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Soil information for on-site effluent management

G.A. Chapman, J.A. Edye, S. Kenway1, H.B. Milford, C.L. Murphy, A.J.E. McGaw and N.A. Simons

New South Wales Department of Land and Water Conservation
P.O. Box 3720, Parramatta NSW 2124
(02) 9895 6172, (02) 9895 7985

Brown & Root Services Asia Pacific
299 Coronation Drive, Milton QLD 4064
(07) 3368 9228, (07) 3368 9229


On-site domestic sewage disposal systems (largely septic systems) service more than 250 000 households in NSW. Experience has shown these facilities fail regularly or periodically due to problems with location, design, or management. This poses a potential environmental threat through off-site and down-stream pollution by sewage pathogens and high-nutrient runoff. The need has therefore arisen for an evaluation scheme to assess current and future risks in areas with decentralised sewage management systems.

Soil is a key determinant of a site’s suitability for on-site sewage disposal. The soil must be of sufficient depth and permeability to filter and adsorb the pathogens and nutrients released from digestion tanks. Septic tank problems can frequently be avoided by careful assessment of soil and landscape resources at the site.

Soil Landscape maps produced by the Department of Land and Water Conservation (DLWC) provide details about soil and landform features across most areas in NSW where on-site domestic effluent failure risks are highest.

This paper outlines a collaborative project, the On-site Sewage Risk Assessment System (OSRAS), managed by the Department of Local Government in consultation with NSW Health. Brown & Root Services undertook the project using Soil Landscape maps anf technical advice provided by DLWC, in a Geographic Information System-based site assessment scheme that classified failure risk levels. These could then provide the basis for an objective risk assessment for on-site sewage management systems. This paper provides a summary of the input of soil data to the project by DLWC.


On-site sewage treatment in New South Wales

Many areas of NSW, particularly rural and remote districts, are not serviced by centralised sewerage systems. Because the expense of constructing the infrastructure makes this prohibitive, households have a number of alternatives by which to dispose of their household wastes. The most widely adopted sewage treatment systems are septic systems and Aerated Water Treatment Systems (AWTS), both of which involve digestion of the household waste in a tank (anaerobic and aerated, respectively) to retain and break down solids, leaving a liquid effluent. This effluent is either removed from the site, treated further, or, in most cases, applied to the land in a soil adsorption system (Department of Local Government et. al., 1998).

The liquid effluent varies in composition, as summarised in Table 1. AWTS systems, with their additional treatment in an aerated tank, achieve a higher-quality effluent than septic tanks if properly managed. However, the effluent remains infectious, often with high nutrient levels.

Table 1. Typical analysis of effluent before and after treatment (Department of Local Government et. al., 1998).


Untreated wastewater

After septic tank treatment

After AWTS treatment

Biochemical oxygen demanda, mg/L




Suspended solids, mg/L




Total nitrogen, mg/L




Total Phosphorus, mg/L




Faecal coliformsb, cfu/100mL




a A measure of the dissolved oxygen required for breakdown of organic matter in the effluent.
A type of bacteria found in human excreta, indicative of contamination.

This material is applied to the soil absorption system. The soil filters the effluent and adsorbs the nutrients and pathogens in the waste. In some cases, material may be added to the soil to increase its absorbency or a watertight membrane installed beneath the soil to direct the flow of effluent.

Soil and site features and effluent disposal

It is clear that the soil’s characteristics play an important role in the eventual removal of the waste. Soil depth and coarse fragment amount will determine the volume of soil available for adsorption. Insufficient soil volume may lead to waterlogging or excessive runoff. Soil permeability will determine the flow of water able to be handled by the soil, and over the long term, must at least match the combined effluent and precipitation inputs. Over-permeable soil may result in contaminants reaching the water table, while impermeable soil may cause water to accumulate or run off the surface. Bulk density and soil texture help determine permeability. Chemical characteristics such as electrical conductivity, exchangeable sodium percentage, cation exchange capacity, and phosphorus sorption will determine the capacity of the soil to store nutrients from the effluent. A summary of important soil characteristics and their limiting levels is given in Table 2.

Table 2. Soil features determining the effectiveness of effluent adsorption.

Soil feature

Capable level

Marginal level

Unsuitable level

Depth to bedrock or hard pan, m

> 1.2

1.0 – 1.2

< 1.0

Depth to water table, m

> 1.2

1.0 – 1.2

< 1.0

Soil permeability

Moderate, Slow


Very slow, Fast

Coarse fragments, %

0 – 20

20 – 50

50 – 100

Bulk density, g/cm3

Sandy loam: < 1.8
Loam: < 1.6
Clay: < 1.4


Sandy loam: > 1.8
Loam: > 1.6
Clay: > 1.4

Electrical conductivity, dS/m

< 4

4 – 8

> 8

Exchangeable sodium percentage

0 – 5

5 – 10

> 10

Cation exchange capacity, cmol+/kg

> 15

5 – 15

< 5

Phosphorus sorption, kg/ha

> 6000

2000 – 6000

< 2000

Emerson Aggregate test (dispersion)

Class 3 or 4

Class 2

Class 1

It is the most limiting soil feature that will determine the site’s suitability for land application of effluent. In some cases, specialised system design or site modifications may be possible to overcome such problems at additional expense (DLG et. al., 1998).

Site characteristics also play an important role on the effectiveness of an on-site sewage management system. Slope must be shallow enough to avoid effluent runoff. Areas prone to regular flooding must be avoided, and site drainage must be sufficiently effective to allow movement of effluent away from the application area. There must also be a buffer distance from waterways and drainage channels.

Many cases exist, however, where on-site sewage systems have been installed without assessment of soil or site characteristics. Aside from malfunctioning of the system components, this is the primary cause of system failure.

Failure of effluent disposal systems

Problems caused by failure of on-site sewage management systems occur when nutrients or pathogens escape untreated from the system. This may be through surface runoff or via the water table or subsoil flow, and is particularly frequent during periods of heavy rain when the hydraulic load on the absorption area is high. Failure rates, however, may be up to 90% regardless of rainfall (Page, 1998). Downstream contamination by pathogens poses a serious health risk through direct contact or contamination of foodstuff. An example of an episode of pathogen contamination occurred at Wallis Lake in 1997, where oyster beds received contaminated flow. Faecal pathogen contamination of the oysters resulted in illness for a number of consumers (Page, 1998). Contamination by nutrients can seriously degrade water quality in waterways, leading to excessive plant and algae growth and ecosystem imbalance. Some environments, such as areas enclosed in National Parks, are particularly sensitive to nutrient contamination. High-nutrient urban and agricultural run-off is a major cause of proliferation of exotic weeds in bushland (Handreck, 1997). Various native plant species, such as Banksia, are intolerant of high nutrient levels in soils, with particular sensitivity to additional phosphorus (Handreck, 1997).

Finding solutions

In view of these problems, it was deemed prudent for more rigorous and regular checks to be carried out to ensure proper functioning of domestic wastewater treatment systems. Depending on the siting of individual systems and the risks and consequences of off-site pollution, however, there will be a varying need for inspections. To assist councils with better risk management, the Department of Local Government (DLG) introduced a system of site and soil assessment known as the On-site Sewage Risk Assessment System (OSRAS). This system involved a spatial analysis of land packages to classify their off-site risk class for contamination by nutrients or pathogens.

In high risk areas the Council may determine that septic systems require regular checks to ensure that they are working properly and that sewage pollution is not occurring. In medium risk areas landowners may be asked to arrange regular function checks themselves and to report the results to the Council from time to time. In lower risk areas Councils may provide long term approvals or conditional exemption from approval provided landowners keep systems well (DLG, 2001a).

The NSW Department of Land and Water Conservation (DLWC) produces maps of soil properties in its Soil Landscape mapping program that cover most areas of NSW where septic system failure rates are potentially highest. The following section outlines the use of Soil Landscape maps in the OSRAS scheme to identify on-site sewage hazards associated with natural features.


DLWC assisted the study team in the development of an appropriate methodology for the OSRAS. Soil Landscape data produced by DLWC were used to produce a number of derivative maps in the process of identifying soil-related hazards. The contribution of these within the general analysis process will be outlined here.

Soil hazard class

Soil hazard class is used, together with slope hazard and climate hazard, to determine the on-site natural hazard class. The analysis process for soil hazard class will be described here. It is summarised in Figure 1.

Figure 1. Process for calculation of soil hazard class (DLG, 2001b).

Soil hazard class is an indication of the site’s likelihood of on-site facility failure based upon the site’s soil limitations. These limitations are supplied in tabular format in the Soil Landscape Reports, with limitations specific to Soil Landscapes and to their component Soil Materials (typical soil horizons or layers) listed separately.

An on-site soil limitation is a property of the site’s Soil Material or Soil Landscape that may increase the likelihood of on-site facility failure (DLG, 2001b). In order to determine the soil hazard class of an area, Soil Material and Soil Landscape limitations are analysed. Application of the classification in a spatial environment provides a map of the relative hazard for on-site facilities.

The following Soil Landscape limitations, published in Soil Landscape Reports, are considered to have particular relevance to failure of on-site facilities (from DLG, 2001b):

  • Flood hazard: Indication of risk of facility inundation and wastewater escape directly to surface waters.
  • Run-on: Indication of enhanced natural risk of facility inundation through run-on from more elevated areas and subsequent wastewater escape.
  • Waterlogging (either permanent or seasonal): Indication of internal drainage characteristics of the landscape which may lead to risk of surface discharge of wastewater.
  • Permanently high water tables: Indication of limitation of available soil moisture holding capacity and risk of groundwater contamination.
  • Shallow soil: Indication of inability of landscape to accept and store wastewater and risk of loss from site.
  • Rock outcrop: Indication of limited ability to assimilate and hold wastewater.
  • Slope: Indication of potential for wastewater runoff. Slope is used at this point in the analysis only where a digital elevation model analysis is not available.

Soil Material limitations included in the analysis were (from DLG, 2001b):

  • Shrink/swell: Soils have limited permeability, may crack to depth when dry and may damage on-site facility leading to failure.
  • Sodicity/dispersibility: Cause of low permeability, and indication that soil permeability may continue to decline in presence of sewage.
  • Low available water holding capacity: Limited ability to hold and assimilate wastewater.
  • Low permeability/High permeability: Indication of potential for surface or groundwater discharge of sewage.
  • Salinity: Restriction of plant growth and indication of seasonally high water table,and low internal drainage.
  • Fertility: Indication of limiting soil layers ability to immobilise nutrients.

Soil Materials are assessed as the average of limitations. This is intended to reflect the manageability of Soil Materials compared to Soil Landscapes, whose features are much more difficult to alter. Averaging this limitation indicates that locations where there are a few limitations will be more manageable, and economic to develop, than locations where there are more limitations (DLG, 2001b).

The limitations noted here are a subset of the limitations listed in the Soil Landscape Report. Where present, each is described as having a localised or generalised occurrence across the soil landscape.

Soil Material and Soil Landscape limitations are placed in a matrix of limitation type against severity to determine the overall soil hazard rating for the map unit. The soil classification is the higher of either any individual Soil Landscape limitation, or the average of Soil Material limitations.

A map can then be made of soil hazard rating for the area of interest.

Soil Landscape data is the primary source of data for the soil hazard classification. The Soil Landscape Mapping program involves the stepwise mapping of soils across coastal and central NSW and is expected to cover most areas where on-site sewage system failure rates and their cumulative impacts are highest. Soil landscape unit boundaries are usually mapped at 1:25 000 scale for publication at 1:100 000 scale. Recently, a higher degree of detail has been added to these maps in the form of Soil Landscape Facet descriptions, where different sub-sections of Soil Landscapes are characterised (at approximately 1:25 000 scale) where they can be further divided by landform pattern or other means (Edye et. al., 2001). This further improves the suitability of Soil Landscape maps for effluent management purposes.

On-site natural hazard classification

The on-site natural hazard classification has been developed using a five-class system similar to that used with Urban Capability Classes (NSW EPA, 1998). The classes developed in the OSRAS are summarised in Table 3.

Table 3. On-site natural hazard classification (DLG et. al., 2001).




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.


Minor likelihood of loss of sewage to surface or groundwater from a well-designed and managed facility, and minor physical limitations to sewage disposal.


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.


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.


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.

After soil, slope and climate have been classified, on-site natural hazard class is determined with an on-site natural hazard matrix. This places each soil hazard class against each slope class, providing the logic to classify the combinations into on-site natural hazard classes (DLG, 2001b). Different matrices are used depending on the climate classification of the area, as this will partially determine the level of limitations existing. An example of this matrix for climates with low rainfall variability (<1000mm) is given in Table 4.

Table 4. On-site natural hazard class matrix for climate classification of low rainfall variability (DLG, 2001b).

Soil classification

Slope classification






Little limitation (1)






Minor limitation (2)






Moderate limitation (3)






High limitation (4)






Severe limitation (5)






The remaining processes in the OSRAS

Once soils data have been classified in terms of natural hazard the OSRAS Handbook (DLG 2001b) provides guidance on the spatial analysis of environmental sensitivity to effluent. It also clarifies GIS applications to analysis of the cumulative effects of catchments and the subsequent estimation of downstream risk. This is further outlined in OSRAS Handbook (DLG 2001b) and an earlier version is summarised in Hillier and Kenway (2000).

The GIS-based identification process for Off-site Risk Classes is summarised in Figure 2.

On-site natural hazard class maps, from the steps described previously, are overlaid with built hazard classes which draw together data about sewage treatment system type, failure rate, lot size, reticulated water and sewerage. The result is the on-site sewage export hazard class. Estimations of environmental sensitivity, from a health and natural environment point of view, are made by mapping sensitive locations such as shallow groundwater bores, drinking water catchments, oyster leases, wetlands, National Parks, and sensitive vegetation. This gives maps of nutrient sensitivity and pathogen sensitivity.

On-site sewage export hazard class maps and catchment/drainage line maps are then overlaid with each of these sensitivity maps. Cumulative contributions from individual systems are calculated for flow paths in catchments. The accumulated result is off-site risk maps for nutrient contamination and pathogen contamination.

Figure 2. Overview of the On-site Sewage Risk Assessment System (DLG, 2001b).

Case Study – Katoomba area

Two case studies, one in the Katoomba area and one in the Eurobodalla Shire, were reviewed during the OSRAS development project.

The Katoomba area is located in the Blue Mountains, west of Sydney, NSW. Due to its proximity to the Blue Mountains National Park, there exist a number of considerable environmental values sensitive to effluent pollution. The area was chosen because of the high level of spatial data relevant to on-site facilities (DLG, 2001b).

Failure of on-site sewage treatment systems in the Katoomba area has a number of local impacts. Pollution of surface- and ground water is a hazard for local drinking water storage and for Warragamba Dam, which supplies drinking water for the Sydney region and is located within the same catchment as the Katoomba area. Ecological degradation by nutrient pollution is a hazard for surrounding National Parks and sensitive vegetation such as hanging swamps (DLG, 2001b).

The Soil Landscape Mapping Program within DLWC includes the Katoomba area (Edye et. al., 2001). Soil landscapes were mapped at a 1:100 000 scale across the study area. Soil Landscapes in the area included Mount Sinai (ms), Medlow Bath (mb), Deanes Creek (dc), Wollongambe (wo), Warragamba (wb), and disturbed terrain (xx). These are represented for part of the Katoomba area in Figure 3.

Figure 3. Soil Landscapes in a subset of the Katoomba area.

A summary of the features of these soil classes is given in Table 5.

Table 5. Summary of limitations of Soil Landscapes in the Katoomba area (King, 1994, DLG, 2001b).

Soil Landscape

Major limitations

Soil hazard class

Medlow bath (mb)

Localised shallow soils
Low fertility
High permeability


Mount Sinai (ms)

Generalised shallow soils
Severe low fertility
High permeability


Warragamba (wb)

Localised shallow soils
Low permeability
Low fertility


Wollongambe (wo)

Generalised shallow soils
Localised sodicity
Low fertility


Deanes Creek (dc)

Generalised waterlogging
Generalised permanently high watertable
Low permeability
Low fertility


The hazard classes of these soils are shown in Figure 4.

Figure 4. Soil hazard classes of Soil Landscapes in a subset of the Katoomba case study area (DLG 2001b).

Soil hazard was combined with slope hazard class in a matrix developed for high climate variability. This means there were large areas of the case study exhibiting high or severe natural hazard. Areas with better soils or shallower slopes had less significant hazards. The majority of minor and moderate hazard areas identified are located within existing urban development areas. Areas of lower hazard class were also identified in limited areas along drainage lines and ridge-tops (DLG, 2001b). These findings are summarised in Figure 5.

Figure 5. On-site natural hazard class for part of the Katoomba case study area, the results of the analysis of soil, climate and landscape data (DLG 2001b).

This classification went on to be combined with environmental sensitivity and catchment/drainage line data to produce off-site risk classes for the Katoomba area. The results of this section of the study can be found in DLG, 2001b.


It has been demonstrated here that soil data is an important input to spatial assessment of suitability for on-site sewage treatment systems. The Katoomba case study displayed the use of Soil Landscape data in the initial assessment of on-site natural hazards (DLG, 2001b). DLWC is in an excellent position to define biophysical risks across much of NSW using this information.

In addition to Soil Landscape data, additional spatial data is required for the built environment as well as parameters for the receiving environment for sensitive receptors. This enables analysis of cumulative off-site or receptor risk areas, which, together with soil hazard information, can be used to guide improved management of on-site facilities.

The spatial nature of these assessments also means that costs involved in the personal assessment of individual systems is greatly reduced. Recommendations can be made and carried out based primarily on data computed by the GIS from a central location.

Recommendations and further steps

These assessments can be furthered to derive potential suggested regimes for inspection and management for use by local councils. Soil Landscape data is readily available for large areas of NSW (Edye et. al., 2001), which means the OSRAS scheme can be applied, as part of the NSW Septic Safe initiative, to many local council areas faced with environmental or health issues due to mismanaged on-site sewage systems.

Soil Landscape data at its varying scales has the potential to offer higher levels of information for such projects as the OSRAS. Sophisticated derivative mapping, analysis and manipulation of Soil Landscape data open the potential for further advances into areas such as water balance modelling.

The utility of DLWC Soil Landscape maps is being further improved by the division of Soil Landscapes into Facets or Sub-landscapes, as previously described. In most cases, the Soil Landscape Report outlines patterns of soil variation across the landscape and gives reasons for these patterns. For example, a single soil landscape may consist of shallow Rudosols on crests, texture-contrasted Chromosols on slopes, and poorly-drained Kandosols in drainage depressions. Facet divisions are often landform-related and therefore mappable on a more detailed level (approximately 1:25 000 scale). Because it is desirable for this to be done semi-automatically through use of Digital Elevation Models and remotely-sensed data, however, development of technology for widespread mapping of this type remains a step to be addressed in the future.


Appreciation is expressed to the NSW Department of Local Government and the project manager of the OSRAS project Mr Robert Irvine for provision of the OSRAS handbook to DLWC from which much of this paper has been drawn. The collaboration between the Department of Local Government and the DLWC soils unit on this project has been highly valued.


Department of Local Government (2001) Septic Safe – Protect your health and environment. At, access date 18th May 2001.

Department of Local Government (April 2001b) On-site Sewage Risk Assessment System Handbook (Consultation Draft). Developed by Brown & Root Services Asia Pacific Pty Ltd for the New South Wales Department of Local Government. ISBN 1 876821 19 1.

Department of Local Government, NSW Environmental Protection Agency, NSW Health, NSW Department of Land & Water Conservation, and Department of Urban Affairs & Planning (1998) On-site Sewage Management for Single Households. February 1998.

Edye, J.A., Murphy, C.L., Chapman, G.A., Milford, H.B., McGaw, A.J.E., Macleod, A.P., and Simons, N.A. (2001) What’s in a Landscape? Describing natural resources with Soil Landscapes. In Proceedings of the Geospatial Information and Agriculture Symposium, Sydney 2001.

Handreck, K. A. (1997) Phosphorus requirements of Australian native plants. Australian Journal of Soil Research, 35, 241-289.

Hillier, H and Kenway, S.J. (2001) On Site Sewage Risk Assessment System. In: Ozwater Australian Water Association Conference Proceedings. April 2001.

King, D.P. (1994) Soil Landscapes of the Katoomba 1:100 000 Sheet. Department of Conservation and Land Management, Sydney.

NSW Environment Protection Authority (1998) Managing urban stormwater: Source control. Draft, Chatswood, NSW. December, 1998.

Page, E. (1998) Preventing Sewage Contamination. A Media Release from the Minister for Local Government, March 9, 1998, Sydney.

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