New Zealand Journal of Geology and Geophysics abstracts
Geology and geochemistry of groundwater within the Taieri Basin, east
Otago, New Zealand
N. Litchfield
D. Craw
P. O. Koons
Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand
B. Edge
Department of Geography
University of Otago
P.O. Box 56
Dunedin, New Zealand
E. Perraudin
B. Peake
Department of Chemistry
University of Otago
P.O. Box 56
Dunedin, New Zealand
Abstract The Taieri Basin, in east Otago, New Zealand,
is a tectonic depression bounded in part by active faults. The basin is floored
and surrounded by Otago Schist that is locally overlain by Cretaceous-Tertiary
sediments and volcanic rocks. Basin fill is predominantly Quaternary gravels,
sands, and silts derived from the Otago Schist, and these sediments are at
least 150 m thick in places. Aquifers are hosted in the Quaternary sediments,
principally gravels and sands. A wedge of estuarine silts and sands (c. 8000-5000
yr BP) extends for 18 km to the downstream outlet of the basin, where it is
25 m thick. Irregularly interfingered fluvial gravels and sands, interbedded
with silts, make up the principal aquifers at the upstream end of the basin.
This complex fluvial sequence dips beneath the estuarine wedge. The estuarine
wedge acts as a confining layer for groundwater in the underlying aquifers,
and artesian water bores occur in the lower reaches of the basin. Alluvial
fans on the western side of the basin are conduits for recharge from adjacent
schist ranges. Alluvial fans on the southeastern side of the basin have their
toes truncated by active faults, and recharge from these fans is structurally
inhibited. Ca, Mg, and bicarbonate contents of Taieri Basin surface waters
and groundwaters indicate chemical interaction with calcite and chlorite of
Otago Schist, in basement and/or aquifer sediment clasts. All waters have
Na and Cl- contents dominated by rainout of NaCl in marine aerosols.
Surface waters have lower Na and Cl- than groundwaters. Numerical
modelling of groundwater flow at the upstream end of the basin can reproduce
field-based inferences of flow patterns and together indicate southwestward
flow beneath the estuarine wedge confining layer. This is significant because
it implies low-concentration NO3+NO2 contamination
of shallow aquifers at the upstream end of the basin will be carried to groundwaters
in the southwest.
Keywords groundwater; chemistry; stratigraphy; Quaternary;
nitrate; Taieri Basin
G01027 Received 6 September 2001; accepted 25 July 2002 ; published 6 December
2002
New Zealand Journal of Geology & Geophysics, 2002, Vol. 45: 481-497
0028-8306/02/4504-0481 $7.00/0 © The Royal Society of New Zealand
2002
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