Delivering the Murray-Darling Basin Plan

Bubble Plume Destratification: Cold Water Pollution (CWP) is an artificial decrease in riverine water temperature downstream of large water storage dams, caused when the bottom cold layer of water is discharged through outlets at the dam base. Releases of cold, stratified water result in persistent suppressed water temperatures during the warmer months which causes pervasive ecological, social, and cultural impacts. Bubble plumes are an established technology that uses off-the-shelf components to destratify reservoirs and to mitigate CWP and poor water quality. Use of bubble plume technology could improve water quality conditions to over 3000 km of mainstem waterway in the Murray-Darling Basin.

In the Murray-Darling Basin (MDB), regulated water is stored and released from large dams. The water released from these dams is characterised by cold water pollution (CWP), which impacts over 3,000 km of mainstem waterways. Cold water pollution has its genesis in the stratification of dam reservoirs during the hotter months from September to April; when warm, buoyant surface waters float on top of the colder, denser layers below. Water releases from this bottom layer result in persistently suppressed water temperatures downstream (commonly 10°C to 17°C colder).

CWP causes pervasive ecological, social, cultural, and economic impacts to regional communities – these impacts are now well documented and acknowledged by state and federal governments and the scientific community.
Ecologically, water temperature is a key driver in freshwater ecosystems. For aquatic fauna, CWP causes increased mortality, reduced spawning and recruitment, and suppressed growth rates that negatively impact riverine carrying capacity and ecological resilience. Regarding native fish, CWP detrimentally impacts all life-cycles 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.

Rivers also provide significant social, cultural, recreational, and aesthetic values for regional communities; however, in many places these values have been diminished because of CWP. Suppressed water temperatures downstream of large dams significantly reduce swimming, boating, and recreational fishing opportunities, which in turn impacts on regional tourism. The loss of native fauna, such as Silver Perch and Golden Perch, from waterways below large dams also reduces the connection of First Nations people with their land and river-country.

Mitigating CWP will improve river health and native fish populations; and bring economic, social, and cultural benefits to regional communities. Moreover, CWP mitigation represents a tangible and enduring commitment by Governments towards the recovery of valued aquatic fauna groups while addressing widely-held environmental and social concerns by the community at large.

Bubble plumes are an established, proven technology that uses off-the-shelf components to destratify reservoirs and to mitigate CWP and poor water quality. Bubble plumes use compressors at the surface to deliver air through a pipe network to diffusers located near the reservoir bottom. As the bubble plume rises through the reservoir water column, cold water is entrained and transported to the surface where its higher density relative to the warm surface layers causes it to sink. The resulting movement of water creates circulation ‘gyres’ that destratify the reservoir that result in consistent temperatures throughout the water column. Any water released from the destratified reservoir is the warmest water available to dam operators, regardless of where the offtake is located.

The use of off-the-shelf components for bubble plumes means that their CAPEX is relatively cheap (< $5 M). Until recently, the main disadvantage of bubble plumes was their high OPEX due to air compressor power consumption. Reservoir size plays a significant role in the initial and ongoing operational costs of bubble plume destratification, with larger reservoirs typically requiring higher compressor air-flow rates and energy consumption for effective destratification. However, advances in green power technology over the last decade (e.g., solar arrays, storage batteries), and changes in the wholesale energy market, provide opportunities to either partially or fully offset the previously high operational costs and associated carbon footprint.

The use of bubble plume technology to destratify reservoirs and mitigation CWP has the following benefits:
• Reduced reservoir evaporation rates due to lower surface temperatures, resulting in water savings (estimated at 5 – 10 % savings)
• Improved water quality conditions within and downstream of the reservoir resulting from reservoir destratification (e.g. dissolve oxygen, nutrients, metals, cyanobacteria)
• Increased riverine water temperatures downstream of large dams which benefits ecological productivity while also resulting in concomitant recreation and cultural benefits
• Meet and exceed Basin Plan Schedule 11 water temperature targets (monthly median riverine temperature to be within the 20th and 80th percentile range of the natural monthly water temperature)
• Potential to offset some or all operational expenditure through the use of modern green technology that takes advantage of the current wholesale energy market

The primary challenge to implementing CWP mitigation via bubble plume destratification is the allocation of CAPEX towards the design and installation of bubble plume systems at priority dams in the MDB for the explicit purpose of mitigating CWP. Past evidence has demonstrated the effectiveness of the systems in destratifying reservoirs and mitigating CWP; however, bubble plumes have rarely been implemented for the primary purpose of mitigating CWP (Tallowa Dam, NSW being an exception).

A key risk for implementing bubble plumes is installing a system that is too small to destratify the reservoir. A review of reservoir destratification by Chaaya and Miller (2022) confirmed the effectiveness of bubble plumes in mitigating CWP in smaller reservoirs. However, as reservoir size increased (i.e., those with deeper and greater storage volumes), the effectiveness of bubble plumes decreased. This finding was attributed to the use of undersized compressors which were unable to maintain destratified conditions, and highlights the need for accurate modelling and assessment of bubble plume components to confirm the effectiveness of installed systems.