Monitoring an Iron-Enhanced Sand Filter Trench for the Capture of Phosphate from Stormwater Runoff

Abstract

This monitoring project was performed on an iron enhanced sand filtration (IESF) trench in the City of Prior Lake. Water from the pond and IESF trench discharges into a wetland that ultimately drains into Upper Prior Lake. In 2002, Upper Prior Lake was listed on Minnesota’s 303(d) List of Impaired Waters for nutrient/eutrophication biological indicators with aquatic recreation being impaired. Water quality has been reduced due to excessive phosphorus loading. According to the TMDL implementation plan developed for Spring Lake and Upper Prior Lake, the total phosphorus load must be reduced by 83% and 41%, respectively, to meet water quality goals. Overall, for 28 monitored natural rainfall/runoff events from 2013-2015, the IESF trench removed 26% of the phosphate mass load it received, though after non-routine maintenance in August 2014 the performance improved to 45% phosphate mass load reduction. These results indicate the importance of maintenance. A newer installation was previously monitored, and found to retain 71% of the phosphate (Erickson and Gulliver 2010). Most of the overall phosphate load reduction was achieved during larger events that had comparatively high influent phosphate concentrations (32.3 – 125.2 μg/L) and mass loads. Many small events in this investigation with low influent phosphate concentrations (3.8 – 38.4 μg/L) or mass loads exhibited negative removal (i.e., effluent mass load > influent mass load). The high effluent phosphate concentrations are suspected to be caused by the degradation of floating plants (primarily duckweed) that were deposited on the surface of the filter trench. As mentioned above, nonroutine maintenance to remove this material resulted in substantial performance improvement. After this maintenance, positive removal was observed for influent concentrations ranging from 6.3 – 44.1 μg/L. Detailed results, maintenance activities, design and operating & maintenance recommendations, and lessons learned are given within this report.