Inverse modeling of water flow and multicomponent reactive transport in coastal aquifer systems

Modeling groundwater flow and geochemical evolution is a useful tool for studying the paleohydrogeology of coastal aquifer systems. Complex geochemical reaction patterns and long freshening time of coastal aquifers make these processes difficult to model. A stepwise inversion method is presented here for the identification of geochemical processes and the estimation of model parameters in two field cases. The first one deals with reactive solute transport through the Llobregat delta aquitard (Barcelona, Spain) in which saline water is being displaced by fresh water moving upwards from a lower aquifer. The inverse methodology provides optimum estimates of geochemical parameters and identifies methane oxidation as a key process controlling the geochemical evolution in the aquitard and leading to a fit to measured concentration profiles which improves previous modeling results. The methodology is applied also to a complex field case in the Aquia aquifer (MD, USA) which involves time scales on the order of 105 years. Available hydrogeochemical data are used to derive optimum estimates of transmissivities, leakage rates, dispersivities, selectivity coefficients, cation exchange capacity (CEC), and initial and boundary concentrations of chemical components. Inverse modeling in this case provides optimum parameter estimates and additional insight into the paleohydrology and paleogeochemistry of the Aquia aquifer.