Geology, groundwater flow and groundwater chemistry of Springston Formation sediment between the Waimakariri River and Christchurch City

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SR_2009-42-pdf
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White, P.A.; Zemansky, G.M.; Hong, Y.S.; Moreau-Fournier, M. 2010 Geology, groundwater flow and groundwater chemistry of Springston Formation sediment between the Waimakariri River and Christchurch City. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/42 128 p.

Abstract: The Holocene Springston Formation (Yaldhurst Member) is an important geological unit for groundwater flow in the study area between the Waimakariri River and Christchurch City because this unit occurs at the land surface and this unit receives groundwater recharge from the Waimakariri River and from the land surface. Yaldhurst Member gravels are relatively permeable between the land surface and approximately 10 m deep in the study area as shown with statistics of lithological descriptions and well depths; relatively fine Springston Formation sediments may occur below these gravels. Groundwater catchment boundaries are proposed in Springston Formation sediments for all major Christchurch City spring-fed rivers and streams in the study area based on maps of historic and pre-historic Waimakariri River floodways, groundwater flow directions and interpretations of Springston Formation geology below Christchurch City. For example the groundwater catchment boundary of Avon River proposed here is mostly coincident with the Harewood Floodway and Airport Floodway. Groundwater may flow into the study area from the west. Therefore the groundwater catchment of Christchurch City may include land to the west of the study area. Some features of groundwater flow in the Springston Formation, and the underlying Riccarton Gravel, are assessed in this study including: groundwater flow in Springston Formation sediments constitute a significant component of flow in Christchurch City spring-fed rivers and streams; horizontal groundwater flow probably dominates in the Springston Formation aquifer in the study area. For example groundwater elevations in Springston Formation wells are generally similar to groundwater elevations in Riccarton Gravel wells in the Harewood Floodway indicating that vertical gradients of groundwater level between these two units are not large. The two main sources of groundwater in the study area are the Waimakariri River and rainfall recharge from the land surface as identified by chloride and nitrate-nitrogen concentrations. (auth/DG)