Arsenic contamination of natural waters in San Juan and La Pampa, Argentina

Arsenic (As) speciation in surface and groundwater from two provinces in Argentina (San Juan and La Pampa) was investigated using solid phase extraction (SPE) cartridge methodology with comparison to total arsenic concentrations. A third province, Rio Negro, was used as a control to the study. Strong cation exchange (SCX) and strong anion exchange (SAX) cartridges were utilised in series for the separation and preservation of arsenite (As-III), arsenate (As-V), monomethylarsonic acid (MA(V)) and dimethylarsinic acid (DMA(V)). Samples were collected from a range of water outlets (rivers/streams, wells, untreated domestic taps, well water treatment works) to assess the relationship between total arsenic and arsenic species, water type and water parameters (pH, conductivity and total dissolved solids, TDS). Analysis of the waters for arsenic (total and species) was performed by inductively coupled plasma mass spectrometry (ICP-MS) in collision cell mode. Total arsenic concentrations in the surface and groundwater from Encon and the San Jos, de Jachal region of San Juan (north-west Argentina within the Cuyo region) ranged from 9 to 357 mu g l(-1) As. Groundwater from Eduardo Castex (EC) and Ingeniero Luiggi (LU) in La Pampa (central Argentina within the Chaco-Pampean Plain) ranged from 3 to 1326 mu g l(-1) As. The pH range for the provinces of San Juan (7.2-9.7) and La Pampa (7.0-9.9) are in agreement with other published literature. The highest total arsenic concentrations were found in La Pampa well waters (both rural farms and pre-treated urban sources), particularly where there was high pH (typically > 8.2), conductivity (> 2,600 mu S cm(-1)) and TDS (> 1,400 mg l(-1)). Reverse osmosis (RO) treatment of well waters in La Pampa for domestic drinking water in EC and LU significantly reduced total arsenic concentrations from a range of 216-224 mu g l(-1) As to 0.3-0.8 mu g l(-1) As. Arsenic species for both provinces were predominantly As-III and As-V. As-III and As-V concentrations in San Juan ranged from 4-138 mu g l(-1) to < 0.02-22 mu g l(-1) for surface waters (in the San Jos, de Jachal region) and 23-346 mu g l(-1) and 0.04-76 mu g l(-1) for groundwater, respectively. This translates to a relative As-III abundance of 69-100% of the total arsenic in surface waters and 32-100% in groundwater. This is unexpected because it is typically thought that in oxidising conditions (surface waters), the dominant arsenic species is As-V. However, data from the SPE methodology suggests that As-III is the prevalent species in San Juan, indicating a greater influence from reductive processes. La Pampa groundwater had As-III and As-V concentrations of 5-1,332 mu g l(-1) and 0.09-592 mu g l(-1) for EC and 32-242 mu g l(-1) and 30-277 mu g l(-1) As for LU, respectively. Detectable levels of MA(V) were reported in both provinces up to a concentration of 79 mu g l(-1) (equating to up to 33% of the total arsenic). Previously published literature has focused primarily on the inorganic arsenic species, however this study highlights the potentially significant concentrations of organoarsenicals present in natural waters.The potential for separating and preserving individual arsenic species in the field to avoid transformation during transport to the laboratory, enablng an accurate assessment of in situ arsenic speciation in water supplies is discussed.