| Summary: | The main aim of this research was to gain a better understanding of the surface and groundwater systems in order to sustainably manage the resource for both current and future generations. Three aquifers are present within the Hinds Rangitata Plain. Aquifer one extends from near surface to approximately 40 - 50 m, aquifer two occurs from approximately 40 - 90 m, and aquifer three occurs from approximately 90 - 150 m. Aquifer one is shown to occur as a series of permeable, iron stained, poorly connected and laterally discontinuous lenses, within and often separated by less permeable sandy or tight claybound gravels. Lenses range from a few centimeters to 20 m wide and from a few centimeters to 1 m thick. These permeable layers are known to be the dominant sources of groundwater from aquifer one. In all three aquifers depth to groundwater and water seasonal water level fluctuations increase with increasing distance inland from the coast. Aquifer one gains and loses groundwater along different sections of the Hinds and Rangitata Rivers. The Hinds Rangitata Plain can be broken into seven distinct zones based on differences in the dominant source (s) of groundwater recharge within each zone. The boundaries for each zone were determined by comparing the short-term seasonal water level fluctuations observed over the course of this study and the long-term water level records, with rainfall, river flows and Mayfield-Hinds Scheme recharge. The majority of the zones also have distinctly different groundwater chemistry and oxygen-18 (d18O) values. Flows in drains and the Hinds River were highly influenced by groundwater levels. Drains and springs within the Mayfield-Hinds Irrigation Scheme were highly influenced by irrigation recharge where as those closer to the coast were more influenced by rainfall. A regional water balance of the Hinds Rangitata Plain was carried out for a one period, between September 2005 and August 2006. During this period, total recharge was 375 m3 x 106, total discharge was 227 m3 x 106, and the outflow was 148 m3 x 106. Data collected during the course of this study showed that rainfall recharge was dominant, accounting for 67 % of the total recharge. The Mayfield-Hinds Irrigation Scheme accounted for 30 % of the total recharge, with a relatively small contribution each from the Rangitata Diversion Race and Hinds River. In terms of discharge, the combined discharge from the drains and Rangitata River terrace springs, accounted for 62 % of the total discharge, with the remaining discharge from coming from groundwater abstraction. There are no overall losses to groundwater from either the Rangitata River or from stockwater race.
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