Mapping Preferential Flow Channels in a Riparian Wetland Using Ground-Penetrating Radar.

Riparian wetlands are complex ecosystems that buffer stream systems from runoff contaminated by neighboring people-based systems like agriculture and urban development. These forested strips of land protect groundwater and stream water quality by reducing surface runoff, binding pollutants to the soil, and removing nutrients through plant uptake. However, lateral near-surface preferential flow can significantly reduce that potential for filtration. Water that is preferentially conducted through by-pass pathways circumvents interaction with the bulk of the soil matrix, reducing elemental adsorption and uptake and increasing the likelihood of groundwater and surface water contamination. Although the outcome of preferential flow has been documented, the extent and origin of the flow channels remain unknown due to the difficulty detecting them without severely disrupting the riparian ecosystem.

To better understand preferential flow mechanisms in wetlands, we developed a novel application of ground-penetrating radar (GPR) technology to non-invasively detect the channels (also called pipes and macropores). Using algorithms created to process the sets of raw GPR scan data and automatically identify and map the contiguous subsurface structures, the first known maps of subsurface preferential flow pathways in a riparian wetland system have been generated.

The presentation will describe the procedures developed and show the soil channel maps created from the first field trial of this application. Soil cores taken from points indicated by the map as containing or not containing soil channels will be shown to validate the mapping results. This innovative application of GPR should provide a practical approach for scientists to study and quantify the impact of preferential flow channels on water quality and integrate these effects into hydrologic models. Resource managers, environmental policy makers, and scientists will benefit from an improved understanding of the hydrological systems underlying these watersheds.