Modeling the potential impacts of CO\u3csub\u3e2\u3c/sub\u3e sequestration on shallow groundwater: The fate of trace metals and organic compounds before and after leakage stops

摘要

Large-scale deployment of CO2 geological sequestration requires understanding and assessing the risks of such an operation. One of these risks is the potential contamination of groundwater by CO2/brine leakage into shallow aquifers. Although our understanding of this issue has improved significantly over the last decade, several questions still need to be better addressed, including the fate of organic constituents, the dominant source of trace metals (are they mainly coming from aquifer sediments, or leaking brine), and whether the trace metals released during the leakage phase recover to background levels if the leakage were to be detected and stopped. In this paper, reactive transport simulations that model the behavior of trace metals and organic compounds in response to the leakage of CO2 and brine into a shallow aquifer are presented to address these questions. Model results show that the metals and organic compounds brought by the leaking brine form a plume at the bottom of the aquifer because the density of the brine is higher than that of groundwater. In contrast, metals are mobilized by CO2 over a larger vertical extent because of the spreading of gaseous CO2 by buoyancy. The concentration of organic contaminants is strongly attenuated by adsorption and degradation, with degradation playing the major role in the modeled scenarios. Although the leaking brine is assumed to contain elevated concentrations of As, Pb, Cd, and Ba, it does not contribute significantly to the contamination of the modeled shallow aquifer by these elements. Once the leakage stops, mobilized organic compounds that undergo degradation vanish, while less degradable compounds linger for a longer time; the dissolved concentrations of trace metals decrease significantly, as a result of re-sorption and reversal of processes leading to Ca-driven cation exchange.