Heretaunga Plains aquifers : groundwater dynamics, source and hydrochemical processes as inferred from age, chemistry, and stable isotope tracer data

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Morgenstern, U.; Begg, J.G.; van der Raaij, R.W.; Moreau, M.F.; Martindale, H.; Daughney, C.J.; Franzblau, R.E.; Stewart, M.K.; Knowling, M.J.; Toews, M.W.; Trompetter, V.; Kaiser, J.; Gordon, D. 2018 Heretaunga Plains aquifers: groundwater dynamics, source and hydrochemical processes as inferred from age, chemistry, and stable isotope tracer data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/33. 82 p.; doi: 10.21420/G2Q92G

Abstract: Hawke’s Bay Regional Council (HBRC) is presently undertaking a range of groundwater science investigations as part of its on-going focus on sustainable management of the hydrologic system of the Heretaunga Plains to inform policy development and a new Regional Resource Management Plan framework. This report provides enhanced conceptualisation of the regional groundwater-surface water system for the development of groundwater flow and transport models for the Heretaunga Plains aquifers. This collaborative study between Hawke’s Bay Regional Council, Hastings District Council, Napier City Council, and GNS Science aims to improve our understanding of the Heretaunga Plains aquifers in regard to groundwater recharge sources, flow dynamics, and interaction between groundwater and surface water. Three main rivers discharge to the sea across the Heretaunga Plains. These rivers have large catchments that extend significantly beyond the Heretaunga Plains. Spring-fed streams and drains form sizeable perennial streams. The Heretaunga Plains is underlain by Quaternary fluvial, estuarine-lagoonal, and marine deposits in-filling a subsiding syncline. Borehole data indicate that the deposition during the low sea level stands of the Last Glaciation was dominated by alluvial gravels accumulated from the bed load of the braided river systems of the Ngaruroro, Tutaekuri and Tukituki rivers. These materials make up the primary aquifer of the Heretaunga Plains. Overlying fine-grained materials deposited subsequently across much of the eastern Heretaunga Plains comprise an aquitard that confines the aquifer. Within the depositional sequence, river-channel gravels form an interconnected unconfined-confined aquifer system containing groundwater recharged from land surface recharge and the Ngaruroro River bed at the inland margin of the plain, 20 km from the coast. At the coast, gravel aquifers extend to a depth of 250 m. The multiple gravel layers are in general highly transmissive. Tritium, CFCs, SF6, 2H, 18O, Ar, N2, CH4, radon and major/minor ion hydrochemistry data are utilised with respect to understanding the dynamics of the groundwater from recharge to discharge and interaction with surface water, and understanding the processes that control the hydrochemical properties (quality) of the groundwater including denitrification. Age tracer and isotope data are available from c. 160 groundwater and surface water sites across the Heretaunga Plains. Hierarchical Cluster Analysis (HCA) results provide context for the main drivers of hydrochemistry including oxic rivers and river-recharged groundwaters with little or no elevation of nutrient concentrations, association with limestone or carbonate geology, oxic rainfall-recharged groundwaters with moderate land-use impact and anoxic groundwater with chemistry typical of natural conditions. If suitable, the combination of these drivers can provide additional evidence for identification of recharge source. A combination of hydrochemistry, stable isotopes, and excess air was able to distinguish between recharge sources. (auth)