HCl degassing from extremely acidic crater lakes: preliminary results from experimental determinations and implications for geochemical monitoring

<jats:title>Abstract</jats:title> <jats:p> Crater lakes are monitored to detect volcanic unrest starting from the assumption that they behave as condensers for magmatic gases. A further assumption is that acidic gases such as HCl are conservative once dissolved in water. This is not true for extremely acidic crater lakes, whose H <jats:sup>+</jats:sup> activity is high enough to induce Cl <jats:sup>−</jats:sup> hydrolysis and consequently HCl degassing. This study presents the results of experimental determinations at 40–45°C demonstrating that HCl degassing from acidic water depends on pH and Cl <jats:sup>−</jats:sup> concentration. HCl degassing starts at pH values <jats:italic>c.</jats:italic> 0.05–0.1 with a rate of 5–10 mg min <jats:sup>−1</jats:sup> l <jats:sup>−1</jats:sup> , increasing up to <jats:italic>c.</jats:italic> 70 mg min <jats:sup>−1</jats:sup> l <jats:sup>−1</jats:sup> at pH<−0.2. This implies that the rate of HCl removal from a crater lake with a volume of 10 <jats:sup>4</jats:sup> –10 <jats:sup>5</jats:sup> m <jats:sup>3</jats:sup> and a seawater-like Cl <jats:sup>−</jats:sup> concentration ranges from 5 to 50 t h <jats:sup>−1</jats:sup> . The estimated HCl/H <jats:sub>2</jats:sub> O ratio in the separated vapour phase (0.01–0.2) is coherent with HCl/H <jats:sub>2</jats:sub> O ratios of fumaroles. Our experiments imply that: (i) the presence of very acidic gas species in fumaroles can be associated with a liquid-dominated feeding system, and (ii) dissolved in extremely acidic crater lakes, Cl <jats:sup>−</jats:sup> behaves as a non-conservative component. </jats:p>