METHODICAL ASPECTS OF MODELING THE WATER AND TRITIUM BALANCE OF THE UNSATURATED ZONE

Detailed investigations of the unsaturated zone in regard to water and tritium balance and tritium input into groundwater are necessary especially in the case of thick and inhomogeneous aeration zone (different soil layers, day horizons etc.) and inhomogeneous land utilization. The paper shows a method to quantify water and tritium movement and storage in unsaturated zone by means of a two-dimensional conceptual box model called BOWAM(T). The model was tested on 4 sandstone aquifers in the Finne region (brink of the Thuringia basin). The results show that water balance depends strongly on surface morphology and infiltration capacity, non- or semiconducting layers within aeration zone, land usage (forest, agriculture, meadow...) and atmospheric conditions. Especially runoff, evapotranspiration and groundwater recharge at Finne region have a sensitive variation. Naturally, the tritium balance is connected with water balance. There is a difference in tritium input functions into groundwater between covered and uncovered aquifers. In the case of a covered aquifer with a deep groundwater table, tritium ''bomb peak'' is within the aeration zone and has not left the aeration zone into groundwater up to now. In the case of uncovered aquifers with a groundwater table near the surface it was found a time different tritium input function by means of the BOWAM(T) model, depends on groundwater distance From soil surface and land usage. A comparison to measured tritium activities in groundwater (wells and springs) shows a respectable correspondence. Further investigation and groundwater modelling is planned.