Multi-chemical fingerprinting of contrasted waters flowing within the unconventional Okavango Delta: evidence of an original ‘island reactor’
Résumé
The 20.000 km2 swamp of the Delta is organized into islands, flood plains and permanent and seasonal channels. Most of these islands display a surprising vegetation distribution composed of tree rings surrounding the islands and limiting an inner domain with scarce vegetation. Whereas the hydrology of the Okanvango wetlands is governed by a series of drivers such as, sedimentation, climate, tectonic and biological processes, the potential of the use of multi-chemical tracing has not been so far much investigated. The conducted study as part of a multidisciplinary project dedicated to the understanding of the functioning of the Delta, involved water samples collected both upstream and downsream the river, close to one of these islands and also recovered within the island as well. The main objective of this geochemical investigation was to better constrain the interactions prevailing in between these islands and the water chemical record. pH, conductivity (C), dissolved organic and inorganic carbon (DOC & DIC) concentrations were measured as well as those of major anion and cation and trace cation concentrations as well. Whichever the tracers are considered, two contrasted groups of samples were evidenced depending on their sampling positioning regarding the island. The samples recovered only within the island displayed pH around and over 9 and higher conductivities, whereas the other showed lower circumneutral pH values and conductivities as well. The high conductivities of the water samples fom the island also correspond to the highest DOC and DIC concentrations. The strong relationship linking the high DIC values and the high pH in the island samples records probably alkaline CO32- et HCO3- -rich waters resulting from water-rock interactions with carbonates. The marked DOC enrichment has mostly to be related to microbial or photo-degradation of plant-derived organic matter and/or hydrological condition variations promoting DOC release. Significant, Cl-, SO42-, NO2-enrichments as well as major cation ones were also evidenced in the same group of samples within the island. However, the most surprising results are sourced in the trace element fingerprinting. This latter includes huge enrichment in heavy, critical metals and metalloids as well (e.g. Cr, Pb, V, REE, U, Th or As). Beyond the only marked REE-spike, Upper Continental Crust-normalized REE patterns displayed markedly contrasted shapes exhibiting two types of waters with circumneutral pH ones with MREE-enrichment, whereas the alkaline waters evidenced a classical continuous enrichment throughout the whole series from LREE to HREE and a positive Ce anomaly. The use of such multi-tracing allowed an efficient fingerprinting of two distint types of waters to get clues to further constrain both the dynamics of such islands and the functioning of the water system. Still in progress, the study will be completed by (i) the stable isotope analysis, (ii) the modeling of the minerals possibly at equilibrium with the waters and of the organic matter-trace element interactions, (iii) the speciation analysis of some enriched elements, (iv) the comparison between water and solid samples analyses and (v) the understanding of the relations in between the concentrations and locations in the hydrological system.