Water residence time by coupling anthropogenic gases (CFC, SF6) and isotopic ratios (234U/238U, 87Sr/86Sr) in the Strengbach catchment (Vosges Massif, Eastern France)
Abstract
Weathering processes are active in surface waters
but groundwater also represents no negligible
chemical fluxes. As residence-time in groundwater
are high, silicate weathering might take place and
control Si, Ca and C fluxes. Weathering processes
can be deduced from U isotopic ratios but the kinetics
of these processes remain relatively poorly
constrained.
In order to better characterize these processes,
residence-times deduced from anthropogenic gases
(CFC and SF6) analysis and 234U/238U isotopic ratios
determination have been coupled. Water samples
were collected in both springs and boreholes in the
granitic Strengbach catchment (Hydro-geochemical
Observatory OHGE, Vosges Massif, Eastern France).
Two campaigns were carried out in May and August
2015 during highly contrasted hydro-climatic periods.
A very clear geochemical distinction is observed
between groundwater from surface springs and
deeper groundwater from boreholes. Springs show
much lower residence time (few years) and specific
chemical composition. Deeper groundwater has
residence time of several decades and different
geochemical composition.
The 234U/238U isotopic ratios confirm this vertical
zonation in the boreholes, with much higher activity
ratios in depth in the borehole than in the surface and
spring waters; such high ratios are indicative of long
water-rock interactions, which is consistent with the
long residence times deducted from the CFC and SF6
data.