Cold water pollution (CWP) has its genesis in the stratification of dam reservoirs during the hotter months when warm, buoyant surface waters float on top of the colder, denser bottom layer. When stratified dams discharge from deep level offtakes, downstream water temperatures artificially decrease. Depending on how a dam is operated during the warmer months, releases of cold, stratified water can result in persistently suppressed water temperatures downstream, or in sudden and severe cold water shocks.
Water temperature is a key driver in freshwater ecosystems. Unfortunately, CWP causes pervasive ecological impacts that have flow on negative outcomes for regional communities. These impacts are well documented and acknowledged by Australian government agencies and the scientific community. For native fish, CWP detrimentally impacts all life-cycle stages and is a key contributor to the loss of over half of expected fish species below the major dams in the Murray-Darling Basin (MDB).
Eight (8) New South Wales (NSW) dams are identified as causing pervasive CWP impacts across 2,000 km of downstream waterways. These high priority dams are the largest and deepest reservoirs that regularly discharge large volumes of cold water during spring and summer to meet downstream irrigation, town water supply, and environmental flow demands.
The NSW Department of Primary Industries and Regional Development, WaterNSW, and the University of NSW Water Research Laboratory are working collaboratively to mitigate CWP at one of the eight high priority CWP sites in NSW. Pindari Dam, which is in the northern MDB, is 73 m deep with a storage capacity of 312 GL. Despite having a multi-level offtake tower, downstream CWP impacts persist due to competing operational requirements associated with low-level releases during persistent algal blooms.
CWP mitigation at Pindari Dam is proposed via reservoir destratification using bubble plume technology. Bubble plume destratification has been used in dams globally for decades to limit cyanobacteria blooms and remediate poor water quality conditions (low dissolved oxygen, high soluble metals, high nutrients) caused by reservoir stratification; however, bubble plumes also offer an opportunity to mitigate CWP. Recent funding by the Commonwealth Government provides the opportunity to demonstrate the efficacy of bubble plumes to destratify a large dam (Pindari) and mitigate CWP to strengthen the case to deploy this technology as an effective option at remaining high priority dams in the MDB. Assessments demonstrating the likelihood and potential benefits of bubble plume destratification at Pindari Dam will be presented.