Methane storage and ebullition in monimolimnetic waters of polluted mine pit lake Vollert-Sued, Germany.

Reliable gas measurements from supersaturated deep waters still remain a challenge. However, good information is mandatory to investigate the limnic carbon cycle, assess the endangerment through limnic eruptions and evaluate a potential source of exploitable energy. We addressed these three points in a heavily polluted mine pit lake in Germany. We quantified the ebullition of methane from deep waters and the sediment below. Exposed to continuous percolation of gas bubbles, the deep (monimolimnetic) water had accumulated high concentrations of gas: directly measured gas pressures indicated the proximity to spontaneous ebullition. Consequently, the possibility of a limnic eruption was assessed by initiating a self-sustained flow through a vertical pipe. Despite the high gas pressures, the flow was slow and the endangerment was considered low. A sampling strategy with bags was developed to achieve a reliable measurement of gas content and gas composition in the monimolimnion. As a result, directly measured gas pressures could be confirmed and were nearly exclusively attributed to methane and nitrogen. Contrary to lakes that had shown limnic eruptions, carbon dioxide played a much subordinate role, and hence the driving force for a violent outburst of gases was missing. Nevertheless the amount of dissolved methane was remarkably high. This investigation closes with some estimates of the commercial value of the deposit and limiting conditions for a possible exploitation.

Quantifying, assessing and removing the extreme gas load from meromictic Guadiana pit lake, Southwest Spain.

High gas charges in deep waters of lakes can represent a hazard to the lives of human beings and animals in the surrounding. As this danger was feared, we quantified the amount of dissolved gas in Guadiana pit lake (Las Herrerías, Huelva; southwest Spain) and documented the temporal evolution over a period of two years. Gas pressure due to dissolved gases, such as carbon dioxide, methane and nitrogen was measured. Based on these data, we assessed the risk and the associated danger of limnic eruptions from the lake and concluded that the present situation cannot be considered safe. By deploying a vertical pipe, the updraft of degassing water was tested and demonstrated: the pilot plant provided enough energy to drive a self-sustained flow. Such a system could be implemented to remove the extreme gas pressure from the deep water. Measurements of discharges could be extrapolated to indicate the size for an efficient plant for the gas removal. The construction of such a system would be technically and economically viable. A reintroduction of degassed water into the monimolimnion would be advisable.