Assessing Iron-Enhanced Swales for Pollution Prevention

Abstract

The treatment of dissolved phosphorus and metals in runoff requires specialized filtration media, which, however, is not accounted for in the typical swale ditch check designs currently employed. In this project, ditch checks with iron-enhanced sand filter insert were developed to increase the retention of phosphate and dissolved metals in roadside swales and ditches. The iron-enhanced swale ditch checks were designed and installed as part of roadway projects of the Minnesota Department of Transportation (MnDOT) and City of Roseville. The effectiveness of the ditch checks was investigated by field testing using synthetic runoff, and field monitoring during natural rainfall events in Fall 2014 and from Spring to Summer 2015. A ditch check containing no enhanced media was also monitored for comparison. The MnDOT iron-enhanced ditch check provided consistent phosphate mass reductions during 15 rainfall events (33% mean; 37% median). The cumulative phosphate mass removal was 35%. Metal reductions were largely negative, possibly due to leaching of metals from the filter media. The Roseville iron-enhanced ditch check exhibited 47%, 43%, 26% phosphate removal and 14% zinc removal under different field testing scenarios. The ditch check without the iron-enhanced sand filter insert showed no phosphate removal but retention of metals in the top soil cover. Since the iron-enhanced ditch check monitoring excluded the effect of top soil, it can be presumed that an iron-enhanced ditch check will retain metals in the soil covering the ditch check and retain phosphate in the filter section. The project results were utilized to develop typical design recommendations for future applications of the iron-enhanced ditch check.