A fifty-year chronicle of tritium data for characterising the functioning of the Evian and Thonon (France) glacial aquifers

Abstract

International audienceUsing lumped models and a transfer function model, this paper deals with the interpretation of exceptionally long (up to 50 years (y)) and precise tritium chronicles characterising the rainfall, recharge (efficient rainfall) and outflow from various types of glacial aquifers from the French Alps (Evian-Thonon area). The efficient rainfall tritium chronicle was computed from tritium measurements performed for 11 years (1969-1979) in a lysimeter. The evapotranspiration induces a mean 15% drop of the annual tritium signal. The three superficial glacial aquifers (two fluvio-glacial kame terraces and a lateral till) provide similar results: a best fit with an exponential flow model (EM) (playing the major role) combined in parallel with a piston flow model (PFM), and a rather short mean transit time (T 5-7 y). The deepest mineral aquifer (Evian) can only be fitted with the in a series combination of a highly dispersive model (DM; T 68 y; DP = 0.5) and a piston flow model (T 2.5 y) or, better, by the in a series combination of an EM (T 8 y) modelling the subsurface aquifer and a DM (T 60 y; DP = 0.75) and the same piston flow model (T 2.5 y) modelling the deep mineral aquifer, this latest combination of models providing the following parameters: T 70 y and median transit time 45.5 y. It is also to be noted that a very small part of the recharge; about 1.3%, avoids both the EM and the DM, and directly enters the PFM (at the Northern limit of the Gavot Plateau). These models are very sensitive regarding the T (±1 y, 0.25 y for the PFM), less so with DP. These results will prompt hydrologists to (re)work historical data to determine if important hydrologic information is available. The interest and limits of such a modelling, also for other constituents than tritium, along with the future for tritium as a tracer are discussed and it also provides new insights on the structure and functioning of alpine paleo glacial hydrosystems

Residence time, mineralization processes and groundwater origin within a carbonate coastal aquifer with a thick unsaturated zone

“…Groundwater from the LA and the UA v (B5, W3, B6 and B7) displays the lowest concentrations, below 1-2 TU. All other groundwater samples displays detectable concentrations but lower than the current atmospheric content, between 7.5 and 8 TU in the Northern Hemisphere (Blavoux et al, 2013), highlighting an important mixing with post-1950's water.…”

“…When groundwater residence times are over timescale of 0-70 years, dating tools such as tritium, chlorofluorocarbons (CFCs) or sulphur hexafluoride (SF 6 ) may be used (Aeschbach-Hertig et al, 1998;Alvarado et al, 2005;Cook and Solomon, 1997;Corcho Alvarado et al, 2007;Delbart et al, 2014). Due to the stabilization of 3 H concentrations around 10 TU in the atmosphere since the 1970s, tritium dating has lost precision (Blavoux et al, 2013). CFCs and SF 6 are anthropogenic atmospheric gases.…”

Aggregation effects on tritium-based mean transit times and young water fractions in spatially heterogeneous catchments and groundwater systems

“…Małoszewski et al, 1983;Stewart and Thomas, 2008;Blavoux et al, 2013;Morgenstern et al, 2015). The binary parallel LPM is given by…”

“…We therefore suggest that the answer to the question in the title of this section may be what has often been practised in the past, even though the term "aggregation error" was not used (e.g. Małoszewski et al, 1983;Uhlenbrook et al, 2002;Stewart and Thomas, 2008;Morgenstern and Taylor, 2009;Stewart et al, 2010;Blavoux et al, 2013;Morgen-stern et al, 2015). This ideally involves evaluation of many types of information about a hydrological system (geological, hydrological, hydrochemical, tritium, and other isotopes) to establish a perceptual model, and experiments with simple and compound LPMs in harmony with the derived perceptual model to fit tritium data (and, if available, other types of chemical or isotopic data).…”

Time series of tritium, stable isotopes and chloride reveal short-term variations in groundwater contribution to a stream

“…Zuber et al, 2005;Stewart and Thomas, 2008;Einsiedl et al, 2009;Manning et al, 2012;Blavoux et al, 2013) and for the assessment of TT in surface water studies (Matsutani et al, 1993;Stewart et al, 2007;Morgenstern et al, 2010;Stolp et al, 2010;Stewart, 2012;Gusyev et al, 2013;Kralik et al, 2014). Most of these studies had to assume stationarity of the observed system by deriving a unique estimate of TT or RT from 3 H time-series data, in order to circumvent the bomb pulse issue.…”