Early assessment of groundwater vulnerability during groundwater exploration is an important precondition for effective protection. Any attempt to assess the “vulnerability” is confronted with the very different factors influencing this property. In this paper we present a new concept to combine different factors on an objective basis. A protection index γ is defined and it is shown that this index can be obtained by the additative combination of a self-cleaning index (γs), a retardation index (γR) and a dilution index (γv). For the estimation of the latter one, isotope techniques are suitable. With lumped-parameter models, easy formulae are derived for the calculation of γvfrom tritium data. The applicability of this approach was tested with data from two field studies. The results obtained show that a reasonable assessment of groundwater vulnerability can be achieved. However, a lot of problems and questions remains to be solved by further investigation.