Evaluating the Nutrient Processing Capacity of Roadside Ditches
Roadside ditches, much like floodplains, pothole wetlands, and drainage ditches, temporarily hold water following rainfall events, and there is potential for surface and subsurface biogeochemical processing to reduce nutrient export. However, the actual amount of water and nutrients these ditches receive has not been well documented.
This study will assess the water and nutrient processing potential of roadside ditches. Objectives include quantifying the catchment area and land use of the contributing areas draining into roadside ditches; quantifying the amount of nutrients and carbon stored in roadside ditch soils; measuring infiltration rates in selected roadside ditches with diverse roadside vegetation communities; measuring groundwater nutrient concentrations in selected roadside ditches; and evaluating the effects of roadside ditches to sequester and process nutrients delivered from agricultural areas.
Roadside ditches in the Lime Creek watershed in east-central Iowa will be evaluated. The watershed is agricultural and contains a range of primary and secondary roads. Elevation data will be intersected with roads to determine the catchment areas that drain directly into roadside ditches. A site selection process that accounts for roadside ditches of different soil types, topography, vegetation and variable vehicle counts will be developed. For each selected roadside location, a series of approximately five soil borings from the roadway to the edge of the right-of-way will be done. Infiltration measurements will be made at each location along the flow path from the field right of way to the pour point from the ditch into a waterway. Water table monitoring wells will be installed, and water samples collected from the wells monthly.
The goal of this study was to assess the subsurface nutrient reduction capacity of roadside ditches in a highly agricultural, eastern Iowa watershed. The specific research objectives were to: 1) characterize vegetation, soil and groundwater conditions in six roadside ditches in the Lime Creek watershed; 2) assess the potential for the ditches to serve as nutrient reduction hotspots; and 3) evaluate the role and scale of roadside ditches to reduce nutrient export at the watershed scale.
Objective 1. A vegetative assessment of the ditches was conducted by the UNI Tallgrass Prairie Center. Two sites were dominated by native species, two by introduced species and two were not dominated by either. Across all the sites, approximately 100 species were present, with about 40 species at each individual site. This vegetative assessment has provided valuable information for the interpretation of the soil and water quality data results.
Seventeen monitoring wells were installed in roadside ditches in the Lime Creek watershed. The water table was found within 10 feet of the soil surface at each site. Therefore, it was unnecessary to install wells to 20 feet as initially anticipated, and actual well depths were approximately 10 to 12 feet. Study results indicated that soil and groundwater conditions in the watershed roadside ditches are favorable for subsurface processing of nitrate nitrogen (NO3-N). Groundwater NO3-N concentrations were <1 mg/l in wells at two transects, but at four transects, groundwater NO3-N concentrations were observed to decrease from upgradient (10.6) to downgradient (4.3 mg/l) locations (average 60 percent reduction). Water table levels were very shallow (<0.3 m) at nearly all sites, and the loamy and organic rich ditch soils appeared sufficiently anaerobic for subsurface processing of NO3-N via denitrification to occur. Groundwater DRP concentrations did not vary systematically among the sites, whereas two of the roadside ditches had chloride (Cl) concentrations above background levels that indicated encroachment of road salt.
Objective 2. The findings showed that approximately 30 percent of the Lime Creek watershed drains directly to a roadside ditch. These areas are mainly located in headwater regions of the watershed where nutrient reductions are critical.
Objective 3. With estimated NO3-N reductions equivalent to typical wetland N reductions (0.2 to 0.4 g/m2/day), researchers recommend that future work should focus on potential use and manipulation of roadside ditches to serve as “linear wetlands” for watershed-scale treatment of nonpoint source pollution.