Examining the Secondary Impacts of Biostimulation on Water Quality and Sustained Bioremediation

Abstract:

Anaerobic bioremediation typically relies on electron donor addition, which supports reductively dechlorinating bacteria, but also stimulates other microbial groups including methanogens, and sulfate- and metal-reducers. This microbial activity can impact groundwater geochemistry and hydrology. For example, ferric iron- and sulfate-reducing bacteria generate ferrous iron and hydrogen sulfide, respectively, causing iron(II) monosulfide (FeS) mineralization that can restrict or block pore throats. The impacts of potential FeS precipitation, and subsequent oxidation caused by infiltration of oxic groundwater, on porous media permeability of were investigated through a series of column and aquifer cell studies. FeS precipitation-caused permeability and porosity losses ranged from 0 – 73% and 1.2 – 7.2%, respectively, and this impact was most apparent in soils with high organic carbon content. In aquifer cell studies, FeS precipitation caused local reductions in permeability, which led to zones of hydraulic isolation and preferential flow paths. Further, microbial activity following biostimulation can also lower groundwater pH through the formation of organic acids and CO₂, and the release of hydrochloric acid during dechlorination reactions. An obvious concern is activity loss of chlorinated ethene-detoxifying Dehalococcoides mccartyi (Dhc) strains, which are active at circumneutral pH, but show reduced dechlorination activity below pH 6.5. A series of batch reactors were established to explore the consequences of pH reduction on dechlorinating isolates and a Dhc-containing consortium. No tetrachloroethene (PCE) dechlorination occurred at pH 5.5 unless solids were added, suggesting that certain solids enable Dhc activity in low pH groundwater. The consortium exposed to pH 5.5 dechlorinated PCE-to-vinyl chloride (VC) upon transfer to pH 7; however, VC-to-ethene dechlorination activity did not recover, suggesting that VC-dechlorinating Dhc strains are particularly susceptible to low pH inhibition.