Quantifying the Performance of Regional Scale Reductions in Nutrient Applications for Source Water Protection through Vadose Zone Monitoring
Department of Earth and Environmental Sciences, University of Waterloo,
In some agricultural areas in Ontario, Canada, excessive application of nutrients have negatively impacted underlying groundwater quality and associated municipal water supplies. Source Water Protection strategies have included the implementation of Beneficial Management Practices (BMPs) involving regional scale reductions in nutrient applications to reduce the risk to municipal water supplies. Assessing and quantifying the effectiveness of these region-scale BMPs, both spatially and temporally, have been problematic due to lengthy response times and multivariate heterogeneity. We have developed a novel approach for rapid assessment of BMP performance by tracking spatial and temporal variability in stored nitrate mass within the vadose zone beneath agricultural lands where significant reductions in nutrient application have been implemented. The project was based on field activities conducted in the vicinity of the Thornton Well Field near Woodstock, ON. Groundwater from several of the production wells at the site have chronically high levels of nitrate, likely as a result of the leaching of excess nitrate from the surrounding agricultural lands. Local county officials purchased land within the wellhead protection area of this municipal drinking water supply in 2002 and implemented a nutrient management plan that involved an average reduction of between 40 percent and 50 percent in total applied nitrogen. To monitor the influence of the reduced nutrient loadings, continuous cores were extracted from a series of locations (field stations) throughout the study site. The cores were sub-sampled and analyzed for nitrate pore water concentration and soil moisture. Cores were repeatedly collected at each of the stations over a period of four years to track changes in subsurface nitrate soil concentrations. At the end of four-year period, the average nitrate concentrations within the vadose zone were initially 22 mg NO3--N/L and subsequently decreased to 8 mg NO3--N/L, a reduction of over 60 percent. Overall total stored mass was documented to have decreased between 20 percent and 60 percent over the field site. Estimated point-scale nitrate mass flux values over the time period show an average 45 percent reduction which compares well to the estimated 40 percent to 50 percent reduction in nitrogen application. Regional-scale flow and transport modeling suggests that a 20 percent decrease in the average municipal well nitrate concentrations will ultimately be achieved, however, the full benefit of the BMPs will not be realized at the wells for between five and 10 years after the initiation of the nutrient reduction. Overall, these field and modeling results indicate that the implementation of BMPs in selected areas within municipal capture zones is a promising strategy to reduce the impacts of agricultural nutrients at supply wells. This approach can be applied to other well fields to provide insight with respect to the magnitude and timing of water quality changes and assist in water resources management decisions.