fisher Site. See Plocus and Rastogi (1997) for a more detailed discussion of the geophysical investigation.…”

“…The prevailing theory in 1993 was that the source of ARD on the Fisher Site was similar to other reclaimed sites: it was primarily located in the buried mine spoil. Unfortunately, conventional surface application of anionic surfactants was believed to be ineffective on revegetated sites as it could not penetrate through subsoil, topsoil, and vegetation (Plocus and Rastogi, 1997). In retrospect, drip irrigation delivery of anionic surfactant solution routinely used in heap leaching operations at metal mines may have overcome this perceived limitation.…”

“…As subsequently determined, neither was it necessary. Plocus and Rastogi (1997) developed an innovative approach to (1) identify the source of ARD on the Fisher Site using geophysical mapping, (2) install subsurface injection wells, and (3) inject sodium hydroxide and an anionic surfactant into the toxic zones within the subsurface strata of the Fisher Site. Among the many uncertainties at the time was the longevity of the bactericide.…”

“…Site vicinity map (AfterPlocus and Rastogi, 1997).Site reclamation consisted of pit backfilling and revegetation; this effort was completed in 1984. The effluent from a small (0.25 ha) passive treatment system, constructed in 1985 to manage the raw seepage, did not meet Pennsylvania Department of Environmental Protection (PA DEP) standards.…”