Model scenarios for a microbial risk assessment tool

Transport of microbial pathogens is a potential risk from various land-use activities, such as application of faecal waste to land, and domestic on-site wastewater treatment system disposal fields. Risk to human health occurs when these wastes, containing pathogens, infiltrate into groundwater resources used for drinking water. In 2010, it was recognised that one particular land use, domestic septic tanks, posed a risk to the quality of groundwater. In response, the Guidelines for Separation Distances Based on Virus Transport between On-site Domestic Wastewater Systems and Wells were published (Moore et al., 2010). These guidelines, from here on referred to as the “2010” guidelines throughout this document, considered appropriate setback distances from septic tanks in order to protect drinking water sourced from wells. The guidelines calculated separation distances for domestic on-site wastewater treatment systems based on virus transport and removal in the subsurface environment.

Since the release of the 2010 guidelines, increased awareness of other potential sources of microbial groundwater contamination, not just from on-site wastewater management systems (OWMS), have become an issue worthy of consideration for many regional councils in New Zealand. The recent delineation of source protection zones for drinking water supply wells has prompted consideration of the risk from a range of activities within these zones. Regional councils need to be able to assist consent planners and rural and peri-urban communities in making decisions about the management of a range of activities near drinking water supply wells. A microbial risk assessment tool is one such tool that could be used in this context, focusing on the risk to human health from drinking-water where microbial pathogens are discharged onto or into land near a drinking-water supply well. Some existing land-use activities fall within designated drinking-water protection zones (often defined retrospectively after the activity commenced), which triggers the requirement for a resource consent. Councils need a defensible method to support any recommendations to grant or decline these consents based on quantitative risk modelling.