Australian studies:::::::::::::::::::::::::::

The following section outlines the main studies that have been conducted in Australia investigating wet weather sewer overflow impacts from SSSs. Two main studies undertaken in Sydney and Brisbane are discussed in detail, followed by a brief discussion of other studies which were mostly inconclusive or preliminary.

Sydney Study- ‘Aquatic ecological and human health risk assessment of chemicals in wet weather discharges in the Sydney region, New South Wales, Australia’ (Bickford et al., 1999)

Sydney Water undertook a comprehensive study to assess risks from wet weather sewer overflows to human health and aquatic organisms in receiving waterways (Bickford et al., 1999). Sites investigated included a wide range of catchments and receiving water conditions and included locations downstream of major sewer overflow sites. The study investigated the risks associated with chemical contaminants from wet weather discharges including sewer overflows. A very comprehensive list of 114 chemicals was investigated, including a wide range of organics, heavy metals and an additional nine conventional compounds such as

ammonia, chloride, and nitrite. Furthermore, some non-chemical physical stressors were also investigated, including dissolved oxygen, salinity, total suspended solids, sedimentation and scouring. Finally bioassays or toxicity testing of receiving water, sewer overflow and sediment samples and field surveys of benthic macro-invertebrates was conducted. The major findings of the studies were as follows:

• Chemicals of concern (COC) were identified at one or more of the receiving water sites evaluated including 3 metals, 5 organic chemicals, 3 conventional chemicals, two physical stressors and an additional 3 organic chemicals for freshwater sites.

• More than 85% of the load of the chemicals which were identified as a concern originated from catchment sources rather than sewer overflows.

• Consequently, most of the aquatic ecological risk is from stormwater inputs rather than wet weather sewer overflow. The exception was for ammonia and nitrite which were associated with both stormwater and sewer overflow inputs.

• Modelling revealed that abatement of wet weather sewer overflows did not significantly reduce the number of COC, or the length of time COC were above toxicity thresholds for most chemicals as they were primarily associated with stormwater run-off. The exception to this was silver, for which, despite being primarily associated with stormwater run-off, the duration of risk was reduced following overflow abatement in some waterways.

Furthermore, the risk posed from ammonia and nitrate, which were associated with both stormwater and sewer overflow, was reduced following overflow abatement in most waterways.

• Depressions in dissolved oxygen from wet weather discharges were predicted to be of concern at 14 of the 37 sites. However it was unclear what portion of this was attributable to wet weather sewer overflows as opposed to stormwater discharge.

• Modelled data showed that following wet weather overflow there is the potential for chronic risk to aquatic life from suspended solids at 17 of the 37 sites evaluated

• Potential loss of habitat of benthic organisms as a result of scouring was identified at five sites. However, this was only found in a small area adjacent to the overflow.

• Sedimentation of suspended particulates was identified at 10 of the 37 sites. However, the contribution of sewer overflow to this impact was small.

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• The field survey of benthic organisms was inconclusive regarding the impacts of sewer overflow as communities were depauperate both upstream and downstream of overflows.

Therefore, even if overflows were abated, it is likely that these sites would remain impacted due to inputs of stormwater run-off.

• The results of toxicity bioassay tests revealed that undiluted sewage from overflows is toxic. However, conclusions could not be drawn on the impacts of overflows on the toxicity in receiving waterways downstream of overflows. The receiving waters upstream of overflows were toxic in wet weather, suggesting that much of the measured toxicity may be attributable to stormwater. Bioassays of sediment collected near overflow sites did not indicate toxicity (for short term exposure). However, as this was only confirmed with amphipods, organisms whose sensitivity differs to amphipods may or may not display effects.

• In relation to human health risk, there was no risk to people engaged in water-based activities from sewer overflow and predicted cancer risks based on consumption of fish in contaminated waters appear to be largely a result of stormwater rather than sewer overflow inputs.

Brisbane Study- ‘Impacts of sewage overflows on an urban creek’ (Pollard et al., 2005)

A study of the impact of both wet and dry weather sewer overflows and stormwater run-off on a tidal waterway in the suburb of Lota in Brisbane examined a wide range of physical, chemical and microbiological contaminants (Pollard et al., 2005). Microbiological contaminants measured in the waterway following these events included faecal indicators, bacterial, protozoan and viral pathogens. Human sterol biomarkers were also measured to determine what percentage of faecal contamination was of human origin, which would contain those pathogens most likely to cause illness. Ecosystem health contaminants such as nutrients, dissolved oxygen and suspended solids and an extensive range of chemical contaminants were also measured in the waterway following sewer overflow and stormwater run-off. Finally, levels of endocrine disruptors, which have more recently been identified as a potential threat from sewage pollution, were measured following sewer overflow.

Major findings of the study were as follows;

• Stormwater was a greater stressor of ecological health than sewer overflow. Impacts of increased turbidity, nitrogen and phosphorous and dissolved organic carbon in the water

column were low from wet weather sewer overflow and restricted to the point of release but unacceptably high from stormwater run-off, which was the main contributor.

• Due to physical re-aeration rates being greater than the microbial respiration rates, wet weather sewer overflow did not cause reduced dissolved oxygen levels in the stream.

This may be different for other streams where physical re-aeration rates are lower.

Another study of dry weather overflow into the Swan and Canning estuary in Perth, Australia also pointed out that whilst dissolved oxygen levels following the spill were acceptable in the lower reaches of the estuary, this may not be the case further up the water course where there is less tidal movement (D.A. Lord & Associates Pty Ltd., 1997).

• Metals were below safe levels in the waterway following sewer overflow due to the dilution effects of the overflow in the waterway.

• Wet weather sewer overflow may result in unacceptable levels of endocrine disruptors in the tributary which received the overflow. Findings relating to endocrine disruptors were an estimate of levels in the waterway based on known levels in raw sewage and hydrological dilutions of raw sewage which would occur during a sewer overflow event, as measured levels were below the detection limit.

• Both wet and dry weather sewer overflow rather than stormwater run-off presented an unacceptably high public health risk to users of the waterway due to a high level of human faecal contamination, as confirmed by the measurement of human sterol biomarkers.

• During wet weather overflow, only 20% of faecal indicators were of human origin compared with 100% following dry weather sewer overflow. However, this still presented an unacceptably high health hazard.

• The high public health hazard following wet weather sewer overflow also resulted in a loss of recreational amenity.

• Whilst faecal indicator levels were high as a result of stormwater run-off (with no sewer overflow), there was a low risk from human faecal contamination.

31 Other minor studies

Some other local studies were undertaken in Melbourne, Australia, although findings were mainly preliminary or inconclusive. One study was aimed at determining a suitable bio-indicator capable of distinguishing the impact on receiving waterways from stormwater drains with and without emergency relief structures (ERSs) attached (Duke and Veenstra-Quah, 1997). Both macro-invertebrates and diatoms were investigated in the study as potentially suitable indicators. The diatom analysis failed to establish any difference in the impacts of the different drains. Results of macroinvertebrate sampling showed a significant difference between sites above and below each drain, indicating that all drains were having a negative impact on the creek. However, it was not possible to distinguish the different impacts from each drain, given the background deterioration in biological community occurring down the length of the stream resulting from these drains but also other inputs. Further monitoring of macro-invertebrates particularly following sewer overflow was recommended to detect direct impacts from ERSs and distinguish between other drains not attached to ERSs.

An early study into the impacts of sewer overflow and stormwater discharges undertaken in various locations in the Sydney area included sampling of sewer overflow and stormwater run-off (Carleton, 1992). Results were reported as preliminary but indicated that pollutant concentrations which included BOD, COD and SS from sewer overflow and stormwater run-off were similar. Another study (Ngo et al., 1992) also in Sydney aimed to investigate the relative contributions of pollution from stormwater run-off and sewer overflow to waterways.

Two overflow events were sampled including levels upstream and downstream of the overflow. Results showed that overflow adds to pollution levels in the waterway including BOD, COD, ammonia, orthophosphate and faecal coliforms with levels at the overflow and downstream site higher than those at the upstream site. However, results also indicated that wet weather conditions in general result in significantly higher pollutant concentrations than in dry weather, and for one event stormwater and sewer overflow appeared to have similar effects on pollution levels in the waterway.