Type of wetland
- ☒ Natural ☐ Constructed
- The effect of flow diversion to the Vasse Diversion Drain and impoundment by elevated land downstream have led to the Lower Vasse River acting as an elongated “lake” area from late spring to late autumn.
Wetland management issue
Owing to increased inputs of nutrients from catchment sources, and the still conditions created by impoundment, the Lower Vasse River is eutrophic. Extremely high nutrient concentrations, particularly phosphorus, and ideal physical conditions drive severe seasonal algal blooms for up to seven months from November to May. Algal blooms cause unsightly water discoloration and scums and unpleasant odours. These blooms are often dominated by blue-green algae (cyanobacteria) which are potentially toxic and close the waters to public use.
- Name of treatment: HT-Clay
- Quantity used: Two of the areas (Area 1 and Area 2) were treated with a total clay amount of nearly 1.5 tonnes.
- Method of application: The dose was split and applied at different times throughout summer, starting in December for Area 1 and February for Area 2 (see treatment image). The control area was left untreated to provide a baseline for the evaluation of the treatment efficiency. The clay was evenly sprayed onto the water surface from a moving barge equipped with a 1000L holding tank, a petrol pump and a spray boom. The tank on the barge was filled directly from the tanker truck that was used to transport the clay to the trial site. To ensure the clay slurry was well-mixed, it was agitated inside the tanker truck by re-circulation with a petrol pump.
- Is there any known scientific research supporting its effectiveness? ☒ Yes ☐No
Date(s) of treatment
In summer 2016-17 DWER scientists completed a small-scale trial by installing 15 large bottomless tanks, or mesocosms, in the river upstream of Causeway Bridge. They added different amounts of clay to these tanks in early December and monitored visual appearance and water quality throughout summer. The trial showed that the clay successfully removed algae from inside the tanks with visual improvements lasting over the summer period.
After the successful small-scale mesocom trial, the trial was up-scaled in summer 2017-18 to treatment Area 1 and Area 2 (each approximately 455 m2), with HT-clay treatments separated by PVC curtains. Area 1 was treated in December 2017 before the onset of algal blooms, and re-treated with smaller top-up clay doses in February and March 2018 after summer rainfall. Area 2 was treated in February after the rainfall events, when an algal bloom was present and received a top-up clay dose in March. The control area was left untreated to provide a baseline for the evaluation of the treatment efficiency, and mimicked the slightly altered and more restricted conditions between the curtains.
Outcome of treatment
- Visual observations: HT-clay treatment effectively reduced algal blooms within hours of application. Phosphorus concentrations were reduced substantially within two to three hours of application and stayed below target thresholds over the entire monitoring period. In Area 2, HT-clay was effective at reducing algal blooms after the bloom was established, which is a major advantage over Phoslock® (needs to be applied prior to a bloom). HT-clay reduced phosphorous release from the sediment. The HT-clay treatment did not have any negative effects on small invertebrate organisms living in the Lower Vasse River.
- Have water parameters specific to the treatment claims been measured before and after application? ☒ Yes ☐ No
- Comments: Water quality and algal growth were monitored by DWER in all three areas until the end of the trial in late March. Aquatic macroinvertebrates were also monitored to see if there was any impact from the clay. Ongoing fortnightly water quality monitoring is being undertaken by DWER.
Contact Officer: Mathilde Breton
Email: [email protected]