The catchment of the Brassus karst spring (Swiss Jura): a synthesis of thetracer tests

Two successive tracer tests were carried out in the Pleine Lune cave which is located in the central part of the Brassus karstic spring catchment area (South-western Jura, Switzerland). During both experiments, the tracers we re not recovered neither. at the Brassus spring nor at the secondary spring s. Following this amazing result, the available data on this spring have be en studied and a synthesis is proposed in this paper. The Brassus karstic spring, situated in the south-western part of Vallee de Joux, is an important resurgence from this part of the folded Jura. The wa ter emerges from Cretaceous limestones covered by a thin layer of moraine: the main alimentation comes however from the underlying Maim limestone aqui fer, Cretaceous limestones bring only a limited part of the total discharge . Within the supposed Brassus spring catchment areal 18 tracer tests were rea lised, but only half of them gave positive results. Positive tracer rests s how low velocities and poor restitution percentage, particularly during low water periods. Such peculiarities are attributed to an important saturated zone, favouring dilution and dispersion of the tracers. The average discharge at the: spring is assumed to be less than 500 l/s and the average specific discharge is about 40 l/s/km(2), following previous d ata on other springs of the area. A calculated catchment area with such val ues would have a surface of 13,5 km(2); but the catchment area derived from the topography of the base of the Maim aquifer (top of the argovian marls considered as an aquitard) covers 56 km(2). This important difference as we ll as the negative results obtained from the Pleine Lune cave tracer tests show how difficult it is to define a catchment area for limestone aquifers: on one hand a delimitation based on water balance calculations tends to un derestimate the catchment area by neglecting outlets as direct infiltration in the alluviums and secondary springs. On the other hand a delimitation b ased on geological considerations seems to overestimate the surface; this i s illustrated by negative tracer tests results with injection points situat ed well inside the catchment area.