Occurrence and distribution of arsenic in the sediments of a carbonate-rich unsaturated zone.

SEBASTIÁN DIETRICH; SERGIO ANDRÉS BEA; PABLO WEINZETTEL; ESTER TORRES; CARLOS AYORA

Environmental Earth Sciences

Springer

Año: 2016

1866-6280

Three combined approaches were used to study the arsenic (As) occurrence in carbonate-rich unsaturated zone from a large plain environment (Pampean plain, Argentina). A sequential extraction procedure (SEP)was used to identify the As-bearing phases and distribution along a2-m-long unsaturated zone profile, whereas XRD and SEM/EDS were usedto determine the mineralogical features of the sediments. In this hydrogeologic environment, groundwater levels tend to be close to the surface (< 5 m) and hydrogeochemical processes that develop during infiltration and recharge through the unsaturated zone may release the As hosted in the sediments to shallow aquifers. According to SEP results, the unsaturated zone was divided in an upper and a lower zones. Disregarding the As contained in the silicates, low crystalline Fe(III) oxyhydroxides were the main As-bearing minerals in the upper zone. Its presence was confirmed by SEM/EDS analysis. In the lower zone, carbonates were the main As-bearing minerals, also supported by XRD analysis. The chemical composition from the resulting water-extractable step of the SEP was constrained usinggeochemical inverse modeling (PHREEQC) to elucidate those potential geochemical processes that may control the As release or retention during, for instance, an infiltration event. They showed that Na/Ca exchange promoted the calcite dissolution which, in turn, raised the pH as a result of proton consumption. High pH and HCO3-contents favored the As desorption from low crystalline Fe(III)oxyhydroxides in the upper zone, where they were present in higher proportion. p { margin-bottom: 0.25cm; direction: ltr; line-height: 120%; text-align: left; }p.western { font-family: "Linux Libertine G",serif; font-size: 12pt; }p.cjk { font-family: "WenQuanYi Micro Hei"; font-size: 12pt; }p.ctl { font-family: "Lohit Hindi"; font-size: 12p&lt;font face="Times New Roman, serif"&gt;Three combinedapproaches were used to study the arsenic (As) occurrence incarbonate-rich unsaturated zone from a large plain environment(Pampean plain, Argentina). A sequential extraction procedure (SEP)was used to identify the As-bearing phases and distribution along a2-m-long unsaturated zone profile, whereas XRD and SEM/EDS were usedto determine the mineralogical features of the sediments. In thishydrogeologic environment, groundwater levels tend to be close to thesurface (&amp;lt; 5 m) and hydrogeochemical processes that develop duringinfiltration and recharge through the unsaturated zone may releasethe As hosted in the sediments to shallow aquifers. According to SEPresults, the unsaturated zone was divided in an upper and a lowerzones. Disregarding the As contained in the silicates, lowcrystalline Fe(III) oxyhydroxides were the main As-bearing mineralsin the upper zone. Its presence was confirmed by SEM/EDS analysis. Inthe lower zone, carbonates were the main As-bearing minerals, alsosupported by XRD analysis. The chemical composition from theresulting water-extractable step of the SEP was constrained usinggeochemical inverse modeling (PHREEQC) to elucidate those potentialgeochemical processes that may control the As release or retentionduring, for instance, an infiltration event. They showed that Na/Caexchange promoted the calcite dissolution which, in turn, raised thepH as a result of proton consumption. High pH and HCO3-contents favored the As desorption from low crystalline Fe(III)oxyhydroxides in the upper zone, where they were present in higherproportion. &lt;br /&gt;&lt;/font&gt;