Freshwater feature: Groundwater aquifers of Christchurch
Groundwater aquifers of Christchurch
Christchurch (population 340 000) has one of the best water supplies in the world. The supply comes from groundwater, is of extremely high quality, and requires no treatment.
The groundwater system that provides this water is dominated in the west by gravels (possibly up to 350 m thick) and sands deposited by the Waimakariri River. Beneath Christchurch, and to the east, is a succession of alternating gravel and marine deposits. Gravel deposition occurred during periods of low sea levels and marine deposition during periods of relatively high sea levels. The marine deposits are fine-grained clays that have low permeabilities and tend to confine the water to the more permeable gravel layers that form aquifers.
Water enters the aquifers from the Waimakariri River, rainfall, and irrigation. Water discharges from the system through spring-fed streams (for example the Avon River), undersea discharge in the coastal fringe, and pumpage.
Christchurch draws groundwater from 150 bores throughout the city. In the year to June 1999 the city used 50.5 million m3 of groundwater: 57% for residential use, 21% for commercial and industrial uses, 17% unaccounted for, and 5% for public use. In addition to the City Council abstractions, private irrigators around the city took 36 million m3 and industries 14 million m3.
Groundwater level has not declined as a result of increasing groundwater pumpage since 1905. The seasonal range of groundwater pressure has increased in the last 20 years as a response to pumping. Growing abstractions have raised concerns about sustainable use of the aquifers. Environment Canterbury, the regional authority responsible, recognises the following important issues for the protection of the valuable resource provided by the aquifers.
- Protection of flows in spring-fed streams. The Styx, Avon, Heathcote, and Halswell Rivers are important recreational features for Christchurch.
- Prevention of salt-water intrusion. Salt water contamination in the Heathcote-Woolston area has been linked to localised long-term over-abstraction of groundwater.
- Protection of groundwater quality. Environment Canterbury aims to protect the aquifer from pollution.
Research by Environment Canterbury and the Institute of Geological and Nuclear Sciences is underway to quantify groundwater recharge, model aquifer geology, and assess groundwater flow of the Christchurch aquifer system. For example, computer models of groundwater flow are used to calculate the effects of groundwater pumpage on flow in the spring-fed Avon and Heathcote rivers. Groundwater pumping can reduce flow in these rivers, and the models identified the sections of river most at risk from this. As part of research into the socio-economic effects of groundwater pumping, Christchurch citizens have been asked for their views on groundwater management options through public consultation documents and questionnaires. One strong message delivered by the community is that they do not want flows in the Avon and Heathcote rivers reduced by groundwater pumping to such an extent that the beds of the rivers became exposed.
Currently, results from research on the physical and socio-economic aspects of use of the groundwater resource in Christchurch are included in Environment Canterbury’s proposed Natural Resources Regional Plan.
Paul White [ [email protected] ]