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

https://doi.org/10.1016/j.scitotenv.2019.133673Get rights and content

Highlights

  • Elevated dissolved As concentrations in groundwater of the southern Pampean plain.

  • Hydrochemistry and sediment geochemistry are linked to explain distribution of As.

  • μ-XRF methods assisted in determination of As distribution in Quaternary sediments.

  • Quaternary soils and paleosoils show highest As concentrations in their pedofeatures.

  • Arsenic desorption from Fe-oxyhydroxides dominate in Quaternary floodplain.

Abstract

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.

Introduction

The increasing demand of water resources has generated attention to harmful elements which can affect groundwater quality. Not only the anthropogenic contaminants can be a problem for humans, but also some geogenic elements (e.g. As, F, Mn, Co, B) can induce a serious threat to the public health via the drinking water pathway (Bundschuh et al., 2012; Rahman et al., 2009). The presence of As in groundwater has been reported in many countries worldwide. Southeast Asia has been cited to host some of the most affected regions, Bangladesh, China, Nepal, Vietnam, Cambodia, and India are some of the most affected countries, both for the severity and scale of the problem (Bhattacharya et al., 2003; Litter et al., 2019; Ravenscroft et al., 2009; Sankar et al., 2014; Smedley et al., 2007; Smedley and Kinniburgh, 2002). High arsenic concentrations have been detected in groundwaters of North and South America, particularly Canada, USA, Mexico, Bolivia, Chile, Argentina, and Nicaragua (Bundschuh et al., 2004; Litter et al., 2010; Manganelli et al., 2007; Nicolli et al., 2012a, Nicolli et al., 2012b).

The Chaco-Pampean plain is an extensive flatland covering ~1000,000 km2 which represents the most productive and populated region of Argentina. It consists of a large mantle of Neogene and Quaternary loess to loess-like deposit (Fig. 1). While several studies (Blanco et al., 2006; Díaz et al., 2016; Dietrich et al., 2016; Esposito et al., 2013; Paoloni et al., 2009, 2005) have reported elevated As concentrations in surface and groundwaters of southern Pampean plain (Buenos Aires province, Fig. 1A), the specific hydrogeochemical processes involved in release of As from sediments to water require further investigation. The volcanic glass shards (VGS), dispersed in the Pampean loess and in distinct layers, is considered one of the main sources of contributing As to the groundwater (Bundschuh et al., 2004; Nicolli et al., 1989; Revenga et al., 2012; Smedley and Kinniburgh, 2002). The oxidizing conditions in the aquifer and the alkaline pH of the groundwater, not only promote the dissolution of the VGS, but also enhance the sorption/desorption processes occurring between As and Fe (Mn, Al)-(oxy)hydroxides (Bhattacharya et al., 2006; Díaz et al., 2016; García et al., 2014). While some of the high As aquifers have been shown to contain minerals like magnetite, clinopyroxenes, carbonates and some silicates, the main sources of As remain VGS and clay minerals containing Fe-(oxy)hydroxides (Dietrich et al., 2016; Nicolli et al., 2012a; Smedley et al., 2002b).

This study aims to understand the water-sediment interactions that lead to elevated levels of As in groundwaters of southern Pampean plain. A detailed mineralogical and chemical characterization of sediments and analyses of inorganic chemical constituents in groundwater were performed to understand processes of As mobilization in a hydrological basin (Fig. 1B).

The Claromecó creek basin consists of a low-gradient plain covering an area of ~ 3200 km2 extending from the northwestern flank of Tandilia range to the Atlantic Ocean (Fig. 1A and B). The absence of slope, an intermittent hydrography and a waterlogging behavior contribute to make the drainage complex (Carbone et al., 2003). Moreover, the presence of high As concentrations in groundwater (Varni et al., 2006), especially in the southern part of the basin, led to define a study area that includes: a) the entire Claromecó fluvial basin; b) areas that drain water to the Quequén Salado river (southwest) and to the Cristiano Muerto creek (southeast). In this study we consider this extended area as Claromecó fluvial basin (Fig. 1B).

The regional climate ranges between humid to subhumid-dry with average temperatures of 12.6 °C in July (austral winter) and 24.1 °C in January (austral summer). The mean annual rainfall varies between the upper and the lower basin from 900 to 1'000 and 700 to 800 mm y−1 respectively as well as the potential evapotranspiration which is comprised between 100 and 110 mm y−1 and 110 and 120 mm y−1 respectively (Carbone et al., 2003; Guanca et al., 2010; Panigatti, 2010). Nevertheless, annual excesses and deficiencies in water availability are quite important and result in drought and floods (Carbone et al., 2003). Formation of wetlands and temporary ponds are common during floods and tend to disappear within a period of 10 to 15 days with an infiltration rate of 3–12 mm h−1 (Carbone et al., 2006). Although these conditions can affect the socio-economic activities, the basin is mostly rural with ~52% of lands being primarily used for crops (wheat, soy and corn) and cattle raising (~45%). Based on the main geomorphic characteristics, the basin can be divided into three sub-basins (upper, middle and lower) (Fig. 1B). The upper basin is made up of a high-altitude flatland where the Claromecó creek springs are located. The middle basin is dominated by an undulating relief with low hills, interfluves and valleys. The lower basin consists of an extensive flatland gently sloping toward the Atlantic coast representing the present-day floodplains.

Aquifers in the studied area, as well as in the rest of southern Pampean plain, are hosted within Neogene to Quaternary sedimentary successions (Blanco et al., 2012). In the Claromecó fluvial basin, these sequences are exposed and identified according to their geomorphological and lithological characteristics (Sosa et al., 2017). The Neogene sedimentary sequence is characterized by intercalated loess layers, reddish silty sand and silty conglomerates generally capped by a ~ 1 m thick calcrete crust. This sequence integrates the subsurface fluvial geology of the highlands, gentle hills and interfluves of the upper and middle basin and represent the bedrock in which the Claromecó fluvial basin was excavated. The Quaternary sequence is dominated by brownish to greenish sandy silts (loess and loess-like) making up the present-day floodplain. Quaternary eolian patchy dunes (loess) are also present and can cover both interfluves and floodplain. This whole sequence crops-out in the middle basin forming narrow floodplains between the Neogene interfluves and hills, while in the lower basin it constitutes the main floodplain (Fig. S1). The mineralogical composition of Neogene and Quaternary sequences is mostly dominated by volcaniclastic material with a high proportion of VGS coming from the Andean Cordillera via eolian processes. Associated minerals are quartz, plagioclase and K-feldspar with minor quantities of amphibole, pyroxene, epidote and magnetite (Sosa et al., 2017; Zárate, 2003).

The southern Pampean plain host multilayer aquifers that can be divided in to post-Pampean (unconfined; depth: 2–10 m) and Pampean (confined; depth 20–30 m or more) (Nicolli et al., 2012b). In the Claromecó fluvial basin, the post-Pampean aquifer is hosted in Quaternary sedimentary sequence and is normally used for domestic, agricultural and farming activities. The Pampean aquifer is a multilayered system consisting of several permeable zones interbedded with lower permeable layers hosted in the Neogene sedimentary sequence, representing the most exploited aquifer of the region (Weinzettel and Varni, 2007). Nevertheless, the salinity tends to increase in depth probably linked to a marine transgression occurred during the late Miocene (Varni et al., 2006). The groundwater flows from north to south and the recharge is mainly represented by infiltration and percolation (Varni et al., 2006). Balances between theoretical and volumetric flows on surface waters configure the Claromecó creek basin as a deficit basin, with an important recharge from infiltration and percolation, favored by the porosity of the soils, flat topography and a poor surface drainage (García Martínez et al., 2008).

Section snippets

Sediment sampling and analytical methods

Geomorphological mapping of the Claromecó fluvial basin was performed through a detailed study of landforms via air-photos supported by field observations. Sedimentary logs (9) were obtained in quarries and natural outcrops registering grain-size, primary structures, discontinuities, pedological features, degree of bioturbation and fossil contents. Sedimentary units and relative ages were defined on the basis of previous studies (Politis et al., 2016; Prieto et al., 2014; Sosa et al., 2017)

Stratigraphy and sedimentary environments

A total of 7 sedimentological units were established and differentiated based on the morphology, stratigraphical, sedimentological and pedological features observed in the field. The first two units are included within the Neogene bedrock while the others are comprised of the Quaternary infill (Fig. 2). The Unit 1 (U1) represents the oldest Neogene sediments of the Claromecó fluvial basin (Sosa et al., 2017) with a minimum thickness of 3 m (base is not exposed) (Fig. 2). Sediments are

Role of depositional and post-depositional processes in Arsenic distribution

The sedimentary sequence that has been studied in the Claromecó fluvial basin clearly shows that As concentrations are higher in the Quaternary deposits than in the Neogene. In particular, soils and hydromorphic paleosoils found in the uppermost part of sedimentary sequences indicate mean As concentrations significantly higher compared with the rest of the sedimentary units. While assessing the relationship between As concentrations and depositional processes, it does not show a remarkable

Conclusions

A detailed geochemical investigation was carried out in the Claromecó fluvial basin, Southern Pampean plain of Argentina to understand occurrence, distribution and potential sources of As in groundwater. Our results demonstrate that the groundwater in Quaternary floodplain aquifer contained higher dissolved AsT than that in Neogene aquifer. Both groundwaters exceeded the WHO guideline for As in safe drinking water. The depositional and post-depositional processes appear to be important in

Acknowledgement

The authors like to thank the Centro de Investigaciones Geológicas (CONICET-UNLP) for the logistical support for the field work. This study was funded by the CONICET doctoral grants awarded to N. N. Sosa. The authors are thankfully acknowledging the people in Tres Arroyos and Claromecó district where field work was executed and for numerous interactions we had with habitants. We also thank the “micro X-ray Lab” of the University of Bari and the Kansas State University for the analyses performed

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