A hydrogeological study of the interaction between Avon River baseflow and shallow groundwater, Christchurch, New Zealand

摘要

The relationship between shallow groundwater levels and Avon River baseflow upstream of Gloucester Street has been investigated. Avon River baseflow is supplied by shallow groundwater-fed springs. Historical and anecdotal information indicate that since European settlement of the Christchurch area in the 1850's, Avon River baseflow has declined. The baseflow decline is attributed to the progressive lowering of the Christchurch area watertable which has caused downstream migration of headwater spring positions and a reduction in spring discharge. Prior to this study minimal historical Avon River flow data existed, and a quantitative estimation of the decline in baseflow is not possible. A management plan for maintaining acceptable baseflow levels in the Avon River is currently being developed by the Canterbury Regional Council. The aim of this study was to provide information on the relationship between Avon River baseflow and shallow groundwater levels to aid baseflow management. The Christchurch groundwater system is characterised by a watertable aquifer that overlies a series of layered confined aquifers. Direct groundwater discharge into the Avon River is considered to be from both the watertable aquifer and upper most confined aquifer. Groundwater was found to enter the river system by two different mechanisms; seepage through stream bed gravel and artesian spring discharge. Groundwater seepage through streambed gravel occurs where the stream channel intersects the watertable aquifer. Artesian springs occur where water-bearing gravels are overlain by between approximately 1 to 10 m of finer-grained confining sediment. Artesian spring water is thought to flow from both the watertable aquifer and the uppermost confined aquifer. Pipes through the confining sediment connect the spring vent to the underlying water-bearing gravels. When the hydraulic head of the underlying gravel aquifer is above the stream stage artesian spring flow will occur. Tributary baseflow and shallow groundwater data were collected for the 11 month period, February 1992 to January 1993. In addition, baseflow was separated from the Avon River flow record. Available flow data indicate that mean A von River baseflow at Gloucester Street from 1980 to 1992 was approximately 1700 1/s. In March 1993 Avon River baseflow was 50% of that in March 1980. Large rainfall events in late-August 1992 caused Avon River baseflow in January 1993 to increase to approximately 77% of the March 1980 value. Regression analysis established a relationship between both hydraulic head in the upper most confined aquifer and unconfined watertable levels, to Avon River baseflow (R² > 0.8). The flow hydrograph showed that the daily abstraction of shallow groundwater from beneath the catchment caused an associated reduction in flow. Seasonal fluctuations in spring discharge and baseflow were found to be greater in the western tributaries than the eastern tributaries. This is attributed to the greater seasonal fluctuation of shallow groundwater levels in the western area of the catchment than in the eastern area. From available data the peak in seasonal groundwater levels occurred throughout the study area during the period of 24-27 October 1992. No observable time delay occurred between the seasonal peaks in shallow groundwater levels and Avon River baseflow at Gloucester Street. In order to sustain acceptable rates of Avon River baseflow, shallow groundwater levels need to be maintained in areas of the catchment were groundwater enters the river. As a first step, the Canterbury Regional Council has placed restrictions on the abstraction of groundwater in areas where springs occur. The information presented in this study on the relationship between shallow groundwater levels and Avon River baseflow confirms the need for management of shallow groundwater levels in areas where groundwater contributes to baseflow. To ascertain the effectiveness of remedial measures continued monitoring of A von River baseflow and shallow groundwater levels are necessary.