An assessment and evaluation of dissolved methane concentrations at a natural gas extraction site in the Free State Province, South Africa

Abstract

A worldwide concern associated with natural gas exploration and extraction is the potential for contamination of shallow groundwater. A natural gas extraction operation is underway near Virginia in the Free State Province of South Africa. The aim of this study is to determine whether ongoing gas extraction activity has affected the chemical composition of surrounding groundwater. This was achieved through resampling of boreholes (water wells) previously sampled in 2015 by the operator, prior to gas extraction. Dissolved methane (CH4) above the laboratory limit of detection was present in six out of seven re-sampled boreholes, at distances between 153 and 1538 m from the production well. Dissolved CH 4 concentrations ranged from < 0.007 to 3.16 mgCH4/l, with an average of 0.234 mg/l. Dissolved CH4 concentrations were positively correlated with manganese (r=0.347, n=55, p=0.010) and negatively correlated with bromide (r=-0.338, n=55, p=0.012), calcium (r=-0.303, n=55, p=0.024), sulfate (r=-363, n=55, p=0.006) and electrical conductivity (r=-0.266, n=55, p=0.049). No statistically significant correlation was observed between dissolved CH4 concentrations and proximity to the gas well (r = 0.221, n = 55, p = 0.105), elevation relative to the production well (r = 0.190, n = 55, p = 0.165) or sampling depth (r = .330, n = 55, p = 0.469). The small observed decrease in the mean concentration of dissolved CH 4 between 2015 and 2017 was not statistically significant (p = 0.812). Thus, no evidence was found that the gas extraction operation has contributed to increased concentrations of CH4 in groundwater by means by stray gas migration from the production well. The methane in the sampled wells is believed to be of biogenic origin, based on the positive correlation between dissolved CH4 and NH4 (p=0.045) and the non-detection of higher-chain hydrocarbons. Comparison of pre- and during-production analyses for 24 water quality variables identified several statistically-significant changes over time: increases in pH (0.195 units), arsenic (0.002 mg/l), lead (0.001 mg/l), potassium (0.875 mg/l) and dissolved organic carbon (7.647 mg/l). The South African drinking quality guidelines were exceeded, both before and during gas production, for electrical conductivity, chloride and manganese.