THE CANTERBURY PLAINS AQUIFER – A SINGLE AQUIFER
Hanson, C.
Environment Canterbury
The Canterbury Plains, located on the east side of the South Island of New Zealand, overlie a thick
sequence of gravelly sediments that were deposited by braided rivers flowing out of the Southern
Alps. Groundwater within these sediments is widely used for domestic and community supply,
irrigation, stock water and industry. The groundwater system is generally described as a set of
multiple, layered aquifers separated by aquitards or aquicludes. I submit that this terminology is
misleading and unhelpful, and that the groundwater system beneath the Canterbury Plains is a
single aquifer.
The sediments that form the plains were deposited in a dynamic, high-energy terrestrial
environment. They are strongly anisotropic and heterogeneous. The depositional environment did
not allow the formation of laterally extensive layers of fine sediments that could become aquitards
or aquicludes.
Broad layering does occur in the northern part of the plains along the coast, including the
Christchurch area, where sea level fluctuations have resulted in layers of fine sands, silts and
organic material alternating with the coarser alluvial gravels. However, the layers are irregular and
discontinuous, and the fine sediments, particularly the sands, are still capable of transmitting
substantial volumes of water, even if they are less permeable than the gravels.
Farther inland and to the south, it is very difficult to distinguish layers in the sediments. Bores do
encounter varying yields at different depths, and in some cases these depth intervals can be
correlated between bores to identify laterally extensive zones of similar yield. However, bore logs
show no clear distinction between the sediments in the different zones that would indicate a strongly
layered structure, and it is not clear that zones of low well yield inhibit the vertical flow of
groundwater across them.
On the contrary, groundwater chemistry data show a strong component of vertical flow beneath the
Canterbury Plains. Nitrate contamination from the land surface extends to substantial depths, over
100 meters below the water table in some areas. Within the layered area near the coast in the
north, oxygen isotope data and major ion chemistry have shown that water from deep in the aquifer
flows upward across the layers.
The evidence, therefore, does not support the presence of multiple, separate aquifers beneath the
Canterbury Plains. The groundwater system is better described as a single aquifer. The aquifer is
unconfined and heterogeneous, and it has an anisotropy that strongly favors sub-horizontal
groundwater flow, parallel to the braided river channels that deposited the sediments. However, the
anisotropy does not prevent vertical flow.
From a management perspective, the description of the groundwater system as multiple, separate
aquifers is misleading. It suggests that the aquifers could be managed separately, and that a stress
on one aquifer would not affect another aquifer. This is not the case. Pumping from anywhere
beneath the Canterbury Plains will eventually affect the rest of the groundwater system. Short-term
pumping from a single deep well may not have a measureable effect on an adjacent shallow well
or stream, but cumulative effects will propagate across the aquifer in the longer term. Contaminants
from the land surface migrate to great depths, and there are no aquitards or aquicludes that prevent
this.
The groundwater system beneath the Canterbury Plains behaves as a single aquifer, and it should
therefore be described as a single aquifer.