Water Journal : Water Journal April 2012
catchment management technical features 110 APRIL 2012 water of which have a slope between 5.0% and 12.9%; 91 of which have a slope between 13.0% and 19.9%; and 89 of which have a slope greater than 20%. Based on these characteristics, slope risk rank was calculated as: 6 multiplied by a risk rank of 1; 81 multiplied by a risk rank of 2; 91 multiplied by a risk rank of 3; and 80 multiplied by a risk rank of 4 -- divided by 267. The same approach was used for the watercourse buffer risk element and an average of the slope and watercourse buffer risk rank determined for the topographic risk rank. Soil risk rank -- An average risk class was defined for a given soil class (for surface texture, susceptibility to water logging and water holding capacity) by multiplying the risk class for each "dwelling property"(which is not connected to sewer or CWMS) by the risk class; and then dividing by the total number of properties. The three individual soil class ranks were then averaged to provide an overall soil risk rank. The water quality risk for each town was then estimated by multiplying the average overall risk rank by the infectious virus load prior to on-site treatment, per day, based on the number of OWTS that have not been surveyed (all surveyed OWTS were considered to not fail either now or in the near future), and where there is no connection to CWMS. The same risk classification approach has been used for individual rural properties outside of town boundaries to enable risk mapping for the Torrens, Little Para and Onkaparinga Catchments. Results Of Risk Assessment The risk assessment results for priority towns have been briefly described within this paper, while the full results can be seen in Billington and Willis (2011). Figure 5 presents the: • Average overall site risk of OWTS failure within each town (broken down into the component parts of topographic, priority area and soil risk rank); • Relative measure of drinking water quality risk based on OWTS failures for each town; and • Estimated average infectious virus load per town per day prior to onsite treatment, to demonstrate the relative microbial loads between towns. The towns of Stirling, Aldgate, Bridgewater and Crafers are identified by the risk assessment to present the most concentrated risk within MLR Watershed. There are approximately 1,200 dwellings within the defined town boundaries that are currently not connected to sewer or CWMS. These 1,200 OWTS represent 55% of the total number of OWTS (2,200) assessed in this Review and represent a major concentration of OWTS within the watershed. The Average Overall Site Risk of OWTS failure within these four towns ranges between 6.1 and 7.0 (maximum theoretical risk of 12). When one takes into account this Site Risk and the number of dwellings with OWTS, these four towns present the highest microbial risk to drinking water quality. The most significant site risk characteristics that impact the potential for OWTS failure in this area are slope (with approximately 308 properties having the majority of the area with a slope greater than 20%), the location of towns within Priority Area 2 of the Watershed, and soil texture, which is predominately sandy loam. A total of 247 properties with OWTS have been audited (21%) within the Stirling, Aldgate, Bridgewater and Crafers area under the Project. Since the audit was completed, 67 properties have been sewered and, while this has removed some of the risk, there remains a considerable number of OWTS (approximately 991) that have not been surveyed and are not currently connected to sewer. Based on a failure rate of 41% (as identified by the Original Project survey), it can be reasonably estimated that some 406 OWTS are likely to be presently failing within these four towns. Conclusions The 10-year review of the MLR Waste Control Project has indicated that the audit of OWTS and subsequent upgrades has reduced the risk viruses by approximately 4-fold or 0.6 log10 orders. Assuming current SA Water treatment practices of 1-log for filtration and 4-log for chlorination, a deficiency of 1.2 log remains in order to meet health-based targets proposed by the USEPA. This level of viruses in the catchment is estimated to equate to 1% of background virus in the receiving community. If current upgrades continue, and long-term targets are reached, the Project would reduce virus levels affecting the drinking water supply by approximately 55-fold or 1.5 log10 orders. Spatial risk assessment has been used to identify future priority towns and rural areas within the MLR Watershed for auditing. Towns where partial sewerage networks occur were identified as a priority. The Authors Karla Billington (email: karla@naturallogic. org) is a water resource management consultant and Director of Natural Logic. Dr Daniel Deere (email: dan@waterfutures. net.au) is Director of Water Futures. References AGWR, 2006: Australian Guidelines For Water Recycling: Managing Health and Environmental Risks (Phase 1). Natural Resource Management Ministerial Council, Environment Protection And Heritage Council and Australian Health Ministers Conference. Web Copy: ISBN 1 921173 06 8. Billington K & Willis D, 2011: Mount Lofty Ranges Waste Control Project -- A ten-year review. Prepared for Adelaide Hills Council. Natural Logic Australia Pty Ltd. Charles K, 2009: Quantitative Microbial Risk Assessment: a catchment management tool to delineate buffer distances for on-site sewage treatment and disposal systems in Sydney's drinking water catchments, University of New South Wales. Deere D, Ferguson C, Billington K, Wood J & Davison A, 2005: Pathogens in the Upper Torrens River Catchment. Water Futures report to SA Water and Torrens Catchment Water Management Board. Deere D, Ferguson C, Billington K, Wood J & Davison A: 2008: Pathogens in the Upper Torrens River Catchment. Water Futures report to SA Water and Torrens Catchment Water Management Board. 40 pages. EPA, 2000: The State of Health of the Mount Lofty Ranges Catchments from a Water Quality Perspective, EPA, Adelaide. Espinosaa A, Mazari-Hiriarta M, Espinosab R, Maruri-Avidalb L, Me´ndezb E & Arias C: Infectivity and genome persistence of rotavirus and astrovirus in groundwater and surface water, Water Research, 42 (2008 ), pp 2618--2628. Ferguson CM, 2005: Deterministic model of microbial sources, fate and transport: a quantitative tool for pathogen catchment budgeting. PhD. University of NSW, Sydney. Ferguson CM, Croke BFW, et al., 2010: Modelling of variations in watershed pathogen concentrations for risk management and load estimations. Denver, Colorado, The Water Research Foundation: 288. NHMRC, 2011: Health-Based Targets For Microbial Safety Of Drinking Water Supplies, Draft Discussion Paper, Australian Drinking Water Guidelines (www.nhmrc.gov.au/ guidelines/consult/consultations/draft_adwg_ guidelines.htm, viewed 20 February, 2011). USEPA, 2006: Long-term 2 Enhanced Surface Water Treatment Rule. United States Environmental Protection Agency, 5 January 2006.
Water Journal May 2012
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