High levels of zinc in the upper kanamaluka/Tamar's sediment could cause "considerable environmental risk" during dredging while the release of ammonia and total nitrogen has the potential for algal blooms, a technical report to the City of Launceston has shown.
The report from March 2020, completed by environmental consultant Marine Solutions Tasmania, was released to The Examiner this week as part of a Right to Information request.
A council tender document assessing sediment management risks was also released, which noted the silt ponds on the West Tamar Highway have limited volume and "alternative locations and ponds" would likely be needed for dredging to occur.
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City of Launceston chief executive officer Michael Stretton refused to release the documents during the election campaign, citing that they were provided to the Tamar Estuary Management Taskforce and it was up to them to release their final report. This prompted an RTI request from The Examiner, approved in full.
The Liberals promised a "targeted" dredging program in the upper Tamar, preempting the TEMT report. The party appears set to retain government following the weekend's election.
Marine Solutions carried out sample testing on sediment in 10 locations in late February 2020 - five in a southern section between Home Point and Paterson Bridge, and five in a northern section downstream of Home Point.
Samples were also taken from centre of three silt ponds on the West Tamar, and three ponds at Ti Tree Bend.
The testing found that the Tamar sediment exceeded the National Assessment Guidelines for Dredging for concentrations of zinc, arsenic, cadmium, copper, mercury and nickel, as well as the highly toxic biocide tributyltin. These guidelines are based on risk to the marine environment during dredging and from pond leaching.
The report noted that zinc accounted for the highest risk.
"These results indicate that the concentration of zinc in the sediment of the dredge area could pose a considerable environmental risk during dredging," it reads.
"Water quality monitoring during dredging is advisable to ensure that the high concentrations of zinc and nutrients [particularly ammonia and total nitrogen] released from the sediment are effectively dispersing."
If ammonia and total nitrogen did not disperse adequately, it could have further impacts.
"The high concentrations of nutrients [ammonia and total nitrogen] in the elutriate samples suggests that dredging is likely to release large amounts nutrients into the water column. This has the potential to stimulate growth of phytoplankton resulting in algae blooms," the report reads.
Pond capacity and leaching
Once the sediment was dredged from the upper Tamar, it would need to be "dewatered" in siltation ponds - a process that can take several years - before it could be disposed of as fill.
The last time the ponds at West Tamar and Ti Tree Bend were used for dewatering was in 2009, according to the report, with testing last year finding the soils remained "moderately to strongly acidic".
Testing at the ponds showed that the EPA Tasmania's guidelines for the management of contaminated soil was exceeded for cadmium, chromium, manganese and the toxic equivalent concentration.
An acid sulfate soil management plan would be required for the disposal of the dredge material.
"Based on this information, the sediment/soil of the dewatering sites would be classified as level 2 low level contaminated soil," the report concludes.
A City of Launceston tender document which assessed risk for Tamar sedimentation management highlighted potential capacity issues with the ponds as part of a potential dredging program.
Written by the council's senior asset management advisor, Randall Langdon, the document noted that the Ti Tree Bend ponds are required "as a wetland under the River Health Action Plan", while the West Tamar ponds have "relatively little capacity above existing surface levels".
"Ti Tree Bend Silt Ponds have a proposed alternative use as wetlands circa 2021 as part of the Clean-up the Tamar project," the document states.
IN OTHER NEWS:
"West Tamar Ponds have limited area hence volume. Principal to investigate alternative locations and ponds."
The Examiner understands siltation ponds must be located near the area being dredged.
The council document notes that the closest undeveloped land further north on the river, Stephenson's Bend - which sits between Riverside Golf Club and the river - is partly a "designated conservation area".
Government committed to dredging
Premier Peter Gutwein announced the Liberals' policy of targeted dredging last month, with $4 million set aside over two years, with continuous funding.
At the time, he said the government would use the existing silt ponds for the dredge spoil.
TEMT was expected to release its report in early 2021, but this was delayed due to the election. The report will assess siltation management options for the Tamar.
When asked whether the government could manage environmental risk given the release of heavy metals during dredging, and where new siltation ponds would be built, a government spokesperson said they were committed to finding solutions.
"We are committed to implementing our election policy to improve the health of the Tamar River and once the new government is sworn in, we will consult with the Launceston City Council, the Tamar Estuary Management Taskforce as well as working closely with the Environmental Protection Authority on the best way forward to implement our policy," the spokesperson said.
Labor also supported targeted dredging, subject to the details in the TEMT report.
Various reports into siltation management were referred to in the Launceston Flood Authority's latest annual report, but City of Launceston chief executive Michael Stretton refused to release these documents when requested during the election campaign.
"That data will be reviewed and considered as part of the TEMT's report, which is expected to be released soon, and is the appropriate mechanism for publication," Mr Stretton said at the time.