Variation in Borehole Gas Concentrations During Long-term Monitoring in Cardiff, South Wales, UK

Temporal variation in methane gas concentrations within a network of monitoring boreholes in Cardiff South Wales is presented. Monitoring was initiated in 1995 and terminated in 2002 and utilised some 64 borehole installations sampling the unconsolidated Quaternary and recent deposits. In addition to concentrations of methane, oxygen and carbon dioxide, the project recorded monthly atmospheric pressure, 24 hour antecedent atmospheric pressure change, relative borehole pressure, borehole temperature, groundwater level and 48 hour antecedent precipitation. These data were used in a bivariate and multivariate regression analysis of variations in gas concentration. Although boreholes responded differently to environmental forcing mechanisms, the overall average concentration of methane demonstrated trends apparently related to Barrage construction and impoundment.