Deoxygenation Prevents Arsenic Mobilization during Deepwell Injection into Sulfide-Bearing Aquifers

Coal seam gas (CSG) extraction generates large volumes of coproduced water. Injection of the excess water into deep aquifers is often the most sustainable management option. However, such injection risks undesired sediment–water interactions that mobilize metal(loid)s in the receiving aquifer. This risk can be mitigated through pretreatment of the injectant. Here, we conducted a sequence of three push–pull tests (PPTs) where the injectant was pretreated using acid amendment and/or deoxygenation to identify the processes controlling the fate of metal(loid)s and to understand the treatment requirements for large-scale CSG water injection. The injection and recovery cycles were closely monitored, followed by analysis of the observations through reactive transport modeling. While arsenic was mobilized in all three PPTs, significantly lower arsenic concentrations were observed in the recovered water when the injectant was deoxygenated, regardless of pH adjustment. The breakthrough of arsenic was commensurate with molybdenum, but distinct from phosphate. This allowed for the observed and modeled arsenic and molybdenum mobilization to be attributed to a stoichiometric codissolution process during pyrite oxidation, whereas phosphate mobility was governed by sorption. Understanding the nature of these hydrochemical processes explained the greater efficiency of pretreatment by deoxygenation on minimizing metal(loid) mobilization compared to the acid amendment.