Waikato on-site wastewater risk assessment - phase two: factors, weighting and rating calibration study

Report: TR 2012/05

Author: A Haigh (Beca Infrastructure Ltd)

Abstract

Phase one of the on-site wastewater treatment system (OWTS) risk assessment project was undertaken by Beca on behalf of Waikato Regional Council in 2010, and modelled the risk of OWTS failure at the land parcel level for the Waikato region using a DRASTIC
modelling approach and six contributing factors:

• system age
• soil type
• lot size
• depth to high groundwater table
• aquifer conductivity and proximity to surface water.

Each contributing factor had a rating (0-5) applied to its characteristic and a weighting applied according to overall influence in the potential for OWTS failure. The factors, ratings and weightings were adopted from previous research undertaken by the Canadian Housing and Mortgage Corporation.

The objective of the phase two calibration study was to take the community results from the phase one modelling, and report and compare them to “real world” activities at a selected number of sample communities, with the purpose of highlighting areas where the model calibration could be adjusted to improve its estimation of community risk.

This report discusses in detail the investigations undertaken in the study, and comments on the findings of the investigations in relation to the calibration of the factors, ratings and weighting used in the phase one model.

Waikato on-site wastewater risk assessment - phase two: factors, weighting and rating calibration study [PDF, 2 MB]