The hydrogeology of the Cheviot Region, North Canterbury, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
In 1967 a pump station was installed on a bore in the north west portion of the Spotswood Plains, and although it has provided an adequate supply to the farms and main township of the region, Cheviot, the water supply has on a number of occasions failed to meet demand. Consequently this study was initiated to examine the geology of aquifer systems, to quantify by hydrological gaugings and pump tests the groundwater system of Spotswood and to determine the groundwater resources of the other major plains area of Mina. The Spotswood Plains cover 28.5 km² and are transgressed by the Waiau River and Leamington Stream which for the 1986-87 period had mean yearly flows of 98676 and 456 ls-¹ respectively. The Mina Plains cover only 16.0 km² and contain three streams, Mina, Swamp and Crystal Brook which do not maintain surface flow from November to March and whose flow rates in the other months range in 1986-87 from 8 to 196 ls-¹. The average number of days where rainfall was recorded for each month during the period November to January 1987 was only five, illustrating the dependence of the areas aquifers on recharge from surface streams. Geological investigations have shown that the aquifer in Spotswood consist of at least four stratigraphic units which in general terms, represent successive aggradational and degradational periods during cyclic climatic variations throughout the Quaternary Period. Geophysical investigations have targeted all of these units apart from the oldest unit, SO in Spotswood and MO in Mina which are not water bearing. Electrical resistivity soundings showed that the aquifer in Spotswood ranges from 10 to 80 metres, on average 40 metres thick, and consists of unconsolidated gravel within a sandy matrix (average resistivity of unit, 422 ohm-m), underlain by a relatively impermeable silty mudstone (average resistivity of 30 ohm-m). The lithologically equivalent unit in Mina was shown to be approximately 10 metres thick ranging from 7 to 16 metres, and consists of unconsolidated alluvium within a silty-clay matrix (average resistivity value of 140 ohms) again resting on silty mudstone, identified as Greta Formation by investigative drilling. Borehole water level monitoring produced potentiometric surveys and flow nets which for the 14th August 1987 showed that 47.23 m³/min was recharged to the aquifer system of Spotswood by the Waiau River and Leamington Stream (38.34 and 8.89 m³/min respectively) and that 28.81 m³/min was discharged back to the Waiau River through subsurface flow. Surface flow gaugings determined that for that same day in August 18.4 m³/min was discharged to the Waiau River. In the Mina Plains 0.815 m³/min was recharged to the aquifer system, 0.047 m³/min was discharged by subsurface flow and 2.00 m3/min was estimated as discharging from surface streams and artesian bores. Pumping tests showed that the alluvial material in Spotswood has a Transmissivity value averaging 5.2 m²/min whilst the best transmissivity value obtained at Mina was 0.04 m²/min. A step drawdown test determined that the suggested maximum drawdown in bore 19, an irrigation water supply bore would occur at 20 weeks based on a calculated long term pumping rate of 0.928 m³/min. A water quality survey showed that the groundwater in Mina is greater than 25 years old and contains concentrations of nitrate, chloride and sodium 2 to 4 times the desirable level set by New Zealand Health Standards. The contaminants originated as a consequence of agricultural practices such as the drilling of offal holes and the application of fertilizer. The groundwater of the Spotswood Plains was dated as less than 5 years old, and contained acceptable concentrations of contaminants primarily due to the dilution effect from recharge waters of Waiau River. The groundwater system of the Mina Plains was proven to be an inadequate substitute to the water supply operation presently in operation in the Spotswood Plains. Further the Spotswood Plains aquifer system is under utilized and remains as an excellent source and potential source of quality groundwater.