Geochemistry of the Maroni River (French Guiana) during the low water stage: implications for water–rock interaction and groundwater characteristics

The Maroni River, draining a basin of 60,000 km2 and flowing down to the Atlantic Ocean between French Guiana and Surinam, was studied during a low flow period using the chemical and isotopic composition of its dissolved load. The dissolved concentrations of major ions (Cl, SO4, NO3, HCO3, Ca, Na, Mg, K), trace elements (Rb and Sr), stable isotopes (O and D) and strontium isotopes (87Sr/86Sr) were determined on samples from the main stream of the river and its tributaries and from groundwaters collected in wells and two boreholes. The total dissolved solid (TDS) and the sum of cations ∑+ agree with those summarised for the major Guyanan rivers, the Orinoco and Amazon. On a TDS vs. Na/Na+Ca diagram, all the data from the Maroni catchment plot in the precipitation dominance field close to the plots of the Tefe and Negro rivers. The groundwaters collected from wells largely mimic the surface waters and may represent alluvial groundwater along the Maroni terraces, whereas the groundwaters collected from the boreholes show clearly higher ∑+ values and reflect an increase in water–rock interaction. The river and groundwaters of the Maroni catchment have been isotopically enriched (δ18O vs. δ2H) through evaporation, and the δ18O and δ2H can be used as mixing tracers of river waters within the catchment. After corrections for rainwater addition were computed for selected elements and for 87Sr/86Sr ratios, using chloride as an atmospheric-input reference, the water–rock interaction along the Maroni catchment was investigated for cations and Sr isotopes.