Cold water pollution: A review

Let’s start at the top. Before we get into the nuts and bolts of cold water pollution, it is important to understand the concept of stratification in large bodies of water. In hydrology, stratification is the term given to a phenomenon through which distinct layers of waters form, separated by varying characteristics. In an estuarine or other saline environment, stratification is observed where fresh and salt waters meet. Salt water is found in the deeper parts of these systems, due to its increased density. In freshwater environments, such as a reservoir, stratification occurs due to variations in the thermal profile through the depth of the water. 

Direct solar radiation and increased air temperatures, especially during summer, heat the surface waters in a reservoir resulting in lower density waters at the top of the water column. As stratification increases in strength, a distinct layering pattern forms characterising a warmer, lower density surface layer (referred to as the “epilimnion”) and a cooler, higher density bottom layer (referred to as the “hypolimnion”). These layers are separated by a thermocline (or “metalimnion”), at which there is a distinct and sharp thermal and density gradient.

Reservoirs do not stay mixed year-round. External mechanisms such as cooling, wind and inflows help to mix these stratified layers throughout cooler periods of the year. Throughout summer, however, the heating mechanisms are much stronger, and natural mixing is often not capable of overcoming the naturally formed stratification.
 

Cold water pollution is a serious environmental issue observed downstream of large storage dams. Dams are commonly fitted with deep offtakes, which withdraw water from below the thermocline. As a result, water discharged into the downstream river environment is often much colder than the natural river temperature. In Australian reservoirs, stratification can commonly result in up to 8 to 12 °C difference between the surface and bed temperatures in a reservoir. In deeper reservoirs, these variations can exceed 12 °C.

This can have significant negative impacts for hundreds of kilometres downstream of the reservoir. Most notable are the ecological impacts on native fish, where unnatural temperature variations can affect their physiology, behaviour, reproduction, recruitment and even lead to mortality. 

Cold water pollution is particularly impactful downstream of large irrigation storage reservoirs, where significant volumes of water are consistently released during irrigation (summer) seasons, when stratification is most prevalent.

As well as cold water pollution, stratification in reservoirs can lead to a variety of other water quality issues.

Dissolved oxygen (DO) is an essential part of the reservoir environment, as it contributes to the metabolism of all aerobically respirating organisms. In a stratified environment, where little to no mixing between the hypolimnion and epilimnion occurs, hypolimnion waters (below the thermocline) can become anoxic due to the biological oxygen demand (BOD) at the sediment-water interface at the reservoir bed. Anoxic waters can lead to a number of ecological impacts, including decreased heart, ventilation and metabolic rate in fish, and in some cases mass fish kills.

Low DO levels can facilitate the release of undesirable soluble iron and manganese. These metals can impact drinking water quality, increasing turbidity, darkening the water colour and causing an undesirable taste. As well as this, high concentrations of both have been linked to ecological impacts on fish populations, including oxidative stress, gill damage and haematological impacts.

Reservoirs are inherently conducive to algae growth, due to the high residence time of water and periods of reduced mixing due to stratification. Cyanobacteria (more commonly known as “blue-green algae”) blooms are of particular concern to water quality. This algae type produces cyanotoxins which can cause illness in humans and can be fatal to fish and other organisms.

Cyanobacteria blooms are most common in warmer months when a reservoir stratifies. They are capable of regulating their buoyancy through gas vesicles, which allows them to dominate the warmer surface layers in reservoirs during periods of low wind and high solar irradiance. This provides a competitive advantage over less toxic phytoplankton such as diatoms and green-algae, by increasing daily light dose and reducing sedimentation losses.

Nutrients released in anoxic waters during periods of stratification contribute to the growth of algae blooms.

Stratification and cold water pollution remains, to this day, a significant issue for larger storage reservoirs. A number of solutions have historically been employed to minimise the impacts of stratification in reservoirs, with varying degrees of success.

WRL’s Brett Miller and Farid Chaaya conducted an exhaustive literature review of the previous work that had been completed to attempt to mitigate cold water pollution and reservoir stratification in general. From this, two main strategies were recognised as the most feasible: selective withdrawal and artificial destratification.

Selective withdrawal involves withdrawing water from different depths of the reservoir using multi-level offtake (MLO) infrastructure. An MLO could be used to withdraw water through one (or more) of a number of offtakes at varying depths in the reservoir for the purposes of cold water pollution mitigation (by withdrawing from above the thermocline) or mitigation the release of cyanobacteria downstream (by withdrawing from below the water surface). 

Compared to artificial destratification, one of the distinct advantages of an MLO are the seemingly minimal operational costs. Conversely, one of the disadvantages are the large capital costs, which increase significantly with an increasing reservoir size. 

A distinct disadvantage of selective withdrawal are the conflicting cold water pollution and cyanobacteria protocols. Where cold water pollution required withdrawal from above the thermocline, mitigating downstream algae release requires water to be withdrawn from below the surface and often below the thermocline. Currently, algae release mitigation protocols trump cold water pollution mitigation operations, which can have significant impacts downstream due to the sudden introduction of cold water to the previously warmed downstream environment (referred to as “cold shock”).

Artificial destratification involves artificial mixing a stratified reservoir to break natural stratification and maintain a destratified reservoir. This would theoretically mitigate the previously outlined issues associated with stratified reservoirs, including cold water pollution and anoxic waters.

Artificial destratification is commonly achieved through one of two methods: bubble plumes or mechanical mixers. Bubble plume destratification is achieved by pumping compressed air to a diffuser located at a deep point in the reservoir, and released as bubbles through small nozzles. These bubbles rise, entraining cold hypolimnion waters to the surface. These waters detrain at the surface, sink to a level of neutral buoyancy (based on density differences) and propagate away from the plume. This sets up large circulation cells, which eventually break down the thermocline and mix the reservoir.

Mechanical mixers utilise propellors to jet water either from the surface-down or bed-up. At the end of the jet, water sinks or rises to a level of neutral buoyancy (based on density differences), much like the bubble plumes. Mechanical mixers at the surface are often accompanied by a draft tube, which directs the jet down to a particular depth.

As part of this review, a library of 125 international examples of destratification was collated. Extensive review of these resources led to the conclusions that in most cases, bubble plume destratification was the more viable option. Mechanical mixers can result in more localised mixing effects, and applications are often limited due to the necessity of a draft tube and the difficulty associated with long draft tubes in very deep reservoirs. 

Compared to selective withdrawal, artificial destratification presents significantly lower capital costs. Operational costs, however, are significantly higher, increasing with the size of the reservoir. One of the main advantages of artificial destratification is the potential benefits to the in-reservoir water quality while concurrently mitigating cold water pollution. By nature of destratifying, DO levels are increased throughout the water column, increasing the health of the reservoir and decreasing the release of undesirable soluble metals and nutrients. Artificial destratification also has the potential to reduce algae growth, by artificially removing the buoyancy advantage it has in a stratified environment. In a deeper reservoir, cyanobacteria should theoretically be mixed to a depth at which it is light deprived, further limiting growth.

With the insights gained out of the literature review, we are looking to the future of mitigating cold water pollution. A number of potential projects will hopefully take us one step further to solving this significant environmental issue, including investigations into:

• Potential feasible sites for in-situ trials of an operational bubble plume destratification systems
• Physical and numerical modelling work to optimise destratification operations
• Renewable energy sources for large scale destratification operations