Water quality in the mid to upper Tweed estuary and within the Cobaki and Terranora broadwaters is poor at the best of times and fails to comply with national guidelines for supporting aquatic life, a new report has found.
The report, tabled at the last Tweed Shire Council meeting, points the finger at land-use impacts and pollutant discharge in the catchment as the main culprits.
It also outlines some strategies to improve quality by reducing the level of nutrients and eroded soil entering waterways.
These include restricting cattle access to waterways to cut down direct fouling of streams with manure, revegetating riparian areas to reduce creek-bank erosion and filtering contaminants from runoff.
The Review of Water Quality in the Tweed Estuary 2007–2011 analyses data collected by Council staff from 29 monitoring sites throughout the Tweed River and its tributaries and is compiled by consulting firm Aquatic Biogeochemical and Ecological Research (ABER).
The five-year time span for the data collection allowed for seasonal variations in rainfall and river flows.
In his assessment of the report, natural resources director David Oxenham said there were ‘clear signs that due to historical development of the catchment and ongoing input of nutrients and sediments from land use and wastewater treatment plants that water quality has not met nationally and state recommended targets for significant proportions of the period 2007–2011’.
Nutrients come from catchment runoff such as eroded soil, cattle manure, agricultural fertilisers, urban stormwater, and the discharge of treated effluent from council’s wastewater treatment plants.
The report says that in the Tweed and Rous nitrogen and phosphorus concentrations exceed Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines for more than 50 per cent of the time in the middle and upper estuary (Chinderah to Murwillumbah).
It says nutrient concentrations are generally acceptable in the lower estuary due to the high rate of tidal flushing.
Nitrogen exceeds ANZECC guidelines by more than 75 per cent of the time in the tributaries of Cobaki and Terranora broadwaters (Duroby, Bilambil, Piggabeen and Cobaki creeks) and around 50 per cent of the time in the broadwaters.
But Mr Oxenham says the objectives in Council’s interim water-quality management plan are less stringent than the ANZECC guidelines for the protection of aquatic life in ‘slightly disturbed ecosystems’, as well as water quality objectives set for the Tweed catchment by the NSW government Office of Environment and Heritage.
Mr Oxenham recommended Council adopt the more stringent water-quality objectives of national and state policy, which had a ‘more appropriate target for the protection of aquatic ecosystem values’.
The data, he said, showed a clear relationship between nitrogen levels, the growth of algae and the levels of dissolved oxygen in the estuary.
‘When nitrogen concentrations are at or less than 0.3mg/L, dissolved oxygen is maintained at approximately 80 per cent saturation, which is the key indicator for the ability of the estuary to sustain diverse and abundant aquatic life,’ Mr Oxenham said.
The water quality objectives adopted by Council in 2000 included a nitrogen concentration of less than 0.5mg/L, whereas the state recommended target is less than 0.3mg/L.
Mr Oxenham says that at times and in various locations, the Tweed estuary showed signs of poor water quality and ecological health, including high nutrients, phytoplankton blooms, poor water clarity and low levels of dissolved oxygen.
‘It must be stressed that these issues are not constant across all parts of the estuary and their occurrence is not static over time. It is also worth noting that some periods of poor compliance relate to natural phenomena, for example floods, where even pristine waterways would “fail” some targets for ambient water quality.’
The report also says there are moderately severe phytoplankton blooms in the middle and upper estuary of the Tweed, including the Rous, and blooms in Terranora Broadwater and estuarine parts of Duroby, Bilambil, Piggabeen and Cobaki creeks.
Mr Oxenham said that despite these water-quality problems, the Tweed estuary sustained large areas of high-value aquatic habitat and ecological communities, and supported a large range of important recreational activities.
He said that due to the influence of climate and the highly dynamic nature of estuaries, monitoring must be ongoing to a detect any long-term trends in condition.
Other recommended actions to improve water quality include: retrofitting urban stormwater quality improvement devices into existing urban areas; modifying agricultural practices to decrease loss of topsoil and fertiliser; and cutting nutrient concentrations in effluent released from council’s treatment plants and reusing as much treated effluent as possible.
The annual cost of the sample collection and analysis is around $125,000, and monitoring is undertaken to assess environmental health and recreational suitability of Tweed waterways, to comply with conditions of wastewater treatment plant effluent discharge licences; and to monitor the quality of drinking water supply.
The ABER report is available on council’s website as an attachment in the last council meeting papers.