Complex confining layers : a physical and geochemical characterization of heterogeneous unconsolidated fluvial deposits using a facies-based approach

Abstract

A proper characterization of physical and chemical heterogeneities in the subsoil is an important condition for successful modeling of groundwater flow and solute transport. This study focuses on the physical and chemical characterization of a complex confining layer in the Rhine–Meuse deltaic plain (The Netherlands) that covers an important production aquifer. The complex confining layer consists of heterogeneous unconsolidated eolian and fluvial deposits, with different textural properties. Using an existing sedimentary model that classifies these deposits in textural and lithologenetic units, heterogeneity was stratified in facies. If there are relations between the physical or chemical properties and facies type, a facies–based approach is an important tool for predicting sediment–water interactions from lithology, being a valuable input for solute transport models. For this purpose, the most abundant facies occurring in the complex confining layer were extensively characterized on grain size, bulk chemical parameters, reactive parameters and hydrochemistry. The data sets were analyzed using existing multivariate techniques – like factor analysis and K–means clustering – with new approaches, which were developed to handle heterogeneous data sets lacking (log) –normality. The results show that all facies have distinct grain size distributions, and that geochemical/ hydrochemical properties partly correlate with grain size and thus depend on syn–depositional sorting processes. Post–depositional processes like secondary mineral forming, decalcification, and degradation or conservation of organic matter are also important factors in determining sediment reactivity and groundwater chemistry. Because both grain size and post–depositional processes are related to facies, the facies–based approach works well to stratify heterogeneity in riverine deposits at a scale that seems appropriate to solute transport modeling. With this solid physical and chemical/ hydrochemical characterization available, transport modeling of reactive contaminants through heterogeneous riverine deposits in the Rhine–Meuse delta should be the next step.