Groundwater dynamics and hydrochemical evolution 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. 2015 Groundwater dynamics and hydrochemical evolution as inferred from Horizon's Regional Age Tracer Data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/32 iv, 55 p. + 1 CD

Abstract: To quantify the impact and lag time of land use and climate changes 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. 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. Most of the groundwater samples around and north of the Manawatu River west of the ranges are extremely old (>100 years), even from relatively shallow wells, indicating that these groundwaters are relatively disconnected from fresh surface recharge. The groundwater wells in the Horowhenua tap into a considerably younger groundwater reservoir with groundwater mean residence time of 10 – 40 years. Groundwater along the eastern side of the Tararua and Ruahine ranges is significantly younger, typically 1 m/y for some wells indicate recharge from the river as the main source. With respect to the redox conditions, the Horizons data set indicates in general decreasing dissolved oxygen with groundwater age, with the younger water more oxic and the older water more anoxic. Old water can, however, also remain oxic, as evident in some water samples from the Foxton area, indicating an absence of organic matter in this aquifer. While young and old groundwater can originate from anoxic conditions, only old groundwater samples have elevated CH4 and ammonium, indicating origin from a highly anoxic environment up to the stage of methane fermentation. Elevated mean rates of oxygen reduction, as deduced from groundwater age, may indicate higher potential for denitrification reactions in the groundwater in the upper Manawatu River catchment; one sample from there actually has excess nitrogen which is likely to originate from such denitrification reactions. Identification of groundwater recharge source from either, local rain or rivers requires a more comprehensive age, gas, and chemistry tracer data set than is currently available. The inference from the existing limited data set points towards that groundwater recharge by local rain is the predominant recharge mechanism. Most of the indicators that could potentially identify river-recharged water are still very scarce but the few available data do indicate several sites are recharged by rivers and streams, in the upper Manawatu catchment and one in the Horowhenua, all in close proximity to a river or stream. Groundwater chemistry data from dated groundwaters covering the last 100 years allow reconstruction of the baseline groundwater quality before land-use intensification 50 years ago, and the impacts of land-use activities on groundwater quality since then. The time range of the last 100 years is, however, still sparsely covered by the Horizons age tracer data set. Land-use impacts on groundwater quality are so far indicated mainly by a sharp nitrate increase over the last 50 years. Old groundwaters indicate that high magnesium, calcium, sodium, bicarbonate, and fluoride occur naturally. Significant stretches of groundwater discharge into the rivers/streams are indicated in the middle to lower reaches of the Mangatainoka River, in the head waters and lower reaches of the Makakahi River, and in the lower reaches of Waikawa Stream. (auth)