Geochemical alteration of shallow groundwater due to injection of CO2

Leakage from subsurface storage sites is one of the main concerns connected with CCS technology. One potential risk associated with CO2 leaking from storage sites is the geochemical alteration of shallow groundwater systems causing pH decrease, mineral dissolution and potential mobilization of heavy metals. As on the one hand leakages into drinking water aquifers within pilot scale CCS projects seem to be very unlikely, but on the other hand a sound risk assessment before implementing CCS is absolutely necessary, the aim of the presented project is to emulate a leakage of CO2 by injecting gaseous CO2 into a shallow aquifer. Two main objectives were pursued by the injection test: firstly, to gain insight into potential geochemical reactions taking place in groundwater systems due to CO2 leakage, especially under the focus whether reactive transport models are capable to predict these reactions. Secondly, to develop and apply different monitoring methods. The focus of the present paper is on the first objective. The test site is located at a former military air field in northeastern Germany. Geologically, the subsurface consists of pleistocene sediments including sandur deposits as well as moraine and basin sediments. The aquifer of concern extends between 10 and 19m below surface and consists mainly of medium to coarse sands free of carbonates. Groundwater flow velocities vary locally within <0.1m/d up to 0.3m/d. At three injection wells at distances of 5m 30L CO2/min (i.e. 54g CO2/min) in total were injected at a depth of 18m below surface. The injection was operated continuously for 10 days in March and April 2011. Groundwater monitoring was performed at 34 monitoring wells covering an area of 80m x 40m. 16 multi-level wells allow for groundwater sampling at three different depths. Ten groundwater sampling campaigns were carried out, two before, three during and five after the injection test. The main results obtained so far concerning the geochemical alteration of groundwater can be summarized as follows: ? The injection of gaseous CO2 and subsequent dissolution into groundwater leads to a decrease of pH down to 4.5, which corresponds to previous model simulations. ? Dissolution of CO2 still continues even 12 weeks after stopping the injection, as TIC concentrations still increase. ? Mobilization of heavy metals is spatially very heterogeneous, at some wells only zinc concentrations increase, at others only concentrations of lead increase, whereas at most wells various heavy metals show significantly increased concentrations. ? The mobilization of heavy metals seems to be reversible: so far, at one monitoring well TIC concentrations, pH and concentrations of all cations including heavy metals have returned to background concentrations. Field studies as the one presented here, especially when combined with modelling studies, are a necessary tool to understand and predict the potential impact of CO2 on natural groundwater systems.