The hydrogeology of the Kaikoura plains, North Canterbury, New Zealand. (1995)
AuthorsSheppard, G. M.show all
In the last decade the Kaikoura Plains have undergone an intensification of dairy farming, and are also currently undergoing an increase in transient population due to a booming tourism industry. This increase in both farming and population have potential in the future to overexploit the Kaikoura groundwater system. The purpose of this study therefore, was to assess the groundwater system of the Kaikoura Plains to facilitate long term management of the groundwater resource.
Hydrogeological and geophysical investigations, combined with baseline data collection, have provided the basis for the development of a conceptual hydrogeological and water balance model of the Kaikoura groundwater system. Geological mapping of the surficial Quaternary alluvial deposits was undertaken, as was the interpretation and correlation of borehole lithologic logs and two transient electromagnetic surveys. These investigations have confirmed the presence of three aquifers, Aquifer 1 which is unconfined, Aquifer 2 which semi-confined to confined, and Aquifer 3 which is confined. Of these three aquifers only Aquifers 1 and 2 are currently utilized. A substantial thickness of unexplored alluvial deposits was also identified, beyond the depths of current wells.
A water balance was calculated for the Kaikoura groundwater system from a very limited database, and as such is subject to a degree of uncertainty. The main inputs and outputs identified for the Kaikoura groundwater system were: recharge from rainfall infiltration; the subsurface leakage of surface flow from the Kowhai River and the streams draining the seaward slopes of Mt. Fyffe; discharge via springs and areas of seepage to streams and drains on the lower surfaces of the Kaikoura Plains; and discharge via offshore springs at the sea bed. Recharge via rainfall infiltration is the main source of groundwater recharge at 1775 1/s, followed by recharge via leakage from the Kowhai River, ranging from 480 1/s to 830 1/s under summer and winter conditions respectively, and recharge from leakage from the streams draining the seaward slopes of Mt. Fyffe, ranging from 180 1/s to 440 1/s. Discharge from seepage to sprmgs and drains ranges from 810 1/s to 1160 1/s while offshore discharge has been estimated from 16351/s to 18651/s.
Chemical analyses of water samples from Aquifers 1 and 2 displayed very similar characteristics and could not be used for differentiation between the aquifers. Piper diagrams of the major ion data show almost identical plot positions for both Aquifer 1 and Aquifer 2,and both aquifers displayed the same evolutionary trends. Comparison of analyses from two separate sampling occasions indicate that microbiological contamination of groundwater is extremely variable and that the unconfined aquifers are the most susceptible to microbiological contamination.
The main conclusions drawn from this study are that:
1. The Kaikoura groundwater system consists of one unconfined aquifer (Aquiferl) and two confined aquifers (Aquifers 2 and 3), with only the unconfined and upper confined aquifers being utilized.
2. The water balance of the Kaikoura groundwater system is in a state of equilibrium under both winter and summer conditions, indicating that a substantial groundwater resource is available for exploitation.
3. The water balance for the Kaikoura groundwater system presented in this study provides a basis for numerical modelling and future management of the groundwater resource of the Kaikoura Plains.
4. Further work is required in baseline monitoring of the Kaikoura groundwater system in order to expand the current database to allow more quantitative and definitive conclusions to be drawn. The drilling of a deep borehole is also required to prove the groundwater resource indicated by geophysical surveys.