Groundwater-surface water interactions along the Waipawa River, Ruataniwha Plains, Hawke's Bay

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SR_2009-37-pdf
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Undereiner, G.; White, P.A.; Meilhac, C. 2009 Groundwater-surface water interactions along the Waipawa River, Ruataniwha Plains, Hawke's Bay. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/37 79 p.

Abstract: The Ruataniwha Plains is one of the most important water resources in the Hawke’s Bay region, New Zealand. Water demand has increased in the last thirty years and there is a general trend of declining groundwater levels in the area and declining flow in streams within the Ruataniwha Plains (Hawke’s Bay Regional Council, 2003). These circumstances underline the need for improved understanding of the dynamics of Ruataniwha Plains hydrology and hydrogeology to ensure wise future management of the water resource. The present study focuses on interactions between groundwater and surface water in a study area on the Ruataniwha Plains between State Highway 50 and the Raukawa Range including Waipawa River and lower reaches of Mangaonuku River, Kahahakuri Stream and Cochrane’s Creek. These interactions are observed at the surface by gains in river flow, when aquifers discharge to rivers, and by losses in river flow when rivers discharge to aquifers. For example, springs are common in the east of the study area and historic data indicate that the Waipawa River is commonly dry in part of the study area. Two aquifers occur in the study area; a shallow unconfined aquifer and a deep confined aquifer. The deep confined aquifer has a hydraulic head above the shallow aquifer, and therefore groundwater may flow from the deep aquifer to the shallow aquifer. The two aquifers are separated by discontinuous clay layers. Springs and seeps occur in the eastern part of the study area where groundwater elevations in the shallow aquifer are very close to the surface. These springs and seeps are fed by losses from the Waipawa River through the unconfined aquifer. Springs and seeps provide water to streams including Mangaonuku Stream, Kahahakuri Stream and Cochrane’s Creek. Water chemistry measurements indicate Mangaonuku Stream also receives groundwater from the confined aquifer. A groundwater budget of the study area indicates groundwater recharge from the Waipawa River above Stockade Road discharges in Mangaonuku Stream, Kahahakuri Stream Cochrane’s Creek. Therefore groundwater management policies are relevant to preservation of flow in these surface water features. A relatively large imbalance between groundwater inflow and groundwater outflows indicates that some groundwater recharge from the Waipawa River above Stockade Road discharges in the Waipawa River downstream of Stockade Road. Further investigations are recommended to improve the understanding of interactions between surface and groundwater and to improve the water budget calculations. (auth/DG)