Unintended Consequences: Stormwater to Ground vs Groundwater Drinking Supply Protection

In Canterbury, the use of infiltration facilities (basins and rapid soakage devices) for stormwater treatment and disposal is actively promoted and is increasingly important for Territorial Authorities (TA) to manage urban stormwater. This is often to avoid or mitigate network capacity issues and surface waterway quality and quantity impacts. Discharging stormwater to ground (instead of surface water) is also generally preferred by local iwi. These infiltration facilities are typically located in areas of free draining alluvial gravels that contain aquifers that supplies drinking water to communities and private individuals.

Infiltrated stormwater has the potential to affect groundwater quality because it contains contaminants (microorganisms and dissolved metals) that can be transported into and through aquifers. Spillages and discharges of toxic substances from spills to a stormwater catchment can be a significant risk to groundwater quality.

When considering this issue for protection of public water supply bores, the findings of the Havelock North Drinking Water Inquiry (the Inquiry) are relevant to stormwater disposal to ground. The Inquiry indicates that an even greater level of conservatism should be used to protect against contamination than has previously been the case.

Protection of drinking supply protection (both public and private) has always been considered when locating stormwater infiltration facilities in Canterbury. Changes to the sensitivity of the environment and legislation changes, combined with increasing awareness of risks to drinking water supply aquifers, has the consequences that reconsenting existing infiltration facilities and consenting new infiltration facilities is becoming increasingly fraught with conservatism and has potential to have increasing costs associated with their implementation.

This paper will provide an insight of the current and increasingly prevalent issues arising in Canterbury from stormwater infiltration facility use. This will be applicable to other regions that use infiltration for stormwater management where underlying aquifers are also used as a drinking water supply. The paper will also discuss a hierarchy of the risk management controls that TA’s will need to consider to allow the long-term security, of stormwater infiltration assets.