The Budos spring, a mixture of old and recent water. Where does the water come from?

International audience

إنجليزي. Budos Springs serve as an important source of water for nearly 160,000 inhabitants of the Bordeaux metropolitan area (Southwestern France). These springs emerge from the karstified limestone aquifer of the Oligocene, outcropping to the east of the Cretaceous Villagrains-Landiras geological structure (Bourbon et al., 2022; Labat et al., 2021; Saltel, 2008). Budos Springs exhibit low mineralization, with electrical conductivities (EC) ranging from 415 to 480 µS/cm, a calcium-bicarbonate geochemical facies, and a high flow rate that is relatively stable over time (Schoeller, 1968). Previous studies have emphasized that Budos Springs are a mixture of recent and older waters (Chery et al., 2001). The oldest water may originate from the Campanian aquifer, which could explain the constancy of the observed flows. To the north of these springs, studies conducted in nearby rivers have revealed a large area of losses located in the Oligocene outcrop in the Tursan and Mouliasse riverbeds (Labat, 2021; Saltel, 2008). Due to the karstic and fractured nature of the carbonate formations in the study area, these losses may also contribute to the recharge of Budos Springs. Thus, this water resource shows a certain vulnerability due to a lack of understanding of this complex karstic environment.Mixing processes, residence times and recharge patterns remain poorly understood. To address these gaps, a multi-tracer method were applied to identify mixing processes between shallow and confined groundwater, water/rock interaction processes, and to estimate residence times and mixing ratios. Several tools were used for this purpose, including physico-chemical parameters, major and trace elements, stable isotopes of water (δ18O, δ2H), strontium (87Sr/86Sr), carbon (δ13C, 14C), and dissolved gases (CFCs, SF6, Ar, Ne, 4He, N2)..Upstream of the springs, initial results show a west-to-east enrichment in Mg and Sr in the Campanian aquifer, indicative of longer circulation times. In addition, 14C activities measured in the same aquifer confirm the increasing age from west to east along the axis of the Villagrains-Landiras anticlinal structure. Finally, a few meters from the springs, the borehole tapping the deeper Cretaceous aquifer has a depleted isotopic content, suggesting an older recharge. The presence of CFCs and SF6 in groundwater sampled at the springs and in the immediate vicinity indicates recharge after 1950, or mixing of older waters with post-1950 waters. According to the binary mixing model, Budos Springs consist of a mixture of 58% older waters and post-1950 waters.From a piezometric perspective, the flow direction in the Campanian aquifer is oriented from west to east toward the springs. Furthermore, piezometric records for the Cretaceous water table are very similar along the axis of the Villagrains-Landiras geological structure. This sector constitutes a homogeneous compartment in terms of behavior, covering an area almost 20 km wide to the west of Budos Springs. Comparing piezometric records in the Campanian aquifer over this area with groundwater outflows measured at Budos Springs shows strong similarity, which could reflect contributions from the Campanian aquifer to the flow of these springs. Some of the water may indeed come from the Campanian aquifer (Bourbon et al., 2022), rising from depth to discharge into the Oligocene aquifer. This could explain the stability of the flow rate of Budos Springs. In the Budos area, hydraulic heads measured in the Oligocene aquifer are lower than those recorded in the Campanian aquifer. Piezometric levels are not identical, suggesting that the contribution of the Campanian aquifer to the flow of Budos Springs is relatively significant. Although the flows show a certain stability, seasonal variations are still observed in the groundwater outflow. Increased flow is associated with higher turbidity and a slight decrease in pH. Electrical conductivity and nitrate concentrations also appear to be increasing. These elements point to a mixed supply, with the proportion of recent water varying according to recharge conditions. These results have provided a better understanding of the flow dynamics within the aquifers contributing to Budos Springs. However, many questions crucial for ensuring the sustainable management of this resource remain unresolved. To provide these clarifications, a research program will begin in 2025. Hydrogeological, hydrochemical, and hydrodynamic methods will be deployed across all surface and subsurface compartments to refine the understanding of mixing proportions and associated temporal variability."