Delineation of the Rangitata riparian zone Report No. R12/65 ISBN 978-1-927210-92-5 (print) 978-1-927210-97-0 (web) Report prepared for Environment Canterbury by Lee Burbery Lincoln Ventures Limited June 2012 Report R12/65 ISBN 978-1-927210-92-5 (print) ISBN 978-1-927210-97-0 (web) 24 Edward Street, Lincoln PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 Delineation of the Rangitata Riparian Zone Prepared for Environment Canterbury Report No 1050-9-R1 June 2012 By Lee Burbery Delineation of the Rangitata riparian zone Community Summary 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. Environment Canterbury Technical Report i Delineation of the Rangitata riparian zone Executive Summary A riparian aquifer zone has been delineated for the lower Rangitata River based on review of available geological, hydrological, hydrogeological and water chemistry data. A water balance has been evaluated for the riparian zone, as have the potential stream depletion effects of consented groundwater takes within the zone. 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 the paleo-channel that is the Kapunatiki Creek. It encompasses Rangitata Island and the Rangitata South Branch, on the true-right of the river. On the true-left of the river, the riparian boundary follows the main river terrace to within 7 km of the river mouth where a relatively small 102 hectare area comprising the lower river terrace directly south of Coldstream is also considered to be riparian. The total area of the riparian aquifer zone is 17,388 hectares, 5,495 hectares of which is active river channel. Although influences of Rangitata River water can be traced beyond the riparian zone, e.g. under much of the Orton plain, groundwater there is not deemed to be strictly riparian. Similarly, the aquifers under Mayfield-Hinds plain have a conceivable natural hydraulic connection with the river, but indications are they are dominated by LSR (LSR) that is augmented by irrigation schemes, which operate using diverted river water. Some water from the Mayfield-Hinds irrigation scheme drains into to the riparian aquifer zone. Rainfall is estimated to provide 15x106 m3 of recharge water to the riparian aquifer annually. This is less than a quarter of the 61x106 m3 of groundwater that is currently consented to be abstracted from the zone for irrigation. The difference in the annual water balance is assumed to be made up by recharge contributions from the Rangitata River system. The general potential stream depletion effect of groundwater abstraction within the riparian zone is estimated to be in the region of 1.4 - 2.0 m3/s. This is a significant proportion of the river’s managed residual low flow of: 20.1 m3/s (summer); 15.1 m3/s (winter). McKinnons Creek is conceived to be most adversely affected by the seasonal pumped abstraction. 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 a 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. A piezometric survey - particularly of deep groundwater beneath the Orton plain - and modern river gauging measurements are identified as a priority. This would help determine the natural character of the hydrological system between the Rangitata and Orari rivers, and might also inform whether the increase in pumped abstraction over the past decade has invoked river losses. It is recommended that such investigations should be completed before the Rangitata South Irrigation Scheme becomes operational in 2014, because this activity will invoke new hydraulic changes to the natural environment. ii Environment Canterbury Technical Report Delineation of the Rangitata riparian zone Table of Contents Community Summary ................................................................................................ i Executive Summary .................................................................................................. ii 1 Introduction ..................................................................................................... 1 1.1 Background to the Rangitata Riparian Project ............................................................... 1 1.2 Previous work and data sources ..................................................................................... 3 1.3 Purpose and scope of work ............................................................................................ 5 2 Geology ............................................................................................................ 6 2.1 Geomorphology .............................................................................................................. 6 2.2 Geology ........................................................................................................................... 6 2.3 Soils ................................................................................................................................ 8 3 Surface water hydrology .............................................................................. 10 3.1 Rangitata River .............................................................................................................10 3.2 Surface water irrigation schemes .................................................................................10 3.3 Springs and spring-fed streams ....................................................................................12 3.4 Hydrologic regime .........................................................................................................12 3.4.1 Rangitata River main-stem ..............................................................................12 3.4.2 Spring-fed surface waters ................................................................................14 3.5 Rangitata River gaugings (flow gains and losses) ........................................................14