Long-term evaluation of an organic-carbon permeable reactive barrier remediating mine-impacted groundwater and the potential of emulsified vegetable oil to increase treatment performance

Abstract

Degradation of water resources by acid mine drainage (AMD) and metal contaminants from the oxidation of mine wastes that are improperly managed is a global environmental concern. An established economical and passive method for managing the transport of oxidation products in groundwater is the installation of an organic-carbon permeable reactive barrier (PRB) to promote the growth of sulfate reducing bacteria (SRB) and dissimilatory sulfate reduction (DSR) in situ. A detailed biogeochemical evaluation was conducted on a PRB remediating AMD from an abandoned Ni-Cu mine after 26 years of treatment, providing the first long-term evaluation of this technology. Pore-water concentrations of Ni decreased from 97µg L⁻1 to < 25 µg L⁻¹ while Fe decreased by 402 mg L⁻¹ (77% of mean influent) through the PRB. Pore-water SO4 decreased by 1,243 mg L⁻¹ (70% of mean influent) coinciding with an increase in alkalinity and pore-water ẟ34Sₛₒ₄, suggesting DSR is actively occurring. There are distinctly different populations of putative SRB present within the sampled PRB material compared to the surrounding aquifer material. Low abundances of S and Fe oxidizing prokaryotes were detected, which may oxidize Fe-sulfide phases; re-mobilizing Fe and S or result in the formation of Fe(III) (oxy)hydroxide phases. A preponderance of S immobilized within the PRB is in the form of acid volatile sulfur with mineralogical investigations identifying FeS phases often replacing organic carbon in plant cellular material and framboidal pyrite. These results demonstrate that the PRB is still operating as designed with complex organic carbon compounds supporting a diverse microbial community that sustain rates of DSR to effectively precipitate Fe sulfides, decrease the acid potential of groundwater and immobilize contaminants. Column experiments were designed to evaluate the incorporation of emulsified vegetable oil (EVO) into solid-phase organic carbon through soaking and injection to promote and sustain treatment performance. Column effluent compositions demonstrate soaking organic-carbon in EVO resulted in high levels of Fe removal for the duration of observation (315 days). Moreover, an EVO injection re-established treatment after removal rates declined, providing a viable alternative to PRB replacement to maintain effective treatment system performance and extend PRB lifespan.