Lincoln Ventures Ltd. Lincoln Environmental Research.
Abstract
Full conference paper presented at the New Zealand Hydrological Society Annual Conference, 28 November-1 December 2005, Auckland, 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
