Using Hydrogeophysics to Monitor Change in Hyporheic Flow around Stream Restoration Structures

Hyporheic exchange is important because water flux through the streambed influences microbial communities and water quality, but little is known about the way in which it changes over time, particularly in response to stream restoration. We used tracer tests and geophysical monitoring, along with streambed characterization, to examine the evolution of the hyporheic zone after construction of a stream restoration structure at Crabby Creek, an urbanized watershed in southeastern Pennsylvania. Changes were assessed by comparing data from 2009 and 2010 over a 13.5 m reach centered on a J-hook. The breakthrough curves from streambed well sampling were remarkably similar in 2009 and 2010, with steep rising and falling limbs indicating rapid exchange followed by a short period (less than 2 hours) of slightly elevated concentration before returning to background levels. The resistivity tomographs of the subsurface showed areas of lingering tracer after the injection stopped. Upstream of the J-hook, there was an area of lingering tracer that persisted for more than 3 hours after the injection stopped. This upstream region of slow exchange was similar for both years. Downstream of the structure, there was lingering tracer in 2009, but a smaller, less intense region in 2010. Streambed characterization showed that 20 cm of erosion occurred in the downstream section between the two years, along with an increased proportion of fines, which was associated with inhibited hyporheic flux. The resistivity profiles showed areas of lingering tracer and helped map changes in the hyporheic zone that were not apparent from well sampling alone.