EXPLORING THE CONNECTIVITY OF MEANDERING STREAMS AND SHALLOW ALLUVIAL AQUIFERS: THE ROLE OF SUBSURFACE ARCHITECTURE AND PALEOCHANNELS

Stream-aquifer interactions play a fundamental role on the quantity and quality of water and ecosystem functioning at the local, reach, and watershed scales. Subsurface architecture modulates connectivity between streams and shallow alluvial aquifers, controlling the amount of water exchange, temperature and biogeochemistry along the river corridor. In this work, we combine electrical geophysical methods and a network of observational wells, continuously measuring stage and temperature, to characterize the alluvium subsurface architecture and its control on stream-aquifer interactions. Electrical Resistivity (ER) and Ground Penetrating Radar (GPR) surveys capture small-scale heterogeneity and a paleochannel crossing the point bar. Areas associated with the paleochannel are characterized by apparent resistivities in the low 1000’s Ω•m, consistent with the presence of wet sands and gravels, whereas the surroundings present lower resistivity values in 100’s Ω•m, characteristic of fine-grained sediments. A GPR survey corroborates these observations, showing zones of low signal attenuation along the paleochannel. These geophysical observations are consistent with the subsurface stage and temperature observations, which evidence a stronger degree of hydraulic and thermal connectivity between the stream and the wells tapping the paleochannel. The presence of this large-scale heterogeneity features modulates the flow field and exchange process, and therefore are fundamental for the interpretation of field measurements and modeling.