Variation of Hyporheic Exchange in Thin and Thick Sediment Using Solute Tracer Tests, Geophysical Resistivity, and Sediment Characterization at a Reconstructed Stream

Abstract

Solute tracer tests, geophysical resistivity surveys, and sediment characterization were employed at Crabby Creek, an urban stream in southeastern Pennsylvania, to study hyporheic exchange. Hyporheic exchange is defined by the paths that stream water takes to travel in and out of the saturated sediment of a river. The hyporheic zone is an active region of biological and chemical reactions in streams. This study focused on two reaches that have thin (0.67 m) and thick (2.3 m) streambed sediment. Delineating the hyporheic zone from these contrasting sediment beds is important to understand groundwater-surface water interaction. Hyporheic exchange was hypothesized to be different between the two reaches because sedimentology and therefore permeability were disparate. It was expected that the thick streambed sediment would show deeper hyporheic exchange than the thin streambed sediment. To further understand hyporheic exchange, stream tracer tests were conducted. Sodium chloride was used as the tracer and approximately 57 temporary monitoring wells were installed in the thin and thick sediment reaches. Geophysical time-lapse resistivity surveys, or surveys that represent a change from an initial one, were also used to detect the tracer in the subsurface. Deeper hyporheic exchange, down to 40 cm, was observed in the thin sediment rather than in the reach with thick sediment. The exchange seemed to be limited in the thick sediment due to lower sediment permeability. In the thin sediment reach, the breakthrough curves showed steep rising and falling limbs due to immediate response to the tracer with an average arrival of tracer seven minutes after injection. Uniformity in exchange across most of the sampling wells was also observed. In contrast, the breakthrough curves in the thick sediment reach showed more gradual rise and fall of the tracer and varied arrival times (between 15 and 35 minutes after injection) across the transects. Modeling using One-Dimensional Transport with Inflow and Storage (OTIS) found that variation in hyporheic thickness and exchange explained why lag, or delay of tracer arrival time, and shallowing of slope on the rising and falling limbs of the breakthrough curves occurred. The thick sediment reach was found to be composed of poorly graded sediment with abundant fine sediment which can discourage groundwater-surface water interaction. Thickness of hyporheic zone differences also affected exchange and tracer arrival times. Geophysics showed tracer that remained in the hyporheic zone after solute injection was discontinued. This lingering tracer was not apparent from well sampling. Zones of remaining tracer were observed in both sediment reaches. These zones decreased in their conductive change over time but were still present several hours after tracer injection was discontinued. Upwelling and downwelling were located by observing zones of conductive change; downwelling occurred both upstream and downstream of a stabilization structure in the thin sediment reach but only downstream of a stabilization structure in the thick sediment reach. Upwelling and downwelling of stream water was driven by sediment permeability, which was different in the two reaches. Sampling wells helped determine depth to which tracer traveled and conductivity over time, while geophysics helped track the tracer zonation during and after injection was discontinued. Both contributed to understanding the hyporheic zone and its complex characteristics.