Microbiological Evidence for the Origin of Acid Mine Drainage at the Green Valley Site, Vigo County, and Friar Tuck Site, Greene County, Indiana, USA

We examine the origin of acid mine drainage (AMD) that forms within coal refuse (gob) piles at the Green Valley and Friar Tuck sites in Indiana, using microbiology, traditional geochemistry, and oxygen and hydrogen isotopes. Reducing the AMD load from these sites has been an historical priority. Our observations indicate that AMD generation at these sites in Indiana is driven by three complementary factors: elevated populations of chemolithotrophic microbes of the species Acidithiobacillus ferrooxidans; a growth substrate that provides ‘food’ (e.g. pyrite) for these microbes, and a gob pile with geometry and other properties conducive to maintaining the thermal window of 25–40°C for optimal A. ferrooxidans growth. In particular, increasing levels of Fe+3 and total dissolved solids (TDS), and decreasing pH for gob waters were found to be highly correlated with increasing populations of A. ferrooxidans. Furthermore, the chemosythetic bacteria population increase correlates with increasing hydrogen stable isotope shift away from the global meteoric water line for gob waters in this study, though it is unclear if this shift is the result of microbial metabolic processes or a secondary effect due to microbially-mediated pH change or electrolysis.