Groundwater dynamics, source and hydrochemical processes as inferred from Horizon's Regional Age Tracer Data

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Morgenstern, U.; van der Raaij, R.W.; Martindale, H.; Toews, M.W.; Stewart, M.K.; Matthews, A.; Trompetter, V.; Townsend, D.B. 2017 Groundwater dynamics, source and hydrochemical processes as inferred from Horizon's Regional Age Tracer Data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/15 63 p.; doi: 10.21420/G2J596


Abstract: To quantify the impact and lag time of changes in land-use and climate on the quantity and quality of the available groundwater resources within the framework of the National Policy Statement for Freshwater Management 2014, Horizons is increasingly focusing research on age tracers that provide new innovative approaches for understanding groundwater dynamics and hydrochemical evolution. The age tracers provide measurable information that is essential for conceptualising the regional groundwater - surface water system and inform development of meaningful groundwater flow and transport models. A first data assessment was undertaken in 2015 (Morgenstern et al., 2015a), which identified a number of knowledge gaps. To fill these gaps, further samples were collected and analysed in 2015 and 2016. This report is an update of the previous report to include the new data from 2015 and 2016, and supersedes the 2015 report. Areas that have wells with age, isotope and gas tracer data available cover the west coast catchment from the Wanganui Region to Manawatu, in the south the Horowhenua District, and in the west the Mangatainoka and Upper Manawatu catchments. Well depths range from shallower wells in the gravel aquifers in the Horowhenua and Tararua districts, and deeper wells in the aquifers between Palmerston North and Wanganui. Around 200 wells in the region have tracer data available, including tritium, CFCs, SF6, 2H, 18O, Ar, N2, CH4 and radon. The Mangatainoka River, the Makakahi River and Waikawa Stream were investigated for areas of groundwater discharge into the rivers. Groundwater ages in rivers and streams provided detailed understanding of the ability of geologic formations to let the rainwater enter into larger groundwater system, providing larger groundwater stores for maintaining river flow at draught. (auth)