Natural arsenic in the groundwater of the alluvial aquifers of Santiago del Estero Province, Argentina Prosun Bhattacharya, Mattias Claesson & Jens Fagerberg Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden Jochen Bundschuh, Angel del R. Storniolo, Raul A. Martin & Juan Martin Thir Facultad de Ciencias Exactas y Tecnologias, Universidad Nacional de Santiago del Estero (UNSE), Santiago del Estero, Argentina Ondra Sracek Institute of Geological Sciences, Faculty of Science, Masaryk University, Brno, Czech Republic ABSTRACT: Natural occurrences of arsenic has been documented in groundwater of the shal- low aquifers of the Chaco-Pampean Plain, Argentina. The distribution of arsenic and mechanisms of its mobilization in the shallow alluvial aquifers was investigated around the city of Santiago del Estero in Northwestern Argentina in order to provide an insight into the complex hydrological and geochemical conditions that yields high As concentrations in groundwater. Significant spatial variations of total arsenic (As tot ) concentrations were observed with an average value of 743 ␮g/L. Arsenate was a dominant species in most samples. Average concentrations of Al, Mn, and Fe were 360 ␮g/L, 574 ␮g/L, and 459 ␮g/L, respectively. The 7M HNO 3 extraction of sediments and vol- canic ash-layer indicated As NO3 concentrations ranging between 2.5–7.1 mg/kg. As NO3 indicated a significant positive correlation with Mn NO3 , Al NO3 , and Fe NO3 . Oxalate extractions revealed sig- nificant fractions of As (As ox ) in the sediments (0.4–1.4 mg/kg) and a dominance of oxalate extractable Al- and Mn. Speciation calculations indicate that Al oxide and hydroxides have the potential to precipitate in the groundwater, suggesting that As adsorption processes may be to some extent controlled by Al oxides and hydroxides. Mobility of As at local scale seems to depend on high pH values, related to the dissolution of carbonates driven by cation exchange, and dissol- ution of silicates. There is a clear relationship of As with F, V, B and Si, suggesting their common origin in volcanic ash layer. Preliminary conceptual model of arsenic input includes release of As and Al from dissolution of volcanic ash layer, precipitation of Al oxides and hydroxides followed by adsorption of As on Al and Fe phases in sediments, and release of As under high pH conditions. 1 INTRODUCTION Arsenic (As) is a natural inorganic contaminant in drinking water, which is known to have caused serious environmental health problems globally (Bhattacharya et al. 2002a, Smedley & Kinniburgh 2002). Elevated concentrations of As from geogenic sources are reported in groundwaters in dif- ferent parts of the world such as Argentina, Bangladesh, China, Nepal, Mexico, Vietnam, and United States among others (Bhattacharya et al. 2002a, Smedley & Kinniburgh 2002, Bhattacharya et al. 2004). Natural occurrences of As has been documented in groundwaters in Argentina from the alluvial aquifers of the Chaco-Pampean Plain, where a population of approximately 1.2 mil- lion mostly in rural settlements are exposed to As from local drinking water sources. The concen- trations of As in the groundwater are mostly above the limit of safe drinking water (10 ␮g/L; WHO 57 Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam (eds) © 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X Copyright © 2005 Taylor & Francis Group plc, London, UK 2001) as well as the local drinking water standard of 50␮g/L. The first symptoms of arsenic related diseases were detected in 1983 within the counties of La Banda and Robles, and in the fol- lowing year investigations by the government agencies confirmed elevated arsenic concentrations (above 1000␮g/L) in groundwater of the shallow aquifers around the provincial capital of Santiago del Estero in North-western Argentina (Martin 1999). In the recent years, chemistry of groundwater of the shallow aquifers within selected areas of the alluvial cone of the river Río Dulce of Santiago del Estero province (Bejarano & Nordberg 2003, Claesson & Fagerberg 2003, Bundschuh et al. 2004) have been studied in order to understand the mechanisms of arsenic mobil- ization in these aquifers. The aim of this study was to investigate the distribution of As in the groundwater of shallow aquifers located within the alluvial cone of the river Río Dulce around the city of Santiago del Estero and the associated shallow sediments in order to develop a better understanding of the com- plex hydrogeological and geochemical conditions that are responsible for the mobilization of As in groundwater. 2 LOCATION AND GEOLOGICAL SETTING 2.1 The study area Santiago del Estero is the provincial capital of the province with the same name. It is located on the dry, western part of the Chaco plain in northwestern Argentina at an altitude of about 200 meters above sea level (Fig. 1). The study area is located to the east of the capital on the alluvial deposits formed by the Río Dulce (hereinafter referred as the Río Dulce alluvial cone), covering an area of approximately 2000km 2 . The area is rural, but densely populated in comparison with surrounding countryside due to its fertile soils and irrigation systems built up by channels from the Río Dulce. Small agricultural settlements dependent on artificial irrigation are common through- out the Río Dulce cone. Hot, rainy summers lasting from November to March and very dry winters from April to October characterize the climate. The average annual precipitation (1938–90) is 532 mm. Evapotranspiration is very high, especially in summer period. The area was originally forested but due to intensive timber harvesting only limited forest areas remain. Vegetation generally consists of low bushes and ground vegetation. The terrain is very flat with few shallow depressions resulting from his- torical flow-paths of the river. Strong winds are common and carry a lot of dust during winter when binding surface water is scarce. 2.2 Geological and hydrogeological characteristics Río Dulce alluvial cone is limited to the west by the Huyamampa fault. A sequence of 30m Pleistocene Pampean loess is found to the west of this fault comprising dispersed volcanic ash and calcareous crusts, which is underlain by lower Pliocene and green Miocene clays with a thickness of 70 m. Holocene to recent sediments of the Río Dulce cone (Post-Pampean formation) occur to the east of the fault, and the thickness of the fluvial and aeolian sediments is nearly 150m close to the fault margin and pinches out about 50 km towards the east. A sequence of alternating layers of gravel, sand, silt and clay is deposited in discordance with the underlying Pliocene sediments. The coarser sediments represent the deposits of the palaeo-channels of the river Río Dulce, which form a multi-layered aquifer system in the region. Santiago del Estero is located just south of where Río Dulce passes the fault of Huyamampa (Fig. 2). The fault runs from north to south and the river passes from northwest to southeast. West of the fault Pliocene and Miocene clays are covered by approximately 30m of Pampean loess (Bundschuh et al. 2004). Southeast of the fault, the river have deposited alluvial sediments making up the Río Dulce cone. Depth to ground water table ranges from 1 to 6m. The upper-most aquifer, the one of major interest in this study, reaches an approximate depth of 15 m (Martin 1999). An 58 Copyright © 2005 Taylor & Francis Group plc, London, UK important fraction of groundwater recharge to all aquifers, including the shallow one, takes place in a limited section of coarse material along the Huyamampa fault in the northwestern part of the study area. Here all aquifers in the alluvial cone are connected to the surface and are recharged by infiltrating river and surface water. The aquifers are considered to be semi confined with little interactions among them. Major groundwater flow is horizontal in the separate aquifers. The upper-most aquifer consists of aeolian and fluvial sediments (Bundschuh et al. 2004) and has important recharge from the river and surface not only near the Huyamampa fault, but all over its area. Irrigation channels are non-lined which may lead to significant infiltration losses. Local groundwater flow patterns yielding long residence times in natural depressions are likely to be of importance for groundwater chemistry. General direction of flow in the upper aquifer is towards SE (Fig. 2) and several localized discharge areas occur in topographic lows. However, data on local groundwater flow patterns and hydraulic conductivity of the upper-most aquifer are very limited. 59 Figure 1. Digital elevation model of the southern part of South America with location of the project area “Río Dulce alluvial cone” located near the city of Santiago del Estero in North western Argentina (digital elevation model modified from PIA03388 image; http://photojournal.jpl.nasa.gov). Other areas with groundwater arsenic are also shown. Copyright © 2005 Taylor & Francis Group plc, London, UK 3MATERIALS AND METHODS 3.1 Groundwater and sediment sampling Groundwater samples were collected during September–October 2002 from the counties Robles and La Banda (Fig. 2), both within a distance of not more than 50 kilometers from the city of Santiago del Estero. Forty well sites were selected where groundwater was mainly abstracted by hand-pumped tube-wells penetrating the shallow aquifers, that reached a maximum depth of 12 m. Sampling points are shown in Figure 2. The well positions at each of the sampling site were determined using Global Positioning System (GPS). The values of pH, redox potential (Eh), temperature and electrical conductivity of groundwater were measured in the field. Water samples collected from each well involved: (i) fil- tered samples for alkalinity and major anion analysis; (ii) filtered, acidified samples for major cation and trace elements analysis; (iii), sample for DOC analysis, and (iv) filtered through a Disposable Cartridge ® for field separation of As(V) and As(III). Sediment samples were collected from two sites, Balsamo Cuatro Horcones (CH 47) and Nuevo Libano (NL 30) (Fig. 2) up to a depth of 1.2m using an auger drill in the immediate vicinity of tube wells to study the sediment-groundwater interactions. Groundwater and the sediment were analysed following the procedure outlined by Bhattacharya et al. 2001, 2002b. 3.2 Analytical methods Anions such as Cl Ϫ and SO 4 2Ϫ were analyzed by a Dionex 120 ion chromatograph, and NO 3 Ϫ and PO 4 3Ϫ were analyzed using Tecator AQUATEC 5400 analyzer at wavelengths 540 nm and 690 nm, respectively. The major and trace metals were analyzed on a Perkin Elmer Elan 6000 ICP-MS. As(V) was calculated as a difference between total As and As(III) in the samples. Certified standards, SLRS-4 (National Research Council, Canada) and GRUMO 3A (VKI, Denmark) and synthetic 60 28 23 El Quebrachal FORRES FERNANDEZ Pozo Suni Mistol 26 24 30 25 Buey Muerto Las Lomitas 27 31 Villa Hipólita Morcillo Cnia. Bobadal Colonia Chingolo Buey Muerto 32 10 1 El Zanjón Rubia Moreno LA BANDA del ESTERO SANTIAGO 2 Villa Robles Maco La Florida Los Arias 14 15 Mili Cara Pujio 22 Cnia. Jaime Romano Sto. Domingo VILMER 4 12 3 La Bajada 11 8 6 7 20 21 13 16 San José 9 Chilca La Mirella BELTRAN 17 18 El Refugio 19 29 Janta BANDA COUNTY ROBLES COUNTY Huyamampa fault Rí o Dulce 64˚10´ 27˚50´ 64˚10´ 64˚ 5 km Sampled Well Paved road Irrigation canel County limit Urban area groundwater flow direction Eastern limit of alluvial cone Figure 2. Detailed map of the Río Dulce alluvial cone, the location of the sampled wells and the generalized patter of groundwater flow within the alluvial cone. Copyright © 2005 Taylor & Francis Group plc, London, UK chemical standards prepared in the laboratory, and duplicates were analyzed after every 10 samples during the runs. Trace element concentrations in standards were within 90–110% of their true values. In case of wider variations, the standards were recalibrated and the preceding batch of 10 samples reanalyzed. Relative percent difference between the original and duplicate samples were within Ϯ10%. Dissolved organic carbon (DOC) in the water samples were determined on a Shimadzu 5000 TOC analyzer (0.5mg/L detection limit with a precision of Ϯ10% at the detection limit). 4 RESULTS 4.1 Groundwater chemistry Groundwater pH ranged between 6.4 and 9.3 with an average of 7.6. Field measured redox poten- tial ranged from Ϫ60 to ϩ348 mV with an average value of ϩ153 mV. Electric conductivity (EC) ranged between 804 and 9800 µS/cm with an average is 2422␮S/cm. Major ion composition indi- cated Na ϩ (average concentration 427mg/L) and HCO 3 Ϫ (581 mg/L) as the dominating ions in groundwaters (Fig. 3). DOC concentrations in groundwater varied between below detection limit and 18.2 mg/L with an average of 7.6mg/L. Total arsenic (As tot ) concentration indicated considerable spatial variations with an average of 743 ␮g/L. Some wells exhibited extremely high values, reaching a maximum of 14,969 ␮g As/L (Fig. 4b). Speciation of As indicated the dominance of As(V) with average concentration of 617 ␮g/L, while the concentration of As(III) averaged around 125␮g/L. Among the other trace elements, dissolved Al concentrations were low (average 360␮g/L, median 17␮g/L), while the concentrations of Mn (average 574␮g/L, median 128␮g/L) and Fe (average 459 ␮g/L, median 140 ␮g/L) were higher. These groundwaters were characterized by high Si concentrations (average 28.1 mg/L). Fluoride concentrations were elevated (average 2.55 mg/L, median 1.26 mg/L), which also exceeded the WHO limit for safe drinking water (1.5 mg/L; WHO 1993) in 16 out of the 40 sampled wells. 61 80 80 20 HCO 3 2 0 40 60 80 Ca 2+ 20 40 60 80 80 Na + + K + 40 f 20 80 60 4 0 60 - 40 40 SO 4 60 20 d b a 40 60 e c 20 g Ca + Mg 2+ 40 20 2- SO 4 + Cl + NO 3 20 - - 80 80 Mg 2+ Cl - + NO 3 - 2- Figure 3. Representation of major ion chemistry in groundwater samples plotted on a Piper diagram. Copyright © 2005 Taylor & Francis Group plc, London, UK 4.2 Sediment chemistry Geochemical investigations have revealed considerable enrichment of arsenic in shallow aquifer sediments. Extraction of the sediments samples collected at two sites (NL30 and CH47, Fig. 2) by 7M HNO 3 revealed As NO3 concentrations in the range between 2.5–7.1 mg/kg which is higher than the average As concentrations in soils and sediments (Taylor & McLennan 1985). The volcanic ash-layer also had appreciable As NO3 content (3.0 mg/kg). A significant positive correlation (Fig. 4) was observed between As NO3 and the concentrations of Mn NO3 , Al NO3 , and Fe NO3 (R ϭ 0.76–0.79). Sequential leaching of sediment samples was performed using deionized water (DIW), bicar- bonate (HCO 3 ), acetate and oxalate media, which extracted As at varying concentrations (Fig. 5). Oxalate extractions reveal significant fractions of extractable As (As ox ) in all the samples ranging between 0.4–1.4 mg/kg (Fig. 5). Comparison of Fe, Al and Mn concentrations extracted by oxalate clearly indicated dominance of Al- and Mn-oxides and -hydroxides as compared to Fe-oxides and hydroxides (Fig. 6). Much higher concentrations at site CH47 seem to be related to the preferen- tial dissolution of volcanic ash layer located at this site at about 1 m depth. 4.3 Speciation calculations Results of saturation indices (SI) calculations for selected minerals calculated with program MINTEQA2 are listed in Table 1. They show that crystalline forms of Al oxide and hydroxides such as gibbsite are stable in groundwater, implying that As adsorption processes may be to some 62 0 2 4 6 8 0 10 20 30 40 50 60 As NO 3 (mg/kg) As NO 3 (mg/kg) Fe NO 3 and Al NO 3 (mg/kg) a 0 2 4 6 8 200 400 600 800 1000 Mn NO 3 (mg/kg) b Figure 4. Bivariate plots showing the correlation of As NO3 with: a) Fe NO3 (ᮀ – dashed line; R ϭ 0.79) and Al (᭡ – solid line; R ϭ 0.79), and b) Mn NO3 (R ϭ 0.76) in the shallow aquifer sediments. 0.0 0.5 1.0 1.5 2.0 0.0 1.0 2.0 3.0 Leached As (mg/kg) a 0.0 1.0 2.0 3.0 4.0 0.0 1.0 2.0 Leached As (mg/kg) b DIW Depth (m) Depth (m) HCO 3 Acetate Oxalat e 3.0 Figure 5. Sequential leaching of As from the sediment samples from: left: Balsamo Cuatro Horcones (CH 47); right: Nuevo Libano (NL 30). Sediments were sequentially leached by de-ionized water (DIW), bicarbonate (HCO 3 ), acetate and oxalate. Copyright © 2005 Taylor & Francis Group plc, London, UK extent controlled by Al mineral phases. This is consistent with an important role of Al mineral phases in oxalate extractable fraction. However, also ferric minerals like goethite are stable and may be important As adsorbents. 5 DISCUSSION AND CONCLUSIONS Potential primary source of both As and Al is volcanic ash layer, which comprises very soluble vol- canic glass. Principal Component Analysis (Bhattacharya et al. 2004, submitted) shows the relation of As with F, V, B and Si; most likely due to their common origin in volcanic ash, indicating its importance as a source of As in shallow groundwater. This is exhibited by a fairly high correlation (Fig. 7a–d) observed between the concentrations of As tot with F Ϫ (R 2 ϭ 0.43; p Ͻ 0.0001), V (R 2 ϭ 0.67, p Ͻ 0.0001); B (R 2 ϭ 0.43; p Ͻ 0.001) and Si (R 2 ϭ 0.43; p Ͻ 0.001) in the analyzed groundwater samples. Redox conditions in the study area are oxidizing or moderately reducing. This is consistent with predominating arsenate. Thus, reductive dissolution of ferric minerals observed, for example, in Bangladesh (Smedley & Kinniburgh 2002, Ahmed et al. 2004) can be ruled out as a principal mechanism of arsenic input. In contrast, high pH values seem to promote desorption of As 63 0.0 0.5 1.0 1.5 2.0 0 100 200 300 400 500 600 a Oxalate leached Fe, Al, Mn (mg/kg) Depth (m) 0.0 1.0 2.0 3.0 4.0 0 100 200 300 400 500 b Depth (m) Fe Al Mn Oxalate leached Fe, Al, Mn (mg/kg) Figure 6. Trends in the distribution of oxalate extractable Fe, Al and Mn with depth of the sediment samples from: left: Balsamo Cuatro Horcones (CH 47); right: Nuevo Libano (NL 30). Table 1. Results of speciation modeling using the MINTEQA2 program (Alison et al. 1991) and the SI val- ues for selected Al, Fe, and Mn phases for ground water samples from five wells. NL30 SC8 23 24 16 (As tot ϭ 8083 (As tot ϭ 1574 (As tot ϭ 669 (As tot ϭ 36 (As tot ϭ 23 Mineral ␮g/L) ␮g/L) ␮g/L) ␮g/L) ␮g/L) SI SI SI SI SI Al(OH) 3 (a) Ϫ1.98 Ϫ2.68 Ϫ2.50 Ϫ3.04 Ϫ1.35 Gibbsite 0.74 0.03 0.27 Ϫ0.33 1.37 Fluorite 1.09 Ϫ0.33 0.81 Ϫ1.46 n.a. Fe(OH) 3 (a) 0.31 2.08 2.18 1.87 Ϫ1.58 Goethite 6.18 7.95 8.0 7.73 4.29 MnOOH Ϫ8.21 Ϫ4.19 Ϫ1.38 Ϫ2.94 Ϫ9.36 Rhodochrosite Ϫ0.32 Ϫ1.14 Ϫ0.21 0.28 Ϫ1.19 Siderite 0.57 Ϫ2.53 Ϫ4.18 Ϫ2.54 Ϫ0.81 SiO 2 (a) Ϫ0.57 Ϫ0.67 Ϫ0.47 Ϫ0.60 Ϫ0.60 Vivianite Ϫ1.00 Ϫ10.37 Ϫ21.04 Ϫ13.65 n.a. n.a. – not available; (a) – amorphous phase. Copyright © 2005 Taylor & Francis Group plc, London, UK adsorbed on to the amorphous oxides of Al, Mn and Fe. In iso-curves plots, maximum concentra- tions of As and high values of pH generally coincide (Fig. 8). High pH seems to be related to the dissolution of carbonates induced by cation exchange. This is consistent with a negative correl- ation between Ca and Na observed in earlier studies (Bundschuh et al. 2004). Another factor con- tributing to high pH values is probably dissolution of silicates in volcanic glass. Smedley et al. (2001) also postulated high pH as a factor contributing to high As concentrations in La Pampa region located southeast of the study area. Tentative mechanism of arsenic mobilization can be summarized as follows: (a) dissolution of volcanic ash layer with resulting release of As and Al; (b) precipitation of Al oxide and hydroxides and adsorption of As on Al and Fe mineral phases; and (c) release of As under high pH conditions. However, the importance of volcanic ash as the source of As still remains unproved and further investigation of the interaction between the ash layer and aqueous chemistry should be prioritized in further studies. 64 y = 0.0048x + 0.658 R 2 = 0.4284 0,0 1,0 2,0 3,0 4,0 5,0 6,0 0 100 200 300 400 500 600 700 800 F Ϫ (mg/L) a y = 0.7583x + 2.1426 R 2 = 0.674 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 80 0 V (µg/L) b As tot (µg/L) y = 0.0035x + 0.7499 R 2 = 0.484 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 0 100 200 300 400 500 600 700 800 B (mg/L) c As tot (µg/L) y = 3.6975 ln(x) + 13.19 6 R 2 = 0.4246 0 10 20 30 40 50 0 100 200 300 400 500 600 700 80 0 Si (mg/L) d Figure 7. Bivariate plots of total arsenic concentration (As tot ) with: ( a) F Ϫ , ( b) V, (c) B and (d) Si in shallow groundwaters. Note: the data sets with exceedingly high As tot concentrations are excluded from the plots to visualize the trends correlation. Figure 8. (a) Distribution of As tot (␮g/L) and (b) pH in groundwater. Kriging is used to interpolate isocurves. Note irregular concentration scales. Copyright © 2005 Taylor & Francis Group plc, London, UK ACKNOWLEDGEMENTS The authors would like to acknowledge the Swedish International Development Agency (Sida- SAREC) for supporting the research on arsenic-rich groundwater in the Santiago del Ester province of Argentina at the Royal Institute of Technology during 2001–2003. MC and JF acknowledge the financial support provided by the Swedish International Development Agency (Sida) in the form of Minor Field Study grants during 2002. We appreciate the constructive criticisms by an anonym- ous reviewer which has helped to improve the manuscript considerably. REFERENCES Ahmed, K.M., Bhattacharya, P., Hasan, M.A., Akhter, S.H., Alam, S.M.M., Bhuyia, M.A.H., Imam, M.B., Khan, A.A. & Sracek, O. 2004. Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh, an overview. Appl. 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