OCCURRENCE OF MINERAL-WATER SPRINGS IN THE STREAM CHANNEL OF THE ALLIER RIVER (MASSIF-CENTRAL, FRANCE) - CHEMICAL AND SR ISOTOPE CONSTRAINTS

The French Massif Central has long been recognized as an area with num erous mineral water springs. It is often very difficult to measure the how rate of mineral springs since they are spread out over extensive areas. Water from the Allier River has been sampled monthly over a 13- month period and a spatial cross-section survey carried out at base fl ow conditions in order to estimate the discharge of mineral water in t he Allier River streambed. For the monthly survey, the Sr-87/Sr-86 rat ios vary between 0.709511 and 0.712656. A good correlation exists betw een Sr-87/Sr-86 and 1/Sr ratios and defines binary mixing between mine ral waters and the Allier River sensu-stricto. The mixing processes ha ve been studied using the Sr isotopic systematic and a conservative sp ecies (chloride). Except for one sample, the mineral water input never exceeds 2%. However, the mineral water discharge into the Allier stre ambed could not be precisely estimated because effluent seepage of min eral water may occur in a part of the channel representing only a smal l volume of the cross-sections. Variations in the solute concentration s across the river channel were measured at 14 sites chosen as being r epresentative of areas where: (a) mineral water emerges, (b) no minera l water emerges, and (c) mixing is complete. A model developed for the monthly study using Cl as a reference was used in the spatial survey. The input of mineral water in the Allier streambed ranges from 0.1-0. 15% in most of the samples and is greater than 2% at two points. The g eneral mass balance of the proportions of mineral water and Allier Riv er water (sensu-stricto) using the sum of cations and anions was calcu lated and the input of mineral water into the Allier streambed was det ermined at the cross-section nearest the Allier gauging station (0.134 %). The discharge of the Allier River at the gauging station was about 17.7 m(3)/s during the sampling period, which gives a discharge of mi neral water of about 0.02 m(3)/s into the Allier channel. (C) 1997 Els evier Science B.V.