Water quality and ecological impacts of treated sewage effluent on a peri-urban stream (11663)
Sewage Treatment Plants (STPs) commonly dispose of wastes to local waterways. Discharge water from STPs consists of treated effluent; this point source impact may adversely modify downstream water chemistry, with an associated loss of aquatic ecosystem health, such as disturbing the diversity and abundance of macroinvertebrate assemblages. The purpose of this study was to assess the impact of a small STP (North Richmond, NSW), and the impact of the treated effluent being released into a local creek; Redbank Creek. This was achieved by conducting chemical, physical and biological investigations, comparing sampling sites upstream and downstream of the STP. This research found that there were significant differences in aspects of all three components. Salinity levels were significantly different, rising from a mean of 560 µS/cm upstream to 704 µS/cm downstream. Nitrate concentrations also increased significantly from 960 µg/L upstream to 1900 µg/L downstream. Water temperature in the receiving waterways also rose more than 1° C downstream of the STP. There were significant differences in macroinvertebrate assemblages below the STP compared to upstream. Higher mean SIGNAL scores of the macroinvertebrates were found upstream. Downstream SIGNAL scores indicated that more pollution tolerant taxa were present. An EPA licence is in place to regulate pollution, with the central aim of protecting the integrity of the natural environment. It is interesting that the EPA licence 190 for North Richmond STP does not include any discharge conditions for salinity, when many sensitive macroinvertebrates may be directly affected by high salinity levels. The EPA licence for the STP does permit the discharge of highly elevated Phosphorous (total) and Nitrogen (total) wastes as the licence specifies 90th percentile concentration limits of 5mg/L and 15mg/L. This is surprising given the chronic eutrophication (and regular problematic blue-green algal growth) problems further downstream in the Hawkesbury River.