A STRONTIUM ISOTOPIC STUDY OF MINERAL AND SURFACE WATERS FROM THE CEZALLIER (MASSIF-CENTRAL, FRANCE) - IMPLICATIONS FOR MIXING PROCESSES INAREAS OF DISSEMINATED EMERGENCES OF MINERAL WATERS

Surface and mineral waters from a geothermal area (Cezallier) in the c entre of the Massif Central, France, were analysed for Sr isotopes, Sr and major elements in order to use Sr isotopes to investigate the dis charge of a mineralised water area, especially in the case of dissemin ate mineralised water emergences. The Sr-87/Sr-86 ratios of mineralise d waters ranged from 0.715541 to 0.716773. The variation in Sr-87/Sr-8 6 values was small for the 5 mineral springs sampled over two periods (5 x 10(-6)-14 x 10(-6)). Regarding the reproducibility of the Sr-87/S r-86 ratio measurements, no differences can be seen and the Sr-87/Sr-8 6 ratio of mineral waters can be considered to be constant with time. The Sr-87/Sr-86 ratios of surface waters collected along the Zagat Riv er ranged from 0.705303 to 0.715091. The lowest values were always obs erved in the headwaters. The difference in the Sr-87/Sr-86 ratios betw een the two sampling periods was similar to 22 x 10(-4) and can be rel ated to the hydrological situation (low or high flow). The Sr-87/Sr-86 ratios of surface waters collected in the drain ranged from 0.715072 to 0.716585. The difference in the Sr-87/Sr-86 ratios between the two sampling periods is similar to 11 x 10(-4) for the drain headwaters an d similar to 13 x 10(-5) at the outlet of this area. Likewise, these v ariations can also be related to the hydrological situation. Plots of Sr-87/Sr-86 vs. 1/Sr ratios show two distinct binary mixing trends and the existence of at least four end-members. A mixing parameter f givi ng the proportions of the two components can be estimated for assumed Sr-87/Sr-86 ratio and Sr content values of the pure end-members. An es timate of the mineral water discharge can be calculated using the resu lts of the mixing model and the measured flow. In the drain area, the mixing model gives an estimate of similar to 0.5-11 s(-1), or 10-13% o f mineral water in the total flow. A similar calculation was done for the river where the mineral water flow was similar to 0.96 I s(-1) (3. 5 m(3) h(-1)) 100 m downstream from the bridge in April 1994, and 6 m( 3) h(-1) 20 m downstream from the bridge in September 1994.