This paper describes the measurement of shallow groundwater levels done during 14 years in the largest Belgian population (Vrijbroekpark, Mechelen) of the rare plant Helosciadium repens (aka Apium repens) which is listed in Annex II and IV of the European Habitat Directive (92/43/EEG). The data come from wells placed in four locations, of which three in lawns with the species and one in a lawn where the species does not grow. These data are analyzed in order to quantify the ecohydrological conditions at both the sites with and the site without the species. Time series, duration curves and statistical parameters of all locations are presented and discussed, as well as their altitude. This demonstrates that the site without the species is somewhat drier than the other three sites, especially in winter. It also shows the dependence of H. repens on a regular supply of shallow groundwater.