Hydrogeology of the Waipara alluvial basin

Abstract

The Waipara alluvial basin, located 50 kilometres north of Christchurch on the South Island of New Zealand is experiencing rapid transformation in land use from pastoral farming to horticulture. In the last five years the use of the groundwater resources has increased significantly. Knowledge is lacking about the availability and sustainability of the groundwater resources. Groundwater resources can be found throughout the basin in the Quaternary Canterbury and Teviotdale Gravels, and the late Pliocene/Early Pleistocene Kowai Formation. The hydrogeological system can be described as a complex network of discrete, lithologically and hydraulically heterogeneous and aniosotropic semipermeable to permeable channels. The physical and hydraulic nature of the aquifers (or water-bearing units) makes identification and characterisation of the resources difficult. However, the resources can be distinguished in terms of the observed hydrogeologic properties (i.e. lithology, yield, transmissivity, and chemistry). Chemical and isotope sampling indicate that recharge to the basin aquifers is occurring through the uplifted and fractured Tertiary sequences formed along the eastern and western margins of the basin, and through infiltration of local rainfall in the unconfined and semi-confined portions of the aquifer. Groundwater residence times are long (20- 40+ years). Long residence times, slow recharge, and low hydraulic conductivity suggests that if the groundwater resources are not properly monitored and managed, there is great potential for 'mining' the resource(s), or in other words for depleting the resource faster than it can be recharged. Long term monitoring and management strategies have been recommended for future work to help gain more knowledge and understanding of the Waipara hydrogeological system, and ensure sustain ability for future development.