Trapped gases and elevated carbon dioxide in the subsurface create major problems near residential areas

Two case studies of trapped gases with elevated levels of CO (sub 2) migrating from the subsurface into the basements of homes in residential areas in northern Illinois and central Pennsylvania will be discussed. Stable carbon isotope and radiocarbon analyses were performed on samples of the elevated CO (sub 2) gas samples in each case to help determine the source of CO (sub 2) so the problem could be mitigated. The problem in Illinois escalated after large snow events began to melt. The concentration was measured at levels greater than 7% by volume in the basement and as large as 11% in soil probes nearby. In the PA site the concentration of CO (sub 2) near the floor in the basement of a new house was measured at 4% with similar concentrations from boreholes in the yard. In both cases several different possibilities for the trapped gases were considered by local authorities including crushed limestone beneath new pavement, illegal dumping of acidic waste chemicals in the area and/or previous burial of organic wastes within the vicinity, and the possible presence of old coal mining operations or coal seams under the property. The isotopic results helped confirm the source of CO (sub 2) at each site. In Illinois the problem turned out to be related to the construction of a retention basin next to a stream nearby. During the construction of the basin the water table was lowered using a ring of dewatering wells. Lowering the water table allowed air to be drawn into the sediments which were usually saturated. Organic matter buried in the sediments began to oxidize increasing the levels of CO (sub 2) . This was not a real problem until after large snow events began to melt and flooding occurred trapping the CO (sub 2) and air between the lowered water table and the influx of water from the surface. The gas pocket became pressurized and began to follow the path of least resistance out toward the surface which included the basement of a local house. In PA the problem was originally thought to be associated with a coal seam beneath the property but isotopic results showed the CO (sub 2) was from buried brush and plant debris from excavation grading processes during the development of the property. In both cases the problems were mitigated once the source of CO (sub 2) was established.