A conceptual model of hydrogeological function and perchlorate transfer in the unconfined Champagne Chalk aquifer (NE France)

Groundwater of the unconfined Chalk aquifer in Champagne-Ardenne (NE France) is contaminated by perchlorate (ClO4−), a persistent water-soluble anion. The Chalk aquifer is a crucial water resource of the region, with complex hydraulic properties. The presence of ClO4− is of concern due to its potential adverse human health effects. In France, three sources of ClO4− contamination are suspected: industrial, military and agricultural. Both a comprehensive understanding of hydrogeological characteristics of the aquifer, and a sound knowledge of sources and behaviour of ClO4− in groundwater are required to allow the sustainable use of this groundwater resource. From data acquired during hydrogeological, geochemical, isotopic (ClO4−) and groundwater age (CFCs and SF6) studies, and historical investigations in a study area located east of Reims, a conceptual model of Chalk aquifer function and ClO4− transfer has been established. High spatio-temporal heterogeneities in the unconfined Chalk aquifer are discussed. Different correlations between ClO4−, major ions and groundwater-level fluctuations are shown and interpreted, highlighting main factors governing the Chalk groundwater geochemistry and ClO4− transfer mechanisms, including water-level fluctuation, groundwater residence time, thickness of the unsaturated zone, superficial formations, distribution of fissure networks, aquifer–river relationships, origin and location of ClO4− in soil and human activity.