Regional Groundwater Flow Dynamics in the Horizons Region
Uwe Morgenstern1, R van der Raaij1, H Martindale1, M Toews1, A Matthews2, V
Trompetter1, M Stewart, and D Townsend1
1
Geological and Nuclear Sciences, 2 Horizons Regional Council
Aims
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.
Method
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.
Results
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 <5 years MRT.
Extremely low vertical groundwater recharge rates are observed in the central coastal area,
consistent with confined groundwater systems, or with upwelling of old groundwater close to
the coast. Consistent very low vertical recharge rates along the Manawatu River west of the
Manawatu Gorge indicate upwelling groundwater conditions in this area, implying
groundwater discharge into the river is more likely here than loss of river water into the
groundwater system. High recharge rates observed at several wells in the Horowhenua area
are consistent with high nitrate concentrations in this area. Similarly, high recharge rates and
therefore active water recharge is observed in the area east of the Tararua and Ruahine
ranges, also accompanied by elevated nitrate. Extremely high recharge rates of >1 m/y for
some wells indicate recharge from the river as the main source.
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.
Conclusions
Despite the current limitations of the Horizons age, gas, and isotope tracer data set, it already
has revealed details on regional groundwater processes that are pioneering on a New Zealand
and international context. Additional targeted samples for the purpose of understanding
groundwater processes on a regional scale will without doubt greatly improve the
understanding of the regional groundwater resources.