At Crabby Creek, an urbanized watershed in northeast Chester County, Pennsylvania, an NaCl tracer test was conducted in 2010 to assess changes in hyporheic flow from a 2009 tracer test around the same stream restoration J-Hook. This project compares the 2009 and 2010 tracer test breakthrough curves and geophysical time-lapse resistivity surveys. This project also compares elevation cross sections and tile probing from 2009 and 2010, both measured upstream and downstream from the J-Hook. To confirm areas of lingering tracer seen in the time-lapse resistivity profiles, sediment cores using the freeze core method were taken to measure pore water for tracer. This project also measured diurnal temperature flux through the streambed at several locations along the sample site to model vertical water and heat flux. The breakthrough graphs constructed from the conductivity of the well water samples shows similar hyporheic flow characteristics from 2009 to 2010. The time-lapse resistivity profiles show an area of lingering tracer upstream from the J-Hook in 2010 that is similar in shape and location to an area upstream from the J-Hook in the 2009 profiles. However, an area of lingering tracer downstream from the J-Hook present in 2009 as a round feature on the profile is now a thin linear feature. The freeze cores show tracer present in the pore water after the end of the tracer injection in the stream sediment, confirming areas of lingering tracer seen in the time-lapse resistivity profiles. The grain size analysis of the freeze cores and the comparison to the 2009 cores taken at Crabby Creek show similar grain size distribution upstream from the J-Hook. Downstream from the J-Hook the grain size analysis shows a redistribution of sediment. Upstream from the J-Hook the tile probe shows both shallower and deeper bedrock, a redistribution of sediment but no net erosion. Downstream from the restoration structure, however, the tile probe data show a sediment loss of 20 cm. Elevation cross section surveys from 2009 and 2010 confirm what the tile probing found, a loss of sediment downstream but not upstream from the J-Hook. Temperature modeling of heat flux through the sediment shows that the diurnal temperature distribution can be accounted for without vertical flux. Thus, the immobile regions upstream and downstream from the J-Hook seem to be related to sediment distribution rather than hydrologic gradient differences. The significance of this study shows the need to use multiple techniques to characterize the immobile zone as a part of hyporheic flow. The immobile zone is an important area of chemical reactions in the streambed. At Crabby Creek the central J-Hook inhibits net erosion patterns upstream from the structure, allowing for the continued presence of an immobile zone. Downstream from the central J-Hook the erosion of the streambed sediment led to a decrease in size and location of the immobile zone. The disturbance of sediment around restoration structures influences the development of a healthy hyporheic flow and needs to be studied for future restoration of impaired streams and riparian corridors. / Geology

Hyporheic water is thought to infiltrate at the head of a riffle which in turn is complemented by upwelling back to the stream at the tail of the riffle in a pool-riffle-pool (PRP) sequence. Heat tracing is a potentially useful method to characterize these hyporheic flow paths and quantify associated fluxes. Temperature was monitored within a PRP sequence for several days. Temperature in the hyporheic zone reflected the diel temperature change in the river but not uniformly. The observed thermal pattern exhibited deeper penetration of thermal oscillations below the head pool and shallower penetration below the tail pool. This pattern is consistent with the conceptual model of hyporheic exchange over a PRP sequence. One-dimensional analytical heat transport models were used at different points below the PRP sequence to estimate distributed vertical fluid fluxes. The calculated fluxes exhibit a trend that follows the expected distribution for a PRP sequence but modified for a losing stream. Deviation of both magnitude and distribution of fluxes from the conceptual ‘downwelling-to-upwelling’ model is partly due to the dominantly losing conditions at the study site but the trends are consistent with a losing stream undergoing hyporheic exchange. Violation of the assumptions in the analytical models most likely adds error to flux estimates. For this study, flux estimation methods using a temperature time series amplitude analysis more closely matched field measurements than phase methods. / text

Groundwater-surface water interactions between streams and shallow alluvial aquifers can significantly affect their thermal and chemical regimes and thus are critical for effective management of water resources and riparian ecosystems. Of particular significance is the hyporheic zone, an area delineated by subsurface flow paths that begin and end in surface water bodies. Although detailed work has examined hyporheic flow in the vertical dimension, some studies have suggested that the drop in a stream’s elevation as it flows downstream can laterally extend the hyporheic zone. This study examines intra-meander hyporheic flow using extensive field measurements in a full-scale experimental stream-aquifer system. Synoptic head measurements from 2008 and 2009 and a lithium tracer test were conducted to determine the extent and nature of hyporheic flow within the meander. Permeability was measured and sediment cores were analyzed from 2008 to 2009 to assess aquifer properties. Finally, transient head and temperature measurements were collected during flooding events to assess the sensitivity of intra-meander hyporheic flow and temperature to stream discharge. Results verify that hyporheic flow through meanders occurs, but show that it is sensitive to whether a stream is gaining or losing water to the subsurface overall. In addition, permeability and core grain size results indicate moderate heterogeneity in permeability can occur in aquifers composed of relatively uniform sediment. Results also demonstrate that permeability in alluvial aquifers can evolve through time. Such evolution may be driven by groundwater flow, which transports fine particles from areas where porosity and permeability are relatively high and deposits them where they are relatively low, thus creating a positive feedback loop. Finally, measurements during flooding indicate that steady-state hyporheic flow and the thermal regime within the aquifer are largely insensitive to stream discharge. Together, these results expand upon previous field studies of intra-meander hyporheic flow and verify previous modeling work, although they demonstrate a level of complexity within these systems that should be considered in future work. / text