Time-Lapse Resistivity Monitoring of a Simulated Runoff Test in a Bioswale, Philadelphia

As part of the reconstruction of Interstate-95 in Philadelphia, a series of vegetated stormwater infiltration basins (bioswales) have been installed to manage highway runoff. To assess these bioswales, we used simulated runoff tests (SRTs), where the bioswale is flooded from a fire hydrant to simulate a major storm event. SRT monitoring relies on point measurements of inflow, outflow, and soil moisture to determine the volume of stormwater the bioswale can handle and the time to recovery. To provide better spatial coverage for site assessment, we tested the use of time-lapse electrical resistivity tomography (ERT) during a SRT. Using an onsite geophysical monitoring station, we performed ERT surveys every 4 h before, during, and after the SRT test. Inflow and outflow measurements taken during the SRT found that a majority of the water did not exit the bioswale via the outlet box. The time-lapse inversion results indicated that runoff uniformly spread throughout the basin before infiltrating into the heterogeneous urban soil below. The ERT demonstrated that the underlying native soil contributed to the overall performance of the bioswale, a contribution which was previously assumed to be minimal. Recovery to pretest soil moisture levels took roughly 2–3 days, according to both soil moisture sensors and time-lapse geophysical data. The SRT results were consistent with natural storm events recorded by our geophysical monitoring station, which have been monitored continuously for over a year. Use of ERT during SRTs characterize the pattern of infiltration and recovery rate of the soil beneath a stormwater control. Additionally, the heterogeneous infiltration observed during the SRT suggested that ERT surveys preconstruction may improve future planning of stormwater controls by guiding the location of infiltration measurements.

To assess green infrastructure performance, we conducted a simulated runoff test by flooding a bioswale designed to accept highway runoff with water from a nearby fire hydrant to simulate a major storm. It is common during simulated runoff tests to monitor soil moisture using buried sensors, but point measurements provide limited spatial coverage. We tested an alternative method for tracking changes in soil moisture, namely time-lapse electrical resistivity, to see whether the continuous spatial coverage provided by resistivity cross sections would improve bioswale performance assessment. Resistivity decreases with increasing soil water content; hence, we saw a significant decrease in resistivity during the test. Our main finding was that water infiltrated past the bioswale fill and into the urban soil below. The bioswale was designed assuming that the urban soil would impede infiltration, but our results showed the bioswale was larger than necessary. We suggest conducting more infiltration tests on the native soil before green infrastructure is constructed because this could lead to smaller, less costly designs. For existing green infrastructure, using time-lapse resistivity imaging during simulated runoff test can provide better spatial coverage within and beneath the fill material than soil sensors alone.