The influence of extraction rate on the reduced sulphur content of Aix-les-Bains’ thermal spring waters: Consequences for resource-quality monitoring

Sulphurous thermal springs account for a few percent of all thermal springs. As each degree of oxidation of elemental S corresponds to a different S species, sulphurous spring waters may contain a variety of S species. Total S content is one of the parameters measured when analysis is carried out for issuing a spa’s operating licence. Under French law these parameters must be stable over time. The two spas in Aix-les-Bains are fed by a number of natural springs and boreholes, whose waters have total reduced S concentrations of between 30 and 800 mg L⁻¹. To ensure compliance with the requirements of the spas’ operating licences, particularly in terms of total reduced S content, official analysis of the waters is carried out every month at random dates and times. Monthly analyses have revealed seasonal variations in the S content of some of Aix-les-Bains’ springs; therefore, more frequent monitoring was carried out in order to investigate the extent and causes of these fluctuations. As well as seasonal variations, this monitoring has revealed daily and hourly fluctuations that appear to be related to extraction rates. For some of the springs and boreholes, S concentrations were correlated with extraction rates; for others, an increase in extraction rate led to a reduction in total S content. Concentrations of sulphide ions (one of the species included in the total sulphur analyses) were monitored at the same time as the total S content. Variations in sulphide ion concentrations and in sulphide concentration/total S ratios did not appear to be linked to variations in flow rate. These tests show that random monthly testing is not suitable for monitoring the stability of S contents, as this parameter can only be considered stable in terms of its yearly mean value.