Sewage Pollution Risk Assessment for Environmental Health

Transcription

1 Sewage Pollution Risk Assessment for Environmental Health Steven Kenway 1 and Robert Irvine 2 1 Senior Environmental Scientist, Brown & Root Services Asia Pacific Pty Ltd (Brisbane) 2 Principal Policy Officer, NSW Department of Local Government ABSTRACT The tremendous potential for Geographic Information Systems to benefit the management of environmental health is progressively being realised. This paper summarises a new approach to mapping the risks associated with on-site sewage facilities. One of the many intended applications is to help predict and thereby manage sources of pathogens contributing to health issues. Over 284,000 on-site sewage facilities exist in NSW of which around 50 90% fail or perform inefficiently. Incidents including contamination of drinking water and aquatic food industries from failing facilities has led to the need for improved tools to manage current and future risks. This paper summarises the On-site Sewage Risk Assessment System (OSRAS) developed by Brown & Root Services for the NSW Department of Local Government. State agencies contributed data and information to case studies undertaken in the Blue Mountains and Eurobodalla. The OSRAS provides a system for classification of natural resource and other data relevant to the failure rates of facilities. These data include soils, landform slope, climate, allotment size, water reticulation and sewerage reticulation are used to rank the risk of each parcel of land and analyse layers of spatial data to identify areas most likely to have failing systems. It also classifies environmental resource data such as water supplies and food industries to map sensitivity to sewage. The OSRAS provides a software-independent and flexible framework for spatial hazard identification and subsequent risk analysis for on-site facilities. The OSRAS will assist long term management of decentralised sewage facilities, catchment management, monitoring and capability planning. It provides a new risk-based tool for improved management of the environmental health of catchments. The OSRAS is also intended to help local government and others utilise a risk-based approach to manage the performance of decentralised sewage management facilities. The system handbook and CD are available through the NSW Department of Local Government. KEY WORDS On-site sewage, risk assessment, geographic information systems, planning, decentralised sewage management, epidemiology. 1

2 WHY IS ON-SITE SEWAGE MANAGEMENT AN ISSUE? Many areas of New South Wales are not serviced by a centralised sewerage scheme. In those areas various decentralised systems of sewage management are used. The most commonly used facilities are septic tanks discharging to deep soil trenches and aerated septic systems (known as aerated wastewater treatment systems - AWTS). Both rely on primary detention, settlement and anaerobic digestion of human waste and wastewater in a tank designed to retain and break down solids and discharge a liquid effluent. Aerated septic systems provide additional aerobic digestion to reduce suspended solids and may include a final disinfection chamber. The resulting effluent is either stored for removal from site, diverted for reuse as irrigation or utility water, or applied to land in a soil adsorption system. Application to soil is intended to provide absorption of the water and nutrient components and filtering of pathogens. The liquid effluent varies in composition, but traditional and aerated septic systems typically produce effluent with up to 10 7 and 10 4 faecal coliform bacteria colony forming units per 100mL respectively. In addition to faecal and other bacteria such as salmonella, shigella and vibrio, sewage effluent may contain viruses (enterovirus), protozoa (entamoeba, cryptosporidium, giardia) and helminths. The power of mapping John Snow, an English physician provided a classic example of how mapping can be used in epidemiological research. John noted the residence of people in London afflicted with cholera on a map and used this to identify the water source responsible for the disease outbreak. Geographic information systems (GIS) combining mapping and information are used in public health for epidemiological studies. By tracking the sources of diseases and the movement of contagions, agencies can respond more effectively to outbreaks by identifying at-risk populations and targeting interventions. The need for better management of on-site sewage facilities and other diffuse pollution sources is now more widely recognised, especially as point sources such as sewage treatment plants improve discharge water quality. On-site sewage facilities that are appropriately sited, designed and managed can provide satisfactory and sustainable sanitation services. However, lack of attention in any of these areas can lead to failure and release of hazardous levels of pathogens and other contaminants. While the OSRAS assesses both health and environmental consequences of seepage and surcharge from on-site facilities, this paper focuses on the health implications. HOW DOES THE SYSTEM WORK? Failure, hazard and risk Risk analysis requires consideration of potential hazards and the consequence of those hazards. The OSRAS defines failure of an on-site sewage facility as unacceptable surcharge or seepage of effluent from a designated land application area. Failure constitutes a hazard for downstream area. Figure 1 presents the failure mode model for on-site sewage facilities and indicates the various failure modes. 2

3 FIG. 1 - OSRAS SCHEMATIC OUTLINE The OSRAS provides a framework for the classification and analysis of spatial data relevant to on-site sewage management. Natural hazard maps and logic matrixes are used to identify and classify the risk of on-site facility failure. Spatial analysis tools are used to track potential pollutants in runoff as they accumulate in drainage lines. The OSRAS handbook includes case study examples undertaken in the Blue Mountains (Katoomba area) and Eurobodalla Shire (Tuross estuary). On-site natural hazard Natural site characteristics determine the natural ability of a site to assimilate effluent without loss to surface or groundwater. Layers of data describing soil, landform and slope are used to classify and map the on-site natural hazard class. Climate is critical to the failure rate facilities. As rainfall variability is highly correlated to surcharge from septic tank trenches variability is used to qualify hazard levels. The natural hazard class indicators reflect the difficulty of adsorbing or assimilating sewage effluent within a facility. The mapping of natural hazard indicators using soil landscape and other information has been described in Chapman, et al (2001). Five classes of natural hazard were developed to augment the urban land capability classes. These are shown in Table 1 below. 3

4 Table 1 On-site natural hazard classification Class Description 1 Minimal likelihood of loss of sewage to surface or groundwater from a well-designed and managed facility, and little or no physical limitation to on-site sewage disposal. 2 Minor likelihood of loss of sewage to surface or groundwater from a well-designed and managed facility, and minor physical limitations to sewage disposal. 3 Moderate likelihood of loss of sewage to surface or groundwater from a well-designed and managed facility, and moderate physical limitations to on-site sewage disposal. 4 High likelihood of loss of sewage to surface or groundwater from a well-designed and managed facility, and high physical limitations to on-site sewage disposal. 5 Severe likelihood of loss of sewage to surface or groundwater from a well-designed and managed facility, and severe physical limitation to on-site sewage disposal. On-site sewage export hazard The likelihood of sewage effluent being exported from an on-site facility is also influenced by allotment size, access to water service infrastructure and other built environment elements. Small allotments have less area to assimilate wastewater, and premises with water supply or surface or bore water sources can produce 30% more wastewater than premises relying primarily on rainwater collection. Export hazard is mapped by overlaying natural hazard maps with relevant infrastructure characteristics including lot size and access to water. Assessment of 'sustainable' effluent land application areas size is an important consideration for settlement planning in unsewered areas and independent modelling is recommended. Spatial data and information on system design, management and performance (e.g. failure rates) are documented as potential elements of OSRAS, however they have not yet been developed due to the paucity of available spatial and performance data. Environmental sensitivity and receptor risk Sensitivity analysis and mapping is required to determine the consequence of failure of on-site facilities in a particular catchment and is a precursor to determining off-site risks. Key values, the guideline for protection of the value and the relative reduction in septic tank effluent to achieve the value are presented in Table 2. Table 2 Indicative environmental sensitivity to pathogens (surface water focus) Pathogen sensitivity class Examples of environmental values Threshold guidelines to faecal coliform bacteria* (cfu/100 ml) Relative reduction required from septic tank effluent** Very high Raw water supplies < High Aquatic foods (oyster <14 7, ,000 leases/shellfish production areas) Moderate Primary contact recreation < ,000 Low Secondary contact recreation areas <1, ,000 * After ANZECC 1992 (sampling frequencies provided). ** Assuming septic tank effluent cfu/100 ml. 4

5 Contamination of raw water supplies with pathogens can constitute significant risk, particularly if a large population is serviced by the storage. Aquatic food industries including oyster, crayfish and crustacea, are sensitive to sewage pollution particularly when located in estuaries with relatively poor water circulation. Groundwater resources are also sensitive to impacts from on-site facilities for both pathogen and nutrient-related factors. Due to the slower movement of groundwater as opposed to surface water, groundwater has a lower spatial sensitivity to surface water values. Cumulative effects in catchments Catchment boundaries influence the flow, accumulation and die-off of emitted pollutants. This in turn affects receptor risk. The cumulative risk in drainage lines is mapped by hydrological (flow-direction) tracking of emitted pollutants using outputs from a Digital Elevation Model. This is used to classify the number of units risk upstream of any point in the drainage network. This can be used to assess factors such as the likelihood of contamination of particular streams, groundwater reserves or areas of environmental value such as wetlands. APPLICATIONS AND BENEFITS OF THE OSRAS The OSRAS has numerous potential uses outlined in Table 3. The OSRAS has been developed primarily to facilitate improved sewage management in a total catchment management context. This will have considerable environmental health benefits. Table 3 Use and benefits of the On-site Sewage Risk Assessment System The OSRAS provides: The benefits are: a process for risk-based management of decentralised sewage facilities. a tool for rapid and repeatable spatial analysis of sewage pollution risks. a process to classify and locate areas likely to contribute sewage pollution. a process to locate areas most likely to be affected by on-site pollution. sewage pollution impact assessment provides key information for settlement capability planning. guidance on performance expectations for on-site systems. information on cumulative catchment impacts of on-site sewage pollution. helps councils to communicate with affected residents and better target resources to achieve desired sewage management outcomes. consistent and economic mapping of risk will help focus management on highest priorities. by identifying high-risk areas inspection programs can be targeted and sewage infrastructure resource allocation justified. by identifying health and environmental risks (e.g. streams, groundwater bores, wetlands) attention can be directed to those most in need of monitoring and management. the open, consistent and accountable nature of the OSRAS gives regulatory authorities and developers a new tool for consistent decisions on settlement capability assessment and sustainable siting and layout of subdivisions. an objective tool for performance requirements tailored to the needs of specific locations. maps of risk are excellent for communication with and canvassing community expectations. 5

6 The OSRAS is not a computerised decision-making system but a way to present complex data as useful information. OSRAS facilitates the collation and presentation of data to inform sustainable decentralised sewage management. Application to environmental health Many of the applications discussed above will have benefits for improved environmental health outcomes. Improved management of diffuse sources including on-site facilities first requires locating these sources and their significance. This is central to the OSRAS. Mapping of risk enables focussed management and subsequent risk control. This may be achieved through helping both regulators and landholders identify locations at risk of on-site pollution. Geographic Information Systems (GIS) present the greatest potential benefits when assessing, storing, retrieving, querying, overlaying and presenting spatial datasets. With the use of integrated logic matrixes, algorithms and hydrological applications, GIS can process complex data in highly informative ways. The OSRAS first maps pathogen sources. This data, for example, coupled with epidemiological data could enable spatial statistical assessment of incidents and is an avenue yet to be explored. OTHER APPROACHES TO ON-SITE SEWAGE RISK ASSESSMENT There are a range water related risk analysis tools and decision support systems in common use at the catchment level. These include generic checklists, spreadsheet based prioritisation models, water quality models, a range of complex black box models and good old seat of the pants decision making. OSRAS takes a spatial analysis approach that builds on councils existing competence in land use and environmental health assessment and management. OSRAS uses existing spatial and built environment data overlays and transparent logic matrixes. These data are used to generate maps that highlight areas where on-site facilities are likely to be exporting concentrations of pollutants capable of harming identified receptors of high value to the community. The OSRAS presents a new approach to mapping of risk which can be used to direct on-ground assessment and management actions. For example, water quality models and other black-box approaches can only guess at locations of potential pollutant sources OSRAS identifies where they are most likely to be. Once located, generic checklists can strategically be used on-ground to confirm risks and prescribe required management measures. Most importantly, the OSRAS provides an independent and accountable approach for those wanting to protect the seat of their pants. The OSRAS is intended to facilitate the transformation of complex data and knowledge into widely accessible 'big picture' information. OSRAS is not a substitute for sanitation surveys and site inspection but it can help show which sites to look at. For councils supervising up to 12,000 septic systems, this is important information. 6

7 Options for further development of the OSRAS methodology The OSRAS was designed to facilitate flexible use and continual improvement. Some of the areas of potential further development include: broadening scope to consider other catchment hazards (e.g. dairy yard wastes); consideration of design related system performance improvements and of risk attenuation processes for emitted pollutants; and linkage with economic models The OSRAS focuses on on-site sewage facilities. Impacts attributable to on-site sewage may not be easily separated from other catchment processes and sources. For example, wastewater treatment plants, intensive agricultural industry, stormwater, pump stations and other areas of human activity can also introduce nutrients and pathogens to waterways. Many other hazards can be added to obtain a more complete picture of catchment hazard and resultant risk. While this is complex, OSRAS provides a conceptual framework for a common water quality risk assessment tool. The current approach to risk assessment is conservative and does not account for attenuation or risk reduction other than through consideration of distance. Die-off or decay of pollutants can be influenced by both pollutant type and local environmental conditions and these processes could be incorporated into the OSRAS if warranted. Dilution is particularly relevant for chemical pollutants, such as nutrients. Decay or die-off is particularly relevant for biological pollutants, such as pathogens. These approaches have not been documented in OSRAS. However, it is important to note their potential power for future drainage line risk analysis. Utilising such approaches within a GIS would further improve the ability to evaluate and better manage risks. Ultimately, decisions on tolerance of risk require socio-economic consideration. Cost/benefit analysis of potential solutions can be undertaken to assess risks and related economic, social and environmental impacts. Further investigation of the socio-economic impact of different risk scenarios will help clarify this. Linkage of the OSRAS to an economic risk model, could assist management decisions regarding supervision and upgrading of facilities. Finally, the quality assurance processes for effective use of GIS datasets are important. This will require genuine management commitment for OSRAS to be applied effectively, to its fullest potential. THE FUTURE OF OSRAS? The On-site Sewage Risk Assessment System (OSRAS) is proposed to provide the basic risk assessment methodology for stage II of the NSW Septic Safe Program, involving a systematic sewage pollution risk assessment in NSW coastal catchments. Regional OSRAS projects will train and coordinate state agencies, councils and regional consultants in relevant data collection, mapping and assessment tasks. The OSRAS handbook on CD is available through the NSW Department of Local Government. The OSRAS handbook may also be downloaded from the Septic Safe website 7

8 The OSRAS as a conceptual model is free to anyone who wishes to use it. The system as described relies substantially on practical application of public health and environmental knowledge and on the use of spatial data sets. The results are closely related to the quality of these sources of information. The OSRAS is a flexible tool that provides a significant step towards improving management of on-site sewage facilities. It is particularly useful for analysing and tracking cumulative risk to downstream sites. The highly graphical nature of OSRAS provides excellent tool for analysis and communication of catchment-related issues. The OSRAS will assist sustainable catchment management, land capability planning and improved environmental monitoring programs. The OSRAS does not preclude the need for sanitation surveys and site-specific investigations for outcome monitoring and assessing development applications or re-zoning, however it greatly facilitates priority setting and delivery of front desk advice to development proponents. Acknowledgements The authors thank their employers, the Department of Local Government and Brown & Root Services Asia Pacific Pty Ltd and all those who have contributed to the development of OSRAS, particularly Helen Hillier and Greg Chapman. Specific thanks to Eurobodalla Shire and Blue Mountains City Council for their assistance and access to data. Acknowledgement is also due to the Department of Land and Water Conservation, NSW Land and Property Information and the Sydney Water Corporation for access to additional data used in the preparation of OSRAS. References AS/NZS 4360:1999 Risk Management, AS/NZS 3931:1998 Risk Analysis of Technological Systems. and AS/NZS 1547:2000 On-site wastewater management. Standards Australia. Homebush, Sydney. G. A. Chapman, J.A. Edye, S.J. Kenway, H.B. Milford, C.L. Murphy, A.J.E. McGaw, and N.A. Simons (2001). Soil information for on-site effluent management. Proceedings of Geospatial Information and Agriculture Symposium. Sydney. CSIRO Land and Water, December 1999, Audit of the Hydrological Catchments managed by the Sydney Catchment Authority: Final Report, NSW Government. Department of Local Government 1998, Environment and Health Protection Guidelines: On-site Sewage Management for Single Households. ESRI 1999 GIS for Health Care Today and Tomorrow. ArcUser April-June NSW Department of Local Government On-site Sewage Risk Assessment System Handbook (Consultation Draft). Main author Brown & Root Services Asia Pacific Pty. Ltd. ISBN Jelliffe, P. 1999, Developments in determining critical lot density for the protection of water quality. Paper presented at On-site 99 Conference. University of New England, Armidale. Hillier, H and Kenway, S.J. 2001, On-site Sewage Risk Assessment System. In: Ozwater Australian Water Association Conference Proceedings. April NSW State Mapping Advisory Committee 2000, New South Wales Mapping and Spatial Data, 1998/99/2000. Report prepared by State Mapping Advisory Committee, Land Information Centre, Department of Information Technology and Management. 8