Imaging Hydrological Processes in Headwater Riparian Seeps with Time‐Lapse Electrical Resistivity

Abstract

Delineating hydrologic and pedogenic factors influencing groundwater flow in riparian zones is central in understanding pathways of water and nutrient transport. In this study, we combined two-dimensional time-lapse electrical resistivity imaging (ERI) (depth of investigation approximately 2 m) with hydrometric monitoring to examine hydrological processes in the riparian area of FD-36, a small (0.4 km ) agricultural headwater basin in the Valley and Ridge region of east-central Pennsylvania. We selected two contrasting study sites, including a seep with groundwater discharge and an adjacent area lacking such seepage. Both sites were underlain by a fragipan at 0.6 m. We then monitored changes in electrical resistivity, shallow groundwater, and nitrate-N concentrations as a series of storms transitioned the landscape from dry to wet conditions. Time-lapse ERI revealed different resistivity patterns between seep and non-seep areas during the study period. Notably, the seep displayed strong resistivity reductions (∼60%) along a vertically aligned region of the soil profile, which coincided with strong upward hydraulic gradients recorded in a grid of nested piezometers (0.2- and 0.6-m depth). These patterns suggested a hydraulic connection between the seep and the nitrate-rich shallow groundwater system below the fragipan, which enabled groundwater and associated nitrate-N to discharge through the fragipan to the surface. In contrast, time-lapse ERI indicated no such connections in the non-seep area, with infiltrated rainwater presumably perched above the fragipan. Results highlight the value of pairing time-lapse ERI with hydrometric and water quality monitoring to illuminate possible groundwater and nutrient flow pathways to seeps in headwater riparian areas.

Preferential Flow in Riparian Groundwater: Gateways for Watershed Solute Transport and Implications for Water Quality Management

“…Because a large proportion of nutrients reach the channel through riparian groundwater (Mei et al, 2012), horizontally oriented preferential flow can allow pollutants from upland areas to bypass the beneficial reactions that can occur in riparian soils. Pollutants then emerge in the channel largely undiminished (Flewelling et al, 2012; Williams et al, 2017). For example, riparian preferential flow can contribute more than half the total nitrate load (Bohlke et al, 2007).…”

Electrical resistivity tomography determines the spatial distribution of clay layer thickness and aquifer vulnerability, Kandal Province, Cambodia

“…Geophysical imaging can overcome this limitation by providing spatial or volumetric distributions of subsurface physical properties (Binley et al, 2015;Loke et al, 2013). Geoelectrical techniques, which image the electrical resistivity of the subsurface, have been shown to be able to provide reliable, spatial estimates of surface watergroundwater interactions and groundwater flow characteristics, which are in agreement with conventional intrusive investigations (Cardenas and Markowski, 2011;Chambers et al, 2015;Uhlemann et al, 2016;Williams et al, 2016). This is facilitated by the fact that the resistivity of earth materials is not only a function of their lithology, where clay content in particular usually causes a significant resistivity decrease, but also a function of formation water content and pore water conductivity (where increasing water content and/or salinity usually reduces resistivity; Archie (1942)).…”

Groundwater contributions of flow and nitrogen in a headwater agricultural watershed

“…Previous research in WE‐38 revealed that nitrate‐N concentrations increase with increasing agricultural land area and decreasing stream order (Kang, Lin, Gburek, Folmar, & Lowery, 2008), groundwater seeps are generally elevated in nitrate‐N concentrations (Pionke et al, 1988), and that the majority (~75%) of nitrate‐N losses in surface water occur during baseflow conditions (Zhu, Schmidt, Buda, Bryant, & Folmar, 2011). Williams et al found that seeps exert strong controls on stream discharge and N chemistry under baseflow conditions and that seep water is mainly sourced from lateral flow in the underlying aquifer that emerges through fractures in the shallow bedrock (Williams et al, 2014, 2016; Williams, Buda, Elliott, Singha, & Hamlett, 2015). Williams et al (2015) also linked seep N concentrations to agricultural practices in upslope recharge areas within the watershed.…”