Geophysical characterization of vacant lots to evaluate stormwater infiltration

Vacant lots have the potential to increase infiltration in urban settings, acting as ad hoc green infrastructure, but because vacant lots are heterogeneous due to urban debris and varying demolition practices, the degree of stormwater control they provide is poorly understood. Geophysical methods are a tool to characterize subsurface heterogeneity, but are also being tested as a way to study infiltration fluxes in these urban areas. Methods such as electromagnetic induction (EMI) and electrical resistivity tomography (ERT) are non-invasive, sensitive to soil heterogeneity and soil moisture content, and can be used to characterize large areas far more quickly than by using point measurements. We used EMI at a former homesite, now a vacant lot on Temple University's Ambler campus, to select a study line that spans three zones identified in historical aerial imagery: a former driveway, remnant house foundations, and the adjacent lawn. Time-lapse ERT and EMI data were collected along this study line before, during, and after irrigation. ERT proved to be more sensitive to soil moisture fluxes in these zones than EM profiling. However, ERT surveys take longer to prepare and implement than EMI, so they are harder to scale up for monitoring large sites. Thus, there is a need to develop faster methods to monitor long-term storm water infiltration fluxes, for both wetting and drying cycles , which may require multiple surveys per day. The continuation of this study will use a permanent ERT system to monitor hourly fluctuations in infiltration at the same field site for multiple weeks. Our goal is to use these data to improve EMI survey methodology by scheduling data collection after rainfall events to coincide with peak resistivity contrasts.