INVERSE MODELING OF WATER FLOW AND MULTICOMPONENT REACTIVE TRANSPORT IN COASTAL AQUIFER SYSTEMS

Modeling groundwater flow and geochemical evolution in coastal aquifer systems can help us to understand the paleohydrogeology of these systems. We are interested in the freshening of coastal aquifers which is accompanied by sequential elution of seawater cations from the sediments. Complex geochemical reaction patterns and long evolution time can make this process difficult to model. A stepwise inversion technique is proposed for identifying geochemical structures and estimating model parameters in two field cases. The first case deals with reactive solute transport through the Llobregat delta aquitard between two aquifers (Barcelona, Spain), in which the saline water is being displaced by fresh water moving upwards from the lower aquifer. The inverse methodology provides optimum estimates of geochemical parameters and defines the methane redox process which improves the fit to the measured concentration curves. The methodology is also applied to a more complex field case in the Aquia aquifer (Maryland, USA) involving time scales on the order of 102 Ka. Available hydrogeochemical data are used to estimate transmisivities, leakage rates, dispersivities, selectivity coefficients, cation exchange capacity (CEC), and initial and boundary concentrations of chemical components. By using this inverse tool we obtain additional insight into the hydrologic and hydrogeochemical history of the Aquia aquifer.