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Geochemical occurrence of arsenic, vanadium and fluoride in groundwater of Patagonia, Argentina: Sources and mobilization processes
Accepted Manuscript Geochemical occurrence of arsenic, vanadium and fluoride in groundwater of Patagonia, Argentina: Sources and mobilization processes María del Pilar Alvarez, Eleonora Carol PII:
S0895-9811(18)30211-6
DOI:
https://doi.org/10.1016/j.jsames.2018.10.006
Reference:
SAMES 2020
To appear in:
Journal of South American Earth Sciences
Received Date: 15 May 2018 Revised Date:
18 October 2018
Accepted Date: 19 October 2018
Please cite this article as: Alvarez, Marí.del.Pilar., Carol, E., Geochemical occurrence of arsenic, vanadium and fluoride in groundwater of Patagonia, Argentina: Sources and mobilization processes, Journal of South American Earth Sciences (2018), doi: https://doi.org/10.1016/j.jsames.2018.10.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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GEOCHEMICAL OCCURRENCE OF ARSENIC, VANADIUM AND FLUORIDE
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IN GROUNDWATER OF PATAGONIA, ARGENTINA: SOURCES AND
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MOBILIZATION PROCESSES
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María del Pilar Alvarez 1 and Eleonora Carol2
Instituto Patagónico para el Estudio de los Ecosistemas Continentales, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) Argentina, [email protected] Centro de Investigaciones Geológicas. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) Universidad Nacional de La Plata (UNLP) Argentina, [email protected]
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Abstract
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Contamination of groundwater in different parts of the world is a result of natural and / or
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anthropogenic sources, leading to adverse effects on human health and the ecosystem. In Península
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Valdés, where groundwater is the only source of supply, high concentrations of As and F- were
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registered. Since it is a region without industrial activity, an analysis of possible natural sources of
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contamination is necessary. The aim of this study is to analyze the hydrological processes that
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determines the presence and mobilization of those elements through the analysis of the mineralogy
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of the aquifer sediments and the ionic water relationships. The productive aquifer, dominated by
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psamites, coquinas and siltstone is located between 29 and 42 meters below ground surface. The
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hydrochemistry studied from 105 sampling points, shows that groundwater is dominated by Na-Cl
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ions and, in the fresh water sectors, the ionic type is Na-HCO3 to Na-Cl. In 17 of these samples,
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Zn, Cr, Mn, As, V, Sr, Fe, F ions were measured and As and F contents above the potability limit
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were recorded. These contents vary between 0.01 and 0.40 mg/L in As and between 0.31 and 4 in F-
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which are both associated with elevated V values. The optical petrographic microscope
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observations and the X-ray diffraction measurements show that the sediments are dominated by
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volcanic lithic fragments, volcanic glass shards and quartz, plagioclase, pyroxenes and magnetite
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clasts. The scanning electron microscopy, combined with the energy dispersive X-ray analysis,
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shows that the highest concentrations of As are associated with volcanic shards and iron oxides. The
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combined analysis of all these elements leads to conclude that the processes which explain the
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presence of those ions are a result of the interaction of groundwater with the components of the
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aquifer sediments. At alkaline pH, the high solubility of the amorphous silica of vitreous shards
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allows the release of As, V and F- ions towards the solution. Thus, adsorption-desorption processes
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can also control the presence of these ions in groundwater. Both As and V (in solution in the form
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of oxyanions) can be adsorbed by iron oxides, while F- anions have more affinity to be adsorbed by
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the carbonate facies, some of them re-precipitated as a result of the increase in pH. The identified
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hydrogeological processes provide information for the planning of water purification measures that
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tend to improve the water resources management in a large arid region of Patagonia.
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Keywords: hydrogeochemical processes, groundwater – sediment interaction, trace
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elements, water supply, Península Valdés.
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1. Introduction
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The presence of minor elements such as arsenic, vanadium and fluoride in groundwater
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above the potability limit is a serious problem in many regions of the world (Brikowski et
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al., 2013, Galindo et al., 2007, Hoque et al., 2017, Mohapatra et al., 2009, Ormachea
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Muñoz et al., 2015, Smedley & Kinniburgh, 2002). In recent years, this has encouraged
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many researchers to carry out studies aimed to explain the processes that condition
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distribution and mobility of such elements in aquifers (Berg et al., 2001, Binbin et al., 2005,
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Diwakar et al., 2015, Mahanta et al., 2015, Sikdar et al., 2008, Smedley et al., 2002,
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Viswanathan et al., 2009, Wright & Belitz, 2010, Zhang et al., 2003). Although these
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processes may be associated with anthropogenic activities (Brandenberger et al., 2004, Ure
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& Berrow, 1982), in most cases the presence of these elements in groundwater is due to
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natural processes resulting from the interaction of water with aquifer sediments (Appelo &
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Postma, 2005, Hallett et al., 2015, Smedley & Kinniburgh, 2002). Geochemical
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experimental studies demonstrated that the As is one of the most mobile elements among
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the rare elements present in volcanic ashes (Ruggieri et al., 2011). Moreover, studies over
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the volcanic ashes of actual Andean eruptions, which have affected most of the Argentinian
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extra-andean region, shows that several potential pollutants are present in the glassy
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products (Daga et al., 2014).
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In arid rural areas where there are no surface courses, such as most of the extra Andean and
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coastal Patagonian region, groundwater is the only resource for water supply. Although the
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main problem in these regions is the presence of saline waters, high levels of arsenic,
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fluoride and vanadium are also usually registered (Caceres et al., 1992, Grimaldo et al.,
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1995, Karcher et al., 1999, Smith et al., 1998). It is important to evaluate the affected areas
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and the processes that condition the mobility and persistence of these elements in water, in
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order to provide the necessary tools forwater resources management.
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Península Valdés, located in the Argentinian Patagonia (Fig. 1) is an arid region of 4000
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km2 inhabited by about 60 rural settlements. The water supply for the inhabitants of
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Peninsula Valdes, comes from the phreatic and/or semiconfined aquifer located at 20-60 m
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depth. Previous studies on groundwater chemistry indicate that one of the limiting factors
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of water quality, for human consumption, is its high arsenic content. (Alvarez et al.
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2010).The aim of these work is to determine the hydrogeochemical processes conditioning
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the presence of arsenic, vanadium and fluoride in the groundwater of Peninsula Valdés, by
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means of the analysis of the minerals in the aquifer sediments and the ionic water
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relationships.
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2. Study area
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Península Valdés is located in the northeast of the Extra-Andean Patagonia (Fig. 1), where
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the precipitations do not exceed 250 mm/year and with a mean annual temperature of 13.6
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°C. The geology of the region is composed of a basement of Paleozoic, Cretaceous and
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Paleogene rocks overlain by the Neogene and quaternary deposits (Haller et al., 2001). The
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aquifers studied are located in the upper portion of the lithoestratigrapic sequence, mainly
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represented by the Miocene sediments of the Gaiman and Puerto Madryn formations
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(Scasso et al., 2001), the Plio-Pleitocene Patagonian Gravels and the Quaternary deposits
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(Table 1 and Fig. 1).
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Fig. 1 a). Location map. b) Geological and groundwater flow map with an ionic type
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classification of the sampling points. c) Groundwater conductivity map. 1 Salina Grande, 2
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Salina Chica, 3 Gran Salitral. d) Cross section showing the geology from B to B´.
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The main edaphic features are represented by aridisoils and entisoils (Rostagno, 1981). The
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aridisols, typically characterized by a limited organic horizon above a calcic or natric
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horizon, are developed over the deposits of the Patagonian Gravels and the Aeolian
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deposits. For its part entisols, soils that do not show a horizon development and that are
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mainly composed of their parental material, can be found above the alluvial and colluvial
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deposits and the Aeolian deposits.
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Table 1: Upper stratigraphy of the Península Valdés extracted from Haller (2017) Geological unit
Quaternary
Holocene
Alluvium and colluvium deposits Aeolian deposits Playa lake sediments and evaporites
Early Miocene
†
San Miguel Formation Caleta Valdés Formation Patagonian gravels Puerto Madryn Formation
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Plio-Pleisocene Late Miocene
Lithology
Gaiman Formation
Sand, gravel and silt Sand and silt Silt, clay, evaporites (halite, glauberite and gypsum) Gravel and sand
Maximum thickness † (m) 2-3 14 1
6
Gravel
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Gravel Sandstone, siltstone, mudstone and coquinas Claystone, siltstone, tuffaceous mudstones, tuffs and sandstones
4 80 (350)
20 (280)
: thickness from subsurface data.
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Neogene
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Pleistocene
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Regarding the geomorphology of Península Valdés, the geomorphological features are
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grouped in major geomorphologic systems represented by Uplands and Plains, Great
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Endorheic Basins, and Coastal Zone (Bouza et al., 2017). Among the Uplands and Plains
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there are terrace levels that are stepped sequences of old fluvial terraces of the Patagonian
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Gravels lithostratigraphic unit, and stabilized and active aeolian landforms which
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corresponds to the Aeolian deposits lithostratigraphic unit (Fig. 1).
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Based on the stratigraphic sequence, the local hydrogeological system is formed by: a) an
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unsaturated zone (UZ) corresponding to the Quaternary deposits and partly to the Miocene
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sediments; b) a phreatic aquifer contained, depending on its spatial position, within these
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same deposits or in the sandstones of the Puerto Madryn Formation, and which is mainly
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exploited in the region; c) one or more semi-confined or confined aquifers, limited by
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clayey or silty-clay strata in the same Puerto Madryn Formation or in the underlying
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Gaiman Formation (Alvarez et al., 2010). Regarding the hydrodynamic, the predominantly
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groundwater radial divergent morphology, located in the central– southern sector, indicates
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that the preferential recharge area corresponds with the Aeolian deposits unit (Fig. 1). The
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groundwater flow is from the recharge sector towards both i) the regional discharge area,
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located over the coastal cliffs of the San José Gulf, Nuevo Gulf, and the Atlantic Ocean and
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ii) towards the Salina Grande, Salina Chica and Gran Salitral (great endorheic basins with
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hypersaline playa lakes at their bottoms), where a local and internal discharge occurs. (Fig.
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1) (Alvarez & Hernandez, 2017). The playa lakes, mostly composed of halite precipitates,
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are originated by the evaporation of the Na-Cl water facies which characterize the
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groundwater of Península Valdés. The groundwater salinity, with Na+ / Cl- ratios close to 1,
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is mainly due to the dissolution of soil salts whose formation is favored by the contribution
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from marine aerosol and the strong evaporation which characterize the arid climate of the
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area. In the Aeolian deposits sector (Fig. 1), where infiltration is faster, these processes
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occur to a lesser extent and, as a consequence, groundwater salinization and chloride
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contents are lower (Alvarez, 2010).
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3. Methodology
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The methodology includes the mineralogical sampling and analysis of the aquifer matrix as
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well as the sampling and chemical analysis of the groundwater. The characterization of the
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aquifer sediments was made by samples collected from rock drill cuttings obtained from
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exploratory drilling wells. The samples were taken at every one meter of depth until the end
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of the well (43 meters). The location of the well corresponds to a representative sector of
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the hydroestratigraphic settings within the study area. The extracted materials were
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analysed under a magnifying glass in order to characterize them lithologically (texture,
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colour by Rock-colour Chart Committee, 1963, mineralogy and degree of consolidation).
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The mineralogy was complemented by the descriptions of loose sediments (250-150 µm
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fraction) with a petrographic microscope. The mineralogical composition was determined
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by X-ray diffraction analysis (DRX) using a Phillips X'pert Pro. Also the minerals were
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observed with a Jeol JSM 6460 LV scanning electron microscope with an EDAX
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PW7757/78 X- ray energy-scattering micro-analyser (SEM-EDS) to determine the
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qualitative composition of certain minerals. To analyse the relationship between certain
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elements, a line in an observed sample at the SEM was defined along which the
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concentration of O, Si, As, V, Fe and P were determined point by point by the EDAX.
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In relation to the quality of groundwater, in situ physicochemical properties were measured
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and 105 samples were extracted for major ion analysis (Ca2+, Mg2+, Na+, K+, Cl-, SO42-,
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HCO3-, CO32-) and NO3-. Based on this survey, large areas with significant salinity
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differences were identified. This served as the basis for a selective sampling, mostly in sites
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with lower salinity, which were being used not only for livestock supplies but also for
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human consumption. 17 samples were taken to determine minority ions (Zn, Cr, Mn, As, V,
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Sr, Fe, F). The laboratory determinations were made following standardized techniques
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APHA and EPA. Ca2+, Mg2+, Cl-, HCO3-, CO32- were made by volumetry, As, Fe, F-, NO3-
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byspectrophotometry UV-Visible and SO42-, Na+, K+, Zn, Cr, Cu and Mn by atomic
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absorption.
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4. Results
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4.1 Aquifer sediment characterization
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The sedimentological profile obtained from the sedimentological data retrieved from de
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studied well is dominated by psamites intercalated with coquinas and subordinate siltstone
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(Fig 2). The identified productive aquifer level is located between 29 and 42 meters below
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ground surface. It is composed of fossiliferous sands below which a claystone develops as
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aquiclude. It is a yellowish - brown sand, of medium to fine granulometry which becomes
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very fine in depth. The clasts are sub-rounded and poorly selected with a silt-clayey
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support-matrix. Levels of yellowish brown clay-silt loam sediments are interbedded.
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Regarding the mineralogical composition, the clasts identified under the petrographic
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microscope correspond mainly to volcanic lithic fragments, volcanic glass shards and
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monomineral clasts of quartz, plagioclase, pyroxenes, opaque minerals (mainly magnetite)
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and secondarily zircon and apatite (Fig. 2). Those observations accord with the obtained
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results from the XRD analysis over the sediments of the stratigraphic column that shows
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that the mineralogy is quite homogeneous (Fig 2), indicating that quartz, plagioclase,
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magnetite and smectite are present in all the analysed levels. There are also K-feldespar,
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pyroxene and hematite present in most of the levels and illite appears only between 5.5 and
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13.5 m depth (Fig 2). As regards to the volcanic shards observed in the microscope, it is
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important to note that there are some zones with loss of isotropy due to its alteration to
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clays or zeolites (Fig 2).
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Fig. 2. a) Sedimentological profile of the drilled water well; b) table with the DRX
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mineralogical determinations at different depths; c) mineralogy of the 105-250µm fraction.
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Qz: cuartz, Kfs: K-feldespar, Cal: calcite, Px: pyroxene, Hm: hematite, Mag: magnetite, Ill:
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illite, Sme: smectite, Op: opaque minerals.
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Concerning the elemental composition, the sediments of the aquifer level (washed and
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partially concentrated in heavy minerals), which were analysed point by point by the
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EDAX along a line in the sample, shows that the major peaks of As match with the peaks
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of Si and O in a clast with a morphology that corresponds to volcanic ash. (Fig 3a).
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Likewise, another semi-quantitative analysis carried out with the EDAX on total sediment
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of the aquifer (measured on a line similar to the previous one) shows that the concentrations
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of As and Si as well as those of Fe and As have a directly proportional relationship (Fig 3b
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and c). On the other hand, in specific determinations on magnetite clasts, it can be verified
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that magnetite has associated As and V contents (Fig 3d).
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4.2 Groundwater hydrochemistry
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The obtained results from groundwater conductivity, which is directly related to salt
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content and major ionic values, indicate that the salinity and the ionic type varie according
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to the geomorphology, the lithology and the groundwater flow (Fig 1).
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The lowest conductivity values, mostly below 2000 µS/cm, correspond to the main
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recharge areas, located at the Aeolian deposits. In the other hand, a conductivity
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groundwater increase occurs towards the discharge areas. Near the Valdés cove the
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conductivity values are above 16000 µS/cm and towards the Nuevo Gulf and San José
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Gulf, as well as near the Gran Salitral, Salina Grande and Salina Chica salt pans
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conductivity values are between 8000 and 16000 µS/cm. Between the recharge and the
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discharge sector, the area represented by the Patagonian Gravels Formation shows values
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between 2000 and 10000 µS/cm. Finally, surface and subsurface waters in salt pans have
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values above 30000 µS/cm.
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Fig 3. a) SEM image with the location of the line (white) on which the EDAX made the
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determinations point by point. The coloured lines indicate the registered intensity of the
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different selected elements b) Scatter diagrams with the values obtained by the EDAX. c)
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SEM Image of a magnetite clast and, d) the corresponding EDAX diffractogram.
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Regarding the groundwater ionic type, Na-HCO3 facies to Na-Cl facies (Fig. 4) are
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recorded in the dunes deposits areas, where recharge occurs. In these areas the electrical
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conductivity of groundwater is generally below 2000 µS/cm. The samples extracted from
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the terraced plains and endorheic basins areas, which function as transit and discharge areas
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for the groundwater flow, have Na-Cl facies (Fig. 4) and conductivities that reach 10000
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µS/cm. As a whole, the pH values in the samples varied between 7.6 and 8.5, showing a
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tendency to increase the Na+/Ca2+ ratio (from 6.5 to 52.7) from pH above 8 (Fig. 4).
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Fig. 4: Piper diagram of the groundwater samples of Península Valdés
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Among the analysed minority ions that are not regulated with respect to their content for
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potability by the World Health Organization (WHO) are Fe, Zn, V and Sr. The values for
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these elements vary between less than 0.05 to 0.55 mg/L for Fe, between less than 0.002 to
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4.575 mg/L for Zn, between 0.1 to 2.5 mg/L for V and between 0.043 to 3.37 mg/L for Sr
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(Table 2). Among the minority ions regulated by the WHO, which have contents below the
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potability limit, are Cr, Cu and Mn. The content for these elements vary between less than
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0.002 to 0.023 mg/L for Cr, between less than 0.005 to 0.099 mg/L for Cu and between less
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than 0.008 and 0.24 mg/L for Mn (Table 2).Values above the potability limit are recorded
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for As and F (Table 2). The contents of As vary between 0.01 and 0.40 mg/L, having
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registered in only two samples acceptable values according to the limit of reference of 0.01
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mg/L. The F- varies between 0.31 and 4.90 with five samples above the potability limit (1.5
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mg/L).
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Table 2: trace element contents in the Tertiary aquifer of Península Valdés
4.575
Min
< 0.002
†
†
0.023
0.099
0.24
0.57
2.5
3.37
0.55
4.9
< 0.002
0.005
0.05
2
< 0.008
0.01
0.1
0.043
< 0.05
0.31
0.4
0.01
††
††
†
1.5
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Zn (mg/l) Cr (mg/l) Cu (mg/l) Mn (mg/l) As (mg/l) V (mg/l) Sr (mg/l) Fe (mg/l) F- (mg/l) Max
Chemical substances for which reference values have not been established (OMS 2003) Chemical substances which are not listed in the quality guides (OMS 2003)
††
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Considering that the waters are mainly Na-Cl and taking the Cl- ion as an indicator of
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salinity, it is observed that as the salt content increases, the concentration of As, V and F-
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decreases (Fig 5), registering the highest contents of As in samples with low salinity.
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From the analysis of the relationships of these trace elements, which exceed the limit of
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potability, with other ions and with the pH, it is observed that there is a direct relationship
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between the contents of As and F- with those of V, as well as a tendency for these three
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elements to increase their concentrations as the pH increases (Fig. 6).
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5. Discussion
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Peninsula Valdes is a broad coastal region where the inhabitants of the farms depend on
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groundwater resources for livestock production and domestic use. Although the main
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limitation is the saline content, with a dominance of Na-Cl facies with salinities above 3500
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mg/L, those areas with low salinity water tend to have the highest concentrations in As and
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F- (Fig. 5), imposing a limitation for its use for human consumption. Given that there are no
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anthropic sources of contamination in the study area, the mobility and concentration of
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these ions will depend on the interaction between the water and the minerals from the
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aquifer matrix.
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The detailed study of the mineral components that constitute the aquifer matrix showed that
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the highest concentrations of As are associated with volcanic ashes (Fig. 3a) and with iron
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oxides (Fig. 3d). A close relationship between the As and V contents was also registered.
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On the other hand, the As and V contents in groundwater have a strong dependence on pH
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(Fig.6), registering the highest concentrations at alkaline pH (above 8). The pH increase is
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mainly associated with the incongruent hydrolysis of silicates (feldspar, plagioclase and
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some mafic minerals), whose reaction liberates OH- ions (Appelo & Postma 2005). Given
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that there is an arid environment with low rates of organic matter decomposition and with a
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deep water table, conditions which do not favor the H+ generation that neutralizes the OH-,
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the pH tends in turn to increase. Furthermore, the solubility of silica increases considerably
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at alkaline pH, even more in mineral species with an amorphous structure. Under these
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conditions, the volcanic shards of the aquifer sediment would tend to dissolve, releasing the
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As and V which they contain as impurities which would be in solution in the water
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(Smedley & Kinniburgh, 2002). The presence and mobility of As and V in groundwater
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are regulated mainly by the pH and Eh conditions (Appelo & Postma, 2005, Lee et al.,
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2007). Under conditions of positive Eh and pH higher than 6.5, these elements are found as
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oxyanions (Rango et al., 2013, Wehrli & Stumm, 1989, Wright & Belitz, 2010, Yan et al.,
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2000).
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Fig. 5: Scatter diagram showing the relationship between chloride and Arsenic (a), Fluoride
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(b) and Vanadium (c) contents in the groundwater.
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On the other hand, iron oxides, where high concentrations of As and V were also registered,
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control the distribution of these oxyanions, mainly through adsorption - desorption
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processes.
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manganese (De Vitre et al., 1991, Sullivan & Aller, 1996, Wright & Belitz, 2010) and, to a
284
lesser extent, by organic matter and clays. The adsorption by iron oxy-hydroxides is
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particularly strong and the adsorbed amounts can be appreciable, even at low
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concentrations of these elements in the solution (Goldberg, 1986; Manning & Goldberg,
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1996). The adsorption in oxy-hydroxides of Mn can also be important if these oxyanions
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are present in high concentrations (Brannon & Patrick, 1987, Peterson & Carpenter, 1983).
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To a lesser extent they can be adsorbed at the edges of clays (Manning & Goldberg, 1997)
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and on the surface of carbonates such as calcite (Goldberg & Glaubig, 1988). The
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adsorption also depends on the pH, which occurs strongly on the surfaces of the oxides in
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acidic conditions or with a pH close to neutral values. There is a desorption of the
293
oxyanions at pH above 8 that remain in solution in the groundwater (Raven et al., 1998).
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Also, the low content of Fe and Mn dissolved in the water can come from the alteration of
295
silicates, such as amphiboles and pyroxenes, present in the mineralogy of the aquifer
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(Bouza, 2012). It is not expected that the Fe and Mn present in the water come from the
297
dissolution of their oxides since they are stable at pH above 8 (Appelo & Postma, 2005).
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Groundwater in the arid regions is predominantly oxidizing with neutral to alkaline pH (Del
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Razo et al., 1990), a characteristic that favors both the dissolution of the silica that
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composes the volcanic shards and the desorption of the oxyanions retained in the iron
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oxides (Nicollet al., 2010). Both processes contribute to increase the concentrations of As
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and V in groundwater and are processes that can occur within the studied aquifer matrix.
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Given that the increase of these ions occurs after the groundwater exceeds pH 8, it is
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The oxyanions of As and V are adsorbed by oxides, mainly of iron and
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expected that the desorption of the surface of the Fe oxide is the main process that brings
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As and V to groundwater and to a lesser extent the dissolution of volcanic glass. This can
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be observed in the scatter charts obtained from the SEM, where it is recorded that both
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silica and iron have a positive correlation with As in the mineral grains (Fig. 3b and 3c).
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However, the Fe vs. As relationship shows a slope in the correlation line that is of an order
309
of magnitude greater than that of the Si vs. As relationship. This shows that, within the
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aquifer matrix, iron oxides are the ones that can potentially contribute more arsenic to
311
water.
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The positive relationship observed between As and V with the contents of F-, and between
313
these three ions and the pH, indicate that all of them have the same origin. In this way, the
314
dissolution of volcanic glass as result of the increase in the solubility of the silica at alkaline
315
pH also contributes F- ions to the groundwater. Unlike As and V, F- can be adsorbed by
316
carbonates (Zack, 1980), which are commonly present in the aquifer matrix (Fig. 2b).
317
However, in alkaline conditions where there is an important availability of OH- in the
318
water, the OH- competes for the exchange sites causing the F- to remain in solution. It
319
should be clarified that part of the carbonates of the aquifer matrix are formed by the re-
320
precipitation of carbonates that occurs as a result of the increase in pH and the availability
321
of Ca2+ ions in predominantly bicarbonated waters. In this way the precipitation of
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carbonates determines a decrease of the Ca2+ ions in solution, causing the Na+ to become
323
the dominant cation, which produces a predominance of Na-HCO3 facies.
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Figure 6: Scatter diagrams of the relationships between Arsenic and Fluoride (a), pH and
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Arsenic (b), Arsenic and Vanadium (c), pH and Fluoride (d), Fluoride and Vanadium (e)
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and pH and vanadium (f) in groundwater.
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The described processes explain why the samples of the zones that have groundwater of
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low salinity and alkaline pH, have dominantly Na-HCO3 facies and how the favourable
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conditions are generated so that As, V and F- are found as ions in solution.
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6. Conclusion
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Península Valdés comprises a broad coastal region under arid climate where there are no
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surface courses and where groundwater is of vital importance for the development of the
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region. In addition, most of the aquifers contain water of high salinity, mainly of the Na-Cl
338
type. However, there is an area, associated with the Aeolian deposits, where rainwater
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infiltration is favored by the high permeability of the soils and where the groundwater
340
recharge occurs with most efficiency, generating an area with fresh water in the aquifer.
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Remarkably, it is in this sector where groundwater is of low salinity that there are high
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concentrations of As, F- and V, which impose a water quality limitation with respect to
343
international regulations for human consumption. The origin of these elements in
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groundwater is strictly natural since there are no anthropogenic activities in this region that
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can provide As, V and / or F- to the environment. Therefore, the natural processes that
346
explain its presence are due to the interaction of groundwater with the minerals of the
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aquifer sediments. The mineralogical and geochemical data obtained from the
348
sedimentological samples indicate that the volcanic glass that constitutes the ashes of the
349
sediments is the one that mainly contains these elements as impurities. At alkaline pH the
350
high solubility of the silica allows volcanic ash to release the ions of As, V and F- to the
351
solution. Adsorption processes can also control the presence of these ions in groundwater.
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The ions of As and V, in solution in the form of oxyanions, can be adsorbed by iron oxides,
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while F- have more affinity to be adsorbed by carbonates, some of which re-precipitated as
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a result of the increase in pH. However, in the sectors with low salinity, groundwater is Na-
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HCO3 and with alkaline pH, conditions that promotes desorption of As, V and F- ions,
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which favors the increase of the concentration of these ions in groundwater.
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The results of this study allow for a better understanding of the processes that jeopardize
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the quality and limit the use of the unique fresh water resource which exists in a wide
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Extra-Andean region. These data are useful for the water management in the region, since it
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is required to know both the chemical limitations to the water use and how they are
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regulated, in order to plan solutions to this problem.
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7. Acknowledgement
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The authors are very indebted to the Agencia Nacional de Promoción Científica y
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Tecnológica (National Agency for Scientific and Technological Promotion for financially
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supporting this study by means of their grants, PICT 2006-1995 and PICT 2012-0867. This
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research is also framed within the P-UE CONICET N° 22920160100044. Finally the
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authors wants to thanks ALUAR SAIC for allowing the use of the MEB-EDAX.
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Highlights
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The As, V and F presence in the groundwater of Peninsula Valdes is due to a natural origin
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Volcanic particles and iron oxides of the Miocene sediments shows the highest As concentrations.
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Groundwater pH conditions the volcanic shards dissolution and therefore the As, V and F release.
S0895-9811(18)30211-6
DOI:
https://doi.org/10.1016/j.jsames.2018.10.006
Reference:
SAMES 2020
To appear in:
Journal of South American Earth Sciences
Received Date: 15 May 2018 Revised Date:
18 October 2018
Accepted Date: 19 October 2018
Please cite this article as: Alvarez, Marí.del.Pilar., Carol, E., Geochemical occurrence of arsenic, vanadium and fluoride in groundwater of Patagonia, Argentina: Sources and mobilization processes, Journal of South American Earth Sciences (2018), doi: https://doi.org/10.1016/j.jsames.2018.10.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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GEOCHEMICAL OCCURRENCE OF ARSENIC, VANADIUM AND FLUORIDE
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IN GROUNDWATER OF PATAGONIA, ARGENTINA: SOURCES AND
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MOBILIZATION PROCESSES
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María del Pilar Alvarez 1 and Eleonora Carol2
Instituto Patagónico para el Estudio de los Ecosistemas Continentales, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) Argentina, [email protected] Centro de Investigaciones Geológicas. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) Universidad Nacional de La Plata (UNLP) Argentina, [email protected]
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Abstract
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Contamination of groundwater in different parts of the world is a result of natural and / or
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anthropogenic sources, leading to adverse effects on human health and the ecosystem. In Península
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Valdés, where groundwater is the only source of supply, high concentrations of As and F- were
16
registered. Since it is a region without industrial activity, an analysis of possible natural sources of
17
contamination is necessary. The aim of this study is to analyze the hydrological processes that
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determines the presence and mobilization of those elements through the analysis of the mineralogy
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of the aquifer sediments and the ionic water relationships. The productive aquifer, dominated by
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psamites, coquinas and siltstone is located between 29 and 42 meters below ground surface. The
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hydrochemistry studied from 105 sampling points, shows that groundwater is dominated by Na-Cl
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ions and, in the fresh water sectors, the ionic type is Na-HCO3 to Na-Cl. In 17 of these samples,
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Zn, Cr, Mn, As, V, Sr, Fe, F ions were measured and As and F contents above the potability limit
24
were recorded. These contents vary between 0.01 and 0.40 mg/L in As and between 0.31 and 4 in F-
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which are both associated with elevated V values. The optical petrographic microscope
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observations and the X-ray diffraction measurements show that the sediments are dominated by
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volcanic lithic fragments, volcanic glass shards and quartz, plagioclase, pyroxenes and magnetite
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clasts. The scanning electron microscopy, combined with the energy dispersive X-ray analysis,
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shows that the highest concentrations of As are associated with volcanic shards and iron oxides. The
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combined analysis of all these elements leads to conclude that the processes which explain the
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presence of those ions are a result of the interaction of groundwater with the components of the
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aquifer sediments. At alkaline pH, the high solubility of the amorphous silica of vitreous shards
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allows the release of As, V and F- ions towards the solution. Thus, adsorption-desorption processes
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can also control the presence of these ions in groundwater. Both As and V (in solution in the form
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of oxyanions) can be adsorbed by iron oxides, while F- anions have more affinity to be adsorbed by
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the carbonate facies, some of them re-precipitated as a result of the increase in pH. The identified
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hydrogeological processes provide information for the planning of water purification measures that
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tend to improve the water resources management in a large arid region of Patagonia.
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Keywords: hydrogeochemical processes, groundwater – sediment interaction, trace
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elements, water supply, Península Valdés.
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1. Introduction
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The presence of minor elements such as arsenic, vanadium and fluoride in groundwater
45
above the potability limit is a serious problem in many regions of the world (Brikowski et
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al., 2013, Galindo et al., 2007, Hoque et al., 2017, Mohapatra et al., 2009, Ormachea
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Muñoz et al., 2015, Smedley & Kinniburgh, 2002). In recent years, this has encouraged
48
many researchers to carry out studies aimed to explain the processes that condition
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distribution and mobility of such elements in aquifers (Berg et al., 2001, Binbin et al., 2005,
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Diwakar et al., 2015, Mahanta et al., 2015, Sikdar et al., 2008, Smedley et al., 2002,
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Viswanathan et al., 2009, Wright & Belitz, 2010, Zhang et al., 2003). Although these
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processes may be associated with anthropogenic activities (Brandenberger et al., 2004, Ure
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& Berrow, 1982), in most cases the presence of these elements in groundwater is due to
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natural processes resulting from the interaction of water with aquifer sediments (Appelo &
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Postma, 2005, Hallett et al., 2015, Smedley & Kinniburgh, 2002). Geochemical
56
experimental studies demonstrated that the As is one of the most mobile elements among
57
the rare elements present in volcanic ashes (Ruggieri et al., 2011). Moreover, studies over
58
the volcanic ashes of actual Andean eruptions, which have affected most of the Argentinian
59
extra-andean region, shows that several potential pollutants are present in the glassy
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products (Daga et al., 2014).
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In arid rural areas where there are no surface courses, such as most of the extra Andean and
62
coastal Patagonian region, groundwater is the only resource for water supply. Although the
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main problem in these regions is the presence of saline waters, high levels of arsenic,
64
fluoride and vanadium are also usually registered (Caceres et al., 1992, Grimaldo et al.,
65
1995, Karcher et al., 1999, Smith et al., 1998). It is important to evaluate the affected areas
66
and the processes that condition the mobility and persistence of these elements in water, in
67
order to provide the necessary tools forwater resources management.
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Península Valdés, located in the Argentinian Patagonia (Fig. 1) is an arid region of 4000
69
km2 inhabited by about 60 rural settlements. The water supply for the inhabitants of
70
Peninsula Valdes, comes from the phreatic and/or semiconfined aquifer located at 20-60 m
71
depth. Previous studies on groundwater chemistry indicate that one of the limiting factors
72
of water quality, for human consumption, is its high arsenic content. (Alvarez et al.
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2010).The aim of these work is to determine the hydrogeochemical processes conditioning
74
the presence of arsenic, vanadium and fluoride in the groundwater of Peninsula Valdés, by
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means of the analysis of the minerals in the aquifer sediments and the ionic water
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relationships.
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2. Study area
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Península Valdés is located in the northeast of the Extra-Andean Patagonia (Fig. 1), where
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the precipitations do not exceed 250 mm/year and with a mean annual temperature of 13.6
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°C. The geology of the region is composed of a basement of Paleozoic, Cretaceous and
82
Paleogene rocks overlain by the Neogene and quaternary deposits (Haller et al., 2001). The
83
aquifers studied are located in the upper portion of the lithoestratigrapic sequence, mainly
84
represented by the Miocene sediments of the Gaiman and Puerto Madryn formations
85
(Scasso et al., 2001), the Plio-Pleitocene Patagonian Gravels and the Quaternary deposits
86
(Table 1 and Fig. 1).
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Fig. 1 a). Location map. b) Geological and groundwater flow map with an ionic type
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classification of the sampling points. c) Groundwater conductivity map. 1 Salina Grande, 2
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Salina Chica, 3 Gran Salitral. d) Cross section showing the geology from B to B´.
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The main edaphic features are represented by aridisoils and entisoils (Rostagno, 1981). The
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aridisols, typically characterized by a limited organic horizon above a calcic or natric
94
horizon, are developed over the deposits of the Patagonian Gravels and the Aeolian
95
deposits. For its part entisols, soils that do not show a horizon development and that are
96
mainly composed of their parental material, can be found above the alluvial and colluvial
97
deposits and the Aeolian deposits.
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Table 1: Upper stratigraphy of the Península Valdés extracted from Haller (2017) Geological unit
Quaternary
Holocene
Alluvium and colluvium deposits Aeolian deposits Playa lake sediments and evaporites
Early Miocene
†
San Miguel Formation Caleta Valdés Formation Patagonian gravels Puerto Madryn Formation
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Paleogene
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Plio-Pleisocene Late Miocene
Lithology
Gaiman Formation
Sand, gravel and silt Sand and silt Silt, clay, evaporites (halite, glauberite and gypsum) Gravel and sand
Maximum thickness † (m) 2-3 14 1
6
Gravel
25
Gravel Sandstone, siltstone, mudstone and coquinas Claystone, siltstone, tuffaceous mudstones, tuffs and sandstones
4 80 (350)
20 (280)
: thickness from subsurface data.
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Regarding the geomorphology of Península Valdés, the geomorphological features are
102
grouped in major geomorphologic systems represented by Uplands and Plains, Great
103
Endorheic Basins, and Coastal Zone (Bouza et al., 2017). Among the Uplands and Plains
104
there are terrace levels that are stepped sequences of old fluvial terraces of the Patagonian
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Gravels lithostratigraphic unit, and stabilized and active aeolian landforms which
106
corresponds to the Aeolian deposits lithostratigraphic unit (Fig. 1).
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Based on the stratigraphic sequence, the local hydrogeological system is formed by: a) an
108
unsaturated zone (UZ) corresponding to the Quaternary deposits and partly to the Miocene
109
sediments; b) a phreatic aquifer contained, depending on its spatial position, within these
110
same deposits or in the sandstones of the Puerto Madryn Formation, and which is mainly
111
exploited in the region; c) one or more semi-confined or confined aquifers, limited by
112
clayey or silty-clay strata in the same Puerto Madryn Formation or in the underlying
113
Gaiman Formation (Alvarez et al., 2010). Regarding the hydrodynamic, the predominantly
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groundwater radial divergent morphology, located in the central– southern sector, indicates
115
that the preferential recharge area corresponds with the Aeolian deposits unit (Fig. 1). The
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groundwater flow is from the recharge sector towards both i) the regional discharge area,
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located over the coastal cliffs of the San José Gulf, Nuevo Gulf, and the Atlantic Ocean and
118
ii) towards the Salina Grande, Salina Chica and Gran Salitral (great endorheic basins with
119
hypersaline playa lakes at their bottoms), where a local and internal discharge occurs. (Fig.
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1) (Alvarez & Hernandez, 2017). The playa lakes, mostly composed of halite precipitates,
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are originated by the evaporation of the Na-Cl water facies which characterize the
122
groundwater of Península Valdés. The groundwater salinity, with Na+ / Cl- ratios close to 1,
123
is mainly due to the dissolution of soil salts whose formation is favored by the contribution
124
from marine aerosol and the strong evaporation which characterize the arid climate of the
125
area. In the Aeolian deposits sector (Fig. 1), where infiltration is faster, these processes
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occur to a lesser extent and, as a consequence, groundwater salinization and chloride
127
contents are lower (Alvarez, 2010).
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3. Methodology
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The methodology includes the mineralogical sampling and analysis of the aquifer matrix as
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well as the sampling and chemical analysis of the groundwater. The characterization of the
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aquifer sediments was made by samples collected from rock drill cuttings obtained from
133
exploratory drilling wells. The samples were taken at every one meter of depth until the end
134
of the well (43 meters). The location of the well corresponds to a representative sector of
135
the hydroestratigraphic settings within the study area. The extracted materials were
136
analysed under a magnifying glass in order to characterize them lithologically (texture,
137
colour by Rock-colour Chart Committee, 1963, mineralogy and degree of consolidation).
138
The mineralogy was complemented by the descriptions of loose sediments (250-150 µm
139
fraction) with a petrographic microscope. The mineralogical composition was determined
140
by X-ray diffraction analysis (DRX) using a Phillips X'pert Pro. Also the minerals were
141
observed with a Jeol JSM 6460 LV scanning electron microscope with an EDAX
142
PW7757/78 X- ray energy-scattering micro-analyser (SEM-EDS) to determine the
143
qualitative composition of certain minerals. To analyse the relationship between certain
144
elements, a line in an observed sample at the SEM was defined along which the
145
concentration of O, Si, As, V, Fe and P were determined point by point by the EDAX.
146
In relation to the quality of groundwater, in situ physicochemical properties were measured
147
and 105 samples were extracted for major ion analysis (Ca2+, Mg2+, Na+, K+, Cl-, SO42-,
148
HCO3-, CO32-) and NO3-. Based on this survey, large areas with significant salinity
149
differences were identified. This served as the basis for a selective sampling, mostly in sites
150
with lower salinity, which were being used not only for livestock supplies but also for
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human consumption. 17 samples were taken to determine minority ions (Zn, Cr, Mn, As, V,
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Sr, Fe, F). The laboratory determinations were made following standardized techniques
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APHA and EPA. Ca2+, Mg2+, Cl-, HCO3-, CO32- were made by volumetry, As, Fe, F-, NO3-
154
byspectrophotometry UV-Visible and SO42-, Na+, K+, Zn, Cr, Cu and Mn by atomic
155
absorption.
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4. Results
158
4.1 Aquifer sediment characterization
159
The sedimentological profile obtained from the sedimentological data retrieved from de
160
studied well is dominated by psamites intercalated with coquinas and subordinate siltstone
161
(Fig 2). The identified productive aquifer level is located between 29 and 42 meters below
162
ground surface. It is composed of fossiliferous sands below which a claystone develops as
163
aquiclude. It is a yellowish - brown sand, of medium to fine granulometry which becomes
164
very fine in depth. The clasts are sub-rounded and poorly selected with a silt-clayey
165
support-matrix. Levels of yellowish brown clay-silt loam sediments are interbedded.
166
Regarding the mineralogical composition, the clasts identified under the petrographic
167
microscope correspond mainly to volcanic lithic fragments, volcanic glass shards and
168
monomineral clasts of quartz, plagioclase, pyroxenes, opaque minerals (mainly magnetite)
169
and secondarily zircon and apatite (Fig. 2). Those observations accord with the obtained
170
results from the XRD analysis over the sediments of the stratigraphic column that shows
171
that the mineralogy is quite homogeneous (Fig 2), indicating that quartz, plagioclase,
172
magnetite and smectite are present in all the analysed levels. There are also K-feldespar,
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pyroxene and hematite present in most of the levels and illite appears only between 5.5 and
174
13.5 m depth (Fig 2). As regards to the volcanic shards observed in the microscope, it is
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important to note that there are some zones with loss of isotropy due to its alteration to
176
clays or zeolites (Fig 2).
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Fig. 2. a) Sedimentological profile of the drilled water well; b) table with the DRX
179
mineralogical determinations at different depths; c) mineralogy of the 105-250µm fraction.
180
Qz: cuartz, Kfs: K-feldespar, Cal: calcite, Px: pyroxene, Hm: hematite, Mag: magnetite, Ill:
181
illite, Sme: smectite, Op: opaque minerals.
182
Concerning the elemental composition, the sediments of the aquifer level (washed and
183
partially concentrated in heavy minerals), which were analysed point by point by the
184
EDAX along a line in the sample, shows that the major peaks of As match with the peaks
185
of Si and O in a clast with a morphology that corresponds to volcanic ash. (Fig 3a).
186
Likewise, another semi-quantitative analysis carried out with the EDAX on total sediment
187
of the aquifer (measured on a line similar to the previous one) shows that the concentrations
188
of As and Si as well as those of Fe and As have a directly proportional relationship (Fig 3b
189
and c). On the other hand, in specific determinations on magnetite clasts, it can be verified
190
that magnetite has associated As and V contents (Fig 3d).
191
4.2 Groundwater hydrochemistry
192
The obtained results from groundwater conductivity, which is directly related to salt
193
content and major ionic values, indicate that the salinity and the ionic type varie according
194
to the geomorphology, the lithology and the groundwater flow (Fig 1).
195
The lowest conductivity values, mostly below 2000 µS/cm, correspond to the main
196
recharge areas, located at the Aeolian deposits. In the other hand, a conductivity
197
groundwater increase occurs towards the discharge areas. Near the Valdés cove the
198
conductivity values are above 16000 µS/cm and towards the Nuevo Gulf and San José
199
Gulf, as well as near the Gran Salitral, Salina Grande and Salina Chica salt pans
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conductivity values are between 8000 and 16000 µS/cm. Between the recharge and the
201
discharge sector, the area represented by the Patagonian Gravels Formation shows values
202
between 2000 and 10000 µS/cm. Finally, surface and subsurface waters in salt pans have
203
values above 30000 µS/cm.
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205 206
Fig 3. a) SEM image with the location of the line (white) on which the EDAX made the
207
determinations point by point. The coloured lines indicate the registered intensity of the
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different selected elements b) Scatter diagrams with the values obtained by the EDAX. c)
209
SEM Image of a magnetite clast and, d) the corresponding EDAX diffractogram.
210
Regarding the groundwater ionic type, Na-HCO3 facies to Na-Cl facies (Fig. 4) are
211
recorded in the dunes deposits areas, where recharge occurs. In these areas the electrical
212
conductivity of groundwater is generally below 2000 µS/cm. The samples extracted from
213
the terraced plains and endorheic basins areas, which function as transit and discharge areas
214
for the groundwater flow, have Na-Cl facies (Fig. 4) and conductivities that reach 10000
215
µS/cm. As a whole, the pH values in the samples varied between 7.6 and 8.5, showing a
216
tendency to increase the Na+/Ca2+ ratio (from 6.5 to 52.7) from pH above 8 (Fig. 4).
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Fig. 4: Piper diagram of the groundwater samples of Península Valdés
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Among the analysed minority ions that are not regulated with respect to their content for
223
potability by the World Health Organization (WHO) are Fe, Zn, V and Sr. The values for
224
these elements vary between less than 0.05 to 0.55 mg/L for Fe, between less than 0.002 to
225
4.575 mg/L for Zn, between 0.1 to 2.5 mg/L for V and between 0.043 to 3.37 mg/L for Sr
226
(Table 2). Among the minority ions regulated by the WHO, which have contents below the
227
potability limit, are Cr, Cu and Mn. The content for these elements vary between less than
228
0.002 to 0.023 mg/L for Cr, between less than 0.005 to 0.099 mg/L for Cu and between less
229
than 0.008 and 0.24 mg/L for Mn (Table 2).Values above the potability limit are recorded
230
for As and F (Table 2). The contents of As vary between 0.01 and 0.40 mg/L, having
231
registered in only two samples acceptable values according to the limit of reference of 0.01
232
mg/L. The F- varies between 0.31 and 4.90 with five samples above the potability limit (1.5
233
mg/L).
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234 235
Table 2: trace element contents in the Tertiary aquifer of Península Valdés
4.575
Min
< 0.002
†
†
0.023
0.099
0.24
0.57
2.5
3.37
0.55
4.9
< 0.002
0.005
0.05
2
< 0.008
0.01
0.1
0.043
< 0.05
0.31
0.4
0.01
††
††
†
1.5
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236 237 238 239
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Zn (mg/l) Cr (mg/l) Cu (mg/l) Mn (mg/l) As (mg/l) V (mg/l) Sr (mg/l) Fe (mg/l) F- (mg/l) Max
Chemical substances for which reference values have not been established (OMS 2003) Chemical substances which are not listed in the quality guides (OMS 2003)
††
240
Considering that the waters are mainly Na-Cl and taking the Cl- ion as an indicator of
241
salinity, it is observed that as the salt content increases, the concentration of As, V and F-
242
decreases (Fig 5), registering the highest contents of As in samples with low salinity.
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From the analysis of the relationships of these trace elements, which exceed the limit of
244
potability, with other ions and with the pH, it is observed that there is a direct relationship
245
between the contents of As and F- with those of V, as well as a tendency for these three
246
elements to increase their concentrations as the pH increases (Fig. 6).
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5. Discussion
249
Peninsula Valdes is a broad coastal region where the inhabitants of the farms depend on
250
groundwater resources for livestock production and domestic use. Although the main
251
limitation is the saline content, with a dominance of Na-Cl facies with salinities above 3500
252
mg/L, those areas with low salinity water tend to have the highest concentrations in As and
253
F- (Fig. 5), imposing a limitation for its use for human consumption. Given that there are no
254
anthropic sources of contamination in the study area, the mobility and concentration of
255
these ions will depend on the interaction between the water and the minerals from the
256
aquifer matrix.
257
The detailed study of the mineral components that constitute the aquifer matrix showed that
258
the highest concentrations of As are associated with volcanic ashes (Fig. 3a) and with iron
259
oxides (Fig. 3d). A close relationship between the As and V contents was also registered.
260
On the other hand, the As and V contents in groundwater have a strong dependence on pH
261
(Fig.6), registering the highest concentrations at alkaline pH (above 8). The pH increase is
262
mainly associated with the incongruent hydrolysis of silicates (feldspar, plagioclase and
263
some mafic minerals), whose reaction liberates OH- ions (Appelo & Postma 2005). Given
264
that there is an arid environment with low rates of organic matter decomposition and with a
265
deep water table, conditions which do not favor the H+ generation that neutralizes the OH-,
266
the pH tends in turn to increase. Furthermore, the solubility of silica increases considerably
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at alkaline pH, even more in mineral species with an amorphous structure. Under these
268
conditions, the volcanic shards of the aquifer sediment would tend to dissolve, releasing the
269
As and V which they contain as impurities which would be in solution in the water
270
(Smedley & Kinniburgh, 2002). The presence and mobility of As and V in groundwater
271
are regulated mainly by the pH and Eh conditions (Appelo & Postma, 2005, Lee et al.,
272
2007). Under conditions of positive Eh and pH higher than 6.5, these elements are found as
273
oxyanions (Rango et al., 2013, Wehrli & Stumm, 1989, Wright & Belitz, 2010, Yan et al.,
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2000).
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Fig. 5: Scatter diagram showing the relationship between chloride and Arsenic (a), Fluoride
278
(b) and Vanadium (c) contents in the groundwater.
279
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280
On the other hand, iron oxides, where high concentrations of As and V were also registered,
281
control the distribution of these oxyanions, mainly through adsorption - desorption
282
processes.
283
manganese (De Vitre et al., 1991, Sullivan & Aller, 1996, Wright & Belitz, 2010) and, to a
284
lesser extent, by organic matter and clays. The adsorption by iron oxy-hydroxides is
285
particularly strong and the adsorbed amounts can be appreciable, even at low
286
concentrations of these elements in the solution (Goldberg, 1986; Manning & Goldberg,
287
1996). The adsorption in oxy-hydroxides of Mn can also be important if these oxyanions
288
are present in high concentrations (Brannon & Patrick, 1987, Peterson & Carpenter, 1983).
289
To a lesser extent they can be adsorbed at the edges of clays (Manning & Goldberg, 1997)
290
and on the surface of carbonates such as calcite (Goldberg & Glaubig, 1988). The
291
adsorption also depends on the pH, which occurs strongly on the surfaces of the oxides in
292
acidic conditions or with a pH close to neutral values. There is a desorption of the
293
oxyanions at pH above 8 that remain in solution in the groundwater (Raven et al., 1998).
294
Also, the low content of Fe and Mn dissolved in the water can come from the alteration of
295
silicates, such as amphiboles and pyroxenes, present in the mineralogy of the aquifer
296
(Bouza, 2012). It is not expected that the Fe and Mn present in the water come from the
297
dissolution of their oxides since they are stable at pH above 8 (Appelo & Postma, 2005).
298
Groundwater in the arid regions is predominantly oxidizing with neutral to alkaline pH (Del
299
Razo et al., 1990), a characteristic that favors both the dissolution of the silica that
300
composes the volcanic shards and the desorption of the oxyanions retained in the iron
301
oxides (Nicollet al., 2010). Both processes contribute to increase the concentrations of As
302
and V in groundwater and are processes that can occur within the studied aquifer matrix.
303
Given that the increase of these ions occurs after the groundwater exceeds pH 8, it is
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The oxyanions of As and V are adsorbed by oxides, mainly of iron and
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expected that the desorption of the surface of the Fe oxide is the main process that brings
305
As and V to groundwater and to a lesser extent the dissolution of volcanic glass. This can
306
be observed in the scatter charts obtained from the SEM, where it is recorded that both
307
silica and iron have a positive correlation with As in the mineral grains (Fig. 3b and 3c).
308
However, the Fe vs. As relationship shows a slope in the correlation line that is of an order
309
of magnitude greater than that of the Si vs. As relationship. This shows that, within the
310
aquifer matrix, iron oxides are the ones that can potentially contribute more arsenic to
311
water.
312
The positive relationship observed between As and V with the contents of F-, and between
313
these three ions and the pH, indicate that all of them have the same origin. In this way, the
314
dissolution of volcanic glass as result of the increase in the solubility of the silica at alkaline
315
pH also contributes F- ions to the groundwater. Unlike As and V, F- can be adsorbed by
316
carbonates (Zack, 1980), which are commonly present in the aquifer matrix (Fig. 2b).
317
However, in alkaline conditions where there is an important availability of OH- in the
318
water, the OH- competes for the exchange sites causing the F- to remain in solution. It
319
should be clarified that part of the carbonates of the aquifer matrix are formed by the re-
320
precipitation of carbonates that occurs as a result of the increase in pH and the availability
321
of Ca2+ ions in predominantly bicarbonated waters. In this way the precipitation of
322
carbonates determines a decrease of the Ca2+ ions in solution, causing the Na+ to become
323
the dominant cation, which produces a predominance of Na-HCO3 facies.
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Figure 6: Scatter diagrams of the relationships between Arsenic and Fluoride (a), pH and
327
Arsenic (b), Arsenic and Vanadium (c), pH and Fluoride (d), Fluoride and Vanadium (e)
328
and pH and vanadium (f) in groundwater.
329
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330
The described processes explain why the samples of the zones that have groundwater of
331
low salinity and alkaline pH, have dominantly Na-HCO3 facies and how the favourable
332
conditions are generated so that As, V and F- are found as ions in solution.
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6. Conclusion
335
Península Valdés comprises a broad coastal region under arid climate where there are no
336
surface courses and where groundwater is of vital importance for the development of the
337
region. In addition, most of the aquifers contain water of high salinity, mainly of the Na-Cl
338
type. However, there is an area, associated with the Aeolian deposits, where rainwater
339
infiltration is favored by the high permeability of the soils and where the groundwater
340
recharge occurs with most efficiency, generating an area with fresh water in the aquifer.
341
Remarkably, it is in this sector where groundwater is of low salinity that there are high
342
concentrations of As, F- and V, which impose a water quality limitation with respect to
343
international regulations for human consumption. The origin of these elements in
344
groundwater is strictly natural since there are no anthropogenic activities in this region that
345
can provide As, V and / or F- to the environment. Therefore, the natural processes that
346
explain its presence are due to the interaction of groundwater with the minerals of the
347
aquifer sediments. The mineralogical and geochemical data obtained from the
348
sedimentological samples indicate that the volcanic glass that constitutes the ashes of the
349
sediments is the one that mainly contains these elements as impurities. At alkaline pH the
350
high solubility of the silica allows volcanic ash to release the ions of As, V and F- to the
351
solution. Adsorption processes can also control the presence of these ions in groundwater.
352
The ions of As and V, in solution in the form of oxyanions, can be adsorbed by iron oxides,
353
while F- have more affinity to be adsorbed by carbonates, some of which re-precipitated as
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a result of the increase in pH. However, in the sectors with low salinity, groundwater is Na-
355
HCO3 and with alkaline pH, conditions that promotes desorption of As, V and F- ions,
356
which favors the increase of the concentration of these ions in groundwater.
357
The results of this study allow for a better understanding of the processes that jeopardize
358
the quality and limit the use of the unique fresh water resource which exists in a wide
359
Extra-Andean region. These data are useful for the water management in the region, since it
360
is required to know both the chemical limitations to the water use and how they are
361
regulated, in order to plan solutions to this problem.
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7. Acknowledgement
364
The authors are very indebted to the Agencia Nacional de Promoción Científica y
365
Tecnológica (National Agency for Scientific and Technological Promotion for financially
366
supporting this study by means of their grants, PICT 2006-1995 and PICT 2012-0867. This
367
research is also framed within the P-UE CONICET N° 22920160100044. Finally the
368
authors wants to thanks ALUAR SAIC for allowing the use of the MEB-EDAX.
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Highlights
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The As, V and F presence in the groundwater of Peninsula Valdes is due to a natural origin
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Volcanic particles and iron oxides of the Miocene sediments shows the highest As concentrations.
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Groundwater pH conditions the volcanic shards dissolution and therefore the As, V and F release.