THE RETARDATION OF HEAVY METALS IN STORMWATER RUNOFF BY HIGHWAY GRASS STRIPS

Investigations regarding the introduction of contaminants to the aquatic environment through highway runoff began in the late 1970s. Interest in this area of study has continued to increase, particularly in the area of design, operation, and effectiveness of contaminant mitigation devices. Contaminants of concern in highway runoff include metals, organics, and suspended solids. To determine the effectiveness of these grass strips as a retention mechanism, a full-scale grass strip model was constructed that allowed control of slope and stormwater contaminant feed rate. The model was 1.2 m wide (perpendicular to flow path) and contained a 3 m grass section. A simulated highway stormwater was developed that contained sediment, lead, cadmium, copper, and zinc. The research approach was divided into three main areas: (i) determination of the hydraulic retention time for various slope and flow combinations, (ii) estimation of retention times for selected metals, (iii) analysis of the fate of the metal contaminants with regard to spatial location as well as plant uptake. Hydraulic detention time over a range of slope/flow combinations was estimated using a bromide tracer technique. The largest portion of metals were retained within the initial 1 m of the grass strip and 10 mm of depth. These analytical findings were supported by visual observations that indicated that the stormwater sediment was retained in the upper 1 m of the grass strip. Overall metal retention was estimated by mass balance and it was determined that 84% of zinc, 93% of lead, and >99% of cadmium and copper applied to the grass strip was retained. Over the duration of nine individual storm events, no significant metal breakthrough was observed at the test strip outlet or in the first set of monitoring wells placed 0.6 m from the stormwater inlet distribution device.