Investigations regarding the introduction of contaminants to the aquatic environment through highway runoff began in the late 1970's. Interest in this area of study has continued to increase, particularly in the area of design, operation, and effectiveness of contaminant mitigation devices. Studies have been directed toward defining the potential impact of contaminant containing stormwater on receiving water bodies. Contaminants of concern in highway runoff include metals, organics, and suspended solids. Although several management practices have been utilized, their implementation can be prohibitively expensive and logistically challenging. The logistical challenges result from space limitations and, more significantly, from the shear magnitude of areas generating runoff and the volume of runoff generated. These difficulties preclude the implementation of best management practices at all locations. Consequently, it would be desirable to understand the contaminant removal potential of existing highway appurtenances and develop information that could be used to define their removal potential under differing system conditions. One such appurtenance is the highway grass strip; a grass strip associated with the highway shoulder.
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. Contaminant fate and retention was estimated by applying stormwater at a fixed slope and flow during nine simulated storm events. In addition, standard metal partitioning information was collected by developing adsorption isotherms for each metal on the stormwater sediment and grass strip soil.
Hydraulic retention times (HRT) ranged from 8.8 minutes at a flow of 3.8 L/min*m and slope of 17%, to 85.3 minutes for a flow - slope combination of 0.38 L/min*m and 5%. The data indicated that an equivalent percent change in flow had a greater affect on HRT than slope. Metal partition information and the Ogata-Banks solution to a one dimensional advection-dispersion equation were used to calculate theoretical metal retardation times, which ranged between 183 days for lead and 30 days for copper. These predictive estimates could not be confirmed with the data collected since no significant metal breakthrough was observed in the grass strip over the duration of experimentation.
The largest portions of metals were retained within the initial 1 m of the grass strip and 1.0 cm 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.
Clover predominated the site during the simulated stormwater period of experimentation and its general health was observed to deteriorate throughout these experiments. The vegetation appeared to be metal excludor species as the dry weight concentration of metals contained in the vegetation was less than the dry weight soil concentration.
Based on the data collected in this study, grass strips along highway shoulders can retain significant sediment and metal concentrations. Future work should include a long-term study at a field site to better assess the impact of time on retention efficiency.