A radiochemical, hydrochemical and dissolved gas study of groundwaters in the Molasse basin of Upper Austria

The Innviertel (Ottnangian) formation of the Molasse zone of Upper Austria is of major importance as a source of potable water. A hydrochemical, radiochemical and dissolved gas study was undertaken to establish the flow pattern in the Innviertel and its relationship to groundwaters in deeper formations in the Molasse basin. The Innviertel groundwaters could be differentiated into three groups on the basis of their chemical compositions. Waters from shallow boreholes which sampled very localised systems are oxidising waters of the Ca-bicarbonate type and may have high 3H contents. A transition to reducing groundwaters occurs in the second and third groups, with Na+ replacing Ca2+ as the dominant cation and sulphate being reduced to sulphide. The uranium solution chemistry also changes systematically with this change in redox conditions. The 234U/238U activity ratios are enhanced for group 2 groundwaters and decrease again in group 3 groundwaters. The low Cl contents of all the Innviertel groundwaters indicate that the marine sediments have been completely flushed by meteoric water and this is confirmed by the stable isotope composition of the water. Groundwater residence times have been estimated from hydrological data, radiogenic 4He accumulation rates and palaeotemperatures of recharge which were estimated from noble gas contents. The 3He/4He ratio shows that most radiogenic He diffuses from the basement granite and the He flux was estimated on the basis of diffusion theory. Groundwater residence times were assessed from this diffusive flux of helium and together with the recharge temperatures, indicate that the envolved groundwaters of the Innviertel were recharged during the interstadial which preceded the last glaciation. (orig.)