The origin of groundwater in Carboniferous and Devonian aquifers at Maastricht

Abstract

Two approximately 500 m deep boreholes were drilled in Maastricht (The Netherlands) and in one piezometers were placed at respectively 240 and 480 m depth. From the piezometric heads, which are several metres above ground level, and the existing groundwater contour maps of the area no firm conclusions concerning the groundwater origin can be drawn. However, oxygen-18 and carbon-14 isotope analyses of the groundwater indicate a meteoric source. The calculated subsurface migration times suggest infiltration in the Middle-Weichselian for water from the upper piezometer and an Early-Weichselian age for water from the lower piezometer. Ionic ratios for bromide and chloride indicate that both groundwater types are influenced by salts dissolved from the rock matrix. Dissolved salts are supposed to be taken up by diffusive transport from pores with stagnant water into the cracks where the main groundwater flow takes place. Well tests performed on both piezometers yielded hydraulic conductivity. It is concluded that the flow velocities in the upper and lower lavers are different. However, the area of origin of the groundwater found in both screens could be the same, taking into account the differences in conductivity and piezometric head.