Evaluating the Performance of Exfiltration Trenches in Shallow Groundwater Regions

Abstract Submission: Exfiltration trenches (ETs) are among the most widely used stormwater control measures (SCMs) for removing pollutants from stormwater runoff and reducing runoff volume in urban areas. However, there are concerns about the effectiveness of these SCMs in shallow groundwater (GW) regions such as South Florida (FL) as well as their impact on GW pollution which can be exacerbated by sea level rise and extreme rainfall events in coastal areas. The objective of this study is to quantitatively evaluate these concerns. The ET of study is in Miami, FL, receives runoff from adjacent roads, and distributes it throughout the trench using an 18-in perforated pipe. We developed a wireless sensor network for high temporal resolution (5-minute) monitoring of this site that includes a rain gauge, soil moisture and temperature sensors, water level sensors at the upstream and downstream manholes, and a cluster of five GW observation wells around the ET with a water level sensor in each well. Water quality samples were taken from all manholes, inlets, and observation wells both monthly and after major storm events, and analyzed for sediments, nutrients, and metals. Field infiltration tests were also conducted monthly using MPD and SATURO infiltrometers. The results show that the distribution pipe has been partially submerged over the monitoring period due to the GW contribution. However, water level rises and drawdowns are also observed for all storm events, although in a much slower drawdown rate than those observed in other regions (e.g., Philadelphia). This dynamic performance together with the water quality results from manholes, inlets, and wells is used to explain the water quantity and quality performance of the ET. This study provides data and actionable insights for improving the design, monitoring, and maintenance of ETs in shallow GW regions and incorporating them in urban coastal resilience plans.

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