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. Study results indicated that soil and groundwater conditions in Lime Creek 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. 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.
Water sampling continued from all wells in October and November. The wells which were dry in August and September experienced fall recharge in October. Wells were removed and plugged in November. Results from water and soil sampling have been compiled and analysis of the collected data is underway. During this quarter, the area of Lime Creek watershed that drains to a roadside ditch was calculated, and it was determined that approximately 30 percent of the watershed (predominately in the headwater regions) drains directly to a roadside ditch. Analysis of project data will continue and a complete draft report is planned for early 2018.
Attempts were made to collect water samples three times from all 17 monitoring wells. Several wells in August and September were dry. Wells that did contain water were sampled and sent for analysis. Preliminary findings of this study were presented at a roadside ditch conference. Water samples will be collected once a month through November, then the wells will be removed and the holes plugged.
The vegetative assessment of the ditches was completed this quarter 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. Water samples were collected three times from all 17 monitoring wells, plus surface water samples were collected within wet ditches.
During this quarter, pressure transducers were installed in each of the six selected ditches to monitor continuous water levels. Water samples were collected twice from the 17 monitoring wells, plus three surface water samples were collected from wet ditches after a rain event. The water table was found within 2 feet of the soil surface at each site this spring. Results for initial soil tests, including heavy metal concentrations and nutrient data, have been gathered and are being summarized.
Seventeen monitoring wells were installed in proposed locations 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. Detailed soil descriptions were created for each well borehole and soil samples were collected. Surface soil samples also were collected in triplicate to be analyzed for heavy metal concentrations. Approximately one week after well installation, wells were pumped for development. Approximately one week after development, wells were sampled for nitrogen, phosphorus, chloride, and sulfur. Soil samples were processed and are being analyzed for multiple soil quality parameters.
The goal of this study is to conduct an assessment of the roadside ditches in an eastern Iowa watershed (Lime Creek) to assess their water and nutrient processing potential. During the last quarter, a GIS assessment of surface flow paths in Lime Creek watershed was conducted to identify flow accumulation in roadside ditches. This evaluation was used to identify potential monitoring well locations. These potential locations were field verified, and included locations along both primary and secondary roads in the Lime Creek watershed. Permission was received from the Buchanan County Road Department for well installation, but not granted by the Iowa Department of Transportation for potential sites along the I-380 or Hwy 20 corridors. Installation of the approved wells was planned for November 2016.