Potential Impacts On Groundwater Quality Caused By Earthquake Induced Vertical Permeability Enhancement

Following recent major earthquakes in New Zealand there has been concern about possible groundwater quality changes due to the infiltration of pollutants from the soil or ground surface. Various lines of evidence have pointed towards the possibility of recharge or sub-vertical flow following recent New Zealand earthquakes including:

Increased turbidity

Changes in water chemistry

E Coli or total coliform detections

Increased nitrate concentrations

Changes in groundwater level pressure between multi-level piezometers

Artificial pathways may develop due to damage to well heads, infrastructure, and casing that could account for some of the observations. Earthquake-induced breaching of aquitards could also result in deeper aquifers becoming hydraulically connected to shallow aquifers. Wang et al (2016) found evidence for co-seismic breaching of aquitards in around 10% of the wells analysed after the 1999 Chi Chi earthquake in Taiwan. Evidence from our studies, including the convergence of groundwater pressures in multi-level piezometers, and modelling results that suggest leakage between layers1 supports earthquake induced vertical permeability enhancement.

Of particular concern is where there is evidence of contamination of aquifers by pathogens. Environment Canterbury reported that some wells in the Christchurch shallow aquifers had positive detections of E Coli and Total Coliforms following the Canterbury earthquakes. Wellington Water Limited observed an increase in Total Coliforms and also E Coli in three of their supply bores located within the Waiwhetu aquifer in Lower Hutt after the Kaikōura earthquake in November 2016. This increase in bacterial detection was also accompanied by increase in turbidity in some wells, and initial work suggested that this may have been a result of breaching of the confining aquitards. Further evidence of the disturbance of the aquitards is through the changes in groundwater level fluctuations in the underlying Taita Alluvium located just below confining layers, which changed from muted fluctuations to full tidally induced ones, more similar to the productive aquifer.

Breaching of aquitards and damage to wells and well heads by earthquakes could have significant implications for security of groundwater resources for potable water supply, as it has the potential to allow rapid transport of solutes and microbes from the surface to depth. Given the reliance of New Zealand on groundwater for municipal supply, it is important to consider that preferential flow paths could develop following large earthquakes. Further research is needed to answer key questions, such as: How do breaches occur? How often do they occur? How long do the increases in vertical permeability last for? Is there are simple measure to monitor or detect the occurrence of aquitard breaching?