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Extraordinary amounts of greenhouse gases can be stored within the monimolimnion of meromictic lakes, that is, in the water body which is excluded from mixing events. Lake Burgsee (Thuringia, Germany) is a shallow (depth <5 m) lake with a approximately 24 m deep sinkhole, which is fed by underground brine sources and has formed such a monimolimnion. We investigated the carbon dioxide and methane dynamics in this meromictic lake, from production potentials in the sediment via concentrations in the monimolimnion and mixolimnion to emissions to the atmosphere. In the monimolimnion, we found one of the highest methane concentrations (up to >5 mmol L−1) ever reported for a natural freshwater lake, while carbon dioxide concentrations in the water and methane production rates in the sediments were rather ordinary and within the range of holomictic eutrophic lakes. At the thermocline, gas concentrations accumulated to approximately 100 μmol L−1 CH4 and 80–230 μmol L−1 CO2. Estimated fluxes to the atmosphere reached considerable 3.5 mmol CH4 m−2 d−1 and 1.5 mmol CO2 m−2 d−1 above the sinkhole and 0.8 mmol CH4 m−2 d−1 and 0.4 mmol CO2 m−2 d−1 above the near‐by shallow lake center in 2018. Our results demonstrate that lakes in natural brine areas may provide significant storages and releases of greenhouse gases and require further investigation. Plain Language Summary: In meromictic lakes, the deepest water layer, the monimolimnion, is stagnant and not included in seasonal water circulation. Organic matter continuously sinks down into the oxygen‐free monimolimnion, where it is decomposed into the final gaseous products carbon dioxide (CO2) and methane (CH4). Lake Burgsee is a meromictic shallow (depth <5 m), brine‐fed lake with a approximately 24 m deep sinkhole. At the bottom of the narrow sinkhole, salinities are as high as in brackish water and cause a chemical stratification of the water body—a monimolimnion—in approximately 18 m depth. CH4 concentrations above the sediment reach >5 mmol L−1, which is more than one order of magnitude higher than at the water surface and among the highest CH4 concentrations found in freshwater lakes worldwide. Further, emissions of CH4 and CO2 from the water to the atmosphere were considerable in 2018, and about four times higher above the sinkhole than above the shallow lake center. These results demonstrate, that lakes in natural brine areas may store and release significant amounts of greenhouse gases and require further investigation. Key Points: In the urban meromictic Lake Burgsee, methane production potentials in the sediment are similar to eutrophic holomictic lakesAt its deepest site, it contains one of the highest methane concentrations (>5 mmol L−1 CH4) ever reported for a natural freshwater lakeLake Burgsee emits up to >3 mmol m−2 d−1 CH4 to the atmosphere above the sinkhole and <1 mmol m−2 d−1 CH4 at a near‐by shallow site [ABSTRACT FROM AUTHOR]

In the framework of a doctoral thesis and in collaboration with the Brandenburg University of Technology Cottbus, Germany, full-scale field tests were carried out in Goldach, Switzerland, to check the functionality and the behavior of a new system with flexible steel components as reinforcement. Considering the aim of reducing risk due to sinkholes and finding a cost-effective solution, the system should be allowed to deform in the case of an event to make the critical section visually perceptible while not endangering road users. Therefore, the maximum allowed deflection depends on project-specific requirements. Compared to a rigid structure, a flexible component can react promptly to prevent a progressive breaking away of the subsoil. In the tests, an asphalt load-bearing layer as well as a concrete slab with a thickness of 0.20 m each were used. The modeled sinkhole exhibited a rectangular hollow with a free span width of 3.0 m. A total loading of 30.4 metric tons resulted in a depression in the center of the slab of ~0.20 m. The sinkhole protection system was installed right below the binder course and consists of a combination of linear bearing elements combined with a high-tensile steel wire mesh for force distribution. [ABSTRACT FROM AUTHOR]

Abstract: The investigated roof region of a salt diapir in Hamburg, northern Germany, suffers sinkhole activity that was accompanied lately by microseismic events in the Gross Flottbek quarter. Thus, a high geohazard potential is present which can only be evaluated if highly resolved structural data are available. The urban environment and high building density required adapted and new, non-invasive geophysical methods for shallow applications (e.g., urban geophysics). Our shear-wave seismic system, under development at LIAG, consists of a small, horizontal vibrator source (ELVIS, v. 5.0) and a 120m long, mobile land streamer equipped with 120 SH-geophones. Thereby, a cost-effective, complete shear-wave seismic survey is possible, which is dedicated to urban applications and accounts for sealed surfaces. This high-resolution system is especially useful on sealed terrain because of the absence of suppressed (Love) surface waves, and it provides in the near surface an up to 10-times higher resolution than a compressional-wave survey. Across the Wobbe See sinkhole in Gross Flottbek, we acquired 500 profile m of high-resolution shear-wave seismic data that enabled urban subsurface imaging with 1m vertical and 5–10m horizontal resolution. Small-scale structures in the sediments and salt are resolved down to 100m depth. We show that it is possible (1) to classify a sinkhole type structurally and (2) to map subrosion areas by the physical property of lowered shear-wave velocities and a chaotic reflection character. Collapse depth is found here at ca. 60–80m depth below surface, and the fault structures observed coincide well with the focal depth and mechanism of the 2009 microseismic events. Additionally, the diapir model is refined locally in terms of rock salt depth suggested here lying as shallow as 80–100m below surface. [Copyright &y& Elsevier]

The creek Jerstedter Bach is part of the Ringelheimer Mulde along the northern rim of the Harz Mountains in Northern Germany, a trough structure comprising steeply dipping limestones. The limestones are well karstified and drain parts of the region efficiently via sub-surface flow. As the entire region is intensively used by agriculture, contamination of the fast-flowing karst aquifer is a potential problem. During 2006, a small sinkhole (Windmühlenweg) developed alongside the Jerstedter Bach close to the city of Goslar. The sinkhole enlarged rapidly, with a small cave within the gravel cover diverting part of the creek to the subsurface karst catchment. Geophysical methods (gravimetry, geoelectrics) were used around the sinkhole to determine its extent and possible evolution. A negative Bouguer anomaly was found over the sinkhole, indicating more voids further downstream. The geoelectric survey helped to identify the old meandering course of the small river, which was straightened during the cultural land reform in the twentieth century. [ABSTRACT FROM AUTHOR]