Exsolution of nitrogen and argon by methanogenesis in Dutch ground water

Abstract

Dutch ground water was sampled under high pressure in stainless steel vessels and analysed by gas chromatography for Ar, N 2, CH 4 and O 2. Results show a deficit N 2 and Ar concentrations combined with a high CH 4 concentration that are attributed to the occurrence of exsolution of Ar and N 2 during methanogenesis (the focus of this article). They also show N 2 and Ar in concentrations exceeding the concentration in equilibrium with atmospheric air that are caused by the entrapment and subsequent (partial) dissolution of excess air; in recent ground water, N 2 excesses also are caused by denitrification. phreeqc model runs of methanogenesis at different pressures, with different amounts of organic matter decomposition and different amounts of initial dissolved N 2 show five features. Firstly, methanogenesis under lower hydrostatic pressures causes more N 2 and Ar exsolution than methanogenesis under higher hydrostatic pressures. Secondly, when little organic matter is decomposed (e.g. 1 mmol/l), the value taken for the fixed total pressure (between 1.05 and 3.0 atm.) influences mostly the N 2 and Ar concentration, and when much organic matter is decomposed (e.g. 16 mmol/l), the value taken for the fixed total pressure influences mostly the CH 4 concentration. Thirdly, in order to explain the observed gas concentrations, not more than 27 mmol of organic matter should be decomposed per litre of soil. Fourthly, the phreeqc run with initial denitrification (initial N 2 pressure=1.0 atm.) fits the data better than the phreeqc run without initial denitrification. Fifthly, the maximum total pressure of bubble formation is 3.0 atm. Analysed gas concentration profiles show that the greatest source of CH 4 is found up to approximately 20 m below surface. However, samples with CH 4 partial pressures >0.25 atm. were found up to a maximum average sampling screen depth of 81 m below surface.