Importance of accessory minerals for the control of water chemistry of the Pampean aquifer, province of Buenos Aires, Argentina

VITAL, MÉLANIE; DAVAL, DAMIEN; CLÉMENT, ALAIN; QUIROGA, SANDRA; FRITZ, BERTRAND; MARTINEZ, DANIEL E.

CATENA

ELSEVIER SCIENCE BV

Año: 2018 vol. 160 p. 112 - 123

0341-8162

The Pampean aquifer, in south east Argentina, is mainlyconstituted of loess-like sediments. These are clastic sediments mainlycomposed of quartz and aluminosilicates and calcrete concretions. Itshydrochemistry is generally studied assuming a chemical equilibriumbetween mineral phases and the aqueous fluids. The phases forming thematrix of this aquifer are considered to be the reactive phasesresponsible for the chemistry of the groundwater. In the present study,batch dissolution experiments were performed on calcrete and loess tobetter understand the source of the Pampean aquifer water chemistry andto measure the benefit of applying water-rock interaction models that usekinetic rate laws instead of thermodynamic equilibria. The differentminerals composing the loess and calcrete samples were calculated usingquantitative Rietveld refinement of X-ray powder diffraction (XRD)patterns. This analysis showed that the major phases of loess are quartz(~30wt%) and feldspars (~70wt%). The main components of calcrete arecalcite (~95wt%) and quartz (~5wt%). Scanning electron microscopy withenergy dispersive X-ray microanalysis (SEM/EDXS) was used to providedetailed information about the chemical composition of the powdersamples, revealing the presence of traces of minerals like halite, bariteand fluorapatite, which were not detected by XRD. The kinetic code KINDISwas used to carry out simulations using the minerals identifiedpreviously in their relative proportions to identify the signature ofthose geochemical phases on water chemistry. Experimental data from batchdissolution experiments were compared to simulated data. This investigation showed that water reached pseudo steady state concentrations due to the presence of fast dissolving phases like halite,barite, gypsum, plant phytolith. These phases appeared to be of majorimportance in controlling the chemical composition of the Pampeangroundwater. Furthermore, this work showed that the KINDIS software canbe used on all kinds of aquifers as it is very easy to modify theparameters of the simulation to adapt it to numerous situations. Themodeling is a very important tool for thermodynamic and kinetic studiesof groundwater chemistry; it enables the prediction of water quality andcan help to understand the impact of anthropic or natural contaminationon the groundwater.