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FIG. 2. Breakthrough curves of He, Ne, and CH4 at the injection–extraction point during three pairs of gas push-pull tests (GPPTs); "a" designates curves from active GPPTs, and "i" designates curves from inactive tests. The part of each GPPT test pair where CH4 consumption was discernible is shown in more detail. Note the different scales of both axes. Noble gases could not be measured for the entire extraction period in GPPT 3 as the analytical method available at the time was less sensitive. In GPPT 1 He dropped below the quantification limit after 1.4 h, and Ne after 2.1 h.
