Capture zone delineation for National Groundwater Monitoring Programme sites in the Southland region

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Gusyev, M.A.; Tschritter, C.; Moreau-Fournier, M.; Daughney, C.J. 2011 Capture zone delineation for National Groundwater Monitoring Programme sites in the Southland region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/31 54 p.

Abstract: This report presents the delineated capture zones for the active NGMP bores in the Southland region. The numerical modelling method and the Time of Travel (TOT) criterion were selected to delineate capture zones as robustly as possible, given available information about the sites. Relevant datasets for capture zone delineation included bore construction details, aquifer test data, groundwater elevations and existing groundwater flow studies, all provided by Environment Southland. In addition, water quality data and trends were calculated by GNS Science. The capture zones were delineated for each NGMP site in accordance with the overall aims of the NGMP outlined above. This information will help to relate observed groundwater quality with past and present land use. The delineated capture zones identify the source the water flowing to each NGMP bore in the Southland region. Calculated areas for capture zones for individual NGMP sites were relatively small, ranging from 0.8 to 13.2 km2, and in all cases were equivalent to less than 6% of the area of the groundwater management zone(s) that supply water to the site (some sites are supplied by recharge from more than one groundwater management zone). Although the NGMP sites on their own are not intended to provide a stand-alone regional monitoring network, it is clear that the seven NGMP sites are monitoring only a very small part of the Southland groundwater resource. It is recommended that capture zones be mapped for other non-NGMP groundwater monitoring sites in Southland, in order to determine what fraction and what specific locations of the total land area are being evaluated through Environment Southland’s State of the Environment network. In this study, the delineated capture zones were also used to estimate the age of water at each NGMP bore. At four of the NGMP sites the modelled groundwater ages derived from the capture zone delineation were in good agreement with ages inferred from tritium measurements. At one NGMP site such a comparison could not be made because no tritium measurements were available. For the remaining two NGMP sites there was a substantial discrepancy between the groundwater age estimates based on tritium measurements compared to the age estimates derived from capture zone delineation. These discrepancies may arise due to aquifer heterogeneities that are not represented in the relatively simple flow models used in this study. It is recommended that additional research should be conducted at these latter two NGMP sites to understand the cause of the discrepancy between the two methods of inferring groundwater age. (auth/DG)