Groundwater dynamics in a near-stream domain: variability of flow directions and subsurface connectivity

Near-stream groundwater table dynamics, subsurface flow pathways, and streamflow dynamic control near-stream groundwater-stream water exchange. These processes are critical for evaluating the extension of the hyporheic zone and water travel time in the stream corridor. This in turn, influences riverine pollutant transport, biotic activity, and nutrient cycling, making near-stream water exchange crucial when addressing the ecological status of a riverine environment.

However, near-stream groundwater hydrology has rarely been investigated in the past as shallow groundwater studies often focus on subsurface water dynamics along hillslopes. As a result, detailed understanding of near-stream groundwater table oscillations, fluxes, and their changes over time across a various spectrum of rainfall and streamflow conditions in the stream corridor is currently missing. This study aims at the following questions:

To answer these questions, we set-up a monitoring network with 36 near-stream wells and 7 in-stream piezometers along a headwater stream section of 55 m in Luxembourg. Based on the recorded data we calculated the changes of the groundwater table and groundwater flux direction. Subsurface structure was evaluated with an ERT survey. We related the GW level dynamics to stream discharge and rainfall event characteristics.

Results reveal that the changes of near-stream groundwater table and groundwater flux direction across different hydrological conditions display three modes of water exchange between near-stream groundwater and the stream channel. During dry conditions, the groundwater table lays in the weathered bedrock up to 0.8 m below the stream channel. During these conditions, near-stream groundwater fluxes direction are strongly affected by sporadic rainfall events and shift between gaining-stream conditions (pre-event) to losing-stream conditions (post-event). Once the system wets-up, the groundwater table rises in more conductive subsurface layers. This leads to near-stream groundwater fluxes to be mostly parallel to the stream channel, with stream sections displaying variable direction of exchange with the groundwater, passing from losing- to gaining-conditions and vice-versa. During wet conditions, near-stream groundwater level rises above the stream channel, which shows persistent streamflow during this period. The groundwater table points constantly toward the stream channel, now displaying continuous gaining-conditions for the stream.