The hydrogeology of the Diamond Harbour region, Banks Peninsula
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The study area comprises an area of about 40 km2 and includes the township of Diamond Harbour located on the sourthern side of Lyttelton Harbour. The area is broken up into two valley systems (Orton Bradley and Purau Valleys) separated by a gently northward dipping slope known as the Diamond Harbour ‘Dip-slope’. Pressure on a water reticulation pipeline due for repair or replacement, combined with an increasing population) formed the basis for this investigation. It was hoped that local groundwater resources could at least supplement the domestic supply coming from Lyttelton. Drill hole and geophysical information confirmed that the sediments that fill the lower Purau and Orton Bradley Valleys consist of river clays and silts, marine/estuarine muds and a number of gravel units. In the case of the Purau Valley two aquifers were located, a first probably representing an infilled river channel or channels immediately overlying volcanic bedrock (Lower Purau Aquifer), and a second river gravel unit which is saturated only within 200m of the coast (Upper Purau Aquifer). While no borehold data was available to confirm geophysical data interpretations for the Orton Bradley Valley, the indications are that a single river gravel unit exists, and that it is saturated near the coast. Pump test results for the Lower Purau Aquifer show that this aquifer has a transmissivity of 11.92m2/day and a storage coefficient of 3.87 x 10-4. Computer modelling indicated the Lower Purau Aquifer possesses two hydraulic boundaries 14 and 50m from the pumped bore I and this is consistent with the interpretation of the aquifer being an infilled river channel of approximately 64m width. Evidence suggests that the alluvial aquifers of both valleys are recharged from deep circulating groundwaters present in fractured bedrock aquifers located within the volcanic formations found in the area. Isotope and chemical evidence suggests that the alluvial and deep circulating groundwaters are similar in their relative concentrations of most ions, and have similar residence times of about 50 years. The deep and alluvial groundwaters are fit for domestic supply provided treatment is carried out for excessive concentrations of iron and manganese, and aeration to bring low pH values to within acceptable limits. An experiment on a known perennial High Altitude Spring indicates that the increased discharge seen following a rain event is composed almost entirely of 'old' stored water, and the increased flow is due to increa.sed pressure head following recharge of the aquifer system by meteoric waters. Superimposed on this event variability is a seasonal discharge variability related to seasonal rainfall patterns. An infiltration- 'head' /storage model is proposed to explain the behaviour of the High Altitude Springs of Diamond Harbour. Isotopic evidence suggests an exponential-piston flow model is consistent with observed results and this indicates the spring groundwaters have residence times of 10 to 25 years. An estimate of the safe yield from all available water resources in the Diamond Harbour area ranges from 660 to 1300m3/day, allowing for sufficient water to maintain acceptable river baseflows.