A hydrochemical and isotopic case study around a near surface radioactive waste disposal

Abstract

As part of the site characterisation program for the near surface radioactive waste treatment and disposal facility (RWTDF) at Püspökszilágy, Hungary, water quality and environmental isotope investigations have been carried out. Water samples for major ion chemistry, tritium, 14C and stable isotope ratio measurements (δ18O, δD, δ34S, δ13C) were taken quarterly from the observation wells, the streams and the precipitation during the period 1999–2001. The chemical composition of groundwaters presented a continuous transition from waters situated on one side to waters on the top and on the other slope of the disposal suggesting the mixing of the three hydrochemical “endmembers”. Most of δD and δ 18O data were situated between GMWL and LMWL (δD = 7.2 × δ 18O − 1‰) with Oligocene aquifer presenting recharge of Pleistocene origin and water on the top and the gentle slope of the hill presenting recharge of Holocene origin. δ 34S values of dissolved sulphates varied in a wide range (-14.2‰ to + 5.4‰). The tritium in precipitation varied between 4.4 and 18.1 TU with an annual weighted average of 10 ± 0.3 TU. The streams showed larger fluctuations than the wells, but the changes of δ 18O, δD and T were small compared to those in precipitation (showing seasonal variation). Stable isotope, tritium and radiocarbon data proved that the replenishment of groundwater is slow on the steeper side and the direction of water movement is toward the gentle slope of the hill. It was judged that this path is the one that is most likely to give rise to high doses and, therefore, was used in the hydrological modelling of the safety assessment that followed the present work. The possibility that there may also be transport through the unsaturated zone and systems of perched water tables in layers 1 and 2 to both the Szilágyi and Némedi streams cannot be excluded; the transport along these pathways is likely to be intermittent.