The effects of denitrification parameterization and potential benefits of spatially targeted regulation for the reduction of N-discharges from agriculture.

Diffuse nitrate leaching from agricultural areas is a major environmental problem in many parts of the world. Understanding where in a catchment nitrate is removed is key for designing effective land use management strategies that protect water quality, while minimizing the impact on economic development. In this study we assess the effects of spatially targeted nitrate leaching regulation in a basin with limited knowledge of the complexity of chemical heterogeneity. Three alternative nitrate reactivity spatial parameterizations were incorporated in a catchment-scale flow and transport model and used to evaluate the effectiveness of four possible spatially targeted regulation options. Our findings confirm that denitrification parameterization cannot be numerically determined based on model inversion alone. Detailed field based characterization using physical and geochemical methods should be considered and incorporated in the numerical inversion scheme. We also demonstrate that there are potential benefits of implementing spatially targeted regulation compared to spatially uniform regulation. Focusing regulation in areas where nitrate residence time is short, such as riparian zones or areas with low natural N-reduction, results in greater reduction of N-discharges through groundwater. Significantly improved efficiencies can be expected when delineation of management zones considers the chemical heterogeneity and groundwater flow paths. These improved efficiencies are achieved by adopting management rules that regulate land use in discharge sensitive areas, where leaching changes contribute the most to the catchment nitrate discharges. In our case study, regulation in discharge sensitive zones was twice as efficient compared to other management options.

Assessing changes in nitrogen contamination in groundwater using water aging: Waikato River, New Zealand.

Groundwater and surface water contamination by nitrogen can produce serious health issues particularly regarding the contraction of methemoglobinemia in infants; prevention of this disorder is a main reason for regulating drinking water quality in New Zealand. To assist the management of water quality in the Upper Waikato Catchment in the light of growing intensification of nitrogen producing land uses, a regional groundwater flow and contamination transport model was developed to assess probable future nitrogen concentrations. Calibration of the steady state model was achieved with a root mean square error of 4.7% for groundwater levels and 7.9% for gains and losses in river flow within the catchment. Model performance was assisted by relationships determined between SiO2 concentrations in groundwater and water age. Modelling coupled with water aging indicates that the mean residence time of contributing water in the Upper Waikato Catchment ranges from 5 to 101 years with an average of 50 years. Some 77% of surface water is sourced from groundwater which is likely to convey the predominant contribution to the total nitrogen load. Nitrogen concentrations are increasing but the long-term total load is difficult to estimate owing largely to lack of information about historical land use and the degree of attenuation of nitrogen. The model and results obtained may be employed to provide information on water travel time distributions, appropriate source location and surface water receiving reaches in rivers and streams all of which is important and necessary for effective water quality management in the Catchment.