Deuterium, oxygen-18, and tritium in Stripa groundwater

Abstract

Deuterium, oxygen-18 and tritium determinations show that groundwaters in the fractures of the Stripa granite are of meteoric origin, not affected by secondary isotope effects (evaporation or isotope exchange). Deep ground-waters have lower 18O and 2H contents than recent, shallow groundwaters. The differences are most likely due to recharge during cooler climatic conditions. However, no age information can be derived because recharge could have occurred before the last major climatic improvement following the last deglaciation, during minor climatic variations during the more recent past, or could be the result of regional groundwater flow. Low 18O contents are paralleled by increasing salinities with highest salinities limited to a narrow fracture zone at a depth of about 800 m below ground surface. Most saline waters sampled appear to represent mixtures of a saline component with an isotopically depleted water and local fresh water. The presence of young groundwaters is indicated by bomb produced 3H. All surface boreholes contain tritium, indicating residence times <35 years. However, tritium is also found in some boreholes drilled at the 300 m mine levels and its presence may be due to flows induced in response to heater tests. Low but detectable 3H contents (about 0.1 to 1 TU) in V2 at about 850 m below groundsurface are discussed either as subsurface production and/or as a result of admixtures of small amounts of recent water to old groundwater.