Occurrence and distribution of high arsenic in sediments and groundwater of the Claromecó fluvial basin, southern Pampean plain (Argentina)

Occurrences of high arsenic (As) in sediments and groundwaters were investigated in the Claromecó fluvial basin, southern Pampean plain, Argentina. This investigation includes sedimentology, mineralogy, and hydrogeochemistry of the Neogene and Quaternary aquifers to determine possible sources and transport mechanisms for As in the Claromecó basin. Characterization of the sediments revealed homogeneous mineralogy in both Neogene highlands and Quaternary floodplains with abundant plagioclase, volcanic glass shards (VGS), K-feldspar, quartz, clay minerals and minor concentrations of clinopyroxenes, orthopyroxenes, hornblende, epidote, Fe-(oxy)hydroxides and fluorapatite. The sedimentary As concentrations ranged between 2.8 and 31 mg kg-1 in both aquifers. The average total dissolved As (dissolved AsT) concentrations was 47.2 ± 30.8 μg L-1 (15.3-110 μg L-1) in groundwater in Neogene aquifer (GW1), while it was 97.1 ± 30.6 μg L-1 (45-144 μg L-1) in Quaternary floodplain aquifer (GW2), with all samples exceeding WHO's guideline for dissolved AsT in safe drinking water of 10 μg L-1. Some GW1 (33%) and all GW2 samples contained high levels of fluoride (F-) ranging from 0.6 to 2.6 mg L-1 (1.37 ± 0.59 mg L-1) in GW1 and 2 to 5 mg L-1 (3.2 ± 0.9 mg L-1) in GW2 which also exceeded WHO's guideline for F- in safe drinking water of 1.5 mg L-1. Elevated concentrations of Na+, Cl- and SO42- in the Quaternary flood plain groundwater (GW2) could indicated some degree of sea water mixing as well as some contribution from inland processes (e.g. high evapotranspiration rates, long residence time and soil-water interactions). Dissolution of As bearing VGS or Fe-(oxy)hydroxides, alkaline desorption or competitive desorption with HCO3- from Fe-(oxy)hydroxides appear to be dominating processes of As mobilization, while desorption from fluorapatite elevate dissolved F- levels. This study provides valuable insights on As mobilization processes in Neogene and near coast Quaternary floodplain aquifer.