Characterization of Non-Fickian Transport in Ground-water and Hyporheic Systems Using Electrical Geophysics

Non-Fickian solute-transport behavior has been observed at research and aquifer-remediation sites in diverse hydrogeologic settings. Anomalous behavior such as concentration rebound, long breakthrough tailing, and poor pump-and-treat efficiency has been explained by rate-limited mass transfer, where transport occurs between (1) a mobile domain, which consists of well connected pores and fractures in aquifers or of the stream-channel component of watershed systems, and (2) a less-mobile domain, which consists of poorly connected pores and dead-end fractures or of a hyporheic zone. Despite recognition of the importance of non-Fickian transport, verification of its occurrence and inference of controlling parameters remains problematic. Conventional geochemical measurements preferentially sample from the mobile domain and thus provide only indirect information for the immobile domain and exchange between domains. Here, we present a petrophysical framework, experimental methodology, and analytical expressions that can be used to infer mass-transfer parameters from co-located breakthrough curves of mobile concentration and bulk conductivity from geoelectrical measurements. We present (1) field-experimental geoelectrical data from an aquifer-storage recovery site showing evidence of mass transfer; (2) results of numerical column experiments demonstrating an approach to estimate heterogeneous mass-transfer rate coefficients and mobile-immobile porosity ratios based on the temporal moments of bulk electrical conductivity measurements and mobile-domain concentration; and (3) proof-of-concept modeling for application of this method to estimate mass transfer between streams and the hyporheic corridor in watershed settings. Our results indicate that geoelectrical measurements can provide valuable, practical insights into heterogeneous mass-transfer parameters.