Assessing the impact of semi-permeable cover on karst with natural tracer and physico-chemical monitoring. Example of the Moulineaux spring (Dordogne, France)

Undercover karst are characterized by limestone formations underneath a variability thick, low-permeability cover. Karst landforms such as sinkholes or swallow holes are thus not very frequent in these environments. This leads to a high inertia of the environment. This makes it complex to use the tools and methods usually employed to characterize a system and complexify in the interpretation of usual chemical methods to understand the role of the cover karst system.

The covered karst system of the Moulineaux spring  is a key resource for the urban area of Perigueux (France) by ensuring the supply of drinking water to more than 60,000 inhabitants. It’s average flow rate is 820 L.s^-1 and can range between 118 L.s^-1 and 4 000 L.s^-1. The karstic system is mostly covered by a thick semi-permeable layer of alternating marly limestone, alterite rocks and sediments dating from the Campanian period (Upper Cretaceous). Its sizeable catchment area spans more than 80 km² more than 50% of which is occupied by agricultural activities.

A year-long monitoring campaign of physical-chemical parameters has been conducted at the spring, complemented by periodic analyses of major chemical elements at several locations within the study area. A combined approach was used to analyze long-residence-time tracers such as magnesium and silica, natural markers of anthropogenic pollution such as nitrates, potassium, sulfates, pesticides, and short-residence-time tracers, including artificial tracers, dissolved organic carbon (DOC) measurements, physico-chemical parameters, and pCO2​. The results were integrated into a conceptual model of the karst spring, highlighting the significant role of the semi-permeable cover in influencing groundwater quantity and quality. While this cover acts as a natural buffer and filter, anthropogenic markers revealed significant variations in water quality linked to the hydrological cycle.