The utility of concentration duration curve in CH4 and CO2 risk assessment

Abstract

Current CH4 and CO2 risk assessment of comparing the single occurrence of worst-case concentration with trigger values of 5% and 1% respectively is often of low resolution but could be improved by the application of the concept of Concentration Duration Curve (CDC). With the aid of the Gasclam (In-borehole continuous gas monitor), four sites were monitored for CH4 and CO2 concentrations, and the time-series datasets used to construct CDC. The result shows that a 5% CH4 concentration is exceeded for 17, 41, 0, and 0% of the monitoring period in sites 1-4 respectively, whilst a 1% CO2 concentration was exceeded for 75, 75.5, 100, and 93% of the time in sites 1-4 respectively. The recorded worst case CH4 concentration are 11.5, 22.1, 2.7, and 1.56% in sites 1-4 respectively while that of CO2 concentration are 8.2, 15.5, 3.3, and 6.71% in sites 1-4 respectively This implies that treating risk in terms of a single occurrence of the worst-case ground-gas concentration rather than any sort of time-weighted function can be defective. While the concept of CDC can be useful in improving risk assessment due to CH4 and CO2, the worst-case ground-gas concentration may not occur during the monitoring period, therefore prediction is required. To predict the worst-case ground-gases concentration requires an understanding of the processes responsible for controlling gas concentration.