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Journal of Hazardous Materials 262 (2013) 960– 969

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal homep age: www.elsev ier .com/ locate / jhazmat

o-occurrence of arsenic and fluoride in groundwater of semi-arid regions inatin America: Genesis, mobility and remediation

aría Teresa Alarcón-Herreraa,∗, Jochen Bundschuhb, Bibhash Nathc, Hugo B. Nicolli d,elida Gutierreze, Victor M. Reyes-Gomezf, Daniel Nunez f, Ignacio R. Martín-Domingueza,ndra Sracekg,h

Centro de Investigación en Materiales Avanzados (CIMAV), Ave. Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109, Chihuahua, Chih., MexicoFaculty of Engineering and Surveying, University of Southern Queensland, Toowoomba, Queensland 4350, AustraliaSchool of Geosciences, University of Sydney, Sydney, NSW 2006, AustraliaInstituto de Geoquímica (INGEOQUI) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), San Miguel, Provincia de Buenos Aires, ArgentinaDepartment of Geography, Geology and Planning, Missouri State University, Springfield, Missouri 65897, USAInstituto de Ecología, A.C., Centro de Investigación Sobre Sequía (INECOL-CEISS-RAS), Chihuahua, Chih., MexicoDepartment of Geology, Faculty of Science, Palacky University, 17. listopadu 12, 771 46 Olomouc, Czech RepublicOPV (Protection of groundwater Ltd), Belohorská 31, 169 00 Praha 6, Czech Republic

i g h l i g h t s

As and F co-occurrence in groundwater is linked to volcanism, geothermal, and mining activities.As and F co-occurrence are particularly pronounced in arid and semi-arid regions.As and F are generally associated to high concentrations of Na+ and HCO3

−.Technology is required to simultaneously remove As and F from drinking water.

r t i c l e i n f o

rticle history:eceived 17 December 2011eceived in revised form 28 July 2012ccepted 2 August 2012vailable online 10 August 2012

eywords:rsenicluorideatin America

a b s t r a c t

Several million people around the world are currently exposed to excessive amounts of arsenic (As)and fluoride (F) in their drinking water. Although the individual toxic effects of As and F have beenanalyzed, there are few studies addressing their co-occurrences and water treatment options. Severalstudies conducted in arid and semi-arid regions of Latin America show that the co-occurrences of Asand F in drinking water are linked to the volcaniclastic particles in the loess or alluvium, alkaline pH,and limited recharge. The As and F contamination results from water–rock interactions and may beaccelerated by geothermal and mining activities, as well as by aquifer over-exploitation. These types ofcontamination are particularly pronounced in arid and semi-arid regions, where high As concentrationsoften show a direct relationship with high F concentrations. Enrichment of F is generally related to fluorite

roundwaterrinking wateremoval technology

dissolution and it is also associated with high Cl, Br, and V concentrations. The methods of As and Fremoval, such as chemical precipitation followed by filtration and reverse osmosis, are currently beingused at different scales and scenarios in Latin America. Although such technologies are available in LatinAmerica, it is still urgent to develop technologies and methods capable of monitoring and removing bothof these contaminants simultaneously from drinking water, with a particular focus towards small-scalerural operations.

. Introduction

The occurrence of As and F in groundwater has been widelyeported in Latin America, especially in arid and semi-arid regionsf Mexico, Argentina, and Chile. Unfortunately, it is often the case

∗ Corresponding author. Tel.: +52 6144391121.E-mail address: teresa.alarcon@cimav.edu.mx (M.T. Alarcón-Herrera).

304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.08.005

© 2012 Elsevier B.V. All rights reserved.

that F is not considered to be a problem, reason for which its pres-ence has not even been determined in many places of Latin America.Due to its high toxicity, As removal techniques have been a majorresearch focus during the last two decades [1]. Interestingly, a geo-logical co-occurrence of these contaminants has been reported by

numerous investigators from El Salvador, Peru, Bolivia, Nicaragua,Ecuador, Colombia, and Guatemala [2–4]. There is a high probabil-ity that F might occur in other Latin American regions, especiallyin places with arid and semi-arid conditions, where As in drinking

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ater has been detected and where the aquifer has been charac-erized by a mixture of calcareous and volcanoclastic sediments,ocks such as shale and sandstone, or alkaline groundwaters ofa–HCO3

– type [5]. Although this study focuses in Latin America,he problem of high F and As concentrations is far reaching. Areashat are not monitoring F may find that it is present in high con-entrations, and new developments in arid areas may strain thevailability of groundwater, drawing water from deeper parts ofhe aquifer where these contaminants can be in greater concentra-ions. In addition, arid areas with reported co-occurrence of As and

in countries other than Latin America may face similar shortagesn economic resources, considering that in developing countries theroundwater resources are prone to contamination [6,7]. In the lastection of this paper, the analysis and discussion on treatments forater contaminated with As and F apply to all arid areas of limited

conomic resources, and are not restricted to Latin America.In many Latin American regions, the dissolved As and F concen-

rations show a significant co-occurrence [7–9], thus enhancing theealth risks posed by HACRE (Spanish acronym for Hidroarseni-ismo Crónico Regional Endémico – Endemic Regional Chronicydroarsenicism) [10] and/or fluorosis, either dental or skeletal

11]. Millions of people in Latin America could be currently exposedo As and F through drinking water [9–14]. In Mexico alone, approx-mately 6 million people are exposed to both pollutants [15–17].ittle is known about the toxic effects stemming from a co-exposureo As and F [6,17], but they could lead to both HACRE and endemicuorosis. The exposure to either As or F has shown to induceeduced IQ levels and decreased intellectual functionality amongsthildren [18–20]. Recent studies explored the effect of exposureo both pollutants on immune cells in human populations withhronic exposure through drinking water [20–22]. The presencef As and F could be seen by comparing gene expression profiles inumans exposed to both contaminants with the profiles from pop-lations exposed to these elements individually. The interaction ofs with other elements could be a key in the complete elucida-

ion of the molecular mechanisms involved in the development ofnflammatory and malignant diseases [21,22].

The solution to the problem of drinking water with As and requires either tapping alternative water sources or contam-nated water treatment. Several conventional processes for theemoval of the individual contaminants exist, but only a few canimultaneously remove both contaminants [1,2,23]. Many of theseethods are a combination of conventional and advanced tech-

ologies, which are mainly used to treat As concentrations in urbanroundwater supplies. Once again, the removal of F has not beenonsidered as important as that of As, even though it can co-occurith As in most of the semi-arid regions of Latin America.

This manuscript presents a short overview of the co-occurrencef As and F in drinking water sources by means of some key studiesonducted in the Latin American countries of Mexico, Argentina,nd Chile. Although the results have been partially and indepen-ently reported elsewhere, the authors aim to show a greatericture of the factors that affect the occurrence of As and F, empha-izing the similarities and differences between the studied areas.long with their co-occurrence, this study also discusses the gen-sis and mobility of these contaminants, addresses the treatmentseing utilized for the removal of As and F in some of the affectedreas of Latin America.

. Co-occurrence of arsenic and fluoride in groundwater

.1. Mexico

In Mexico, ∼75% of the total population relies on groundwateror drinking. Some communities have been diagnosed with HACRE

rdous Materials 262 (2013) 960– 969 961

and hydrofluorosis, especially in arid and semi-arid regions of cen-tral and northeastern Mexico [3,4,8,15,24,25]. The presence of As inthese areas has been reported over the past decade, while F deter-minations have only been recently performed. Many areas containAs and F levels above the Mexican drinking water standards of 0.25and 1.5 mg/L, respectively [24–31].

The predominant geology of the arid and semi-arid areas ana-lyzed in this work is characterized by the outcrops of Paleozoicand Mesozoic sedimentary rocks, Tertiary igneous rocks, and marshand lake deposits. Along the coast of Sonora, aquifers are com-prised of sedimentary rocks of marine origin. The windblowndeposits from the Quaternary period formed the filled valleys withconglomerates and alluvial soils [32–35]. The igneous material,together with the conglomerates and the Quaternary alluviumderived from it, is believed to be the source of As and F in sev-eral aquifers in the States of Chihuahua, Sonora, Durango, and SanLuis Potosí [36,37]. The aforementioned geological compositions,together with conditions of aridity, generate greater concentrationsof As and F that are intensified by the high evaporation, rain short-age, and fluctuation of water levels that result from excessive waterextraction [38].

Arid and/or semi-arid climates dominate from northern Mexicoto the central states of San Luis Potosi, Jalisco, and Michoacán;whose annual rainfall varies between 200 mm in the northernregion and 900 mm in the central part [39].

This study identified three main hydrogeological environmentsthat contain high As and F concentrations: areas of geothermalactivity (central and northern Mexico), alluvial aquifers (northernMexico), and areas of mining activity (north-central Mexico).

2.1.1. Areas of geothermal activityMost of the high As and F concentrations occur where there is

upwelling of water characteristic of geothermal and volcanic areasin closed basins. This water upwelling indicates a possible corre-lation between As and F due to the water quality conditions thatfavor the migration of both elements [40].

Los Azufres, Michoacán (Fig. 1, location 13), shows a natural con-tamination of geothermal As and F (5.1–24 mg/L and 9–17 mg/L,respectively). In addition to this, the concentrations in Los Azufreshave increased since 1982, after the establishment of a power plantthat uses evaporation ponds and sometimes re-injects wastewaterinto the aquifer. A two-year study showed the increased levels ofAs (from 2 to 32 mg/L) and F (10–90 mg/L) in the groundwater fromLos Azufres [41]. Although this water is intended for industrial useand has different conditions, it shows how evaporation results inincreased As and F concentrations; which can be extrapolated toexplain the evaporation occurring in arid areas.

Las Tres Vírgenes (Fig. 1, location 1) is also found under anarid climate and encompasses an alluvial aquifer that lies above agranular basement [32,33]. The concentration of As in Las Tres Vir-genes is relatively constant (6.5–6.7 mg/L) and its probable originhas been reported as a dissolution of primary minerals followedby a dispersion of As [41]. In both Los Azufres and Las Tres Vír-genes geothermal reservoirs, data shows relatively constant Asconcentrations throughout varying temperature conditions; thesetemperatures range between 230 and 250 ◦C, providing optimaland stable conditions for As mobility [41].

The Araró hot springs region is located in the northern part ofMichoacán (Fig. 1, location 12). This region has a moderate to semi-arid climate, with an average annual precipitation of 906 mm. Thesite was included here because of the high As and F found in its

waters, which is due in part to geothermal activity [33,34]. Arsenicconcentrations varied from 0.01 to 63 mg/L, and F concentrationsrange from 0.7 to 4.2 mg/L. The highest As and F concentrationswere found at sites where water temperature was close to its

962 M.T. Alarcón-Herrera et al. / Journal of Hazardous Materials 262 (2013) 960– 969

al env

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asflwartip0[pcos

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Fig. 1. Hydrogeologic

oiling point (89–93 ◦C). Additionally, As concentrations in theseaters showed a direct relationship to F and Cl–concentrations [42].

.1.2. Alluvial aquifers of northern MexicoReyes Gómez et al. [37] studied the occurrence and possible ori-

in of As and F in the aquifers of Tabalaopa–Aldama–Laguna deormigas (GTALH), in the state of Chihuahua (Fig. 1, locations 6–8).he exposed rocks are mainly rhyolites composed of quartz, ortho-lase, sanidine, illite, and volcanic glass (>10 and a lesser quantitiesf fluoroapatite (0.1–1.0%). The As content in rocks and sedimentsanges from 2.3 to 540 mg/kg. Fluorapatite was detected in 8 outf 14 analyzed samples (0.1% and 1% proportion) with an esti-ated 41 mg/kg F in average (assuming 0.1%). The highest water

emperatures (32 ◦C) were found near the town of Aldama. The F-ontaminated wells coincided with the As-contaminated wells. Theelationship between As and F and the variation in water tempera-ure produced a positive trend that asserted a co-occurrence of Asnd F [37].

The presence of As and F in the GTALH aquifer is most likelyttributed to geogenic origins, such as the presence of rhyolite andhale. The former contains As in its matrix and the latter containsuorapatite and As-bearing minerals [43,44]. The deep ground-ater extraction contributes to the release of As and F which is

lso influenced by high evaporation rates [45]. Few studies haveeported the co-occurrence of As and F in groundwater; amonghese, correlation coefficients (with their corresponding probabil-ty level p) between As and F has been reported for Sonora (r = 0.92,

< 0.05), San Luis Potosí (r = 0.68, p < 0.001), Chihuahua (r between.55, and 0.66, p < 0.001), and geothermal waters (r = 0.68, p < 0.001)37,42,44,46]. Alluvial aquifers in Sonora (Fig. 1, locations 2–5)resented As concentrations between 0.018 and 0.031 mg/L and Foncentrations between 1.5 and 7 mg/L [33,46]. The co-occurrencef these pollutants was also observed in the northwestern andouthern parts of Sonora (r between 0.65 and 0.92, p < 0.05) [46].

In the arid areas of Valle del Guadiana (Durango) and Comarcaagunera (Durango-Coahuila) (Fig. 1, locations 9–10, respectively),

he As and F concentrations in the alluvial aquifers vary from 0.02o 0.4 mg/L and 1.2 to 16 mg/L, respectively [24–26,30,31]. In bothnstances, the origin of As and F has been associated to magmaticrocesses, and possibly to the presence of an ancient hydrothermal

ironments in Mexico.

groundwater systems that provided large concentrations of oligo-elements such as Li, B, As and F. The area between the cities ofTorreón and Francisco I Madero has already been affected by a pointsource of As, while the northwestern part of the Villa de Juárezvalley shows signs of contamination by As, F, nitrate, and sulfate,among others [24,25,31].

2.1.3. Mining areas in north-central MexicoThe presence of mine tailings and mineral deposits of hydrother-

mal origin may be a contributing factor to As in soils, sediments,and even in groundwater [47]. As is generally attached to the solidphase and has little mobility; however, under some circumstances,it may be released into the aquifer [48]. The identification of anoma-lous concentrations on the soil and sediment surface may help toexplain the presence of contaminants in groundwater [49,50].

Exploratory data analysis (EDA) technique was applied to sedi-ment geochemical data (n = 2,046) from northern Mexico. The areais dominated by large basins filled with alluvium, and pierced bymany wells. A number of these wells have a high As and F content[47,51]. Tertiary volcanic (composed of ignimbrites and rhyolites)outcrop was observed throughout the study area, alternating withlimestone and shale outcrops. The region contains economicallyimportant hydrothermal skarn deposits with silver, lead, and zincmineralization [52]. A strong As and F correlation was observed inthe localities where As has been reported in high amounts (Table 1)(correlation coefficients > 0.5, p < 0.001) indicating co-occurrencesof these elements. Arsenic in sediments correlated to Al (r2 = 0.42,p < 0.0001), Fe (r2 = 0.44, p < 0.0001) and to a lesser degree with Cu(r2 = 0.30, p < 0.001). The background and anomaly levels of As con-tent in sediments using the Tuckey diagram are included in Table 2.The locations of As anomalies were plotted in Fig. 2. After visuallysuperimposing As, Fe and Cu anomalies to locations of mines, a pat-tern emerged in which anomalies of all three elements clusteredaround and directly downstream of the large hydrothermal-typemineral deposits (e.g., San Antonio mine, which produces Ag andPb) [52,53].

High concentrations of As (up to 0.12 mg/L) and F (up to 16 mg/L)have been reported in the northeastern part of the Independencebasin (Fig. 1, location 11) [54]. The high As and F concentrationswere associated with high Na+ and HCO3

– concentrations [54,55]

M.T. Alarcón-Herrera et al. / Journal of Hazardous Materials 262 (2013) 960– 969 963

Table 1Pearson correlation coefficients (p < 0.001, 2-tailed) for As and F in water samplescollected from different municipalities in Chihuahua, Mexico, (data from Alarcón-Herrera [84]).

Municipality (No.of samples)

Correlationcoefficients

% of samples>0.025 mg/L As

Julimes (n = 9) 0.99 100Coronado (n = 10) 0.97 40SF Conchos (n = 5) 0.97 40Meoqui (n = 27) 0.78 67Jiménez (n = 32) 0.68 50Saucillo (n = 37) 0.67 74Rosales (n = 13) 0.61 69.2Delicias (n = 22) 0.47 54.5Allende (n = 18) 0.43 33.3López (n = 10) 0.31 30.0Camargo (n = 34) 0.29 44.1La Cruz (n = 10) 0.10 70.0

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ercentage of samples with As concentrations above the Mexican norm of.025 mg/L are also noted.

ainly derived from the dissolution of minerals present in thehyolite-ignimbrite rocks that conform the fractured aquifer and

shallow granular aquifer above it. Dissolution of As-bearing min-rals was considered as the secondary process for As enrichment;hereas F enrichment was related to the dissolution of fluorite, theresence of Li+, Cl–, Cs, and Br– in high concentrations, and to theigh groundwater temperature [54].

.2. Argentina (Chaco-Pampean plain)

The Chaco-Pampean plain is the largest and most populatedegion in Argentina. The area extends from the Paraguay border inhe north to the Patagonian plateau in the south and the Pampeanills to the east (Fig. 3). Groundwaters with high salinity are thenly sources of water in wide areas, hindering their use for humanonsumption. Many areas of the basin show significant correlationsetween dissolved As and F concentrations, posing a health riskecause of the probability of developing HACRE and/or fluorosis,ither dental or skeletal [9,10].

.2.1. Geology and hydrogeologyAeolian loess and loess-like deposits mantled the Chaco-

ampean plain, forming a series of thick sedimentary sequenceshat are now acting as a groundwater aquifer. Loess sediments arealcareous in nature and contain volcanoclastic mineral associa-ions, with volcanic glass contents varying between 20 and 50 wt.%sometimes up to 63 wt.%). Loess sedimentation started during thepper Miocene and continued until the lower Pleistocene. The Pam-ean loess is crowned with an abundant secondary carbonate inhe form of an undulating calcrete layer (locally known as “tosca”).

he calcrete layer (few centimeters to several meters thick) can beonsidered as a potential source or sink of fluoride, which also isn indicator of semi-arid conditions. The most important “tosca”evelopment, between Pliocene and lower Pleistocene [56], marks

able 2hresholds and number of anomalies for sediment data (n = 2,046).

Al (mg/L) As (mg/L) Ca% (mg/L

Median 2.0 5.8 1.8

Inter-quartile range IQR 1.2 3.3 5.7

Lower inner fence LIF −0.23 −0.60 −8.0

Lower outer fence LOF −2.0 −5.6 −17

Upper inner fence UIF 3.3 9.4 9.2

Upper outer fence UOF 5.0 14 18

No. of mild anomalies 271 188 314

No. of extreme anomalies 11 92 7.0

oncentrations in mg/L, unless otherwise stated. All anomalies correspond to the upper f

Fig. 2. Locations of As anomalies in Mexico.

as a palaeo-topographic relic of a palaeo-climatic event more aridthan the present climate.

The pseudo-sedimentary loess sequence hosts exploitable mul-tilayered aquifers that consist of several alternating sandy silts andfine grained layers [57]. Hydraulic conductivity is low to medium

(maximum of 0.5 m/day). Due to the transitional passage of thesesediments to the underlying Miocene formations, the base of theloess-type sediments becomes very difficult to identify. A flat

) Cu (mg/L) Fe% (mg/L) Sr (mg/L) Zn

13 2.4 112 6914 1.2 155 34

−13 0.11 −180 3.5−33 −1.7 −413 −47

29 3.6 286 10449 5.4 519 154

256 204 181 199125 33 88 41

ences.

964 M.T. Alarcón-Herrera et al. / Journal of Hazardous Materials 262 (2013) 960– 969

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eomorphology and groundwater recharge, amounting to about0% of the total precipitation, characterize the subsurface hydro-ynamics.

.2.2. Hydrochemistry of Chaco-Pampean plainMany studies have been conducted within the Chaco-Pampean

lain to find out the reason for widespread occurrences of As and in groundwater. Table 3 compiles some of these studies andheir results with respect to the co-occurrence of As and F inroundwater.

In southern Buenos Aires province (Fig. 3, location 1), low-alinity groundwater was observed, which is good for humanonsumption. In contrast, an incremental salinity was observedowards the basin margin close to the Atlantic slope. Groundwaterxhibits an analogous hydrochemical trend, being slightly alka-ine (pH ∼ 7.4) and Ca–HCO3

– type at the recharge areas while

a–HCO3

– type with alkaline pH (up to 9.0) at the discharge areas58]. This trend was interpreted as a consequence of the cationxchange process together with the dissolution of carbonates, evenhough the dissolution of silicates may be significant, especially at

ents in Argentina and Chile.

high pH [59]. Generally, the increments in salinity and alkalinitywere correlated to high As and F concentrations that restrict humanconsumptions over an extended area [60,61].

In Santiago del Estero province (Fig. 3, location 6), shallowgroundwaters and groundwater in the Río Dulce alluvial cone arehighly variable in composition. The water chemistry is dominatedby Na–HCO3

– type, which was prevalent in nearly 73% of the stud-ied wells [62].

In the Córdoba province (Fig. 3, locations 2–4), F concentration inshallow and deep wells shows a strong positive correlation with As(r = 0.72, p < 0.001). In shallow groundwater, a negative correlation(r = –0.91, p < 0.001) was observed between F and Ca2+, which canbe explained by the low solubility of CaF2 [63,64].

2.2.3. Sources of arsenic and fluoride in groundwaterBundschuh et al. [62] found a good correlation between As and

F, noting the occurrence of As–F complexes in groundwaters. InRobles county, (Argentina) high As and F concentrations in shal-low groundwaters are related to a layer of volcanic ash depositswithin the pseudo-sedimentary sequence [62]. The groundwater is

M.T. Alarcón-Herrera et al. / Journal of Hazardous Materials 262 (2013) 960– 969 965

Table 3Groundwater quality in the Chaco-Pampean region, Argentina (N = North, S = South, E= East).

Province region pH As (mg/L) F (mg/L) As versus F EC (�S/cm) Reference

S. Buenos Aires 6.3–8.4 Not detected 0.1–0.5 n/a 100–400 [67]Buenos Aires, Atlantic region 7.9–8.8 0.01–0.10 0.9–3.9 n/a n/a [67]Buenos Aires, shallow wells n/a n/a 5–7 n/a 800–4000 [66,67]N. La Pampa 7.0–8.7 0.004–5.3 0.9-3.9 n/a 770–17,500 [62,63]S.E. Córdoba 7.0–8.3 0.02–3.8 0.26–6.3 n/a 845–13,000 [60,63,64]S. Córdoba n/a 0.05–0.30 1.4–6.8 n/a n/a [50,63]Córdoba, shallow wells n/a 0.001–4.5 (av. 0.001) 0.4–10 n/a [60,61]Córdoba, deep Wells n/a 0.001–0.2 (av. 0.04) <1.5 r = 0.72 p < 0.001 n/a [48,61]Santiago del Estero, Robles County 6.9–8.8 0.01–2.4 0.1–4.7 n/a n/a [62]Santiago del Estero, Rio Dulce Alluvial Cone 6.4–9.3 0.01–15 0.7–22 r = 0.87, p < 0.0001 804–9,800 [65]

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f Na–HCO3– type with an average pH: 7.7, and has an As concen-

ration that correlates negatively with Ca2+ and Mg2+ and positivelyith F and Na+. In Los Pereyras, Tucumán province (Fig. 3, location

), Warren et al. [65] found high F concentrations only in the upper0 m of the weathered loess deposits. Arsenic concentrations werelso high in the upper aquifer and decreased downwards. A similars stratification was reported by Garcia et al. [66].

Geographic distribution of sediments derived from loess anduviatile loess reflected the spatial variability of As and F in ground-aters [60]. Additionally, As and F partitioning into loess granules

s also a critical factor for their transference from solid to liquidhases. Excessive As and F concentrations in groundwater of theouthern Chaco-Pampean plain are associated with salinity andlkalinity. Potentially toxic contaminants like As, F, and/or oth-rs (B, U, Se, Sb, Mo) were observed to increase near the riverouth, the groundwater discharge zones, and in the endorehic

akes or lagoons. Such variability in the distribution of As and F inroundwater appeared to be controlled by the combined interac-ions between geology, geomorphology and local landform features60]. In addition to evaporation and cation exchange (Ca2+ and Mg2+

ersus Na+), adsorption of K+ on clay minerals modifies the waterhemistry along the groundwater flow path and enhances the sol-bility of As and F [67].

High salinity, along with the presence of high As and F con-entrations in irrigation waters, constitute a strong limitation forgricultural productivity, leading to a decrease in sensitive cropields. Highly saline waters with concentrations of As (>0.2 mg/L)nd F (>2 mg/L) are also toxic for animal consumption [68].

The aquifers of the Chaco-Pampean plain are comprised of Ter-iary aeolian loess-type deposits, with water soluble volcanic glasss the principal As source. The common features of As-enrichedotspots are of Na–HCO3

– type groundwater, with pH > 8, high elec-rical conductivity and frequent presence of high concentrationsf B, F, Mo, V, U, and Mo; indicating their common origin in theolcanic glass. Low As concentrations are found in the zones witha-HCO3

– type groundwater with neutral pH.

.3. Chile

.3.1. Geothermal waters of El Tatio, ChileIn Chile, the co-occurrence of As and F has been reported from

he sites of geothermal activities. The best known example is Elatio geothermal system located in the Antofagasta region (Fig. 3,ocation 7), northern Chile [69]. El Tatio is located at an altitudef about 4,200 m above sea level, where many geothermal springs,umaroles, geysers, and boiling and mud pools can be observed. The

rincipal hydrothermal reservoir is confined within the permeableuripicar Formation and the Salado Member. The recharge takeslace about 15 km east of the field [70]. The water is heated at theaguna Colorado ignimbrite complex, and the producing aquifer

has reservoir temperatures of 265 ◦C and is located at a depth of800 m. Discharging water is of Na–Cl– type and has temperature ofabout 86 ◦C [71].

High dissolved silica in El Tatio geothermal system resultsin a massive precipitation of siliceous sinters in the dischargechannel, which are mostly composed of amorphous silica [69].Extremely high aqueous silica concentrations aid the formation ofsiliceous coating on the ferric oxyhydroxide surfaces that inhibitAs(V) adsorption and maintain As in the dissolved phase severalkilometers downstream [72]. The discharge of As from El Tatiogeothermal field has a strong impact on the river “Rio Loa” waterquality, which is used as a water supply by the Antofagasta town[73]. The dissolved As concentrations were observed between 1.0and 2.0 mg/L. These concentrations are a consequence of evapora-tive enrichment with alkaline pH and high salinity values, whichdo not favor As(V) adsorption [68]. The behavior of F in El Tatiogeothermal system was much less studied. The reported F concen-trations ranged from 0.8 to 8.5 mg/L [70]. However, no informationis available on the fate of F in the Rio Loa river or its tribu-taries; but the evaporative enrichment has resulted in an increasingNa+/Ca2+ ratio and precipitation of calcium carbonate [74], whichactually contributes to maintain high F concentrations furtherdownstream.

3. Arsenic and fluoride removal processes

The most commonly available technologies for As and F removalfrom water include processes that can be used alone or in com-bination. In Latin America, removal technologies used for Asand F removal have been membrane technology and coagula-tion/filtration [2,75,76]. A widely tested removal technology forboth contaminants is adsorption on activated alumina. Despite itssimplicity, this method has not been implemented in Latin Amer-ican countries yet, even though it’s been thoroughly tested andproven to be successful at the laboratory- and pilot-scale in Mexico,Canada, and the USA [77–80].

3.1. Membrane technologies

In Mexico and Argentina, reverse osmosis (RO) is the processused for the successful removal of As and F from drinking water.Reverse osmosis is a membrane technology that uses pressure toforce water through a semi-permeable membrane, thereby remov-ing dissolved solutes from solution based on particle size, dielectriccharacteristics, and hydrophilic/hydrophobic tendencies [81]. RO

water should be pre-treated for particle removal. The RO systemsmay have significant water losses, typically between 35% and 65%,which constitute the concentrated brine discharges that must betreated before discharged [81].

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The RO technique has a removal efficiency of over 92% for As(V)nd As(III) and 85–95% for F [80–83]. In the state of ChihuahuaMexico), more than 280 small RO plants have been installed inural communities to alleviate the problem of As and F pollution inrinking water. However, a new environmental problem is createdecause there are no provisions available to treat or safely disposef the rejected brackish water [84]. In Argentina, several RO plantsave been installed in the provinces of Santa Fe, Cordoba, and Laampa [76,85].

.2. Coagulation/filtration techniques

Coagulation/filtration is a common water treatment processsed to remove suspended and dissolved solids from water78,80,81]. A pilot scale experiment was performed in Argentina onhe application of coagulation/filtration for the removal of As (rang-ng from 0.15 to 0.20 mg/L) and F (ranging from 1.8 to 2.0 mg/L).he treatment system had an average aluminum polychloridePAC) dose of 100 mg/L and pH of 6.9–7.0. The system’s treatmentfficiency made it possible to obtain average As and F concentra-ions of 0.02 and 1.5 mg/L, respectively, with operating costs of3.36 USD cents/m3 of treated water. The system has been installednd proved in two communities: (Villacanas city and López com-unity, both in Santa Fe Province) [85]. This process has been

uccessfully implemented in full-scale plants in Argentina, withuoride and total arsenic concentrations in raw water up to 2 mg/Lnd 200 �g/L, respectively. The process is cost effective; however,he optimal operating doses are highly dependent on the quality ofhe source water [86]. Nevertheless, long-term monitoring is nec-ssary for the pilot scale experiments to determine the method’spplicability and treatment costs [75,86]. In Mexico, other labora-ory studies report that the process is effective when aluminumulfate is used as a coagulant for the removal of As and F, as long asheir concentrations do not exceed 0.19 and 6 mg/L, respectively87,88]. Another laboratory study was performed by combininglum and polymeric anionic flocculant (PAF) for the removal ofs and F from drinking water. The field data revealed that Asnd F concentrations (0.134 and 5.9 mg/L, respectively) may beeduced up to 99% and 77%, respectively, at a cost of US$38 cents/m3

f treated water [89]. In northern Chile, coagulation technologyas been used since 1970 for removing arsenic from drinking-ater [90]. It is currently possible to reduce arsenic from 400 to

0 �g/L at a rate of 500 L/s, assuming pH, oxidizing and coagulationgents are strictly controlled. The reported cost was US$ 2.3 ¢/L.he Chilean experience with the removal of arsenic demonstrateshat the water matrix dictates the selection of the arsenic-removalrocess [90,91].

. Discussion

The occurrence of As and F in groundwaters of central-northernexico, El Tatío, Chile, as well as in the Chaco-Pampean plain of

rgentina are mostly geogenic. Their primary source is associatedith volcaniclastic material present in loess and alluvium deposits

26,37,59,69], shales, and/or geothermal activity [4,60,70]. A sec-ndary source is also recognized, consisting of solid materials thatdsorb As and F, such as Fe−, Mn−, Al− oxides/hydroxides, and clayinerals. Groundwater with high As and F concentrations is com-on in arid and semi-arid areas, and it reportedly results from

he combined effect of chemical weathering and high evaporationates. The latter concentrates the solutes in water and produces

igh salinity, shifting the pH to alkaline values which are moreonducive to desorption of As and F from the secondary sourceaterials [40,52,92]. High pH values promote the dissolution of

ilicates, which may be one of the processes releasing As into the

rdous Materials 262 (2013) 960– 969

groundwater for the sites discussed here. Once in solution, As andF adsorb to Fe−, Mn−, and Al− oxides/hydroxides and clay miner-als present in the sediments [36]. The alkaline pH conditions reducethe adsorption capacity of these solids, releasing anion-forming ele-ments F, V, B, Mo and U [40] from loess and alluvium back to thewater.

The areas described here had the following common attributes:an arid or semi-arid climate, limited recharge, saline groundwaters,high pH values, and abundant volcanic glass in the sediments (loessor alluvium). Arsenic content in volcanic glass has been reported byOnishi and Sandell as 5.9 mg/kg [93], in basin-fill material of Ari-zona as 6–7 mg/kg [94], and surface basin-fill material in northernMexico as 6–35 mg/kg [47]; all three showing a significant enrich-ment relative to an As crustal abundance of 1.8 mg/kg.

Other important mechanisms that release As and F are desorp-tion from hydrous Fe− and Mn− oxides/hydroxides and oxidation ofminerals such as arsenopyrite (FeAsS) and fluorite (CaF2), which arefound to be associated with hydrothermal mineral deposits [40,92].In arid and semi-arid regions, evaporation plays an important rolein limiting the recharge, thus precluding flushing of As and F richwater and promoting a longer water–rock interaction. This interac-tion may continue to higher stages, which leads to the salinizationof sediments and the formation of Na-rich deposits. If carbonaterock is present, it will dissolve; buffering the water from neutral toalkaline pH, and forming a predominant Na–HCO3

– type of ground-water. Competition for limited sorption sites with other oxyanioniccontaminants such as V may further increase dissolved As and Fconcentrations [40].

In Mexico, the main sources of F are fluorite and F-apatite.A mixture of these minerals in sediments and anthropogenicfragmentation is a known source of As and F in groundwa-ter [37,43]. The observed anomalies of As and F contaminationsare attributed to natural sources, with lesser contributions fromanthropogenic factors like mining runoff, sewage, fertilizers, andpesticides [47]. Other sources of F in the Chaco-Pampean plaininclude the loess deposits containing abundant secondary car-bonate in the form of calcrete layers, as well as veins orconcretions typical of arid/semi-arid climates, and volcanic glassin volcanic ash present as a distinct layer or dispersed in loesssediments. In many areas of Latin America, As and F are alsolinked to geothermal waters [4,41,64]; as geothermal activity holdsa greater potential for dissolving minerals in contact with hotsolutions.

Chemical precipitation/filtration is the most widespread processused in Argentina and Chile for the removal of As and F in large-scalecommunity water treatment and distribution facilities [85,89,91].In Latin America, activated alumina adsorption has been success-fully tested in laboratory and pilot-level tests, but its application incommunities hasn’t been reported [77]. Reverse osmosis is also awidespread treatment method in México and Argentina, both at asmall and large-scale [76,84]. Nonetheless, there are economic con-cerns because of its high operating costs and environmental issuesrelating to the management and discharge of the produced brines[81,84].

An important issue, which has only been recently addressed, isthe effect of As and F on agricultural activities [95,96]. The presenceof these contaminants can decrease agricultural yield, production,and quality of animal products. Researchers studying this effecthave reasons to believe that these secondary agricultural effectsmay be at least as harmful to society as those of direct As and Fconsumption through drinking water [97,98]. However, As and Fpollution of agricultural products is a problem of a different mag-nitude because it is unfeasible to treat all the water that is supplied

to crops and livestock. The joint removal of As and F will require agreat research effort and the development of innovative treatmenttechnologies.

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. Conclusions

The origin of As and F in groundwater in Latin America is mainlyeogenic by nature, with the primary source identified as vol-anic glass and, to a lesser extent, hydrothermal minerals. Theecondary source is the Fe−, Mn−, and Al− oxides/hydroxides andlays towards which As, F, and other trace elements have a greatdsorption affinity. High evaporation rates, typical of arid and semi-rid climates, generate saline groundwaters (e.g., Na–HCO3

– typeater) and alkaline pH, releasing As and F from both the primary

nd the secondary material sources, resulting in water rich in Asnd F.

The principal methods for As and F removal used in Latin Amer-ca are chemical precipitation/filtration and reverse osmosis. Inrder to reduce their exposure to humans, it is imperative to imple-ent risk communication programs that address the health risks

osed by the combined presence of these contaminants. Furthertudies urgently require addressing the behavior of these contam-nants, their health impact; the development and implementationf affordable treatment technologies, and the appropriate disposalf the generated waste.

Presented data demonstrate clearly that co-occurrence of Asnd F is common in many aquifers in Latin America especially inxidized and alkaline environment. When one of these contami-ants is present, ground water should be analyzed also for anotherontaminant and their health impact should be evaluated simul-aneously. This study may help explain and remediate problemshat may occur in arid areas other than the regions reported here,specially if they have similar geologic conditions.

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