An Exploration of Stream-Riparian Groundwater Exchange During Baseflow Recession: Integration of Hydrologic and Geophysical Data

Stream-groundwater interactions are critical to ecosystem structure, function, and water quality in many streams; it is therefore desirable to quantify exchange rates and volumes of water in these coupled stream-groundwater systems.  In this work, we explore the controls on riparian subsurface hydrology, which, in turn, influences hyporheic exchange.

We investigate the changes in relative magnitudes of down- and cross-valley hydraulic gradients in two steep, narrow headwater catchments at the H. J. Andrews (HJA) Watershed in Oregon across three distinct timescales: (1) throughout an extended period of baseflow recession (approximately two months in 2010), (2) during a large storm event, and (3) over a complete diurnal cycle of rising and falling streamflow and groundwater levels. We demonstrate that subsurface hydraulic gradients result in spatially complex patterns of subsurface flow and stream-groundwater exchange than would be predicted by the assumption of groundwater table position based on surface topography.

Coupled with this work, we explore the use of near-surface geophysical methods, specifically electrical resistivity (ER), to inform on groundwater-surface water exchange.  Traditional characterization of hyporheic exchange (connectivity between streams and near-stream aquifers) relies upon solute tracer studies, and a spatially sparse set of observations in streams and shallow monitoring wells. We demonstrate the use of ER methods to image hyporheic exchange in both two and three-dimensions, and identify flowpaths at different temporal scales of stream connectivity. These data are an improvement over traditional methods, which would otherwise provide only reach-averaged values, or single observations in space.  Temporal moments of solute and ER data are used to compress trends into descriptive statistics and identify the distribution of dominant solute transport processes (e.g., transient storage dominated versus advection dominated) in the subsurface.