Extreme Methane Emissions from a Swiss Hydropower Reservoir: Contribution from Bubbling Sediments

Abstract

Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methane sources and losses. Methane diffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance. Finally, knowing the contribution due to bubble dissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH(4) m(-2) d(-1), which is the highest ever documented for a midlatitude reservoir. The substantial temperature-dependent methane emissions discovered in this 90-year-old reservoir indicate that temperate water bodies can be an important but overlooked methane source.

Intense methane ebullition from open water area of a shallow peatland lake on the eastern Tibetan Plateau

“…The results showed that the rate of methane ebullition in Lake Medo was one to two orders of magnitude higher than that from most lakes in northern region (Mattson and Likens, 1990;Huttunen et al, 2003;Bastviken et al, 2004), including that from the Lake Villasjön in Sweden which is most similar functionally, climatically, and morphologically to Lake Medo (Wik et al, 2013), but lower than that from subtropical and tropical lakes (Keller and Stallard, 1994;Martinez and Anderson, 2013). When compared with wetland ecosystems other than lake, the methane ebullition rate from Lake Medo was much higher than that from tropical floodplain (Marani and Alvala, 2007) and that from rice paddies in temperate or even tropical regions (Ziska et al, 1998;Meijide et al, 2011); Lake Medo on the Tibetan Plateau presented a methane ebullition rate much lower than tropical reservoirs (Sturm et al, 2014;Delsontro et al, 2011), but much higher than even the highest methane emission rate from temperate reservoirs reported so far (Delsontro et al, 2010). Therefore, it could be concluded that the methane ebullition from Lake Medo is much higher than that from wetlands ecosystems in temperate and northern regions (Table 3).…”

Projecting hydropower production under future climates: a guide for decision‐makers and modelers to interpret and design climate change impact assessments

“…Large hydropower reservoirs can have an impact on the local climate but hydropower is generally assumed to be emission-free during the production stage. Locally, this might not entirely hold if considering hydro-chemical processes occurring due to land flooding and leading to emission of greenhouse gases (Chanudet et al, 2011;Hertwich, 2013;DelSontro et al, 2010). Finally, HPP systems can increase or decrease the flood risk downstream of water restitution points.…”

Greenhouse Gas Emissions from Hydroelectric Reservoirs: What Knowledge Do We Have and What is Lacking?

“…Even though the formation of CH 4 bubbles is favoured by high temperatures, temperate reservoirs may also emit significant amounts of CH 4 through bubbling (e.g. Delsontro et al, 2010).…”

“…67 from 2 to 8 millimetres (Delsontro et al, 2010). The bubbles are usually formed in the sediment of reservoirs, under anoxic conditions.…”