Water Journal : Water Journal December 2011
small wastewater systems refereed paper 74 DECEMBER 2011 water technical features Abstract In recent years, oyster harvesting in a number of rivers, estuaries and coastal lakes on the mid-north coast of NSW has been adversely impacted by reduced water quality. Since 1997 Wallis Lake, Tilligerry Creek (part of the Port Stephens estuary) and, more recently, the Kalang River, have been closed to oyster harvesting. In particular, following periods of heavy rain, water quality in the estuaries has diminished and a number of incidents of pathogen contamination of estuarine waters have occurred. Such incidents have significant adverse impacts on the oyster industry and may result in long periods of closure to harvesting with consequent implications for oyster supply and the livelihoods of those employed in the oyster industry. On-site wastewater management systems (OWMS) on properties close to the estuary have often been considered possible sources of contamination of estuarine waters. State Government agencies have broad responsibilities for public and environmental health and in the oversight of shellfish quality, while local Councils are responsible for the approval and ongoing operation of OWMS. Studies by the authors and State Government agencies, such as the NSW Food Authority which manages the Shellfish Quality Assurance Program along with local Councils, have attempted to use a range of methods to demonstrate connectivity of failing OWMS to the estuaries. Systematic audits of these systems in some catchments have quantified the extent to which they do not meet current regulatory requirements and have been used to develop a risk-based approach to assessment of the wider extent of the problem. Pathogen die-off modelling has been used to assess risk and determine the need for upgrading or replacement of existing OWMS. Standard designs have been developed and implemented in some catchments to ensure a higher level of performance and there is clear evidence that, where these designs have been implemented, impacts have been reduced. Introduction Wastewater management in small communities may involve a reticulated system with centralised treatment and water-based discharge or, in some cases, land application involving reuse of the treated effluent. Where the cost of a reticulated system is prohibitive to small communities, wastewater may be treated and disposed of on-site at individual properties (OWMS systems). Typically in these situations, a septic tank and land application system involving subsurface trenches or beds is commonly used, although other designs are available. There are over one million OWMS in Australia (Gardner et al., 2006) and surveys and audits of their performance often demonstrate that a proportion (15--40%) may periodically perform poorly or even fail due to poor construction, undersizing with respect to hydraulic loads, or soil and land capability constraints for on-site effluent disposal. Where these failures occur there is concern with respect to public health and the environment; however, there are very few studies that are able to demonstrate direct linkages between OWMS failures and any adverse impacts to human health and receiving waters. The inability to discern such linkages and widespread contamination which may be anticipated due to these failures is partly due to effluent dilution, difficulties which exist in differentiating effluent pathways in the field, and the attenuation of faecal contaminants. Case Studies In NSW there have been a number of highly publicised cases of contamination of estuarine waters used for oyster growing along the coastline north of Newcastle. In each case possible sources of human faecal and elevated nutrient concentrations have been found to come from agricultural areas, waterway users, runoff from urbanised areas and unsewered small communities. It has proven difficult to separate the overall impact that these small communities have to estuarine water quality in comparison with agriculture; PM Geary, JH Whitehead Standard designs for on-site wastewater systems have been developed and, where implemented, impacts have been reduced WATER QUALITY IMPACTS ON ESTUARINE AQUACULTURE: A REVIEW Oyster beds in Tilligerry Creek Estuary, Port Stephens, NSW.
Water Journal April 2012
Water Journal November 2011