P. M. Geary

Geotechnical aspects of domestic on‐site effluent management systems

Geotechnical aspects of domestic on‐site effluent systems (septic tanks) and their impact on groundwaters are discussed and the limited relevant literature is reviewed. While there are few Australian case studies, the early stages of an ongoing study at Dodges Ferry, Tasmania, show a connection between shallow aquifer quality, number of residences and domestic on‐site wastewater disposal practices. Of 26 groundwater samples analysed, a number fail to meet established criteria for potable use. Most samples were highly or very highly saline. This renders them unpalatable and has an adverse impact on vegetation if the water is used for irrigation. Several samples had pH less than 6.0 and those taken from shallow wells were discoloured by leached soil organic matter. Elevated nitrate levels, an indicator of contamination by sewage effluents, were found at nine locations and these were usually associated with small clusters of residences. One very high sample was clearly contaminated by effluent from an adjacent on‐site wastewater disposal system. One odorous, black organic seep was found at the foot of the dunes backing a bathing beach and this gives cause for concern that failing on‐site wastewater disposal systems are contributory to reduced bathing water quality. Faecal coliforms and Escherichia coli were not found, even in samples with the highest nitrate concentrations, suggesting that residence time has been sufficient for bacterial die‐off. Contamination of shallow aquifers is greatest where there is a high density of residences with small lots.

Ecological Sustainability and On-Site Effluent Treatment Systems

In non-sewered urban and rural residential developments, domestic wastewaters are treated and disposed of on-site. Surveys indicate that the performance of domestic systems needs to be significantly improved to overcome potential public health and nuisance problems caused by failing systems. Many current effluent disposal practices are clearly not sustainable according to the accepted definitions of ecological sustainability. It is essential that the on-site disposal of domestic effluent is considered as an integral part of the land development process. Individual soil and site assessment needs to be undertaken in the design and sizing of effluent disposal systems. An important implication from the consideration of land capability criteria for on-site effluent disposal is that individual lot sizes will need to be larger in future developments of this nature.

Ground water contamination from on-site domestic wastewater management systems in a coastal catchment

A catchment management and groundwater monitoring program has been undertaken inseveral small coastal communities approximately 35 km south east of Hobart, Tasmania,Australia. Domestic wastewater disposal is on-site with absorption systems in sandy soils,while the shallow groundwater is increasingly being used in the community for non-potableuses. Lot sizes are typically less than 800 m 2 . The coastal waters are commonly used forprimary contact recreational activities such as swimming and surfing in the summer monthsand bathing water quality during this period is of some concern.This paper presents the results from part of the groundwater monitoring program. Lowlevels of bacterial indicators have been identified in groundwater samples from 50 mmdomestic wells. Some high nitrate concentrations have been recorded, usually associatedwith clusters of houses and, from this, several high-risk areas have been identified. Inparticular some linkages between on-site systems and effluent contaminated seeps along thebeachfront have been confirmed.Whilst it does appear as though the failure of on-site systems is affecting, to a limiteddegree, the quality of shallow groundwater in the area, a clearer picture is emerging of thecapacity of this area to adequately treat the effluent load from the relatively high lotdensity. The results from this study will afford the local council and planning agenciessome useful guidelines for the future planning of developments.

Surface wetlands for the treatment of pathogens in stormwater: three case studies at Lake Macquarie, NSW, Australia.

The treatment of stormwater using surface constructed wetlands has become common in the last decades. However, the use of constructed wetlands for stormwater management has not been thoroughly evaluated in their capacity to treat microbial loads. The case studies presented in this paper are situated at Lake Macquarie, a large estuarine lagoon located approximately 150 km north of Sydney, Australia. To protect the lake ecosystem from the impact of increasing urban development, the local Council constructed numerous stormwater quality improvement devices (SQIDs) at selected locations. The SQIDs typically consisted of trash racks, gross pollutant traps and surface constructed wetlands. To evaluate the effectiveness of three of these devices in reducing faecal contamination, water samples were collected for faecal coliforms (FC) during and following rainfall at inlets and outlets of the structures. Results indicated one of the SQIDs as the most efficient for bacterial reduction, while the other two provided low or non reduction of FC. Results also illustrated dependence of bacteria reduction on flow conditions. Comparison of devices suggested that hydraulic residence times and other design parameters strongly influenced the capacity of each device to reduce FC counts during different weather conditions.

Distinguishing Wastewater Contamination From On-site Systems In Mixed Land Use Watersheds

Part of a large estuary along the eastern Australian coastline (150 kms north of Sydney, NSW) used for shellfish production has been closed to harvesting for over 18 months. Monitoring has shown viral contamination of oyster tissue on a number of occasions and surveys of drainage channels and the estuary indicate regular exceedances of bacterial standards set for shellfish growing waters, particularly following heavy rainfall. The contamination has been attributed to urban runoff, agricultural wastes and possibly failing on-site wastewater systems.

Contamination of estuaries from failing septic tank systems: difficulties in scaling up from monitored individual systems to cumulative impact

Aquaculture in many coastal estuaries is threatened by diffuse sources of runoff from different land use activities. The poor performance of septic tank systems (STS), as well as runoff from agriculture, may contribute to the movement of contaminants through ground and surface waters to estuaries resulting in oyster contamination, and following their consumption, impacts to human health. In monitoring individual STS in sensitive locations, it is possible to show that nutrients and faecal contaminants are transported through the subsurface in sandy soils off-site with little attenuation. At the catchment scale however, there are always difficulties in discerning direct linkages between failing STS and water contamination due to processes such as effluent dilution, adsorption, precipitation and vegetative uptake. There is often substantial complexity in detecting and tracing effluent pathways from diffuse sources to water bodies in field studies. While source tracking as well as monitoring using tracers may assist in identifying potential pathways from STS to surface waters and estuaries, there are difficulties in scaling up from monitored individual systems to identify their contribution to the cumulative impact which may be apparent at the catchment scale. The processes which may be obvious through monitoring and dominate at the individual scale may be masked and not readily discernible at the catchment scale due to impacts from other land use activities.

On-Site Wastewater Systems: Investigating Dynamics and Diurnal Patterns Impacting on the Performance of Mound Systems

Many dynamics and diurnal patterns impact upon the performance of effluent disposal areas. The timing of household water use and conveyance to a septic tank impacts on the available volume for application to an effluent disposal area. The timing and method of effluent application will impact on the performance of the effluent disposal area in assimilating nutrients and maintaining adequate infiltration rates. Rainfall patterns will impact on soil saturation at different times during the year; and threshold rainfall events may increase groundwater levels that will impact on the performance of the effluent disposal area. However, the dynamics and diurnal patterns comprising the “whole-of-on-site wastewater- system” are rarely discussed in the literature. This paper uses data obtained from a study of two allotments in a non-sewered subdivision at Salt Ash (NSW) to highlight these dynamics and diurnal patterns in order to improve the performance evaluation of on-site wastewater systems, in particular Mound systems. Results indicate that under existing septic tank-collection well design criteria the variability in average daily indoor water use and average diurnal water use patterns will impact on the temporal comparative performance of effluent disposal areas (Mound systems) at similar sites.