Groundwater contamination in the Heathcote/Woolston area, Christchurch, New Zealand

Abstract

Christchurch City (population 360,000) depends entirely on an underlying stratified, leaky, confined, artesian aquifer system to provide untreated water for its residents and industries.

Concerns have been raised by the Canterbury Regional Council about brackish water entering the aquifer system in a localised area in the south-eastern part of the City (Woolston/Heathcote). Due to the coastal, urban, and geological, setting of the area several possible groundwater contaminant sources exist and needed to be investigated.

These include: seawater, urban wastes, thermal groundwater, and connate seawater. A potentiometric survey carried out in the area, combined with water quality sampling, hydrogeological information from previous studies, and previously obtained water quality data, provided the basis for a conceptual model of groundwater contamination. Downward leakage of estuarine water through the confining layer appears to be the dominant contaminant source.

In the past, the potential risk of seawater intrusion has been regarded as low for the Christchurch artesian aquifer system. The freshwater/seawater interface was considered to be located 40km offshore where the uppermost confined aquifer intersects with the sea at its submarine outcrop. To enhance the understanding of freshwater and saltwater flow dynamics of the aquifer system, a steady-state crosssectional finite-difference model along the coast of Christchurch has been constructed and calibrated. The modelling indicated that the location of the freshwater/seawater interface is dominated by leakage from the sea through the confining layer and not, as presumed before, by lateral inflow of seawater through the offshore outcrop. Consequently the interface location is to be expected much closer to the shoreline at approximately 3km offshore.

Groundwater contamination in a localised area in Christchurch has demonstrated that the uppermost confining layer does not act as an effective barrier towards seawater intrusion where the hydraulic gradient between the sea and the aquifer is directed downward. A groundwater level and quality monitoring network, and a groundwater model specific to the study area, have been constructed to facilitate the future management of the resource. Immediate pumping restrictions are needed on 3 major abstraction wells to increase potentiometric heads that currently sit below sea level. An upward hydraulic gradient between the uppermost aquifer, the estuary, and the confining layer, is essential to protect the aquifer from ongoing downward leakage of saline contaminant sources. Ongoing monitoring of water levels and groundwater quality is recommended. This data will allow more refined modelling of management scenarios.