Groundwater Recharge Interface and Nitrate Discharge: Central Canterbury, New Zealand

Regional surveys of groundwater quality in Central Canterbury, from 1977 to the present, show widespread occurrence of nitrate contamination primarily from agricultural land use. Two reported analyses of these surveys, in 1984 and 2002, describe a general decrease of nitrate concentration with depth from the groundwater table to uncontaminated water at about 50-80 m. This high quality water is considered to be recharge from the large rivers that originate in alpine catchments. The occurrence and location of the interface between these distinctly different groundwater bodies is important for issues of access to high quality drinking water and for the quality of groundwater-fed surface waters in the down-slope areas of the Central Canterbury Plains. This paper describes the application of a prototype regional-scale model of nitrate transport in groundwater to investigation of the likely nature of this groundwater interface and the implications for quality of surface waters in the groundwater discharge zone. Results from a 2-D horizontal groundwater flow model indicate that river recharge is the major source of groundwater, from leakage rates per kilometre of river reach that are less than 1 % of mean annual river flow. The vertical distribution of groundwater contaminant transport was examined with a combination of stream-function analysis of flow, and mixing-cell model simulation of contaminant dispersion. The results of a model demonstration with a realistic land use pattern, for a typical groundwater flow path, illustrate the formation of a dispersive, concentration interface between the two groundwater bodies. The demonstration example also shows how average nitrate-N concentration of about 8 mg/L in recharge from agricultural land use contributes to mean concentrations of 2 3 mg/L in the groundwater discharge zone, due to the influence of river recharge.