Mean residence time of groundwater in the Vistula and Narew basins on the basis of tritium measurements

An issue of mean residence time (MRT) evaluation, i.e. the period of time from rainfall until drainage of groundwater by rivers (Danube River Basin, Vistula River Basin and Narew River Basin), is discussed Asimple method of evaluating the groundwater MRT for the Danube River Basin based on tritium data is introduced. It has been confirmed that the historical tritium level in the rivers, being higher than in precipitation, has its end and is reversed, opening a new pathway in understanding the surface-groundwater interaction and specifically in evaluating MRT of groundwater. The authors have defined a separate notification for mean residence time of the whole water in the basin (τ) and mean residence time for groundwater in the basin (τwp). Based on the tritium study the obtained τ values are 6.5 and 5.7 years for the Vistula River Basin (VRB) and the Narew River Basin (NRB), respectively. In turn, the obtained τwp values are 8.0 and 6.0 years, respectively. NRB is a lowland plain and a morphologically, geologically and hydrogeologically uniform basin. The obtained τ wp of groundwater is thus representative for the whole basin. Instead inmorphologically non uniform VRB basin groundwater τwp is a generalized value,i.e.averaged over contributed parts of the basin