Field Scale Assessment of Contaminant Transport From an On-site Wastewater Management System: Risk to Groundwater

Bronwyn Humphries, Erin McGill, Andrew Pearson, Louise Weaver - ESR

Michael Dawson (Hynds), Fiona Ambury (Whiterock Consulting Ltd), Andrew Dakers (ecoEng Ltd), Lisa Scott (Environment Canterbury)

Municipal wastewater treatment plants service nearly 75% of New Zealand’s current population with the remaining population (>1 million) relying on on-site wastewater management systems (OWMS) (GDH, 2021). This number increases throughout the year when those who typically dwell in reticulated wastewater urban areas visit peri-urban, rural or sensitive environments (e.g. holiday hotspots and national parks) where on-site wastewater systems are the only available treatment system. In New Zealand on-site wastewater systems range from servicing a single dwelling (e.g. rural home or holiday house) to a rural school, holiday park or marae with systems that can service several hundred people and experience the stress of high, variable and seasonal loads. Inadequate treatment, inappropriate land application systems, aging, poorly serviced and unmaintained systems, can result in a risk to environmental and public health. On-site wastewater systems can be located overlying shallow groundwater systems in which nearby shallow wells are utilised for drinking water, creating a potential public health issue.

The paper will present results from a pilot study tracer experiment conducted in Canterbury alluvial gravels from an on-site wastewater management system (OWMS) land application system (LAS) which allows the exploration of:

      • the fate and transport of chemical and microbiological tracers within a newly formed LAS which has not yet been             conditioned with human effluent.
      • the risk to groundwater quality down gradient from the LAS as indicated by the microbial tracers.

For years to come this research site will provide an opportunity to monitor groundwater quality down gradient from a domestic on-site wastewater system testing five different design stages in one of New Zealand’s most vulnerable and utilised groundwater aquifer types: alluvial gravels. The paper contributes to the knowledge of residence times within the unsaturated zone (vadose zone) of alluvial gravels which is crucial to understanding the fate and transport of chemical and microbial contaminants in the vadose zone where opportunities for removal occur. Information from the study site will assist and enable councils to determine the impact of on-site wastewater systems on the environment and potential risks to drinking water and ultimately public health.


579 KB
03 Nov 2022