Hydrogeological investigation of the Christchurch City aquifer

Abstract

The Christchurch City aquifer is a coastal groundwater system, consisting of a sequence of terrestrial and marine hydro-stratigraphic units. It is recharged from the west, by groundwater and surface-water that passes through agricultural land before reaching the groundwater reserves beneath Christchurch or discharging into coastal springs. Critical understanding of the aquifer system’s hydrogeological properties can help develop an understanding of potential groundwater flow, and aid in the effective management and utilisation of this groundwater resource.

The principal aim of this thesis is to establish a vertical profile of hydraulic conductivity in an extension to the Christchurch City aquifer system about Kaiapoi by utilising an array of multi-tier wells installed across two sites. Additionally, this thesis will incorporate an examination of model complexity by evaluating the range of methods used to achieve the principal aim.

Initially, aquifer sediments extracted from boreholes at the field sites were analysed for particle size distribution. A range of empirical models were then applied to the data to obtain hydraulic conductivity estimates. The results showed great variation between the range of empirical models, signifying the large margins of error associated with the modelling approach. This method produced the least reliable results in this study.

Following this, slug tests were conducted on the multi-tier wells at both sites. Analytical and numerical models were applied to the slug test data to obtain hydraulic conductivity estimates. These two modelling approaches yielded similar results. However, the numerical model results are considered more reliable as the method requires the least amount of assumptions and simplifications and allows a more accurate representation of the wells construction and boundary conditions.

Overall, variation in hydraulic conductivity both vertically and horizontally between the two sites illustrated the heterogeneous nature of the aquifer system. Additionally, the system was found to be highly conductive.