Delineation of the Rangitata riparian zone.

Abstract

A riparian zone aquifer is a groundwater system that is closely related to a surface water body. Water resource management rules within a riparian zone might be tailored differently from rules outside such a zone, to account for the strong surface water/groundwater connection. The riparian aquifer zone of the lower section of the Rangitata River which divides the Mayfield-Hinds groundwater allocation zone (GWAZ) and Rangitata-Orton GWAZ has been delineated based on the review of available geological, hydrological and water chemistry data. The Rangitata riparian aquifer zone is conceived to include both shallow and deep groundwater that underlies the margin of land between the Rangitata River and Kapunatiki Creek (encompassing Rangitata Island and the Rangitata South Branch). The historic flood plain on the north side (true-left) of the Rangitata, directly south of Coldstream, is also considered to be part of the riparian zone. Covering 17,388 hectares, the riparian aquifer zone is approximately three times the area of the active Rangitata River channel. 61 million m3 of groundwater is currently consented to be pumped from the riparian aquifer zone, annually. This is four times more than what can conceivably be supplied by rainfall recharge, the deficit of which must be made up from river water flow losses. Consequently, groundwater abstractions from within the riparian zone have potential to significantly impact flows in the Rangitata River system and in particular the spring-fed McKinnons Creek. McKinnons Creek and Ealing Springs constitute features of the Rangitata River system that are protected under the Rangitata River Conservation Order for their salmon spawning properties and cultural significance to Ngāi Tahu. Elevated nitrate levels in these surface waters pose a potential environmental risk to the ecological qualities for which these spring-fed water systems are recognised. There is technical merit in defining a riparian aquifer zone for the Rangitata River, although the resource management implications of doing so are not clear, particularly given the Rangitata River is already subject to a conservation order. The relatively small spring-fed McKinnons Creek would likely stand to benefit the most from the establishment of a riparian aquifer management zone on the Rangitata. There is an obvious need for further field investigation work to be undertaken that would assist in the technical refinement of the Rangitata riparian zone, and is required before any changes to groundwater allocation resource management in the Rangitata region might be made.