16The Upper Rhine alluvial aquifer is an important transboundary water resource. However, as 17 in many alluvial systems, the aquifer inflows and outflows are not precisely known, due to the 18 difficulty in estimating the river infiltration flux and the boundary subsurface flow. To 19 provide a thorough representation of the aquifer system, a coupled surface-subsurface model 20 was applied on the whole aquifer basin, and several parameter sets were tested to investigate 21 the uncertainty due to poorly known parameters (e.g. aquifer transmissivity computed by an 22 inverse model, river bed characteristics). Twelve simulations were run and analysed using 23 standard statistical criteria, as well as a more advanced statistical method, the Karhunen 24 This quantity is larger than estimated in previous studies, but also in agreement with some 32 results obtained during low water periods. This important conclusion highlights the 33 vulnerability of the Upper Rhine Graben aquifer to pollution from the rivers and to climate 34 change since it is highly probable that the rivers' regime from the neighbouring mountain 35 ranges will be affected by a reduced snow cover. 36 37
The combination of managed aquifer recharge (MAR) with soil-aquifer treatment (SAT) has clear advantages for the future sustainable quality and quantity management of groundwater, especially when using treated wastewater. We built a Marthe flow and transport model of an MAR–SAT system located in a near-shore sand aquifer, for quantifying the influence of environmental factors (climate, tides, and operational conditions) on the coastal hydrosystem with regard to the fate of trace organic compounds (TrOCs). The simulations show the impact of these factors on flow rates and dilution, and thus, on the potential reactivity of TrOCs. The dilution of secondary treated wastewater (STWW) is variable, depending on the operations (feeding from infiltration ponds) and on shore proximity (dilution by saltwater). We show that, close to the ponds and during infiltration, the attenuation of TrOC concentrations can be explained by reactivity. At the natural outlet of the aquifer, the simulated average residence times ranged from about 70 to 500 days, depending upon seasonal dynamics. It is important to study TrOCs at site scale in order to anticipate the effect of natural variations on the SAT and on the fate of TrOCs.
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