Response of the Christchurch groundwater system to exploitation: Carbon-14 and tritium study revisited

The Christchurch groundwater system is an exceptional water resource with very high drinking water quality supplying all the water requirements of the city. The groundwater system has changed over the years because of rising groundwater abstraction due to increasing population and development. The present (2017) data revealed slightly older ¹⁴C ages and increasingly steep west-east age gradients compared to the earlier work from 1976 to 2006, showing continued upflow of deep water into the exploited aquifers which is much older on the east (coastal) side than on the west (inland) side. In addition, the ³H ages for wells on the west side of the system are often much younger than their ¹⁴C ages showing that there is input of young shallow water to the wells in addition to the deep water input. Application of a binary model identifies the ages and amounts of the two components, showing that the young component is becoming younger although smaller as a proportion of the flow, and the old component from depth is becoming larger. Newly completed wells near the Waimakariri River have allowed identification of the young component, which is almost entirely composed of very young Waimakariri River seepage at all depths and therefore has very little chemical loading. Instead any chemical input (e.g. chloride, nitrate) to the Christchurch aquifers is being brought in by the old deep component which on the western side of the Christchurch system is derived from rainfall recharge on the developing Ashley-Waimakariri Plains area (plus river seepage). Chemical traces of this deep input from the northwest are at present very subtle, although more appreciable signals are seen in some wells further to the north of Christchurch. In the future, slowly increasing chemical input to the Christchurch aquifers on the west side of the system is to be expected as abstraction increases.