ASSESSING THE INFLUENCE OF URBAN KARST ON THE SUSTAINABILITY OF STREAM FLOW

In urban watersheds, subsurface infrastructure can intersect with stream networks, potentially modifying natural patterns in stream flow. For small, urban stream catchments in Hamilton County, Ohio (approximately 1058-km of streams draining <10-km2), it is estimated that 11% of stream channel centerlines are within 5-m of a sanitary sewer line. To assess potential correlation between proximity to sewer lines and enhanced stream flow loss, we established a monitoring network in a headwater stream reach that is near a sanitary sewer. Historical visual observations indicate that a portion of the monitored stream reach exhibits diminished or cessation of flow more frequently and for extended periods of time relative to immediate upgradient portions. A piezometric monitoring network was established to track both hyporheic flows under the stream channel along with shallow groundwater flow over the intervening distance between the stream channel and the trench in which the sanitary sewer line was installed. These measurements were supplemented with continuous tracking of stream flow intermittency using transects of resistivity sensors installed at the streambed surface, along with continuous flow monitoring within the adjacent sewer pipe. Analysis of spatial and temporal trends across the monitoring network were used to identify enhanced loss in stream flow in an area where the sewer line was closest to the stream channel. Given the common historical practice of installing sewer networks adjacent to stream networks, it is anticipated that strongly losing conditions may be a common phenomenon in urban settings. In addition to flashy flows and decreased groundwater recharge associated with impervious surfaces, downwelling to subsurface infrastructure contributes to deterioration of hydrologic conditions that impact beneficial ecosystem functions in an urban watershed. The results of this study are informing protocols for characterizing anthropogenic downwelling zones and developing cost-effective intervention strategies to restore natural stream flow conditions.