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

Abstract

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.

Assessing and Managing Large‐Scale Geochemical Impacts From Groundwater Replenishment With Highly Treated Reclaimed Wastewater

“… The first scenario involved the deoxygenation of the injectant to suppress pyrite oxidation, as recently proposed (e.g., Prommer et al, 2018) to prevent arsenic mobilization during (re)injection of coal seam gas coproduced water into the Precipice aquifer in Queensland, Australia. In this scenario, dissolved oxygen concentrations in the injectant were assumed to be zero, while all other water quality parameters were left similar to the current injectant composition. The second scenario simulates the amendment of bicarbonate with the aim of enhancing the pH buffering capacity of the injectant (e.g., Robinson et al, 2009).…”

“…The oxidation of ferrous to ferric iron was simulated following a previously established rate expression (Eckert & Appelo, 2002;Sun et al, 2018). The iron(III) produced from the reaction was allowed to precipitate as Fe(OH) 3 , which was modeled as an equilibrium reaction (Prommer et al, 2018;Sun et al, 2018). Seibert et al (2016) suggested that in response to acidity generation, proton buffering was the major pH buffering process during the GWRT.…”

“…Briefly, acidity generation during GWR at the Beenyup recharge site occurs mostly through the oxidation of pyrite (FeS 2 ), primarily by oxygen that is contained in the injectant and potentially also by nitrate. The pyrite oxidation reactions were included in the VISM based on a previously developed rate law (Eckert & Appelo, 2002;Prommer et al, 2018):…”

“…The aforementioned cases also highlight that the planning and implementation of MAR schemes requires a thorough, site-specific geochemical understanding, as well as the necessary tools for predicting the potential for groundwater quality risks and identifying technical options to mitigate these risks. Such a site-specific understanding is typically developed by a combination of hydrogeological and geochemical characterization activities (e.g., Rathi et al, 2017), laboratory tests (e.g., Fakhreddine et al, 2015;Schafer et al, 2018) and short-term field injection trials (e.g., Seibert et al, 2014Seibert et al, , 2016, or push-pull tests (e.g., Fakhreddine et al, 2020;Prommer et al, 2018;Rathi et al, 2017). Short-term field experiments can often provide some early warning signs of geochemical disequilibrium, and the interpretation of these results can be used to identify the geochemical mechanisms that control the water quality evolution as the injectant migrates through the native aquifer sediments (Rathi et al, 2017).…”

Redox Dependent Arsenic Occurrence and Partitioning in an Industrial Coastal Aquifer: Evidence from High Spatial Resolution Characterization of Groundwater and Sediments

“…These conditions favor the reductive dissolution of Fe-oxides/hydroxides, releasing associated As into porewater [22]. Aqueous As concentrations can also increase due to the oxidation of As-bearing iron sulphide minerals, such as pyrite or arsenopyrite [23,24].…”