The hydrogeology and hydraulics of artesian springs in Canterbury
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The increasing demand for water in the Canterbury region, and the realisation that spring flow plays an important role in many wetland and river systems, requires methods for predicting changes in spring behaviour as Canterbury's groundwater resources are utilised. This study into the flow of groundwater through artesian spring systems provides a better understanding of the impacts of changes in artesian aquifer pressure. The aquifer system of the central coastal Canterbury Plains consists of gravels separated by successive layers of fine sediment. The fine sediment acts as a confining aquitard, creating artesian pressures in the gravel aquifers where the piezometric surface is above ground level. Artesian springs occur in the confining aquitard through localised zones of weakness. Piezometric levels associated with artesian springs in gravel aquifers are not well documented and have never been observed in Canterbury. Examination of near-spring groundwater flow patterns should confirm the shallow artesian aquifer as the main source of water to artesian springs. Little information is available on the relationship between artesian aquifer pressure and artesian spring discharge. Groundwater flow equations indicate that turbulence occurs in the high velocity flow encountered in artesian spring systems. Energy losses should thus be proportional to velocity squared and the pressure - flow relationship is expected to be non-linear. A non-linear relationship would buffer spring discharge against changes in aquifer pressure induced by groundwater abstraction. Field investigations were carried out at two spring sites near Christchurch. Near-spring water levels were observed in the aquifer directly below the spring to confirm it as the principal source of water to the spring. Variations in artesian pressure were then induced via groundwater abstraction from nearby wells in the source aquifer, and changes in spring discharge measured. The upper confined aquifer was confirmed to be the primary source of water for Christchurch's artesian spring systems. The relationship between artesian aquifer pressure and artesian spring discharge was found to be entirely linear for the range of pressures and flows observed. Although theoretical analysis indicates that turbulent flow is occurring at, and close to, the spring vent, the distance over which it occurs is small enough that energy losses due to rapid flow in the groundwater system are negligible. The results imply that any reduction in artesian pressure due to groundwater abstraction will have a direct, linear impact on artesian spring flow.