Monitoring the performance of urban green infrastructure using a tensiometer approach.

Little is known about how stormwater exfiltrates from green infrastructure and few efforts have been undertaken to address this question. This study used tensiometers to monitor water exfiltration from an aggregate-filled storage gallery installed under permeable pavement. An 80-space parking lot was built at Seitz elementary school in Fort Riley, KS under an agreement between EPA and US Army during the summer of 2015. A network of twelve tensiometers and twelve monitoring wells was installed under and south of the storage gallery. Tensiometers were installed a various depths and distances to monitor soil moisture tension. The installation was used to monitor subsurface water flow patterns from the storage gallery under the permeable pavement site. The results of the study showed that soil moisture tension is larger at the shallower depths, decreasing with depth from the ground surface. Larger soil tension was associated with increased distance from the permeable pavement storage gallery. The results showed exfiltration from both the sidewalls and the bottom of the gallery while the changes in soil tension were larger for the tensiometer monitoring exfiltration from the side walls. Both the accumulated water depth inside the storage gallery and groundwater level rise were positively correlated with total rainfall depth. The calculated vertical flow rate was larger than the horizontal flow. The soil water tension change associated with storage gallery exfiltration was measured in a radius of <5 m from the storage gallery. Long-term peak groundwater level rise should be considered for design of the storage gallery depth to maintain the effective exfiltration. Understanding the exfiltration pathways aids with the placement and design the storage gallery. Additional research is necessary to understand how specific local parameters and vadose zone characteristics would affect the long-term exfiltration process.