Arsenic enrichment in waters and sediments of the Rio Loa (Second Region, Chile

Arsenic enrichment in waters and sediments of the Rio Loa(Second Region, Chile)

L. Romeroa, H. Alonsoa, P. Campanoa, L. Fanfanib, R. Cidub,*, C. Dadeab,T. Keeganc, I. Thorntonc, M. Faragoc

aUniversidad Catolica del Norte, Av. Angamos 0610, box 1280, Antofagasta, ChilebUniversita di Cagliari, Dipartimento Scienze della Terra, Via Trentino 51, 09127 Cagliari, Italy

cImperial College of Science, Technology and Medicine, London SW7 2BP, UK

Abstract

The Second Region of Chile (126,500 km2) is extremely arid, with a dramatic scarcity of water. The only waterresource for the population (about 420,000 habitants) and the mining industry (the most important economic activityin the region) is the 440 km long Rio Loa. Moreover, this is highly enriched in As. In order to assess As concentrations

and sources, and to evaluate the impact of mining activity on the water quality in the Rio Loa basin, water and sedi-ment samples were taken at strategic points along the river and its major tributaries. The water in the whole basin isquite saline (total dissolved solids up to 11 g/l) and heavily enriched in As (average: 1400 mg/l) and B (average: 21,000

mg/l). These values are up to 300 and 100 times higher than the respective guidelines suggested by the WHO fordrinking water. The quality of water is extremely poor along the tributary Salado, mainly fed by the El Tatio geo-thermal waters that are very rich in As (up to 27,000 mg/l) and other components. Sediments from the Rio Loa and itstributaries have As contents in the range of 26–2000 mg/kg (mean value of 60 samples: 320 mg/kg), and reach 11,000

mg/kg at El Tatio. Sequential extraction analyses show the As to be mainly associated with Fe–Mn oxy-hydroxides andresidual phases, but part of the As (about 20%) is readily available being extracted from the exchangeable and car-bonate phases. This result is in agreement with the correlation observed between As content in sediments and As con-

centration in waters in the area. The extreme arid conditions, high evaporation, and the lack of low-As tributariescontribute to maintain high concentrations of As and other components in the Rio Loa water to the mouth. Due to theoxidising conditions, neutral to alkaline pH, high salinity and high As concentrations, adsorption of As-species is not

favoured. The main As source in the Rio Loa basin is considered to be natural, i.e. linked to the lithologies in the area.Smelter emissions and mining wastes, as well as the As-rich effluents from the water treatment plants, possibly repre-sent additional sources.# 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

The adverse effects of As exposure on human health

are well known, though not all populations or indivi-duals have the same response to As (Hopenhayn-Richet al., 1996). The consumption of water with high

concentrations of As over a prolonged period of timecauses serious diseases, including arsenical dermatosis,hyperkeratosis and arsenicosis (Bhattacharya et al.,

1998), especially in rural and semi-urban areas, wheredrinking water is often used without treatment.Therefore, the occurrence, origin and mobility of As

in natural waters have received significant attention inrecent years. Arsenic concentrations above maximumpermissible national limits for potable water have been

reported in groundwater in several parts of the worldincluding the USA (Robertson, 1989; Fuji and Swain,1995; Welch and Lico, 1998), Mexico (Del Razo et al.,1990), Argentina (Nicolli et al., 1989), Chile (Caceres et

al., 1992), Taiwan (Kuo, 1968), Mongolia (Smedley et al.,2001), India (Acharyya et al., 2000) and Bangladesh(Bhattacharya et al., 1997). The most commonAs-species

0883-2927/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0883-2927(03)00059-3

Applied Geochemistry 18 (2003) 1399–1416

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* Corresponding author. Tel.: +39-070-67-57-724; fax:

+39-070-282-236.

E-mail address: [email protected] (R. Cidu).

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in water are As(V) as arsenate and As(III) as arsenite,the latter being more mobile and toxic for living organ-isms. Methylation of inorganic As, associated with bio-logical activity in water, may mitigate the adverse effects

because inorganic As is transformed into organic forms(e.g. monomethyl- and dimethylarsenic acids) that areless toxic (Wood, 1974; Cullen and Reimer, 1989).

Arsenic enrichment in water of the Second Region ofChile (capital town: Antofagasta) is well known (e.g.Caceres, 1999; Queirolo et al., 2000, and references

therein). Here the streams are characterised by a highcontent of As (100–1000 mg/l), mainly associated with theQuaternary volcanic and present-day geothermal activity

in the Andes Cordillera. The relationship between As andgeothermal activity is well known in several parts of theworld (e.g. Ellis andMahon, 1977; Ballantyne andMoore,1988; Yokoyama et al., 1993; Robinson et al., 1995).

The high concentration of As, associated with highsalinity and B, compromises the use of the water in theregion of Antofagasta as safe drinking water (Smith et

al., 1998). In the urban supply, the As is removed to alarge extent (<50 mg/l), but not in the supplies to ruralareas nor in the water used for irrigation. In the Second

Region of Chile, adverse health effects have been recog-nised in rural populations since 1962 (Smedley et al.,2000, and references therein), and include As-induced

skin lesions (Smith et al., 2000), and cancers of thebladder and lung (Smith et al., 1998).This paper aims to assess the concentrations and

sources of As enrichment in the Second Region of Chile

and understand the chemical partitioning of As in thesediments of the basin, as a basis for planning adequatetechnologies to reduce the enrichment in water resour-

ces. This study, in fact, is part of a project supported bythe European Community devoted to provide innova-tive strategies for the remediation of water quality in

mining areas of Latin America.

2. Field area and environmental situation

The region of Antofagasta (Second Region) is locatedin northern Chile at the border of the Pacific Ocean,

between 20�50 and 26�50 S. It covers an area ofapproximately 126,500 km2 (16.7% of the national ter-ritory). The population (approximately 420,000 habi-

tants) is organised in nine urban centres. Economicactivities in the region are directly or indirectly relatedto mining. The region is extremely arid, with a dramatic

scarcity of water. The rainfall decreases from the AndesCordillera (300 mm/a above 3000 m) to the Pacific Ocean(1–2 mm/a). The only permanent superficial water is the440 km long Rio Loa which flows in a westerly direction

to the Pacific Ocean (Fig. 1). The Rio Loa and its maintributaries (San Pedro, Salado and San Salvador) providewater both to the cities and to the mining activity in the

region. Furthermore, they support irrigation for limitedagricultural development and the survival of someaquatic and terrestrial flora and fauna around a fewsmall villages.

Water in the Rio Loa basin is not only scarce, but isof poor quality due to high salinity, and high dissolvedAs and B (Orellana, 1985). The chemical composition of

water in the Loa is strongly influenced by the inflow ofthe Salado tributary that originates in the geothermalfield of El Tatio. The enrichment is observed down-

stream even at a considerable distance from the con-fluence, through the important mining area ofChuquicamata, to the mouth. The high salinity in the

Loa has been related to the inflow of groundwater andthe intense evaporation occurring in the AtacamaDesert (Orellana, 1985). Seasonal variations in salinityhave been associated with irrigation in the Calama area

that causes the leaching of saline soils (Alonso et al.,1992). The poor quality of water in the Second Regionof Chile has forced the construction of treatment plants

to supply drinking water. The Cerro Topater plant islocated upstream of Calama (Fig. 1). Here, the waterfrom the Rio Loa is processed by a conventional tech-

nique using FeCl3 to remove the As. The processingcomprises oxidation, adsorption in settlement tanks, andfiltration to achieve concentrations of <50 mg/l As. The

Cerro Topater effluents contain As in the range of 20–500mg/l. Until 2000, these effluents were discharged directlyinto the Rio Loa with a daily volume of 1044 m3.The flow of water in the Rio Loa usually shows sea-

sonal variations in the range of 500–3000 l/s (GARHIN,1994). During rare heavy-rain events occurring in theAndes Cordillera, its flow may exceptionally increase up

to four orders of magnitude. The last storm eventsoccurred in March 1997, February 2000 and February2001, and were related to the El Nino phenomenon.

Due to the high hydraulic gradient, the water flowsdown with a great destructive force, removing andredistributing stream sediments. During the 1997 event,the content of As and heavy metals in the waters of the

Rio Loa increased considerably and As concentrationsup to 30 mg/l were observed at the Sloman reservoir(CONAMA, 1997). The agricultural activity around

some towns and villages and the fishing industry in thecoastal region were seriously damaged, and the supplyof drinkable water to the main towns became difficult. A

large contribution to the pollution increase during theseextreme events is ascribed to the downstream dispersionof the sediments deposited in three artificial basins

(Sloman, Santa Fe and Santa Teresa) constructed inthe 1920s and located in the Lower Loa (Fig. 1). Atpresent, these reservoirs are almost 80% filled withsediments heavily contaminated by the wastes of the

mining activity developed in the first half of the 20thcentury (Arroyo et al., 1999). At that time, manymining activities and small-extraction plants (metallic

1400 L. Romero et al. / Applied Geochemistry 18 (2003) 1399–1416

and non-metallic) operated near the Rio Loa con-taminating the river water. Only in 1994 was the dis-posal of mining wastes covered by environmental law.

3. Morphology and geology of the Loa basin

Fig. 1 shows the geographical subdivision of the RioLoa basin into four well-defined sectors: Upper Loa,Salado, Medium Loa, and Lower Loa. Fig. 2 shows the

main geological and structural features of the region.The Upper Loa extends from its source at the foot of

the volcano Mino to the confluence with the Salado.

The Loa canyon cuts limestone deposits, volcanic(rhyolitic and liparitic), alluvial and lacustrine rocks

(Miocene-Holocene). The canyon formation is probablyassociated with the tectonic N-S longitudinal structuresextending along the upper course of the river.The Salado, mainly fed by the geothermal springs of

El Tatio located in the Andes Cordillera, flows in an E–W direction into a canyon and cuts into volcanic rocks,mainly andesite and rhyolitic ignimbrite of Miocene-

Holocene age (Boric et al., 1990).The Medium Loa comprises the area between the

confluence with the Salado and that with the San Sal-

vador. The Medium Loa flows westwards and parallelto the river San Salvador. In this sector, both rivers cutmainly Miocene to Holocene alluvial sediments and

minor Pliocene to Holocene evaporitic deposits whichfill the extended basin of the deserted Pampa.

Fig. 1. The Rio Loa Basin, Northern Chile, and location of water sampling sites.

L. Romero et al. / Applied Geochemistry 18 (2003) 1399–1416 1401

The Upper and Medium Loa sectors geologicallyinclude the Calama, Loa and ChiuChiu Formations (Mayet al., 1999). The Oligocene Calama Formation is formedby a series of alluvial conglomerates. The LowerMiocene-

Lower Pliocene Loa Formation covers almost the entirearea of the Calama basin, and consists of a succession offluvial and alluvial facies comprising limestone, calcar-

eous sandstone, and clay. The overlying Pliocene-LatePleistocene Chiu Chiu Formation comprises a sequenceof clay with intercalations of diatomite, gypsum and

alluvial deposits. The alluvial, fluvial, and fluvio-lacus-trine sediments host intercalations of volcaniclasticdeposits in the Calama Basin (May et al., 1999).

The Lower Loa comprises the zone downstream ofthe Medium Loa-San Salvador confluence to the mouth.The northward course of the river changes downstreamwestwards and flows to the Pacific Ocean through a steep

canyon cutting Mesozoic and Palaeozoic formations ofthe coastal mountain range Cordillera de la Costa(GARHIN, 1994).

4. Ore deposits and mining activities

Mining activity in the Loa Basin takes place in theintensively mineralised porphyry-Cu belt with develop-

ments in three large Cu deposits: Chuquicamata, Rado-miro Tomic and El Abra (Fig. 1).The mineralisation in the Cu (Mo–Ag–Au) deposits

of Chuquicamata (2�3 km in area and 810 m deep;located 17 km north of Calama, 2900 m a.s.l.) andRadomiro Tomic (25 km north of Calama, 3000 m

a.s.l.) may be divided in four zones: (1) a leached zoneconstituted of Fe oxides; (2) an oxidised zone consist-ing of atacamite [Cu2(OH)3Cl], chrysocolla [Cu2H2-

Si2O3(OH)4], and other secondary Cu minerals; (3) asulphide-rich zone, mainly constituted of chalcocite(Cu2S), covellite (CuS), and (4) a primary sulphidezone, including mineral associations such as chalco-

pyrite–bornite–digenite, pyrite–molybdenite–chalcopyr-ite and pyrite–chalcopyrite at Chuquicamata, andchalcopyrite–bornite, and pyrite–chalcopyrite at

Fig. 2. Schematic geological map (May et al., 1999, modified) and location of the sediment sampling sites in the Rio Loa Basin.


Radomiro Tomic. The migration of Cu-rich solutionsoriginated the Exotica deposit (2 km south of Chu-quicamata), mainly constituted of chrysocolla andatacamite.

The Cu(Mo) mineralization of the El Abra deposit(42 km north of Chuquicamata at 4000 m a.s.l) com-prises two zones: (1) an oxide zone mainly constituted of

chrysocolla; and (2) a mixed zone constituted ofchrysocolla, cuprite, native copper and chalcocite.Open pit exploitation of the Chuquicamata-Exotica

deposit started in 1920. In 1995, the Codelco ChileDivision Chuquicamata processed about 150,000 tonsper day (t/day) of Cu sulphide, 28,000 t/day of Cu

oxide, and 100,000 t/day of ancient wastes, resulting inan annual production of 600,000 tons of Cu (about 30%of the Chilean national production). The ratio of eco-nomic ore to waste materials is approximately 1:2.7.

One third of the reserves with an average content of1.6% Cu had been exploited by the year 2000, and onethird more (about 1.7 billion tons) with an average

content of 0.84% Cu are planned to be exploited by2025.Water used in the mining activity at Chuquicamata is

extracted from the Loa Basin (the Codelco abstractionplant is shown in the inset map of Fig. 1). In 1980, thewater consumption at Chuquicamata was 1760 l/ton of

mineral processed. In 1995, water consumption wasreduced to 910 l/ton, water abstracted totalled 1960 l/sand the recycled water was 7200 l/s. Tailings from themineral processing are discharged in the Talabre

impoundment (Fig. 1) located in the Salar of Talabre at12 km NE of Calama. This impoundment with an areaof approximately 40 km2 is located in a depression ori-

ginated by fluvial erosion cutting the limestone of theLoa Formation and partially exposing a level of slimeand clay. An impermeable argillaceous parapet isolates

the limestone and prevents the infiltration and drainingof water. In 1999, the water recycled from Talabre toChuquicamata was 690 l/s.

5. Sampling and analytical methods

Sampling of water and river sediments was performedin May 1999, September 2000 and September 2001.Because of the large extent of the area and the difficulty

of access to some remote sites, sampling was undertakenat specific points located at the Upper Loa, Salado,Medium Loa, Lower Loa, and the main tributaries. The

location of water and sediment sampling sites is shownin Figs. 1 and 2, respectively. Waters at sites No. 4, 34,36, 38, 41, 42 and 55 were sampled both in 1999 and2000. Sediment samples were taken at 5 cm depth. The

35 waters and 60 stream sediment samples are con-sidered representative of the environmental situationalong the entire basin.

5.1. Analytical methods for water

The pH, Eh, conductivity, alkalinity, and temperaturewere measured in situ. The Eh was measured by Pt

electrode and values were corrected against ZoBell’ssolution (Nordstrom, 1977). Water samples were filteredin situ at 0.4 mm and acidified with HNO3 (1% v/v,

suprapure grade) for metal analyses. Major, minor andtrace elements in solution were analysed by ion chro-matography (IC, Dionex DX-120), inductively coupled

plasma atomic emission spectrometry (ICP-AES, ARL-3520), and inductively coupled plasma mass spectro-metry (ICP-MS, Perkin Elmer Elan-5000). An aliquot

filtered and acidified to 0.5% v/v HCl was used for thedetermination of As by ICP-AES and flow injectionhydride generation ICP-MS (Cidu, 1996). Data qualitycontrol comprised field blank solutions, analysis of cer-

tified reference solution (NIST-1643d), and comparisonof results derived from different techniques. Accuracyand precision were estimated at �7% and �12%, or

better, at the mg/l and mg/l concentrations, respectively.Charge balancing computed by the PHREEQC com-puter program (Parkhurst, 2001) was 45%. This pro-

gram was also used for speciation and equilibriacalculation.

5.2. Analytical and sequential extraction methods forstream sediments

Samples were dried at room temperature, dis-

aggregated with acetone-washed pestle and mortar, thensieved using a nylon 80 mesh (<170 mm). Only the<170 mm fraction was used for the bulk analysis and

sequential extraction procedure. A portion of eachsample was further ground in an agate mortar and usedfor mineralogical analysis by X-ray powder diffraction

(XRD, Philips PW1710). Bulk analysis (Li, Na, K, Be,Mg, Ca, Sr, Ba, Al, La, Ti, Cr, Mo, Ni, Cu, Ag, Zn, Cd,Pb, and P) was performed by HNO3/HClO4 attack (4:1ml) on a 0.25 g sample. The solutions were heated in an

Al block and analysed by ICP-AES.For the determination of As and Sb, the sieved sam-

ples (0.25 g) were attacked using 1 M Mg(NO3)2 solu-

tion, HCl and KI with a volumetric ratio of 1:5:5,respectively. The solutions were heated in a furnace for6 h at 450 �C and analysed by ICP-AES.

A further characterisation of the samples was under-taken on 1 g of material using the multi-step sequentialextraction scheme by Li et al. (1995). With this method

it is possible to distinguish 5 operationally determinedchemical fractions:

� Exchangeable fraction: extraction with 8 ml of

0.5 M MgCl2 at pH 7, at room temperature;� Fraction bound to carbonates and specifically

adsorbed: leaching with 8 ml of 1 M sodium


acetate (NaOAc) at pH 5, at room temperature;� Fraction bound to Fe–Mn oxides: extraction

with 20 ml of 0.04 M hydroxylamine hydro-chloride (NH2OHHCl) in 25% of acetic acid

(HOAc) at 96 �C;� Fraction bound to organic matter and sulphides:

addition of 3 ml of 0.02 M HNO3 and 5 ml of

30% H2O2 at pH 2, at 85 �C; then addition of asecond 3 ml aliquot of 30% H2O2 at pH 2, andfinally 5 ml of 3.2 M ammonium acetate

(NH4Oac) in 20% HNO3;� Residual fraction: digestion with 4 ml of 70%

HNO3, 3 ml of 60% HClO4 and 15 ml of 40%

HF. Samples are taken to dryness by heating at90 �C (6 h), 120 �C (10 h) and 190 �C (6 h).

After the attack and extraction procedures the result-ing fractions were analysed by ICP-AES. In bulk ana-lyses and sequential extractions, trace element accuracy

evaluated by international standard material (NBS-2709and NBS-2710) was �20%; precision evaluated by dupli-cate analysis was usually <10%. In sequential extrac-

tions, the accuracy of As determined at the 100 mg/kglevel was �8%, but at the 10 mg/kg level it was �20%.

6. Results and discussion

6.1. Chemistry of the water

Table 1 reports the physico-chemical parameters andthe most significant major and minor inorganic compo-

nents in the waters of the Rio Loa basin. Overall, sur-face waters show near-neutral to alkaline pH, oxidisingredox conditions, a wide range of total dissolved solids

(TDS: 0.3–10 g/l), and a dominant Na–Cl composition.The water samples collected during different campaignsshow small variations in their chemical composition.The water in the Upper Loa has a relatively low TDS

(1–2 g/l) with a prevalent Na–Cl character and a rela-tively low content (180–250 mg/l) of dissolved SO4. TheAs (120–300 mg/l) and B (6–9 mg/l) concentrations are

the lowest ones in the Rio Loa course, but they greatlyexceed the As limit (50 mg/l) required by the Chileanlaw, and the guidelines (10 mg/l As, and 500 mg/l B)

proposed by the World Health Organisation for drink-ing water (WHO, 1993).Thermal waters in the El Tatio geothermal field

(samples No. 10 and 11) have emergent temperaturesclose to 90 �C, variable TDS (2.6–9.6 g/l) but prevalentNa–Cl character, very low SO4 (37–57 mg/l), high SiO2

(up to 210 mg/l), and extremely high As (27 mg/l), B

(130 mg/l) and Li (27 mg/l). High concentrations of Cs(11 mg/l), Rb (2.4 mg/l) and Sb (1.4 mg/l) are alsoobserved. Ellis and Mahon (1977) reported similar

characteristics and concentration ranges in thermalsprings at El Tatio (Table 2). The thermal waters flowinto the Rio El Tatio and after mixing with surfacewater from the Andes (e.g. samples No. 20, 21) are the

source of the Salado (see inset map in Fig. 1). The geo-thermal input to the Salado is attenuated only at theCodelco abstraction plant (about 30 km from El Tatio,

sample No. 31) and the chemical composition of theSalado remains similar to that of the thermal water aftermoderate dilution as far as the confluence with the

Upper Loa (about 70 km downstream of El Tatio,sample No. 34).In the Medium Loa basin, the dilution effect observed

downstream of the Upper Loa -Salado confluence per-sists over a short distance only. In fact, TDS graduallystarts to rise downstream of Calama, and the As con-centration reaches 2.9 mg/l, B 42 mg/l, and Li 10 mg/l in

the Lower Loa.The general features of the water chemistry in the Loa

basin are summarised in Figs. 3–6. Both upper arms are

sketched in the figures since the confluence site isapproximately equally distant from both sources. Fig. 3shows the variation of Cl in the Salado-Loa basin: the

thermal water from El Tatio is an important source ofCl, but additional sources occur in the Medium andLower Loa. Fig. 4 shows the SO4/Cl ratio and Sr con-

centration. Sulphate and Sr concentrations, low at ElTatio, significantly increase in the downstream part ofthe Medium Loa and in the Lower Loa. Fig. 5 showsthe water to be oversaturated with respect to calcite and

strontianite along the entire course but at El Tatio, andundersaturated with respect to gypsum and celestite.The dissolution of SO4 solid phases explains the uncon-

trolled increase of dissolved SO4, and the moderateincrease of Ca, as well as Mg and Sr (see Table 1); thesein turn are further controlled by the equilibrium with

solid carbonate phases. Fig. 6 represents the behaviourof SiO2, B, and Li, whose concentrations in the Loawaters seem to derive from the El Tatio geothermalfield. The tributaries flowing into the Salado and the

Upper Loa water cause a marked dilution of the geo-thermal components. However, while SiO2 continues todecrease until the mouth, B and Li reach a minimum

value downstream of the Upper Loa-Salado confluenceand then increase again.When looking at the modifications in the water

chemistry in the entire Rio Loa it is apparent that theyreflect the interaction with different rocks in the basin.The interaction of water with young volcanic rocks,

distinguished by the high SiO2, Li, and B contents, isvery strong in the water circulating at depth in the ElTatio geothermal field, and is important in the Mediumand Lower Loa basin too. On the other hand, the increa-

ses of SO4/Cl and Sr, as well as B and Li, in the Medium-Lower Loa are related to the predominance of sedimen-tary formations (see Fig. 2) hosting evaporitic minerals.


Table 1

Physico-chemical parameters and chemical components of waters from the Rio Loa Basin

No. Name Altitude

(m a.s.l.)

T

(�C)

pH Eh

(V)

Cond.

(mS/cm)

TDS

(g/l)

Ca

(mg/l)

Mg

(mg/l)

Na

(mg/l)

Cl

(mg/l)

SO4

(mg/l)

A

(

SiO2

(mg/l)

B

(mg/l)Li

(mg/l)Sr

(mg/l)As

(mg/l)

Rio Loa Upper course

1b Loa Lequena 3320 16 7.0 nd 1.5 1.05 150 35 140 260 180 64 5900 550 750 120

2b Loa before Conchi 3040 17 6.9 nd 2.9 1.95 200 78 340 670 200 83 8000 910 2000 190

3b Loa after Conchi 2950 15 8.3 nd 2.7 1.78 170 72 310 640 200 72 7400 850 1900 210

4a Loa before Salado 2640 15 8.2 0.44 3.5 2.02 150 105 430 830 250 66 8900 1100 1900 300

4b Loa before Salado 2640 15 8.3 0.45 3.3 1.96 150 89 380 760 240 64 8300 1100 1900 300

El Tatio geothermal field

10c El Tatio geyser 4300 87 *7.2 0.25 4.8 2.63 52 2.7 800 1400 37 213 33 000 6500 650 7600

11c El Tatio spring 4290 84 *6.2 0.11 17.3 9.59 250 2.2 3120 5600 57 192 130 000 27 000 3600 27 000

12a Rio El Tatio 4250 22 7.7 0.36 12.4 6.62 170 6.8 2200 3900 58 170 92 000 21 000 2400 21 000

Rio Salado - Rio Loa Medium-Lower course

31c Salado Chico 3130 14 8.1 0.40 6.9 3.99 120 13 1200 2200 79 130 50 000 10 000 1500 10 000

32a Salado Curti 3120 17 7.7 0.40 7.0 3.96 190 42 1200 2100 90 100 35 000 7800 3000 6800

33a Salado Aiquina 2870 16 7.4 0.42 6.5 3.67 270 65 980 1900 92 86 16 000 4200 5200 1200

34a Salado before Loa 2640 13 7.8 0.44 7.1 3.98 200 88 1100 2200 110 67 15 000 4600 5500 1200

34b Salado before Loa 2640 14 7.7 0.47 6.4 3.53 160 80 1000 1900 120 69 13 000 4200 5400 1100

34c Salado before Loa 2640 18 7.9 0.44 6.3 3.55 170 77 960 1900 110 84 16 000 4200 5600 1200

35b Loa Angostura 2490 15 7.4 0.41 6.1 3.47 210 87 900 1700 190 68 12 000 3500 5000 1100

36a Loa Yalquinca 2300 11 7.8 0.47 6.8 3.93 230 110 1000 2100 200 65 14 000 3600 5000 1100

36b Loa Yalquinca 2300 12 7.8 0.47 6.6 3.80 230 91 950 1900 210 68 13 000 3800 5200 1100

37a Loa Finca 2050 15 8.2 0.46 7.6 4.42 270 130 1200 2300 290 71 17 000 4300 5300 1600

38a Loa Coya Sur 1200 14 8.1 0.40 11.9 7.03 350 190 1900 3600 650 57 26 000 6700 8400 1900

38b Loa Coya Sur 1200 13 7.8 0.45 10.1 6.46 300 170 1700 3400 540 45 24 000 6800 8000 1700

39a Loa Crucuero 1150 13 8.0 0.43 11.6 6.59 360 190 1800 3500 620 62 27 000 6800 8300 2200

40b Loa Santa Fe 1100 15 8.0 0.40 12.9 7.52 370 200 2000 3800 720 50 32 000 8500 9300 1800

41a Loa Sloman 980 17 8.1 0.39 15.9 8.89 450 270 2400 4700 910 52 38 000 9000 11 000 2900

41b Loa Sloman 980 15 8.1 0.43 12.9 7.61 370 210 2000 3900 720 50 33 000 8800 9300 1900

42a Loa Quillagua 790 19 8.1 0.40 15.2 9.44 450 290 2600 5000 990 49 42 000 10 000 11 000 2900

42b Loa Quillagua 790 17 8.1 0.43 13.5 8.22 370 220 2100 4200 780 46 34 000 9200 9500 1800

43b Loa mouth 1 17 8.0 0.34 16.9 10.6 520 240 2600 5200 1400 43 37 000 10 000 12 000 1700

(continued on next page)

L.Romero

etal./Applied

Geochemistry

18(2003)1399–1416

1405

lk

mg/l)

410

690

570

480

460

72

45

120

170

480

570

370

350

340

480

470

460

480

370

360

420

490

460

420

420

410

240

1406
Table 1 (continued)
No. Name Altitude

(m a.s.l.)

T

(�C)

pH Eh

(V)

Cond.

(mS/cm)

TDS

(g/l)

Ca

(mg/l)

Mg

(mg/l)

Na

(mg/l)

Cl

(mg/l)

SO4

(mg/l)

A

(

SiO2

(mg/l)

B

(mg/l)Li

(mg/l)Sr

(mg/l)As

(mg/l)

Tributaries to Rio Salado

20c Andean stream 4390 6 7.5 0.49 0.4 0.26 41 16 13 12 110 70 140 9 190 13

21c Andean river 4280 4 7.7 0.45 1.2 0.78 43 42 130 150 270 62 4300 560 260 630

22a Caspana stream 3340 12 7.8 0.46 0.6 0.33 32 9.8 55 81 47 49 230 34 220 19

23c Rio Toconce 3140 18 8.1 0.48 2.2 1.24 88 25 300 470 78 76 5800 1400 1300 850

24c Ayquina stream 2990 14 8.0 0.45 2.2 1.24 130 33 230 410 130 90 2200 850 2000 240

Groundwater

25c Spring on Salado 3100 24 6.2 0.40 9.2 5.58 380 110 1500 2500 150 1 100 13 000 5400 9500 300

26c Bano Turi 3110 22 6.0 0.35 3.0 1.68 110 40 400 730 86 85 3800 1500 2900 540

27b Angostura spring 2490 19 6.9 0.42 6.5 3.80 240 85 950 1900 190 83 13 000 4100 5100 1700

28b Ojo Opache spring 1960 18 7.9 0.37 6.6 4.03 240 91 930 1800 660 65 17 000 4000 4700 1000

Chuquicamata

50b Talabre impound 2480 18 9.7 0.35 10.2 7.48 900 4 1400 2000 2800 34 25 000 6100 5600 13 000

51b Angostura drillhole 2500 20 7.3 0.22 11.4 7.33 670 180 1500 3000 1600 42 22 000 5000 12 000 38

Rio San Salvador

52b Quetena up 2250 18 8.2 0.39 7.0 4.09 230 120 1000 1700 730 58 19 000 4100 4900 1800

53b Quetena 2220 17 7.9 0.43 12.4 7.84 490 210 1900 2900 1900 48 35 000 5600 9000 1500

54b Ojo Opache 2030 20 8.5 0.44 8.0 4.67 230 120 1200 2400 390 82 20 000 5100 5900 1600

55a Coya Sur 1160 13 8.2 0.44 9.8 5.65 340 170 1500 2800 610 72 24 000 5800 7200 2400

55b Coya Sur 1160 11 7.9 0.47 10.9 5.95 230 160 1500 3000 610 80 25 000 6500 7600 2600

TDS: total dissolved solids; Alk: alkalinity as HCO3�; nd: not determined; a, b, c: sampling in 1999, 2000, and 2001, respectively

* pH in thermal waters measured at 35 �C.

L.Romero

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Geochemistry

18(2003)1399–1416

lk

mg/l)

96

120

110

340

400

600

400

560

250

230

310

270

470

250

480

490

.

The weathering of these rocks adds a significant con-tribution to the dissolved solids in the Rio Loa water,clearly distinct from the weathering of volcanic rocks.Fig. 7 shows As concentrations in the two upper arms

(Upper Loa and Salado), in tributaries and groundwaterof the Salado, in the Medium-Lower Loa and Rio SanSalvador, in two springs in the Medium Loa basin, and

in the Talabre impoundment which collects the wastewater from the Chuquicamata mine and plant. The RioEl Tatio thermal water shows the highest As concen-

tration (21 mg/l), but this is rapidly diluted downstreamin the Salado by the inflow of groundwater and super-ficial water from tributaries (concentration range:

13–900 mg/l. Queirolo et al., 2000, reported a similarrange for stream waters located southward of El Tatio).However, at the confluence with the Upper Loa, thedissolved As in the Salado (1200 mg/l, sample No. 34) is

much higher than that of the Upper Loa (300 mg/l,sample No. 4). The As content in the Medium Loadownstream of the Upper Loa-Salado confluence is

1100 mg/l, but a significant increase is observed down-stream of Calama and in the Lower Loa (up to 2900 mg/l at the Sloman reservoir). Values similar to those in the

Medium Loa are observed in the Rio San Salvador (animportant tributary of the Medium Loa) and in thegroundwater of the area (see Table 1).

Fig. 3. Plot of Cl� concentration in waters from the Salado and Loa versus distance from El Tatio geothermal field.

Table 2

Chemical composition of spring waters at El Tatio (statistical parameters calculated from data reported in Ellis and Mahon, 1977)

T

(�C)

pH
Ca
(mg/l)

Mg

(mg/l)

Na

(mg/l)

Cl

(mg/l)

SO4

(mg/l)

HCO3

(mg/l)

SiO2

(mg/l)

B

(mg/l)

Li

(mg/l)

Cs

(mg/l)

Sr

(mg/l)

As

(mg/l)

Mean
85 7.2 250 2.2 3600 6500 36 46 180 150 38 11 1.2
SD
1 0.4 35 2.2 710 1280 5.7 27 53 30 10 0.8 0.1
n
12 12 12 12 12 12 12 12 12 12 11 6 6
Max
86 7.8 310 7.2 4600 8200 50 110 270 190 47 13 1.3 50
Min
82 6.2 170 0.3 2300 4000 27 12 120 91 28 10 1.0 –
SD: standard deviation (1s); n: number of samples; –: not reported.


Fig. 4. Plot of SO4/Cl ratio and Sr concentration in waters from the Salado and Loa versus distance from El Tatio.

Fig. 5. Saturation index with respect to calcite, strontianite, gypsum and celestite in waters from the Salado and Loa versus distance

from El Tatio.


Fig. 6. Concentration of SiO2, B and Li in waters from the Salado and Loa versus distance from El Tatio.

Fig. 7. Concentration of As in waters from the Rio Loa Basin versus distance from El Tatio.


It is remarkable to note that concentrations of 1–2mg/l As in the Medium-Lower Loa persist over 280 kmaway from the Upper Loa-Salado confluence. Severalfactors may concur to maintain these high-As con-

centrations in water: the extreme arid conditions favourevaporation processes, especially at the Santa Fe andSloman reservoirs; the lack of low-As tributaries in the

Medium-Lower Loa does not allow dilution; the neutralto alkaline pH, high salinity and high-As concentrations

do not favour the adsorption of As(V)-species (Smedleyand Kinniburgh, 2002, and references cited). Indeed,HAsO4

2� and H2AsO4� appear to be the dominant

species by PHREEQC calculations taking into account

the values of dissolved O2 (2–8 mg/l) measured in theMedium-Lower Loa waters.Though the geothermal activity, the lithologies in the

area, and the climatic factors explain the As enrichment inwater as a natural phenomenon, possible anthropogenic

Table 4

Mean, standard deviation (SD: 1s), maximum and minimum values (mg/kg) of selected chemical components in stream sediments of

the Rio Loa Basin

n
Al Ca Na K Li Sr Ba Ti V Mn Fe Co Ni Cu Zn Mo S As
Rio Loa

Upper Course
10 Mean 27 000 48 000 2200 4800 32 330 170 1700 90 13 33 000 9 13 270 90 10 2600 120
SD
11 431 34 793 830 1810 13 186 62 568 35 4 13 648 3 4 108 22 3 1936 71
Max
69 000 140 000 3300 25 000 44 690 290 2600 190 920 95 000 16 21 1600 120 60 24 000 350
Min
13 000 4300 620 2400 10 50 60 560 27 250 11 000 4 4 32 72 2 49 7
Medium Course
7 Mean 22 000 120 000 3900 4800 50 1100 140 1200 70 11 24 000 7 11 130 90 6 5300 380
SD
14 548 65 510 1985 3007 19 721 20 379 19 6 8361 4 6 138 34 5 2175 185
Max
52 000 220 000 7600 11 000 33 2200 170 1600 81 5200 38 000 15 22 350 160 14 7700 650
Min
6600 39 000 4000 1500 78 400 110 730 35 470 13 000 3 4 31 67 2 2300 150
Lower Course
13 Mean 23 000 125 000 5600 5900 70 1200 140 900 60 14 24 000 9 14 70 90 7 8400 700
SD
18 333 84 877 2327 4646 32 863 38 559 32 10 11 375 5 10 105 44 4 6407 523
Max
67 000 340 000 9800 18 000 120 2700 200 1700 98 1500 44 000 17 42 410 160 14 20 000 2000
Min
90 500 000 3100 350 26 400 90 15 3 410 590 1 1 5 9 3 1800 180
El Tatio
1 4400 13 000 6500 2800 120 60 120 200 18 340 6100 2 2 64 20 2 1500 11 000
Salado
14 Mean 21 700 19 400 2800 5100 36 190 170 1700 100 950 39 500 12 14 150 110 6 1400 250
SD
6810 12 622 1529 1955 28 138 38 301 41 948 15 170 4 4 199 49 4 1642 288
Max
40 000 56 000 6300 9600 87 590 230 2100 180 3700 72 000 21 20 760 250 17 4900 930
Min
14 000 10 000 1500 2900 12 70 110 1500 52 270 24 000 7 9 27 54 3 150 32
Salado tributaries
13 Mean 21 000 26 000 3900 4600 50 400 160 1600 79 440 29 000 8 12 93 100 5 1400 120
SD
6543 38 027 2265 1753 45 763 64 502 45 173 11 810 3 4 88 43 2 1458 128
Max
33 000 150 000 10 000 7100 140 2900 250 2600 210 720 59 000 14 21 360 210 11 4500 480
Min
11 000 6900 1500 2000 9 76 60 520 24 190 8900 3 5 30 48 3 260 30
San Salvador
2 Mean 18 000 86 500 2300 3600 38 580 140 1500 80 380 27 000 7 11 80 90 5 1300 250
SD
5657 61 518 141 919 14 262 6 495 44 120 15 556 4 5 71 45 3 42 134
Max
22 000 130 000 2400 4200 48 760 140 1800 110 460 38 000 10 14 130 120 7 1300 340
Min
14 000 43 000 2200 2900 29 390 130 1100 48 290 16 000 4 7.2 30 57 3 1300 150
n: number of samples.

Table 3

Selected major and trace components in waters of the Chuquicamata mining area

No.
Name pH Eh
(V)

TDS

(g/l)

SO4

(mg/l)

As

(mg/l)
Sb
(mg/l)
Fe
(mg/l)
Mn
(mg/l)
Cu
(mg/l)
Zn
(mg/l)
Mo
(mg/l)
Pb
(mg/l)
Hg
(mg/l)

50
Talabre impoundment 9.7 0.35 7.48 2800 13 000 170 650 30 90 50 2700 2.1 5.1
51
Angostura drillhole 7.3 0.22 7.33 1600 38 6 5000 2400 160 8700 570 110 1.3
27
Angostura spring 6.9 0.42 3.80 190 1700 16 50 10 2 88 7.3 0.5 0.4
35
Loa Angostura 7.4 0.41 3.47 190 1100 9 190 50 6 120 6.7 1.4 <0.3
28
Ojo Opache spring 7.9 0.37 4.03 660 1000 9 50 10 4 17 22 0.6 <0.3

sources must be considered too. They include emissionsfrom the Cu smelting plant (Codelco Chile DivisionChuquicamata), the mine wastes deposited in the pastalong and near the bed of the river, and inadequate

treatment of the effluents from mining activities, as wellas the As-rich effluents from the water treatment plantin Cerro Topater near Calama.

Since one of the aims of this study was to evaluate theimpact of the present-day mining activity, as manywater samples as possible were collected in the area of

Chuquicamata. Table 3 reports the 4 water samplesclosest to Chuquicamata (see Fig. 1), plus the OjoOpache spring in the Rio San Salvador basin reported

for comparison. This table only lists the chemical com-ponents which could be derived from the weathering ofore deposit minerals (major components are reported inTable 1). The Talabre impoundment (sample No. 50),

which collects the waste water from Chuquicamata,exhibits high pH (9.7), due to the use of alkalisingmaterial in the mineral processing, and high concentra-

tions of SO4, As, Mo, Sb and Hg. Data from the drill-hole (sample No. 51, water table 45 m below thesurface) show concentrations of Fe, Mn, Zn, Pb and Cu

much higher than in Talabre. The lower Eh and/or near-neutral pH in the drillhole as compared with Eh - pH

conditions in Talabre, may increase the mobilisation ofFe, Mn, Zn, Pb and Cu (Stumm and Morgan, 1996).The very low content of As in the drillhole may berelated to sorption processes involving Mn- and Fe-

bearing solid phases (e.g. Wilkie and Hering, 1996).Both sets of data from Talabre and the drillhole areconsistent with a circulation in mine wastes or miner-

alised rocks; specifically, the relatively high contents ofSO4, Mo, Cu, Zn, Pb and Hg might testify an impact ofthe mining plants around Chuquicamata. On the other

hand, the values of these components in the Angosturaspring and in the Rio Loa at Angostura, are much lowerthan those from Talabre, and are fairly similar to the

concentrations observed in the Ojo Opache spring(about 50 km from Chuquicamata). Therefore, anydirect impact of the mining activity on the surroundingenvironment should be of limited extent. It may be

argued that the lack of rain in the area does not allowmobilisation and transport of contaminants from thesolid phases into the Loa river.

6.2. Chemistry of stream sediments

The XRD analysis on 12 stream sediment samplesfrom the Loa basin reveals the most abundant minerals

Fig. 8. Content of As in stream sediments from the Rio Loa Basin versus distance from El Tatio.


Fig. 9. Factor loading of elements in stream sediment samples from the Rio Loa Basin.


Fig. 10. Distribution of Al, Fe, Cu, and As derived from sequential extraction

L.Romero

etal./Applied

Geochemistry

18(2003)1399–1416

1413

s.

to be plagioclase, muscovite and quartz, with minoramounts of brushite (Ca–phosphate), and gypsum. Atotal of 60 samples were analysed for 18 elements; theresults are summarised in Table 4 as average values and

relative statistical parameters in the different areas ofthe basin.Average values of Fe, Al, Ca and Cu range from

24,000–40,000, 17,000–25,000, 9000–95,000, and 50–220mg/kg, respectively. The average Fe and Al amountsappear negatively correlated to Ca amounts. Assuming

that Fe and Al are mainly bound to alumo–silicate spe-cies, and Ca to sulphate–carbonate species, it is sug-gested that sediments from volcanic origin clearly

prevail in the Upper Loa and Salado, while Ca-richsedimentary formations contribute largely to the sedi-ment chemistry in the Medium and Lower Loa and SanSalvador. This is in agreement with the lithologic fea-

tures of the region where the extent of sedimentary for-mations increases downstream (see Fig. 2). On a similarbasis it could be argued that the high Cu amounts in

sediments derive from the porphyry Cu deposits in theregion. This is generally true when it is considered that,excluding the outliers, the mean Cu content in the

stream sediments of the Rio Loa basin is 130 mg/kg,compared with world-wide average values of 80, 60, and25 mg/kg Cu in basalt, andesite, and soil, respectively

(Wedepohl, 1974). However, a precise association ofhigh Cu contents with the localisation of the maindeposits (e.g. Chuquicamata in the Medium Loa basin)is not observed. Indeed, the wide spread of mineralised

bodies in the area and the dissemination of mine wastesmay be responsible for the relatively constant high valueof Cu observed everywhere.

The average content of As in stream sediments fromdifferent regions of the Loa Basin ranges from 120 to700 mg/kg; the extremely high value (11,000 mg/kg)

observed at El Tatio reflects a direct As supply from thegeothermal area. Fig. 8 shows the As content in streamsediments versus the relative distance of samples fromEl Tatio. In the upper part of the basin, the sediments

along the Salado are richer in As than those of theUpper Loa. Downstream of the Upper Loa-Saladoconfluence, the As content in sediments increases show-

ing two sharp steps downstream of Calama in the Med-ium Loa (510 mg/kg), and downstream of the Slomanreservoir in the Lower Loa (up to 2000 mg/kg). Then,

As concentrations in sediments do not change sig-nificantly until the mouth. This behaviour indicates thepresence of an initial As source (El Tatio geothermal

field), as well as additional sources in the entire LoaBasin with two specific inputs.In order to identify the element associations, and

possibly distinguish the potential As sources, factor

analysis using the software SPSS 6.0 (Norusis, 1996)was performed. Five main factors which explain the86% of the variance expressed by the data matrix have

been defined. Fig. 9 shows the graphical representationof these factors. Factor 1 which explains 43% of thevariance, includes the following components: Ni, Fe, Ti,Co, Al, V, Zn, and K, and probably represents the

mineralogical alumo–silicate component in the sedi-ment. Factor 2 (23% of the variance) groups Li, Na, Asand S and may reflect the impact of geothermal activity

on the sediments in the area. Factor 3 (10% of the var-iance) associates Cu, Mo, Zn and S, is strictly related tothe porphyry Cu mineralisation, with K and Al prob-

ably derived from its alteration. Factor 4 (5% of thevariance) includes Ca, Sr and S and seems to reflect thechemical component in the sediments linked to evaporitic

minerals. Factor 5 (5% of the variance) is essentially dueto Mn and Ba and may reflect the presence of Mn-oxideswhere Ba can be an important constituent (Wedepohl,1978; Baturin and Chelishchev, 1998).

Results of the sequential extractions performed on 38samples are shown as mean values of Al, Fe, Cu and Asin Fig. 10. More than 90% of Al and approximately 80–

90% of Fe are present in the residual phase, thus con-firming that both elements mainly belong to alumo–sili-cate clastic phases. Part of the Fe (approximately 10%)

is bound to sulphide phases and to crystalline oxides.The plot for Cu shows its predominant distribution insulphides and residual phases (from 56 to 84%); a sig-

nificant amount is associated with crystalline oxidephases.Though mainly extracted from residual phases (aver-

age 39%), sulphides (average 18%), and oxides (average

23%), As is extracted from the carbonate (average 18%)and exchangeable (average 2%) fractions too, indicatingthat part of the As is readily available. This result is in

agreement with the correlation observed between Ascontent in sediments and As concentration in waters inthe Rio Loa study area (R2=0.70; n=18, both sediment

and water samples being collected at the same site). Thesignificant percentages of As extracted in the first threesteps of the sequential extraction procedure from theSalado samples, directly influenced by the El Tatio geo-

thermal field, may be related to the presence of crystal-line and amorphous secondary Fe–Mn oxides, observedin the mineralogical study, with a strong capacity to co-

precipitate or sorb elements (Bowell, 1994; Kavanagh etal. 1997).

7. Conclusions

Water and sediments in the basin of the 440 km long RioLoa are enriched in many elements besides As. Arsenic hasbeen discussed in more detail because it represents the mainchemical source of risk in the whole basin.

The high concentration of As (mean value 1400 mg/l)in the Rio Loa water is generally associated with highconcentrations of B and Li. This apparently indicates a


common source of these elements. This common sourcehas been identified in the El Tatio geothermal fieldlocated in the Andes Cordillera, with a relevant impactin the Salado tributary. The extremely high As (up to

27,000 mg/l) in the El Tatio water decreases down to1200 mg/l As about 70 km downstream (at the UpperLoa-Salado confluence). However, the As concentra-

tions significantly increase in the Medium and LowerLoa water and persist to the mouth in the Pacific Ocean.Hence, a direct supply of contaminants from El Tatio

cannot be the only factor governing the river chemistry.The very dry climate and strong evaporation in the area,and the lack of any dilution from unaffected streams,

may only partially explain this fact. Indeed, groundwaters interacting with volcanic rocks, with compositionanalogous to those in the geothermal field, also showrelatively high As, Li and B, and their inflow into the

Medium and Lower Loa may contribute to keep highconcentrations of these components in the riverwater.

High As contents are observed in stream sedimentsamples collected at several sites in the whole Loa basin.The average As in these sediments is 320 mg/kg, i.e. at

least 10 times higher than world-wide average values (5–10 mg/kg As) reported for similar sediments and rocks(Smedley and Kinniburgh, 2002, and references cited).

Results from sequential extractions show that the As inthe Loa sediments is mainly associated with Fe–Mnoxides and residual phases. However, a significantamount of As is also extracted in the first steps of the

procedure indicating that part of the As is readilyavailable.Emissions from the Cu smelting plant at Chuquica-

mata, the mine wastes deposited in the past in the bedand on the banks of the river, inadequate treatment ofthe effluents from mining activities, as well as the As-

rich effluents from the water treatment plant at Calama,represent other potential sources of As. However, thepoor quality (high TDS, As and B) of the water cannotbe directly ascribed to the intense mining activity. The

very dry climate may explain the low mobilisation ofcontaminants from these sites even when contaminatedmaterials are piled up in the environment without spe-

cific devices. More detailed studies in the area aroundChuquicamata and in the Lower Loa would be neces-sary for a better evaluation of effects derived from these

anthropogenic sources.Presently, the remobilization of contaminants at very

high levels in connection with heavy-rain events on the

Andes Cordillera, as was observed in 1997, 2000 and2001, is the major environmental hazard. During theseevents, the reworking of contaminated sediments andmining wastes abandoned in huge amounts along and

nearby the river course represents a very dangerousadditional source of contamination in the Loa Basin.Then, the continuous use of untreated or not adequately

treated water for drinking and irrigation, as observed inrural areas and small villages during the field campaigns,is a serious risk for the health of the population in theregion.

Acknowledgements

The authors thank the European Community forsupporting the research (WAQUAMINAR Project, EC

Contract no. ERBIC 18CT980284). Two anonymousreferees are gratefully acknowledged for their usefulsuggestions. Thanks to Laura Rundeddu for help with

the illustrations.

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Arsenic enrichment in waters and sediments of the Rio Loa (Second Region, Chile

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