investigation of sedimentary filling in the Germont-Buzancy marsh : impact on local hydrogeology and hydrology
Résumé
The Germont Buzancy fen, located at the head of the Bar watershed, represents the largest alkaline peatland in the Ardennes. Classified as a NATURA 2000 site and ZNIEFF type I area, it constitutes a key biodiversity hotspot, hosting emblematic taxa such as Castor fiber and Coenagrion mercuriale, as well as characteristic helophytic communities (Carex, Cladium). The system is nevertheless affected by recurrent summer edaphic dewatering in its central sector. Sedimentological and hydrogeological investigations were undertaken to reconstruct the fen’s long-term dynamics. Radiocarbon dating indicates the accumulation of a continuous 1.8 m peat sequence since the Late Glacial–Holocene transition, providing a valuable palaeoenvironmental archive. Beneath the peat, a 6–7 m thick succession of clayey-sandy silts, deposited as rhythmites, testifies to lacustrine sedimentation during the final Cataglacial stages. This unit hydraulically isolates the shallow peat aquifer from an underlying coarse calcareous alluvial aquifer resting on valley bedrock. ESR dating of the latter yielded an age of 450 ± 100 ka, correlated through geomorphological reconstruction with terrace formations associated with the hydrographic capture event of the Aire–Bar system. The resulting sedimentary architecture defines a multilayer aquifer system. Groundwater recharge of the fen is complex: the deep confined aquifer is hydraulically disconnected from surface processes, whereas the peat layer is recharged by karstic springs, surface runoff, and overbank flows from the Bar River, and the Germont ditch. However, anthropogenic drainage and catchment reconfiguration since the 18th century, combined with global climate change, have modified the recharge regime. Due to its domed morphology, the central sector is predominantly ombrogenous, while marginal areas are sustained by lateral inputs, including runoff and karstic resurgence, reflecting topogenous and soligenous processes. The recent recolonization of the site by several beaver (Castor fiber) populations induces localized and temporary rewetting. Dam-building activity raises upstream water levels, generating overbank flows that periodically flood the fen and adjacent plots. These hydrological alterations are sources of conflicts in land use with local stakeholders, particularly farmers and livestock breeders, who are affected by the resulting inundations.