Cold lake during a winter morning

50 per cent cut in freshwater methane emissions possible, say researchers

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Methane emissions from lakes and other freshwater sources could be reduced by 50 per cent or more using a phosphate-binding method, according to a study by researchers in the Netherlands.

Lakes and other freshwater systems emit large quantities of methane, the second-most important greenhouse gas worldwide after CO2. Dredging and the use of Phoslock (a phosphate-binding clay particle) could reduce these lake emissions by over 50 per cent, according to a study by Radboud University researchers, published today in Science of The Total Environment.

Methane largely originates in natural systems, with 49 per cent of all emissions coming from freshwater systems. Worldwide methane emissions also continue to increase as a result of global warming and eutrophication (an excess of nutrients in water). Radboud University researchers investigated whether reducing this eutrophication could help curb methane emissions.

The study used a system of 16 testing ponds: some of the ponds were dredged, some were treated with Phoslock, and in the last group both techniques were used. “Dredging sediment from the ponds’ bottom reduced methane emissions by approximately 52 per cent,” said Tom Nijman, an aquatic ecologist and author of the paper. “In the ponds where we used Phoslock, emissions were even reduced by 74 per cent.”

Both methods appear to dramatically reduce emissions. “Phoslock leads to less phosphate in water, which in turn reduces the number of floating plants," Nijman added. "Invasive plant species such as Azolla (also known as water fern) grow less fast with Phoslock, resulting in less methane being produced in the water. Dredging also allows us to remove organic material from the bottom, i.e. the carbon that produces methane. Our study shows that both methods reduce eutrophication in a lake, thereby also reducing emissions.”

The researchers say it is premature at this point to extend the approach to as many lakes as possible. “These experiments first have to be repeated on a large scale. The first results are promising, but we want to measure the effect in more locations and over a longer period of time to see whether the positive effects lasts,” Nijman said.

In addition, the approach is not suitable for every situation, as it becomes too expensive for that, as Nijman showed with a simple calculation. “An approach based on the use of Phoslock can easily be six to ten times more expensive than an approach that focuses on the environment surrounding the lake. This is not always possible in the Netherlands: think of lakes that are surrounded by trees or agricultural land, in some cases with little to no water flowing through. In such places you cannot simply overhaul the entire environment, in which case it might be useful to use dredging or Phoslock.

"In places where the environment can be adjusted, the latter is always preferable. Prevention is still better than a cure.”

Phoslock is the commercial name for a bentonite clay in which the sodium and/or calcium ions are exchanged for lanthanum, which reacts with phosphate to form an inert mineral known as rhabdophane. It was originally developed in Australia in the late 1990s by Dr Grant Douglas as a way of utilising the ability of lanthanum to bind phosphate in freshwater natural aquatic systems.

Phoslock has been used in lake restoration projects worldwide to remove excess phosphorus from aquatic systems, thus improving water quality and inducing biological recovery in impaired freshwater systems. 

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