IAHS2022-373, updated on 23 Sep 2022
https://doi.org/10.5194/iahs2022-373
IAHS-AISH Scientific Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Thermonuclear 36Cl, a new tritium-like tracer of the hydrologic cycle. Application to the water residence time in Cameroon volcanic lakes

Souleyman Abba1,2, Bruno Hamelin1, Pierre Deschamps1, Benjamin Ngounou Ngatcha2, Yannick Garcin1, and A.s.t.e.r Team1
Souleyman Abba et al.
  • 1Aix Marseille Université, IRD, CNRS, INRAE, Collège de France, CEREGE, France (abba@cerege.fr)
  • 2The University of Ngaoundéré, Faculty of Science, Department of Earth Sciences, Cameroon

Chemical and isotopic tracers are powerful tools to study the water cycle and may constitute complementary methods in areas where instrumental records are insufficient. Tritium has been used routinely up to the 2000’s, but because of its short half-life, the bomb-3H spike of the 1960’s is today hardly distinguishable from the natural background level. Bomb-produced 36Cl offers an alternative tool, due to the conservative behavior of chloride and the large amounts of 36Cl released during the marine nuclear tests of the 1950’s.

In this study, we use the bomb-36Cl pulse to assess the residence time of water in five volcanic lakes (Mbalang, Tabere, Tizon, Gegouba and Baledjam) of the Adamawa plateau in the northern region of the Cameroon Volcanic Line. Sediment cores from Mbalang and Tizon have been studied previously for paleo-environments, but otherwise, the Adamawa lakes received little attention so far.

The lakes chloride content is low (0.009-0.035 meq/L) but higher than that of the rainwater samples (0.0019-0.0095 meq/L). In all the lakes, the 36Cl/Cl ratio (1*10-12 to 3*10-12 at/at) is higher by at least one order of magnitude than the natural atmospheric ratio (0.2*10-12 at/at) as determined previously from groundwater measurements in the lake Chad basin and in the Northern Saharan CT aquifer (Bouchez et al.2019, Hadj Ammar (2016)). These high ratios are a definite signature of the nuclear imprint, and thus of the lakes memory of atmospheric deposition, as already observed by our group in lake Chad, Iro and Fitri (Bouchez et al. 2019, Poulin (2019)). These data can then be used to determine the hydrological budget of the lake, independently of any water flux measurement, based on a simple one-box model assuming steady state for water and chloride. Indeed, simulating the transient response of the lake’s 36Cl to the bomb pulse allows us to estimate simultaneously the water residence time and the evaporation/precipitation ratio. Results from this model, as well as discussion of their comparison with classical hydrologic data will be shown at the meeting.

References:

Bouchez C. et al. 2019, (Paper in Scientific Report)

Hadj Ammar F. 2016 (Thesis)

Poulin C. 2019 (Thesis)

How to cite: Abba, S., Hamelin, B., Deschamps, P., Ngounou Ngatcha, B., Garcin, Y., and Team, A. s. t. e. R.: Thermonuclear 36Cl, a new tritium-like tracer of the hydrologic cycle. Application to the water residence time in Cameroon volcanic lakes, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-373, https://doi.org/10.5194/iahs2022-373, 2022.