Sources and flow of north Canterbury plains groundwater, New Zealand

Abstract

Geological, hydrological, isotope (tritium and 18O) and chemical (mainly nitrate and chloride concentrations) evidence is interpreted to give a mutually consistent picture of the recharge sources and flow patterns of the important groundwater resource in the deep glacial and interglacial deposits of the sector of the Canterbury Plains between the Selwyn River and Ashley River. Particular attention is paid to the confined gravel aquifers which presently provide about 300,000 m 3 daily of mainly very high-quality water for the needs of Christchurch city. The study period for tritium measurements extends over 27 years, encompassing the peak and decline of thermonuclear tritium fallout in this region. Major rivers emerging from the hill and mountain catchments to the west of the Plains are depleted in 18O relative to average low-level precipitation. Most of the groundwater is river-recharged, but some areas of significant local precipitation recharge contribution are clearly identified by 18O and chemical concentrations. The pressure distribution, tritium and chemical data reveal that the artesian ground-water underlying Christchurch ascends from deeper aquifers into the shallowest aquifer via gaps in the confining layers; much of this flow is induced by withdrawal, and the data reveal nothing about possible offshore discharge through the seaward extension of the shallowest aquifer, which is known to outcrop 40 km beyond the coast. The Christchurch aquifers are recharged by infiltration from Waimakariri River in its central Plains reaches, and the resulting flow regime is east- and southeast-directed; satisfactory water quality of the deeper Christchurch aquifers appears to be guaranteed for the future provided the river can be maintained in its present condition. Shallow groundwater, and water recharged to depth by other rivers, irrigation and local precipitation on the unconfined western areas of the Plains, are more susceptible to agricultural and other pollutants; none of this water is encountered in the deeper aquifers under Christchurch, but only in shallow aquifers and surface discharges, or else flowing to the southeast, well away from the city area.