How to Address Low pH During in situ Bioremediation
Management of aquifer pH must be closely monitored during in situ remediation projects as it has a major influence on contaminant mobility and attenuation especially during biodegradation of chlorinated solvents. However, adjusting aquifer pH can be challenging due to strong buffering by clays, iron oxides and sorbed Al3+. Additionally, there few options available to calculate and design an approach for pH adjustment. Recently SERDP/ESTCP published a base-addition design tool to help practitioners develop a comprehensive pH management plan utilizing a verity of commonly used bases. These commonly used bases like NaOH and KOH can result in excessively high pH while others such as Na2CO3, NaHCO3 offer relatively low alkalinity/lb. Mg(OH)2 has many advantages including lower equilibrium pH, greater alkalinity/lb., and slow release over time, providing a long-term source of buffering capacity. Despite these benefits, distribution of an aqueous Mg(OH)2 suspension in situ can be extremely difficult. A method was developed to alter the surface charge of Mg(OH)2 to improve subsurface transport and injectability. Laboratory studies validated that a colloidal suspension of Mg(OH)2 could be transported through sand packed columns without significant permeability loss. This proprietary formula of colloidal Mg(OH)2 was commercialized into the product CoBupH™. Using the base-addition design tool from ESTCP along with laboratory titration data a challenging chlorinated-impacted site in NC was successfully remediated. CoBupH™ and potassium bicarbonate were selected as the buffers, with EOS® Pro co-injected as the electron donor. The site had two distinct water-bearing zones; the shallower zone contained groundwater with an average pH of approximately 4 standard units (SU) due to an acid release from an off-site source. Post-injection monitoring since 2016 shows almost complete conversion of chlorinated solvents to ethene while maintaining a neutral aquifer pH.