Degassing, dating and deeper insights into the groundwater dynamics of an arsenic contaminated aquifer in Vietnam

Arsenic affected alluvial aquifers in South and Southeast Asia commonly exhibit strongly reducing, methane-rich conditions, where in-situ gas production can induce substantial degassing of groundwater. Such conditions complicate groundwater dating calculations using the tritium–helium-3 ( ) ingrowth method, which is widely applied to infer groundwater flow velocities and contaminant transport timescales. This study evaluates groundwater from a methane-rich aquifer in Van Phuc, Vietnam, with the aim of applying the method to date groundwater. Dissolved noble gas concentrations were analysed to account for degassing and excess air components required for effective isolation of the tritiogenic fraction. Groundwater from 24 of the 28 sampled wells shows clear evidence of degassing, with up to 98% depletion of dissolved gases relative to expected atmospheric equilibrium concentrations. After correcting for degassing, residence times range from 35 to 65 years and are highly sensitive to both the magnitude and timing of degassing. In addition, a comparison between He, Ar, and Kr concentrations measured on-site using membrane-inlet mass spectrometry (GE–MIMS) and those obtained from conventional copper tube samples revealed substantial discrepancies in He concentrations—of up to cm g -1 —whereas comparable differences for Ar and Kr were considerably smaller. Discrepancies were observed in wells with elevated methane concentrations and total dissolved gas pressures exceeding 1 bar. These differences are interpreted as resulting from sampling-induced degassing during copper tube groundwater collection, superimposed on in-situ subsurface degassing, further complicating groundwater age determination. Finally, degassing is shown to be more widespread than originally assumed, contrary to previous assumptions, necessitating revision of earlier groundwater age estimates. The revised understanding of degassing processes and groundwater age distributions is applicable to similar aquifers where in-situ gas production may influence contaminant transport. • Two analytical methods for noble gas analysis diverge in a CH4 producing aquifer. • A simple degassing model with excess air bounds 3 H- 3 He groundwater ages. • Methane production influences noble gas behaviour in aquifers in unpredictable ways. • Degassing affects groundwater beyond methane-producing zones.