THE HYPORHEIC ZONE - HYDROGEOLOGICAL AND GEOCHEMICAL STUDY OF A STREAM IN THE PYRENEES MOUNTAINS

The hydrogeological and geochemical characteristics of the hyporheic z one were studied for a year on a 30 m(2) site within a stream in the P yrenees Mountains. This included the drawing of potentiometric maps, m easuring hydraulic conductivity and porosity, and the monthly analysis of major chemical elements in water samples. Use of the combined data led to the clarification of certain aspects of the characteristics, f unctioning and development of the hyporheic zone. Even though the area studied lies in a zone of groundwater recharge, the direct vertical e xchange between surface-and groundwater does not seem to be determinan t in the potentiometric and chemical development of the area. However, the state of reserves, which can be reduced or reconstituted, seems t o play an essential role in flow dynamics, and thus of the chemistry o f this area. Flow dynamics in the hyporheic zone apparently are of the longitudinal-transversal type, related to the local stream morphology . The results show how difficult it is to define the hyporheic zone in a structural sense, even though the zone represents an entity that is clearly characterized by specific and identifiable dynamics.