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.