Hydrology and Earth System Sciences
www.hydrol-earth-syst-sci.net
1027-5606
1607-7938
12
1
2008
10.5194/hess-12-1-2008
http://www.hydrol-earth-syst-sci.net/12/1/2008/
http://www.hydrol-earth-syst-sci.net/12/1/2008/hess-12-1-2008.html
http://www.hydrol-earth-syst-sci.net/12/1/2008/hess-12-1-2008.pdf
1
19
2008-01-18
A conceptual model of flow to the Waikoropupu Springs, NW Nelson, New Zealand, based on hydrometric and tracer (<sup>18</sup>O, Cl,<sup>3</sup>H and CFC) evidence
M. K. Stewart
mk.stewart@gns.cri.nz
J. T. Thomas
GNS Science, Lower Hutt, New Zealand
Tasman District Council, Richmond, New Zealand
now at: Aquifer Dynamics Ltd, Lower Hutt, New Zealand
The Waikoropupu Springs, a large karst resurgence 4 km from the coast, are
supplied by the Arthur Marble Aquifer (AMA) underlying the Takaka Valley,
South Island, New Zealand. New evidence on the recharge sources in the
catchment, combined with previous results, is used to establish a new
recharge model for the AMA. Combined with the oxygen-18 mass balance, this
yields a quantitative description of the inputs and outputs to the aquifer.
It shows that the Main Spring is sourced mainly from the karst uplands
(74%), with smaller contributions from the Upper Takaka River (18%)
and valley rainfall (8%), while Fish Creek Spring contains mostly Upper
Takaka River water (50%). In addition, much of the Upper Takaka River
contribution to the aquifer (58%) bypasses the springs and is discharged
offshore via submarine springs. The chemical concentrations of the Main
Spring show input of 0.5% of sea water on average, which varies with flow
and derives from the deep aquifer. Tritium measurements spanning 40 yr,
and CFC-11 measurements, give a mean residence time of 8 yr for the Main
Spring water using the preferred two-component model. Our conceptual flow
model, based on the flow, chloride, oxygen-18 and age measurements, invokes
two different flow systems with different recharge sources to explain the
flow within the AMA. One system contains deeply penetrating old water with
mean age 10.2 yr and water volume 3 km<sup>3</sup>, recharged from the karst
uplands. The other, at shallow levels below the valley floor, has much
younger water with mean age 1.2 yr and water volume 0.4 km<sup>3</sup>,
recharged by Upper Takaka River and valley rainfall. The flow systems
contribute in different proportions to the Main Spring, Fish Creek Springs
and offshore springs. Their very different behaviours, despite being in the
same aquifer, are attributed to the presence of a diorite intrusion below
the surface of the lower valley, which diverts the deep flow towards the
Waikoropupu Springs and allows much of the shallow flow to pass over the
intrusive and escape via submarine springs.
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