Characterization of Storm Water–Suspended Sediments and Phosphorus in an Urban Catchment in Florida

Characterizing the incoming pollution load from contributing watersheds is a basic requirement to assess the functioning of storm water treatment systems. These systems are often built with assumed design performances, but their pollution loads and functioning are seldom checked, making their efficacy questionable. This paper presents a case study of a storm water system built in a 40-ha residential catchment in Gainesville, Florida. Despite its promising performance and assumed low pollution load, a receiving wetland basin overfilled with sediments, and a polishing pond was overgrown with algae. It was hypothesized that the system was not functioning as designed as a response to high sediment loads and poor treatment performance; hence, an investigation was carried out to quantify the suspended sediments through the storm water system during runoff events. Ten events were collected and analyzed for suspended sediment concentration (SSC), sediment mass load, and first flush intensity. In addition, total phosphorus (TP) associated with different particle fractions was analyzed. Results from this study showed that SSC event mean concentration (EMC) was 23.7–194.6 mg/L in untreated runoff, 8.8–65.1 mg/L after a hydrodynamic separator (HDS), and 4.6–107.5 mg/L in the receiving water. A large first flush intensity was found in most events, with 43–86% of the total mass of particulate matter (PM) coming during the first 20% of the storm water volume. Total phosphorus concentration in runoff was typical of similar land uses in Florida, ranging from 0.162 to 0.834 mg/L. Event mean concentration and load of PM were not significantly greater than estimates for similar catchments, but the HDS removal efficiency (46–88%) was considerably lower than the design estimates. These findings support the need for proper monitoring and maintenance of storm water treatment units despite their pollution loading intensity.