Hyporheic zone size from 3D geophysics

The hyporheic zone is an ecologically critical region of a stream where surface waters transit the subsurface. Spatially defining the hyporheic zone from in-stream solute dynamics models is indirect, where individual model terms describe overlapping regions of the solute breakthrough curve. Alternatively, direct measurements of the hyporheic zone from geophysical imaging techniques, such as electrical resistivity tomography (ERT), combined with additions of an electrically conductive tracer (e.g. sodium chloride; NaCl) have provided insight to the extent and spatial heterogeneity of hyporheic exchange in two dimensions. While 2D ERT produces a planar representation of the hyporheic zone, characterizing all three spatial dimensions is critical to better describe its distribution and role in biogeochemical processes along a reach. We applied 3D ERT imaging combined with constant rate NaCl additions to three headwater streams to yield spatially comprehensive representations of dynamic solute concentrations volumetrically within the hyporheic zone. Vertical exchange of the tracer with the streambed was the dominant vector of hyporheic exchange in all streams. The average depth of penetration of the tracer into the streambed was similar for streams and ranged from 0.77 to 0.80 m. The movement of solutes through the hyporheic zone was quicker than the 3D ERT acquisition speed (<45 min) in all three streams, where the hyporheic zone both reached a steady enriched state and returned to ambient conditions in a single measurement cycle. The rapid hyporheic exchange at all of the streams did not permit the assessment of their temporal dynamics. Due its direct measurement of subsurface electrical properties, the use of 3D ERT provided a better representation of the hyporheic zone extents and heterogeneity with estimates of the cross-sectional area of the hyporheic zone 2-4x larger than estimates from transient storage modeling.