Geochemical indicators to determine relations between spring behaviour and water losses of a divergence tunnel

Hydraulic and geochemical relations between a subthermal, SO4-rich spring (Q6) and water losses of a divergence tunnel indicate that both features are hydraulically connected. The divergence tunnel, crossing the evaporite rocks of the Penninic Pontis-Nappe (anhydrite, gypsum, dolomitic limestones) in the Rhone River valley (Central Wallis, Switzerland), deviates water from the Rhone River at a rate of up to 62 m(3)/s. The recharge rate of weakly mineralized water exfiltrating from the divergence tunnel and flowing to the Q6 spring varied between 0.5 and 5 l/s, depending on seasonal variations of the discharge in the divergence tunnel. Methods used to determine the degree of connection included discharge measurements, continuous and monthly water quality measurements, and geochemical modelling. Hydrochemical indicators of particular significance include major inorganic ions, as well as temperature, pH, calcite and gypsum saturation indices. Following the divergence tunnel decommissioning in August 17 1998, simultaneous rises in spring temperature from 13.0 to 13.7 degreesC and electrical conductivity from 2430 to 2600 muS/cm were recorded. A decline in spring discharge from 10 to 6.7 l/s was also observed.