Coal seam gas production involves generation and management of large amounts of co‐produced water. One of the most suitable methods of management is injection into deep aquifers. Field injection trials may be used to support the predictions of anticipated hydrological and geochemical impacts of injection. The present work employs reactive transport modeling (RTM) for a comprehensive analysis of data collected from a trial where arsenic mobilization was observed. Arsenic sorption behavior was studied through laboratory experiments, accompanied by the development of a surface complexation model (SCM). A field‐scale RTM that incorporated the laboratory‐derived SCM was used to simulate the data collected during the field injection trial and then to predict the long‐term fate of arsenic. We propose a new practical procedure which integrates laboratory and field‐scale models using a Monte Carlo type uncertainty analysis and alleviates a significant proportion of the computational effort required for predictive uncertainty quantification. The results illustrate that both arsenic desorption under alkaline conditions and pyrite oxidation have likely contributed to the arsenic mobilization that was observed during the field trial. The predictive simulations show that arsenic concentrations would likely remain very low if the potential for pyrite oxidation is minimized through complete deoxygenation of the injectant. The proposed modeling and predictive uncertainty quantification method can be implemented for a wide range of groundwater studies that investigate the risks of metal(loid) or radionuclide contamination.
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.
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