Abstract. Understanding the variability of the chemical composition of surface waters
is a major issue for the scientific community. To date, the study of
concentration–discharge relations has been intensively used to assess the
spatiotemporal variability of the water chemistry at watershed scales.
However, the lack of independent estimations of the water transit times
within catchments limits the ability to model and predict the water
chemistry with only geochemical approaches. In this study, a dimensionally
reduced hydrological model coupling surface flow with subsurface flow (i.e.,
the Normally Integrated Hydrological Model, NIHM) has been used to constrain
the distribution of the flow lines in a headwater catchment (Strengbach
watershed, France). Then, hydrogeochemical simulations with the code KIRMAT
(i.e., KInectic Reaction and MAss Transport) are performed to calculate the
evolution of the water chemistry along the flow lines. Concentrations of
dissolved silica (H4SiO4) and in basic cations (Na+, K+,
Mg2+, and Ca2+) in the spring and piezometer waters are correctly
reproduced with a simple integration along the flow lines. The seasonal
variability of hydraulic conductivities along the slopes is a key process to
understand the dynamics of flow lines and the changes of water transit times
in the watershed. The covariation between flow velocities and active lengths
of flow lines under changing hydrological conditions reduces the variability
of water transit times and explains why transit times span much narrower
variation ranges than the water discharges in the Strengbach catchment.
These findings demonstrate that the general chemostatic behavior of the
water chemistry is a direct consequence of the strong hydrological control
of the water transit times within the catchment. Our results also show that
a better knowledge of the relations between concentration and mean transit time (C–MTT
relations) 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 times
provided by the hydrological simulations also shows that it is possible to
capture the chemical composition of waters using simply determined reactive
surfaces and experimental kinetic constants. The results of our simulations
also strengthen the idea that the low surfaces calculated from the
geometrical shapes of primary minerals are a good estimate of the reactive
surfaces within the environment.
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