Origin, evolution and residence time of saline thermal fluids (Balaruc springs, southern France): implications for fluid transfer across the continental shelf

Abstract

Thermal fluids in the Balaruc-les-Bains peninsula, on the northeastern edge of the Thau lagoon (southern France), supply the third largest spa in France. These thermal fluids interact with karst water in the Upper Jurassic aquifer composed of limestone and dolomite, forming two massifs to the east and north of the lagoon. These calcareous formations extend under the western end of the Thau lagoon. Geochemical and isotope analyses were carried out in 1996 and 1998 on the thermal wells of the Balaruc-les-Bains peninsula to determine the origin of the thermal fluids and their interaction with subsurface karst water. The thermal fluids are a mixture of karst water and water of marine origin. 3H and NO 3 concentrations show that the proportion of present-day karst water in certain thermal wells is small (<5%), thus enabling us to define a “pure” thermal end-member. The thermal end-member is itself a mixture of seawater and meteoric paleowater. Ca and Sr concentrations indicate a lengthy interaction with the carbonate substratum of the deep reservoir. Sr isotope signatures are very homogeneous and associated mainly with the dissolution of Jurassic carbonate, but also to evaporitic minerals. δ 13C contents indicate that this dissolution is linked to deep inflow of CO 2. 87Sr, trace element and rare earth element (REE) concentrations indicate that there is also a component, with a systematically minor participation, whose origin is deeper than the Jurassic carbonate and attributed to the Triassic and/or to the crystalline basement. 36Cl concentrations are extremely low, indicating a residence time of around a hundred thousand years. The outflow temperature of the thermal fluids reaches 50 °C, and geothermometers indicate a reservoir temperature of around 80–100 °C, locating this aquifer at a depth of between 2000 and 2500 m. The geometry of the geological formations indicates a thrust plane associated with major basement faulting that separates the two calcareous massifs and seems to control the rise of deep thermal fluids from the Jurassic carbonate reservoirs and the participation of a deeper component from the basement and/or the Triassic. The present study shows that seawater can infiltrate at great depths and reside for long periods of time compared to the subsurface groundwater cycle. Compared to other highly saline fluids encountered in basement zones, these waters have a relatively well-preserved marine signature, probably due to the carbonate nature of the aquifer in which the fluids resided and their short residence time.