Near-surface and deep groundwaters

Abstract

Groundwater occurs in all climatic zones and differs in hydrodynamic, isotopic and chemical terms according to its activity within the water cycle and its interaction with rocks. Among the many chemical components in groundwaters, tritium concentrations allow differentiation between groundwater of high and low mobility and thus permit the distinction between shallow and deep groundwaters. In Germany shallow groundwaters range in depth between 10 and 100 m and account for more than 85% of groundwater recharge. Deep groundwaters, in contrast, participate in the water cycle only at intervals of hundreds or thousands of years and reach depths of several hundred metres. Both types of groundwater overlie connate waters that have been isolated from the water cycle since the accumulation of the sediments in which they occur. The exploitation of deep groundwaters mostly causes long-term non-steady-state conditions, large-scale changes of flow patterns of the groundwaters and hydraulic short circuits to shallow aquifers. Thus, persistent contaminants may penetrate extraction wells in deep groundwaters even if the wells are believed to be well protected against pollution. In deep groundwaters it is difficult to apply the conventional use of groundwater protection zones to prevent this kind of intrusion of contaminants.