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  Services » Publications » Technical reports - by year » tr201205

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

On this page: Abstract, table of contents

Report: TR 2012/05

Author: A Haigh (Beca Infrastructure Ltd)


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 (2 mb)

Table of contents

  Executive summary 7
1 Document references 8
2 Objectives of the calibration study 9
3 Sources of information for the calibration study 10
4 Review of memo "Communities service by on-site wastewater systems in the Waikato region" 11
4.1 Overview 11
4.2 Review results 11
4.2.1 Review of phase one influencing factors and any new factors 11
4.2.2 Comparison of observations in report to the phase one model 12
4.2.3 Commentary onthe application of the memo information to the calibration study 15
4.2.4 Memo review results and comparison to phase one modelled results for each community 16
5 Review of on-site wastewater/sanitary survey/reports 20
5.1 Overview 20
5.2 Review of reports 20
5.2.1 Whale Bay sanitary survey 2004 20
5.2.2 Appendix B of the Piopio onsite survey 21
5.2.3 Tauwhare sanitary works subsidy application. Preliminary AEE and health impact assessment (September 2004) 22
5.3 Commentary on OWTS surveys and reports 23
6 Results of field surveys for two communities 25
6.1 Overview 25
6.2 Location plan 26
6.3 Field survey data 27
6.3.1 Field surveys 27
6.3.2 Data supplied 27
6.3.3 Processing data for use in GIS 27
6.4 Ngarimu results 31
6.4.1 Overall results 31
6.4.2 Looking at contributing factors to system performance 35
6.4.3 Looking at indicators of system non-performance 40
6.5 Hatepe results 47
6.5.1 Overall results 47
6.5.2 Looking at contributing factors to system performance 51
6.5.3 Looking at indicators of system non-performance 56
6.6 Commentary on field survey results in relation to calibration study 62
7 Responses to household surveys for two communities 63
7.1 Overview 63
7.2 Household survey data 63
7.2.1 Questionnaire 63
7.2.2 Processing data for use 63
7.3 Ngarimu results 65
7.3.1 Questionnaire response spread 65
7.3.2 Response broken down by question 66
7.4 Hatepe results 71
7.4.1 Questionnaire response spread 71
7.4.2 Response broken down by question 72
7.5 Commentary on household survey results in relation to calibration study 77
8 Investigation into the relationship with the Deprivation Index 78
8.1 Overview 78
8.2 The Deprivation Index 78
8.3 Applciation of the Deprivation Index to the OWTS phase one model 79
8.3.1 Consideration of weighted averages 79
8.3.2 Application of weighted averages to communities 79
8.4 Commentary on the application of the Deprivation Index to the calibration study 86



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