Hydrogeological studies of springs in Akaroa County, Banks Peninsula
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Akaroa County, which is situated on the eroded remnant of the basaltic Akaroa Volcano, is developing as a tourist and recreational area, with consequent increasing demands on water supplies. Improved utilisation of the springwater resource of the area is a possible solution to these demands and this thesis develops hydrogeological models for the springs to assist in their future management. Methods used in this study include detailed hydrogeological mapping of two specific areas, isotope and chemical analyses of springwater, and spring discharge monitoring over a one year period. Groundwater discharge as springs and seeps is common in Akaroa County, with more than 200 springs mapped in the French Farm study area and 470 springs in Pigeon Bay Valley. Spring discharge may occur directly from defects in the basaltic lavas, but more commonly occurs through the surficial cover that mantles most of the area. Relatively impermeable beds (eg. tuff and unjointed lava) within the volcanic sequence are the dominant influence on spring distribution because of their perching effect. A “head”/storage groundwater model is proposed whereby water contained in heterogeneous lava "aquifers" is displaced to springs by infiltrating rainwater because of a pressure head applied to the irregularly shaped perched groundwater bodies. Most springs flow at less than 2.5 litres per minute and discharges of greater than 15 litres per minute are uncommon. All monitored springs show high discharge variability (1780% for one measured spring), with peak discharge occurring in winter and the lowest flows in autumn. The "Abattoir Spring" in the summit region shows rapid (within 24 hours) response to storm events with subsequent peak flows occurring 2 to 6 days later. Groundwater recharge is the result of infiltration of local precipitation, and the greatest potential for recharge occurs in the summit regions. Chemical quality of the springwaters is generally within the N.Z. Standards for Drinking Water, although nearly all samples show low pH values (pH = 6.0 to 7.3) and some show high iron and turbidity contents. Proposed utilisation of springs for water supply will require preliminary discharge monitoring. If excavation of the spring is employed to increase yield then water budget analysis of the recharge area should be undertaken so that safe yields are not exceeded. Reticulation systems should incorporate treatment facilities to counteract low pH, turbidity, or high iron content where these exceed Standards limits. Future study should include detailed water budget work to determine the magnitude of the groundwater resource of Akaroa County.