required to 'ensure that the best practicable measures are taken for environment protection' (s7(2) POEA Act) but in reality, approval and licence standards for STPs appear to be a tradeoff between compliance costs and environmental quality benefits. General lack of environmental monitoring along the NSW coastline means that successful determination of real environmental benefits within this system may be difficult to measure. Recent EPA acceptance of the stringent ANZECC Australian Water Quality Guidelines for Fresh and Marine Waters (ANZECC, 1992) may appear to provide a good basis for environmental protection, though from a practical standpoint, compliance costs associated with attainment of 'acceptable' pollutant levels and the monitoring of outcomes are likely to be high.
Coastal Effluent Systems Development and introduction of strategies for change to wastewater disposal methods are usually the province of state governments, but individual communities are increasingly active in promoting the need for change. Heightened community concern is a response to overt environmental changes that have resulted from increased seasonal and permanent population along our coastline. Reticulated sewerage networks in small coastal villages, often designed and built twenty or more years ago , are now required to process effluent volumes that far exceed design capacity. Examples that typify how coastal populations have outrun disposal infrastructure are Anglesea in Victoria and Tathra in New South Wales where systems were designed to cater for maxima of around 2000 people (with treatment nominally to secondary level) . Permanent and summer holiday populations in these two coastal areas have more than quadrupled over the last twenty years (since initial system design and installation in the 1970s) to over 12000 at Anglesea and 6000 to 8000 at Tathra during summer. There has been little or no infrastructure upgrade during this time. Wastewater disposal is still effectively to the ocean (via a submerged pipeline at Anglesea and by infiltration into dune sands and ultimately into a low tidal intrusion estuary and wetland at Tathra). This is a typical disposal means for most sewered coastal communities in Victorian and New South Wales (see Figures 1 and 2). Some communities have recently developed limited water re-use (eg. local golf course at Tathra, recreation areas at Lakes Entrance and wood lots at Alberton in Victoria) but re-use of significant effluent volumes is not typical. The current capacity of treatment and disposal systems to cope with highly variable discharg-e volumes and variable pollutant loads (variable with season) is highly suspect. Examples of discharge variability are given in Figure 4 where peak
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summer load at Anglesea has been recorded at 2.63MUd in contrast to a winter flow rate of 0.55MUd (280MUyr) (Gutteridge Haskins and Davey 1991). A similar seasonal variation of over 250% can be shown for Tathra ( Figure Sa, b).
Environmental Impact Depending on local dispersion conditions, seasonal sewerage system overload like that described above can result in elevated concentrations of pollutants in areas surrounding outfalls. While effluent quality is usually monitored at the outfall (for instance end of pipe monitoring at Anglesea shows typical effluent mean phosphorus and nitrogen concentrations of 3mg/l and 48mg/l respectively; Gutteridge Haskins and Davey 1991), distribution of pollutants and their concentrations within dilution zones or within groundwaters surrounding outfalls are not usually well understood. The efficiency of any dilution effects around an outfall depends upon a range of hydrologic conditions including swell and current conditions for open ocean outfall (eg. Anglesea) or tidal/riverine/ groundwater flow interactions for wetland disposal situations (eg. Tathra). These interactions are complex and demand considerable broad area monitoring to enable confident prediction of any dispersion effects, but broad area monitoring does not seems to be a common part of traditional effluent outfall management. Attempts have been made at some locations to record baseline data within receiving surface and/ or groundwaters (eg. Tathra Sewerage Augmentation
Groundwater Study: Groundwater Technolog"J 1994) though typically, studies seem to be very focused . They examine short-term effects and therefore fail to provide sufficient understanding of long-term disposal consequences (eg. Tathra Sewerage
Augmentation Groundwater Study: Groundwater Technolog"J 1994 and Geelong and District Water Board, 1991). Choosing between disposal options, however, even when monitoring data of this sort is available, is likely to be difficult, particularly if the physical and biological framework of the disposal area is complex and consequently not well understood. Tathra provides an example of how, despite considerable field data gathering (sampling and monitoring of over sixty bores), understanding of the physical processes around the outfall is still insufficient for clear choice between the various disposal options. Tathra effluent enters a small low tidal intrusion estuary where a complex set of highly variable and seasonal land and ocean processes (flushing mechanisms, wetland biological processes and aquifer movement) are relied upon to effectively process effluent pollutants. These natural estuarine processes are further complicated by variable but generally elevated
freshwater input from the STP which has formed a substantial fresh groundwater mound about the outfall (Groundwater Technology 1994). The natural wetland equilibrium is obviously disturbed (evidenced by nearby development of a reed swamp, Pragmites australis), but the exent of this disequilibrium is unclear (no further overt changes to the swamp ecology have been observed: SWC Wetlands, 1994) and the threat posed by low numbers of Microsystis (blue-green algae) cannot be predicted. To address the broader question of impact to the whole wetland by effluent ~osal, a communityled pollution momtoring program is attempting to plot seasonal as well as longer term changes to the swamp water and sediment chemistry. Leakage of nutrients from the infiltration pond into the surface waters of the swamp can now be shown. (Phosphorus levels over 4 mg/I and nitrogen at 10 mg/ I have been detected within the groundwater at the edge of the swamp and within the surface waters). However, not enough is known of the wetland ecology to characterise any risk posed by disposal. Similar uncertainties exist at many locations along the New South Wales coast. Direct ocean disposal presents a different, but no less complex set of environmental variables. Anglesea, for instance, relies on strong easterly running diurnal tidal currents to disperse discharges from its outfall which is located below low tide level. There are times, though, when discolouring of the receiving waters is significant (Geelong and District Water Board, 1991) and dispersion is poor. Ocean waters are demonstrably subject to significant vertical as well as horizontal stratification which depends on season and weather. This affects effluent dispersion (AGSO Research Newsletter May 1995). While the tools to resolve ocean dispersion effects are available (see AGSO Research Newsletter May 1995), traditional attitudes regard dispersion within the ocean as adequate, the ocean being self cleaning, ipso facto by nature. Certainly, visual inspection of seagrass beds (H ta.smaniai) opposite the effluent discharge point (above the 7 to 8 metre depth contour) showed no overt signs of biological degradation (Geelong and District Water Board 1991), though pollutant retention in local or far afield marine sediments or benthic and planktonic plants and animals are beyond investigation at this point. There is potential for concentration of pollutants at other locations along the coast.
Disposal Choices , Today, the range of potential disposal choices and eventual selection of alternatives is determined by a combination of community needs as well as natural physical systems (climate, geomorphology and geology). In the past, physical constraints WATER JANUARY/FEBRUARY 1996