Using streamlines to simulate stochastic reactive transport in heterogeneous aquifers: Kinetic metal release and transport in CO₂ impacted drinking water aquifers

A Lagrangian streamline approach that stochastically represents uncertainty in spatial hydraulic conductivity distribution is coupled to kinetic reactive transport in a heterogeneous 3-D domain. This methodology is designed to efficiently account for uncertainties inherent in subsurface reactive transport while maintaining hydro-geochemical processes. A hypothetical CO₂ leak from a geological carbon storage site into an overlying aquifer is used to simulate reactive transport where contamination may occur. Uncertainty in subsurface hydraulic conductivity is accounted for using correlated, Gaussian random fields in a Monte Carlo approach. In this approach 100 realizations of each ensemble were simulated with variances of the natural log of hydraulic conductivity (σ²_lnK) of 1, 3.61, and 16. Peak ensemble lead concentrations were found at σ²_lnK of 3.61, the middle of the variances simulated. σ²_lnK within an aquifer was found to influence chemical residence time, which in turn determined the equilibrium state of the plume along the flow path and at the pumping well thus driving geochemical conditions. However, macrodispersion due to heterogeneous flow paths caused lower contaminant concentrations at the pumping well due to dilution with uncontaminated water. Furthermore, a strong link between σ²_lnK and the probability of well-capture was found, suggesting that proper characterization of the σ²_lnK within an aquifer will help to quantify the impact of uncertainty on risks of groundwater contamination.