Measurement of methane leakages from lakes in the Siljan Ring

Rapport, 2025

A study was conducted to measure natural methane (CH₄) leaks from lakes in the Siljan Ring—an ancient meteorite crater system where natural gas is present several hundred meters below the surface. The objective was to identify CH₄ emission sources, particularly in areas where local inhabitants have reported recurring winter ice holes. Measurements were carried out during two three-day campaigns, spaced seven months apart, in 2023 and 2024.

This study, for the first time, revealed strong, localized and persistent lake emissions of CH₄ around the Siljan Ring, forming detectable gas plumes in the air (ebullition). Active and highly localized ebullition sources were identified in Lake Orsa (4 locations), Fudalsviken (1 location), and Vikarbyn (2 locations). Elevated concentrations were also observed at Ickviken and around Stumsnäs, both in air and water, although these appear to represent more classical ebullition from lakebed sediment. A few additional potential sources were noted but not followed up. The lakebeds in the region consist mainly of sand, and the leaks were typically found in areas with circulating water and relatively shallow depths (2–5 meters).

Floating chamber measurements indicated CH₄ emissions ranging from 0.05 to 0.2 µg·m⁻²·s⁻¹, while carbon dioxide (CO₂) emissions were approximately 100 times higher. These diffuse surface CH₄ emissions are up to four times higher than typical ebullition rates observed in lakes. Furthermore, measurements using a novel trace gas method revealed ebullition rates ranging from 20 to 300 g/h—significantly exceeding the diffuse emissions measured by floating chambers and surpassing typical lake ebullition rates by two to three orders of magnitude. This indicates that the studied sources are not only dominated by ebullition but are also exceptionally strong and likely unique.  Although the emissions at the hotspot locations are exceptionally strong, their total contribution from a national perspective is negligible. Total CH₄ emissions from the identified hotspots were estimated at approximately 3.5 tons per year (equivalent to 85 tons of CO₂), with additional diffuse emissions contributing around 50 kg per year (1 ton CO₂ equivalent). These emissions represent a very small fraction—about 20 parts per million—of Sweden’s total CH₄ emissions, according to the Swedish National Greenhouse Gas Inventory.

However, this study only targeted hotspots along the eastern shores of Lake Siljan and the southern part of Lake Orsa, and the findings may represent just the tip of the iceberg. Many more sources may exist along other parts of the lakes, in nearby water bodies, at greater depths, or with lower but still significant emission strengths.

Due to the limited number of measurements and potential sampling artifacts, it remains unclear whether the gas is of thermogenic or biogenic origin. Further investigation, including detailed analysis of VOC content and δ¹³C isotopic composition, is needed.

The emission sources in the Siljan Ring are unique in being localized, persistent and in source strength —an uncommon characteristic for lake ebullition, which typically occurs sporadically over broad areas with emission several order of magnitude lower. These types of emissions are difficult to detect and quantify using traditional methods, such as floating chambers or eddy covariance systems.

Further research is needed to assess temporal variability, identify additional sources in the Siljan Ring (which may be numerous), and evaluate their impact on Sweden’s natural climate budget. Additionally, similar lake systems elsewhere should be investigated, and mitigation strategies may need to be developed.