Pinpointing Unknown On-site Wastewater Systems in Canterbury Region

Weaver L1, Orsi A1, Humphries B1, Pearson A1, Wills M2, Qiu R1

1 Institute of Environment Science and Research (ESR) 27 Creyke Road, Ilam, Christchurch 8041 

2 Environment Canterbury 200 Tuam Street Christchurch 8011

Throughout Aotearoa the locations of decentralised on-site wastewater systems are largely unknown if they were installed prior to the year 1998, because installing an on-site wastewater system did not require a resource consent before that time. Those systems are presumed to be primary treatment systems which sometimes date back to the 1950s or older. Their performance and risks to environmental and public health are poorly understood. Even for more recently installed secondary treatment systems, their risks are also poorly understood and require further research.

This paper investigates the application of Geographical Information Systems (GIS) to create a pipeline of methods to estimate the locations and number of unknown on-site wastewater systems on a regional scale based on rating units (property), building outlines, rating information alongside information of consented on-site wastewater systems. A case study of the Canterbury Region, which contains 10 districts, shows how geospatial data can assist in pinpointing the locations of on-site wastewater systems. The investigation revealed that approximately 34,265 rating units are serviced by on-site wastewater systems. Out of these, only 23.61% have active wastewater discharge consents. Around 76.39% of rating units which potentially have on-site wastewater systems were unknown to the Canterbury regional council before the investigation. The findings also revealed that on-site wastewater systems are not only located in communities without reticulated wastewater service, but also within town boundaries where reticulated wastewater infrastructure exists. Additionally, the research uncovered 862 previously unknown on-site wastewater systems in drinking-water protection zones.

The locations and number of the unknown on-site wastewater systems is vital to inform potential future locations where reticulated wastewater infrastructure investment may be required. It also provides insights into those locations which may be at risk from on-site wastewater contamination, particularly drinking water protection zones and those environments that are sensitive to changes in our climate (e.g., shallow groundwater and coastal environments).