Effect of leaking natural gas on soil and vegetation in urban areas

Leakage of natural gas from the gas distribution system affects the physical, chemical and biological processes in the soil. Particularly the microbial oxidation of methane is then of predominant importance for the composition of the soil gas phase. The rate of methane oxidation was measured under varying conditions of gas phase composition, temperature and nutrient supply. Computation models were evolved with which it is possible to calculate the effect of these and other factors on the distribution of methane, oxygen and carbon dioxide around a leak. Experiments with actual and artificial leaks as well as the calculations showed that the extent of the gas zone largely depends on the leakage rate, the depth of the groundwater table, the soil moisture content and the extent of the pavement. The soil temperature also proved to have a significant influence by its effect on the microbial methane oxidation. At low temperatures this microbial process is limited and consequently the anaerobic zone, which is invariably present in summer, may then disappear completely, thus making the probability of injury to vegetation negligible in winter. After repair of the leak the poor aeration conditions in the soil may persist for quite a long time. This is caused by the high consumption rate of oxygen required for the oxidation of organic substances and reduced anorganic compounds accumulated in the soil during gas leakage. The oxygen overdemand and the oxidation rate were determined for various gassed soils. Measures can be taken to accellerate soil recovery processes and to improve conditions for regeneration of injured trees and before planting new trees. Both experiments and calculations with computation models proved that installation of open ventilation channels is very effective, even if the leak cannot be immediately repaired. So ventilation channels can also be installed as preventive measure