Ameliorating Water Quality in Mildura and Wentworth Weir Pools

"2004".

Project Number: MDFRC Technical Report.

MDFRC item.

Under current operating conditions, the Mildura and Wentworth Weir Pools are prone to experience blue-green algal blooms. These blooms represent a potential public health risk and may have adverse economic and social impacts on the local tourist and agricultural industries. The summer of 2001 provided ideal conditions for the development of a blue-green algal bloom. The conditions that led to the formation of the bloom were 1. Flows that allowed water to reside in the weir pool for more than 10 days 2. A prolonged period of hot weather 3. These factors allowed the formation of persistent thermal stratification 4. Stratification led to the formation of anoxic conditions on the bottom of the weir pool. These conditions resulted in a mid level alert bloom whose cell counts peaked at 7716 cells.ml-1 in January and 6642 cells.ml-1 in February. By chance the weather conditions also provided an opportunity to evaluate the effectiveness of elevated flows as a mechanism to disperse blooms. It would appear that elevated flows do act to disperse blooms through; 1. The breakdown of thermal stratification 2. Re-oxygenation of the water at the bottom of the weir pool 3. Reduction of residence time within the weir pool 4. Reduced surface water temperature and increased turbidity Due to the difficulties associated with providing these flows, further work is required to development-effective and practical control measures for the Mildura and Wentworth weir pools. This work is being undertaken by the Mallee CMA in conjunction with the CRC for Freshwater Ecology. The priority areas of research include; 1. influences on sediment nutrient dynamics in the weir pools 2. The potential for upstream management to influence blue-green algal dynamics 3. Hydraulic modelling of the Mildura and Wentworth Weir pools 4. Blue-Green algal modelling It may also be possible to improve monitoring through the use of telemetry and remote sensing that would enable better prediction of blue-green algal blooms and better advice to water users about ways to minimize exposure to potentially harmful algae.