GROUNDWATER MOVEMENT AROUND THE WAIMAKARIRI RIVER
Neil Thomas1
1
Pattle Delamore Partners Ltd.
Aims
The Eyre River and Christchurch – West Melton Groundwater Allocation Zones (GAZ) are
separated by the Waimakariri River, which is often thought to represent a hydraulic boundary to
groundwater flow. Therefore, it is assumed that there is little interaction between the two zones.
In general, groundwater in the Eyre River GAZ to the north has higher concentrations of some
dissolved parameters, such as nitrate-nitrogen which reflects its land surface recharge source. In
contrast, groundwater on the south side of the river and particularly in the Christchurch aquifers
has low concentrations of nitrate-nitrogen and dissolved parameters, which reflects its dominant
recharge source as seepage from the Waimakariri River.
However, various reports have put forward the hypothesis that groundwater is moving from the
Eyre River GAZ, beneath the Waimakariri River, and into the Christchurch – West Melton GAZ.
Those hypotheses are primarily based on groundwater quality and isotope data, with some
similarities observed between groundwater on the north and south sides of the river, particularly in
north-east Christchurch.
Water for public supply in Christchurch is almost exclusively sourced from untreated groundwater,
so groundwater movement from the north side of the river to the south side, together with a decline
in groundwater quality would be an issue of potential concern to city water managers.
The aim of this work was to review the hypotheses to determine whether cross boundary flow is
likely to occur. The project was carried out for Environment Canterbury.
Method
A variety of datasets were assessed as part of this investigation including:
 Groundwater level data (piezometric contours, groundwater level timeseries and cross
section data;
 Groundwater quality data (oxygen 18 ratios, nitrate-nitrogen concentrations, chloride
concentrations, cluster analysis); and
 Assessment of aquifer parameter information.
Results
Based on groundwater level and groundwater quality data from both sides of the river, the plains
section of the Waimakariri River can be split into three overall sections. The most upstream reach
extends from the gorge to around Courtenay Road and groundwater movement under the river
within that reach is possible if the river does not provide a hydraulic barrier to such flows. That
potential exists because groundwater levels are generally deeper than the river and not obviously
connected to it. A hydraulic gradient from the north side to the south side of the river may also be
present, although there is a lack of clearly defined data to define groundwater flow directions.
The middle reach extends from around Courtenay Road to downstream of Crossbank and
groundwater movement under the river is unlikely in this section of the river. The river has a
noticeable effect on groundwater levels adjacent to the river and is likely to act as a barrier to
groundwater flow. Groundwater quality data indicate that the groundwater chemistry is noticeably
different on the north side of the river compared to the south side, reinforcing the idea that the
Waimakariri River acts as a barrier to groundwater movement, or if any movement is occurring it is
making a very minor contribution.
The most downstream reach extends from downstream of Crossbank to the coast. The coastal
confining layer is present in this section and groundwater at depth is apparently isolated from direct
effects from the Waimakariri River. However, groundwater level data does not indicate that a
hydraulic gradient from north to south is present and therefore groundwater movement from north
to south is unlikely. To the contrary a deep basement trench to the north-east of Kaiapoi is likely
to draw groundwater in that direction.
The various datasets reviewed are not exhaustive and the flow directions cannot be precisely
defined. However, the data, and in particular the groundwater quality data, imply that if movement
does occur, it is of a minor scale for the Christchurch aquifers in comparison with the much greater
quantities of seepage that occur from the Waimakariri River.