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. Understanding the variability of the chemical composition of surface watersis a major issue for the scientific community. To date, the study ofconcentration–discharge relations has been intensively used to assess thespatiotemporal variability of the water chemistry at watershed scales.However, the lack of independent estimations of the water transit timeswithin catchments limits the ability to model and predict the waterchemistry with only geochemical approaches. In this study, a dimensionallyreduced hydrological model coupling surface flow with subsurface flow (i.e.,the Normally Integrated Hydrological Model, NIHM) has been used to constrainthe distribution of the flow lines in a headwater catchment (Strengbachwatershed, France). Then, hydrogeochemical simulations with the code KIRMAT(i.e., KInectic Reaction and MAss Transport) are performed to calculate theevolution of the water chemistry along the flow lines. Concentrations ofdissolved silica ( H4SiO4 ) and in basic cations ( Na+ , K+ , Mg2+ , and Ca2+ ) in the spring and piezometer waters are correctlyreproduced with a simple integration along the flow lines. The seasonalvariability of hydraulic conductivities along the slopes is a key process tounderstand the dynamics of flow lines and the changes of water transit timesin the watershed. The covariation between flow velocities and active lengthsof flow lines under changing hydrological conditions reduces the variabilityof water transit times and explains why transit times span much narrowervariation ranges than the water discharges in the Strengbach catchment.These findings demonstrate that the general chemostatic behavior of thewater chemistry is a direct consequence of the strong hydrological controlof the water transit times within the catchment. Our results also show thata better knowledge of the relations between concentration and mean transit time ( C –MTTrelations) is an interesting new step to understand the diversity of C – Q shapes for chemical elements. The good match between the measured and modeled concentrations while respecting the water–rock interaction timesprovided by the hydrological simulations also shows that it is possible tocapture the chemical composition of waters using simply determined reactivesurfaces and experimental kinetic constants. The results of our simulationsalso strengthen the idea that the low surfaces calculated from thegeometrical shapes of primary minerals are a good estimate of the reactivesurfaces within the environment.

hydrology and earth system sciences

Crossing hydrological and geochemical modeling to understand the spatiotemporal variability of water chemistry in a headwater catchment (Strengbach, France)