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Interactions between groundwater and surface water are often overlooked in lake water budgets, even though groundwater can significantly contribute to the total annual water inputs to a lake. Isotope mass balance models have seen significant developments in the last decade for assessing the spatial and temporal variability of hydrological processes in lakes but are generally applied assuming steady-state. While this assumption is generally acceptable for long-term water balances of large lakes, it may be less appropriate for lakes which undergo strong seasonality of hydrological processes and meteorological conditions. In this study, a volume-dependent transient isotopic mass balance model was developed for an artificial lake (named Lake A) in Canada, and in a context where direct measurement of surface water fluxes is difficult, if not impossible. This lake typically receives important inputs of flood-water during the spring freshet period, as a hydraulic connection with a large watershed establishes each year. Quantification of the water fluxes to Lake A allowed to highlight the impacts of flood-water inputs over the annual water budget. The isotopic mass balance model revealed that groundwater and surface water inputs respectively account for 71 % and 28 % of the total annual water inputs to Lake A, which demonstrates its dependence on groundwater. An important part of these groundwater inputs is likely to correspond to flood-derived surface water due to bank storage. On an annual timescale, Lake A was found to be resilient to surface water pollution and sensitive to groundwater quantity and quality changes. There is however a likelihood that the resilience to surface water pollution is lower from April to August, as important water inputs originating from Lake DM contribute to the water balance via direct and indirect inputs (i.e., from bank storage). This suggests that the surface water fluxes between Lake DM and Lake A did not only have an impact on the dynamic of Lake A during springtime but also significantly influenced the long-term dynamics of Lake A. These findings can help anticipating the impacts of variation in the intensity and/or duration of future flooding events on lakes' water quality. From a more global perspective, this knowledge is useful for establishing regional-scale management strategies for maintaining water quality at flood-affected lakes in a context of land-use and climate changes.

Due to the abundance of surface water in the province of Quebec, Canada, it is suspected that many groundwater wells are pumping a mixture of groundwater and surface water via induced bank filtration (IBF). The regulatory framework in Quebec provides comprehensive guidelines for the development and monitoring of surface water and groundwater drinking water production systems. However, the regulations do not specifically address hybrid groundwater-surface water production systems such as IBF sites. More knowledge on the use of IBF in the province is needed to adjust the regulations with respect to the particularities of these systems. In order to provide a first evaluation of municipal wells potentially using IBF and the corresponding population served by these wells, a Geographic Information Science framework (GISc) was used to implement an IBF spatial database and calculate the distance from each well to the nearest surface water body. GISc is based on open source GIS programs and openly available data, to facilitate the reproducibility of the work. From this provincial scale approach, we show that nearly one million people are supplied by groundwater from municipal wells located &lt, 500 m from a surface water body, and half a million have a significant probability to be supplied by IBF wells. A more focused look at the watershed scale distribution of wells allows us to improve our interpretations by considering the aquifer type and other regional factors. This approach reveals strong spatial variability in the distribution of wells in proximity to surface water. Of the three selected regions, one has a high potential for IBF (Laurentides), one requires additional information do draw precise conclusions (Nicolet), and the third region (Vaudreuil-Soulanges) is unlikely to have widespread use of IBF. With this study, we demonstrate that extensive use of IBF is likely and that there is a need for improved understanding and management of these sites in order to properly protect the drinking water supply.

Fractured volcanic aquifers are the main water resources in the arid climate of the Republic of Djibouti. Nonetheless, these strategic reservoirs are overexploited and their comprehensive understanding is therefore a pre-requisite for a sustainable use. A geochemical and isotopic survey, including major ion chemistry, 2H, 18O, 13C, 3H, 87Sr/86Sr, 15N was performed and combined with existing 14C data to study recharge, contamination processes and groundwater flow patterns inside and between the compartments of a complex aquifer system composed by basaltic rocks and by alluvium located in Petit Bara, Grand Bara, and Mouloud areas (Southwest of Djibouti). A main feature was the common trend from a fresh Na-Cl-HCO3 water type (alluvium groundwaters) to an intermediate water type (alluvium and basalt groundwaters) and finally to a Na-Cl-SO4 water type (most mineralized basalt groundwater). Elementary and isotopic nitrate evidenced and located anthropogenic and geogenic origins of nitrate. Alluvium groundwaters had δ2H and δ18O signature of modern precipitation while basalt groundwaters were significantly depleted and enriched in δ13C due to water-rock interactions. Modern radiocarbon and tritium were evidenced in the alluvium groundwaters, while recalculated radiocarbon ages located recharge of the basalt groundwaters in the early to mid-Holocene. These features revealed a common evolutionary pattern, with a recharge from wadi-rivers to the alluvium and a downward circulation to the basalt through major faults, combined with a mixing with a more geochemically evolved groundwater. Accordingly, highly saline groundwater at the outlet of the Petit Bara plain was found to be diluted by modern recharge in the alluvium. Two major basaltic aquifer compartments were found to be connected (Grand Bara and Mouloud), revealing a global northeastward flowpath below the endorheic Grand Bara plain.