Earthquake has little impact on Christchurch Groundwater

An investigation by Environment Canterbury has found there is unlikely to be any significant long-term impact on the status of groundwater as a result of September and February’s earthquakes.

The report, the third to be released since September relating to groundwater, used updated information to look at groundwater levels and quality.

It has confirmed earlier findings that showed no clear evidence of significant permanent change to groundwater levels or groundwater quality in Christchurch public supply wells.

Environment Canterbury’s Director Investigations and Monitoring, Ken Taylor said despite damage to the city’s sewerage system, Christchurch’s groundwater quality has not been adversely affected.

“In Christchurch, while our sewage pipes are relatively close to the surface, our public supply wells are between 16 and 177 metres deep.

“This means contamination of the deep aquifers is unlikely unless contaminated water enters the well itself and we have not found evidence of this occurring,” he said.

The report also looked at the presence of many new springs and their impact on the water available to the city. These springs may have emerged from old sealed bores, be reactivated old springs caused by higher water pressures, disruptions developed in confining layers and permeability developed in consolidated rocks.

Ken Taylor said the new springs have the potential to cause some localised disruptions.

“Springs have arisen along the margin of the loess at the base of the Port Hills which may reflect increased permeability from fracturing of the consolidated rocks of the Port Hills and where liquefaction disrupted confining layers.

“Our information suggests the new springs have the potential to cause considerable localised disruption and possible damage to some structures. Nevertheless, the combined flow of these springs is not expected to be a significant proportion of total groundwater flow in the Christchurch area.

“This means the water available to the city from its authorised allocation is unlikely to be negatively impacted by this new leakage in terms of flow rates and quantity. We believe the threat to the long-term supply of Christchurch’s groundwater is relatively low,” he said.

The latest report features more comprehensive readings following repairs to recorder wells. Ken Taylor says many wells and recorder installations suffered damage following September’s earthquake and subsequent aftershocks.

“The effort to rehabilitate wells has been significantly disrupted by the frequent aftershocks. Additional field inspection work is underway to repair wells that may have been blocked by sediment flowing into the well at depth.

“The June aftershocks caused further installation damage and the recorder network is currently operating with 50% of the capacity that existed prior to the September earthquake,” he said.

Environment Canterbury will continue to monitor groundwater sites throughout Christchurch but this will be the last report on the impact of the earthquakes on groundwater

Update #3 can be viewed on the ECAN website at: Effects on groundwater.