Capture zone delineation of community supply wells and State of the Environment monitoring wells in the Greater Wellington Region

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Toews, M.W.; Donath, F. 2015 Capture zone delineation of community supply wells and State of the Environment monitoring wells in the Greater Wellington Region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/06 63 p.

Abstract: Greater Wellington Regional Council (GWRC) commissioned GNS Science to delineate capture zones for community supply wells and for State of the Environment (SOE) wells. A capture zone (CZ) is defined as the total source area that contributes groundwater to a well. The objective of delineating CZs for community supply wells is to help form policy for a natural resource plan. CZs for SOE wells can help understand geochemistry and trends in water quality by evaluating the land use in the CZs for each well. Existing calibrated groundwater flow models for Hutt Valley, Kapiti Coast and the Wairarapa Valley were used delineate CZs for wells within these modelling domains. The transient models were largely used “as is”, with modifications to implement additional wells to delineate CZs. Particle tracking techniques were used to trace the outline of CZs around each well, using a combination of forwards tracking particles on the water table and/or backwards particle tracking around each well screen. The simulations were run several times in order to conduct a sensitivity analysis, by varying hydraulic parameters and particle release times. A maximum CZ shape was obtained by aggregating pathlines from all sensitivity simulations. In addition to tracing the total CZ shape, each zone was subdivided into shallow and deep zones, based on a 3-dimensional evaluation of particle pathlines below the water table. A shallow CZ is the areal extent on the surface that contributes to a well’s water source, while a deep CZ is the source region found below the water table. Travel times of pathlines were used to delineate microbial protection zones (PZs) based on a 1-year travel time to each well. The resulting maps show four different zones of shallow and deep CZ and PZ polygons. (auth)