THE BIMODAL PH DISTRIBUTION OF VOLCANIC LAKE WATERS

Abstract

Volcanic lake waters have a bimodal pH distribution with an acidic mode at pH 0.5–1.5 and a near neutral mode at pH 6–6.5, with relatively few samples having pH 3.5–5. To investigate the reasons for this distribution, the irreversible water–rock mass exchanges during the neutralization of acid SO4–Cl waters with andesite, under both low- and high-temperature conditions, were simulated by means of the EQ3/6 software package, version 7.2. Reaction path modeling under low temperature and atmospheric PCO2 and fO2, suggests that several homogeneous and/or heterogeneous pH buffers exist both in the acidic and neutral regions, but no buffer is active in the intermediate, central pH region. Again, the same titration, under high-temperature, hydrothermal-magmatic conditions, is expected to produce comparatively infrequent aqueous solutions with pH values in the 3.5–5 range, upon their cooling below 100°C. Substantially different pH values are obtained depending on the cooling paths, either through boiling or conductive heat losses. These distinct pH values are governed by either HSO4− and HCl(aq), in poorly neutralized aqueous solutions, or the CO2(aq)/HCO3− couple and the PCO2 value as well, in neutralized aqueous solutions. Finally, mixing of the acid lake water with the aqueous solutions produced through high-temperature titration and cooled below 100°C is unlikely to generate mixtures with pH values higher than 3, unless the fraction of the acidic water originally present in the lake becomes very small, which means its virtually complete substitution. Summing up, the evidence gathered through reaction path modeling of the neutralization of acid lake waters with andesite, both at low and high temperatures, explains the scarcity of volcanic lake waters with measured pH values of 3.5–5.