heavy metal contamination of water, soil and produce

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0048-9697/03/$ - see front matter� 2003 Published by Elsevier B.V.doi:10.1016/S0048-9697(03)00485-6

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3 Heavy metal contamination of water, soil and produce within4 riverine communities of the Rıo Pilcomayo basin, Bolivia´

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6 J.R. Miller * , K.A. Hudson-Edwards , P.J. Lechler , D. Preston , M.G. Macklina, b c d e

78 Department of Geoscience and Natural Resources Management, Western Carolina University, Cullowhee, NC 28723, USAa

9 Research School of Earth Sciences at UCL-Birkbeck, University of London, Malet St., London WC1E 7HX, UKb

10 Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USAc

11 School of Geography, University of Leeds, Leeds LS2 9JT, UKd

12 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion, Wales SY23 3DB, UKe

1314 Received 16 February 2003; accepted 13 August 2003

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25 Abstract26

27 The Rıo Pilcomayo heads on the Cerro Rico de Potosı precious metal–polymetallic tin deposits of Southern´ ´28 Bolivia. Mining of the Potosı deposits began in 1545 and has led to the severe contamination of the Pilcomayo’s´29 water and sediments for at least 200 km downstream of the mines. This investigation addresses the potential human30 health affects of metal and As contamination on four communities located along the upper Rıo Pilcomayo by´31 examining the potential significance of human exposure pathways associated with soils, crops and water(including32 river, irrigation and drinking water supplies). The most significantly contaminated agricultural soils occur upstream33 at Mondragon where Cd, Pb and Zn concentrations exceed recommended guideline values for agricultural use. Further´34 downstream the degree of contamination decreases, and metal concentrations are below Dutch, German and Canadian35 guideline values. Metal and As concentrations in agricultural products from the four communities were generally36 below existing guidelines for heavy metal content in commercially-sold vegetables. Thus, the consumption of37 contaminated produce does not appear to represent a significant exposure pathway. A possible exception is Pb in38 carrots, lettuce and beetroots from Sotomayor and Tuero Chico; 37% and 55% of the samples, respectively, exceeded39 recommended guidelines. Most communities obtain drinking water from sources other than the Rıo Pilcomayo. In´40 general, dissolved concentrations of metals and As in drinking water from the four studied communities are below41 the WHO guideline values with the exception of Sb, which was high at Tasapampa. The inadvertent ingestion of42 contaminated water from irrigation canals and the Rıo Pilcomayo represents a potential exposure pathway, but its´43 significance is thought to be minimal. Given the degree of soil contamination in the area, perhaps the most significant44 exposure pathway is the ingestion of contaminated soil particles, particularly particles attached to, and consumed with45 vegetables. The risks associated with this pathway can be reduced by thoroughly washing or peeling the vegetables46 prior to consumption. Other exposure pathways that are currently under investigation include the consumption of47 contaminated meat from livestock and poultry, which drink polluted waters and the ingestion of contaminated wind-48 blown dust.49 � 2003 Published by Elsevier B.V.5051 Keywords: Bolivia; Heavy metal; River; Human health; Mining; Vegetables; Water; Agricultural soil531214

1215 *Corresponding author. Tel:q1-828-227-2269; fax:q1-828-227-7647.1216 E-mail address: [email protected](J.R. Miller).

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4 Fig. 1. Map of the Rıo Pilcomayo basin of Southern Bolivia showing the location of the four sampled communities(modified from´5 Miller et al., 2002).

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1. Introduction

56 A considerable number of the world’s rivers are57 contaminated by the release of heavy metals from58 present-day and historic mining operations(e.g. 59

Lewin et al., 1977; Leenaers, 1989; Marron, 1992;60 Macklin et al., 1994; Swennen et al., 1994; Miller,61 1997; Hudson-Edwards et al., 2003). In spite of62 this fact, very little is known about the effects of63 mining contamination on human populations that64 live beside, and rely on these rivers for food and65 livelihood, particularly within developing countries66 (seeAlbering et al., 1999; Harada et al., 2001for

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exceptions). Excess consumption of non-essential68metals such as Cd and Pb can result in neurologi-69cal, bone and cardiovascular diseases, renal dys-70function, and various cancers, even at relatively71low levels (Calderon, 2000; Watt et al., 2000;72Jarup, 2002). In riverine communities, the primary73pathways of metal accumulation in humans are74through the ingestion of contaminated drinking75water, vegetables, fruits, fish and soil(both from76hand-to-mouth contact and through the ingestion77of soil-particles clinging to unwashed food). Thus,78an initial step in any evaluation of the risks posed79by metal mining-related pollution to riverine com-

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munities is the analysis of water, agricultural soil81 and foodstuffs grown on contaminated materials.82 The Rıo Pilcomayo is contaminated with metals´83 derived from both historic and modern mining84 operations within the Potosı mining district of´85 Southern Bolivia(Macklin et al., 1996; Garcia-86 Guinea and Huascar, 1997; Hudson-Edwards et87 al., 2001; Miller et al., 2002; Smolders et al., in88 press). This paper presents the preliminary results89 of the first stages of a research program aimed at90 documenting(1) the concentrations of heavy met-91 als, arsenic and other select elements in drinking92 and irrigation waters, agricultural soils, and pro-93 duce collected from four riverine communities94 within the upper Rıo Pilcomayo basin; and(2) the´95 potential risks that these contaminants pose to96 human health. The latter objective is assessed by97 examining the potential significance of human98 exposure pathways associated with soils, crops and99 water (including river, irrigation and drinking100 water supplies). The upper reaches of the Rıo´101 Pilcomayo(examined in this study) are essentially102 devoid of fish. Thus, fish are not a significant103 component of the local diet and concentrations of104 metal contaminants within them will not be dis-105 cussed here.

106 2. Geologic and geographic setting

107 The Rıo Pilcomayo(208S 658W) rises at an´108 elevation of approximately 5200 m in the central109 Andes and flows in a Southeasterly direction for110 approximately 670 km until reaching Argentina’s111 Northern boundary with Paraguay(Fig. 1). The112 Pilcomayo basin encompasses an area of 272 000113 km , of which 98 100 km lies in Bolivia. Near2 2

114 the Argentine border, the river has an estimated115 average discharge of 80 m s at low stage and3 y1

116 3600 m s in flood(Wilkinson and Mohler,3 y1

117 1995). In the lower reaches, the Pilcomayo enters118 the Chaco Plains where it has constructed a119 210 000 km , low-declivity, fan-shaped body of2

120 sediment(Iriondo, 1993; Wilkinson and Mohler,121 1995). Between its headwaters and the Chaco122 Plains, the Rıo Pilcomayo traverses six major´123 North–South trending structural units consisting124 of Ordovician, Silurian, Cretaceous, Tertiary and125 Quaternary volcanic and sedimentary rocks, which

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are locally intruded by dacites, quartz porphyries127and adamellites(Perez, 1996).´128Near its headwaters, the Rıo Pilcomayo drains´129the large Cerro Rico de Potosı precious metal–´130polymetallic tin deposits characterized by miner-131alised hydrothermal veins. The veins are developed132in Ordovician slate, dacitic tuff and tuff breccia,133and rocks of a dacitic stock(Cunningham et al.,1341991). The veins are zoned outwards and consist135of a high-temperature, cassiterite-rich core sur-136rounded by zones of Ag and base metal sulphides,137oxides such as wolframite and gangue minerals138including quartz, tourmaline, siderite and kaolinite.139This paper focuses on data collected from water,140soils and crops from four riverine communities141including Mondragon, Tasapampa, Tuero Chico´142and Sotomayor located along the upper reaches of143the Rıo Pilcomayo(Fig. 1). Mondragon is located´ ´144approximately 26 km from the city of Potosı and´145has a population of 176(INE, 2002). The small146area of flatland within the valley-bottom is partly147irrigated from springs rather than from the Rıo´148Tarapaya. Maize, potatoes and some other cereals149are grown on aereally extensive alluvial terraces150of the Rıo Pilcomayo, largely for domestic con-´151sumption; cattle and sheep are grazed on the152surrounding hillsides.153Further downstream(near Tasapampa and Soto-154mayor; Fig. 1), historic terraces are poorly pre-155served, but small remnants exist upstream of156alluvial fan deposits created at the mouth of157tributaries. In these areas, both the fans and the158alluvial terraces are utilized for agriculture. Tasa-159pampa (379 inhabitants) and Tuero Chico(180160inhabitants) are located approximately 110 km161downstream from Mondragon on the left(North)´162bank of the Rıo Pilcomayo and approximately 9´163km upstream from the main highway connecting164Potosı and Sucre. Irrigation water is used intensi-´165vely to grow a range of vegetables for sale in166Sucre and via wholesalers elsewhere in Bolivia.167Livestock are grazed on hillsides but are used for168domestic consumption more than for commercial169sale.170Sotomayor, approximately 32 km downstream171of Tasapampa is larger than the other villages172studied, with some 676 inhabitants. It has an173extensive area of irrigated valley-bottom land

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farmed intensively for vegetables that are sold in175 Sucre and throughout highland Bolivia. Carrots are176 the principal crop although grapes and tree fruit177 are produced in small quantities.

178 3. Mining-related contamination of the Pilco-179 mayo basin

180 Mining activity began at Cerro Rico in 1545181 with the exploitation of the very rich(4.25–182 25.5%) silver (Ag) ores in the upper part of the183 mountain. Following exhaustion of these highly184 enriched ores in 1572–1573, Ag was extracted185 using an Hg amalgamation method in which Hg186 was mixed with pulverized ore to form dense Hg–187 Ag amalgam grains. The amalgam grains were188 subsequently separated from the rest of the mate-189 rials and then heated to drive the Hg off as a190 vapor. During the process, Hg and other heavy191 metals associated with the waste materials were192 released to the upper tributaries of the Rıo Pilco-´193 mayo, including the Rıo Tarapaya. Amalgamation´194 mining ceased around 1900.195 In 1952, the national government nationalised196 all major mines, transferring ownership to the197 Corporacion Minera de Bolivia(COMIBOL)´198 (MMDEC, 1999) and tin replaced silver as the199 main mineral mined at Cerro Rico. Subsequently,200 Bolivian mining suffered from the adoption of201 neo-liberal policies in the 1980s and COMIBOL202 was effectively broken up and the small- and203 medium-sized mines became the dominant produc-204 ers. In addition, falling tin prices led to a shift in205 mining from tin–silver to zinc–silver ores. Ore206 concentration(beneficiation) also changed to flo-207 tation processes. Currently, the extraction of sale-208 able minerals from the ores of Cerro Rico entails209 beneficiation of Ag, mainly by cyanidation, and210 Zn and Pb by froth flotation.211 Studies byHudson-Edwards et al.(2001) and212 MMDEC (1999) demonstrate that there are several213 sources of metals to the headwaters of the Rıo´214 Pilcomayo. The most significant sources are the215 30–40 ingenios(mills) that release flotation efflu-216 ent and tailings materials directly into the Rıo de´217 La Ribera, a headwater tributary to the Rıo Pilco-´218 mayo close to Potosı. The released effluent exhibits´219 a dark gray color that has been observed more

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than 175 km downstream at Sotomayor(Fig. 1).221In addition, headwater streams receive acidic, met-222al-rich drainage from the extensive mine shafts223within Cerro Rico and by the flow of water through224hydraulic mine tailings called Sucu, which are225found at the base of the mountain.226Hudson-Edwards et al.(2001) presented data227collected from three types of sedimentary deposits228found along the Rıo Pilcomayo downstream of´229mining operations at Cerro Rico. The deposit types230include: (1) low-water channel sediments which231are perennially inundated by flows and in upstream232areas are continuously affected by mine effluent;233(2) high-water channel bed materials that lie above234the water surface during the dry season, but which235contain sediments that are actively reworked by236the river during periods of high water; and(3)237historical deposits that comprise the floodplain and238terraces. Concentrations of As, Sb, Cd, Cu, Pb,239Hg, Ag, Tl and Zn in low-water channel sediments240collected between Potosı and Uyuni(near Icla;´241R4) in both 1998 and 2000 are significantly242elevated above background values(determined243from the analysis of pre-mining alluvial deposits).244Elevated levels appear to be associated with pyrite-245and other sulphide mineral-bearing tailings mate-246rials transported downstream from the Potosı´247mines. Significant downstream declines in elemen-248tal concentrations occur within 15 km and again249between 150 and 200 km from Cerro Rico(Hud-250son-Edwards et al., 2001). Decreases in concentra-251tions are due to the dilution of tailings effluent252with sediment from tributary channels to the dep-253osition and storage of heavy mineral grains close254to the mine sites and downstream of RQQ(Fig.2551), to the storage of contaminated sediment in the256channel bed as a result of long-term aggradation257(Hudson-Edwards et al., 2001). Downstream of258Uyuni (R4; Fig. 1) metal concentrations of Cu,259Zn and Pb are only slight elevated, whereas Ag,260Cd, Sb and Tl cannot be distinguished from back-261ground(pre-mining) values.262With the exception of Hg, the concentrations of263metals within the historic(post-1545) floodplain264and terrace deposits are lower than those found in265the contemporary channel bed sediments. Data266collected at sites R2, R4a and R6(Fig. 1) dem-267onstrate that Hg values in sediments dated younger

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than approximatelyAD 1220–1435 are significant-269 ly elevated above regional background values,270 suggesting that formation of these sediments coin-271 cided with the period of Hg amalgamation mining272 (ca. AD 1573–1900).

273 4. Methods and materials

274 Soil samples were collected from selected agri-275 cultural fields in August and September of 2001276 in Mondragon, Tasapampa, Tuero Chico and Soto-´277 mayor (Fig. 1). Two collection procedures were278 followed. The most detailed sampling protocol279 involved the collection of soils from the upper 5280 cm of the ground surface at multiple locations281 along transects oriented perpendicular to drainage.282 Sample transects were generally positioned near283 the irrigation inlet(at the upper end of the field)284 and near the water outlets(at the lower end of the285 field). At Sotomayor, samples were also collected286 along transects in the middle of the fields. The287 samples obtained along each transect were com-288 bined and mixed to create a single composite289 sample with presumably reduced variability in290 heavy metal concentrations resulting from cross-291 field spatial influences. The composite samples292 were subsequently subdivided for different analy-293 ses. The second sampling procedure involved the294 random collection of soils from the upper 5 cm of295 the ground surface at a single location within a296 selected field. These samples can be expected to297 exhibit more variability in metal concentrations298 than observed in the samples obtained using the299 previous procedure because, local variations in300 metal values within the soils have not been reduced301 by mixing of the materials, collected from multiple302 locations. All of the samples were placed in303 polypropylene sampling containers, and packaged304 for shipping in plastic bags.305 Analysis involved the digestion of 200 mg of306 dried and homogenized sediment,-2 mm in size,307 in 125 ml polypropylene screw-top bottles contain-308 ing 4 ml of aqua regia. These were sealed and309 held in a 1008C oven for 60 min. The leaches310 were then transferred to 200 ml volumetric flasks,311 brought up to volume with deionized water and312 stored until analyzed by ICP-MS or ICP-AES. Pb,313 Zn, Hg, Cu, Sb, Cd, As and Ag were chosen as

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analytes because some(e.g. Pb, Cd, As) have315well-known human health effects, and others were316identified in earlier studies as being elevated in317the Pilcomayo alluvium and river water(Hudson-318Edwards et al., 2001). Both precision and accuracy319were within 10%(seeLechler, 2003for details on320analytical procedures).321Samples of beans, beetroots, carrots(including322tops), maize, onions, lettuces and potatoes were323collected from the four communities. These were324stored in plastic bags and shipped to the Nevada325Bureau of Mines and Geology in Reno, Nevada326(USA). In the laboratory, the vegetables were327peeled(if possible), washed with Nanopure water328and 200 mg samples were digested in 4 ml of329aqua regia in a CEM microwave digestion appli-330ance at 2008C for 30 min. Leachates were diluted331to 200 ml with Nanopure water and analyzed with332a Micromass Platform ICP-HEX-MS. Instrument333calibration was performed with commercial mul-334tielement standards from CPI and Claritas; data335quality was assured with NIST 1547(peach336leaves) (Lechler, 2003). Analytical data for vege-337table samples are reported on a wet weight basis.338Water samples were obtained from the irrigation339canals, the low-water channel of the Rıo Pilco-´340mayo, and the drinking water supplies of each of341the riverside communities. A water sample from342the Rıo Pilcomayo was also collected at Yocalla,´343upstream of the main mining area, to provide344insights into the background concentrations of345metals within river waters. The samples were346filtered through a 0.2mm filter, collected in poly-347propylene bottles and those destined for cation348analysis were preserved with nitric acid to pH-2.349Field blanks were opened and acidified in the350same manner as samples. Pb, Zn, Cd and Cu were351analyzed by ICP-AES(Philips instrument) at the352UK NERC Facility at Royal Holloway and As and353Sb by furnace—AAS(Unicam Soker system) at354the UCL-Birkbeck Wolfson Laboratory for Envi-355ronmental Geochemistry.356The sampling of water and sediments was par-357alleled by interviews with village leaders as well358as other men, women and children from each of359the communities. The interviews focused on their360perceptions of risks arising from the use of water361and riverine soil for agriculture, as well as their

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11 Fig. 2. Photograph of the sampled fields at Mondragon. The Rıo Tarapaya is located in the foreground; village is behind and to the´ ´12 left of the photograph.

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opinions of how the fluvial environment appeared363 to have changed within living memory. Data were364 gathered by informal discussion, often during sam-365 pling or during discussions of the variability in366 productivity of different parts of a field.367 Guidelines governing the acceptable levels of368 metal within the sampled media do not exist in369 Bolivia. Thus, published guidelines from other370 countries were used to interpret the results.

371 5. Results and discussion

372 5.1. Contamination of agricultural soils

373 5.1.1. Mondragon´374 Mondragon is located approximately 26 km´375 from the mills at Potosı(Fig. 1). The river valley´376 at Mondragon is relatively narrow and the Rıo´ ´377 Tarapaya, which connects the mines and mills to378 the Rıo Pilcomayo(Fig. 2), is incised into alluvial´379 valley fill deposits. Three fields located at the

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Northern end of Mondragon were sampled in the´381summer of 2001(Fig. 2). The fields are developed382on a suite of alluvial terraces that increase in383height from approximately 2 m(F1) to more than38410 m (F3) above the channel(Fig. 2).385Irrigation of the fields is primarily limited to386waters derived from small springs and overland387flow from the surrounding hillslopes. The most388significant source of water to the sampled fields is389a small tributary located immediately to their390south, which flows into the Rıo Tarapaya(Fig. 2).´391With the exception of Zn, dissolved concentrations392of the analyzed metals in the tributary were below393minimum guideline values for irrigation waters394(Table 1). In contrast, the analysis of sediments395from one of the adjacent irrigation canals(sample396MD-I1) exhibited concentrations of Zn, Cd and397Ag that exceeded regional background levels and398concentrations of Sb that exceed global concentra-399tions in shales(Table 2). It is likely, however, that400the high values result from the construction of the

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17Table 1

18 Irrigation water analyses19

Trace metals Pb Zn Hg Cu Sb Cd As20Health guidelines mg ly1 mg ly1 mg ly1 mg ly1 mg ly1 mg ly1 mg ly1

2127 Canadian recommended maximum 10 2000 n.g. 200 n.g. 10 10028 concentrations in irrigation water1

29 Australia–New Zealand long-term trigger 2000 2000 2 200 n.g. 10 10030 values in irrigation water2

31 Australia–New Zealand short-term trigger 5000 5000 2 5000 n.g. 50 200032 values in irrigation water23334 1. Mondragon´35 Drinking and irrigation water(MN5-6) -0.2 2500 0.119 1.9 -0.1 1.7 5.23637 2. Tasapampa38 Irrigation water(TP 2) 0.8 -10 0.124 5.4 -0.1 -0.1 1.03940 3. Tuero Chico41 Irrigation water(TCW 1) 40.0 11.8 0.245 16.2 -0.1 0.4 16.94243 4. Sotomayor44 Irrigation water(SOT3-4) -0.2 24.1 0.166 15.2 11.4 0.9 19.545 Irrigation water(SOT5-6) 81.4 27.2 0.209 16.5 20.8 0.3 23.546 Irrigation water(SOT7-8) -0.2 38.0 0.075 13.9 9.9 0.3 22.147 Irrigation water(SOT9-10) -0.2 -10 0.260 17.6 19.1 0.3 21.24849 Values in bold are those analyses that exceed the minimum guideline values for irrigation waters.50 Extracted fromWateResearch Corp and Agriculture and Agri-Food Canada(1999).1

51 Extracted fromANZECC (1992).2

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canal in contaminated sediments, and not from the402 accumulation of metals from irrigation waters403 extracted from the Rıo Tarapaya.´404 With respect to the agricultural soils, all three405 of the fields are contaminated with Pb, Zn, Hg,406 Cu, Sb, Cd and Ag, although metal concentrations407 decrease with increasing elevation of the fields408 above the Rıo Tarapaya(Table 2). This is in spite´409 of the fact that the highest terrace(F3) is located410 more than 10 m above the channel bed of the Rıo´411 Tarapaya and is unlikely to have been inundated412 by flood waters in the past several decades. For413 example, while most samples exceed regional414 background values, Pb values in the lowest terrace415 (F1) are nearly 15 times greater than regional416 background concentrations, and are triple the val-417 ues in the upper two fields(Table 2). Zn concen-418 trations are at least 10 times greater than regional419 background values in all three fields, and Hg420 concentrations exceed regional background values421 by at least an order of magnitude. Soils within all422 three of the fields exceed the Dutch standard for

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Zn (720 mg kg ), whereas the lowest fieldy1

424adjacent to the Rıo Tarapaya also exceeds the´425Canadian standards for Pb(200 mg kg ), Cuy1

426(100 mg kg ) and Sb(20 mg kg ), and they1 y1

427German standard for Cd(2 mg kg ) (Table 2).y1

428Preliminary geomorphic data suggest that the429upper reaches adjacent to Mondragon have under-´430gone significant incision during the past 2000431years, creating the observed suite of terraces,432perhaps much of it occurring within the past few433centuries. If this is the case, the elevated concen-434trations of metals within the highest terrace may435largely be due to the historic deposition of contam-436inated sediments within the alluvial deposits. The437elevated values may also be due to metal accu-438mulation from irrigation waters or extreme historic439floods.

4405.1.2. Tasapampa441The agricultural fields at Tasapampa are located442on a large alluvial fan constructed along the443Northern margin of the Rıo Pilcomayo valley´

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56Table 2

57 Summary of agricultural field data(all values reported inmgyg)58

Pb Zn Hg Cu Sb Cd As Ag5964 Typical conc.(rocks)1

65 1. Granite 17 50 0.03 20 0.2 0.13 – 0.0466 2. Sandstone 7 16 0.03 2 0.4 – – 0.0767 3. Shales 20 95 0.40 45 1 0.3 – –6869

Rıo Pilcomayo basin´ 18 66 0.03 9 – 0.13 – 0.0470(Regional background) (6–34) (17–132) (0.01–0.087) (3–24) (-0.05–3) (-01–22)71

72Health guidelines731. Dutch guidelines 530 720 10 190 – 12 – –74(Action levels)75Canada guidelines 200 400 – 100 20 8 – –76(Maximum limits)277Germany 500 300 – 500 – 2 – –78(Maximum limits)279

801. Mondragon´81MD-F1U (adjacent to river) 295 1295 3.50 47 120 2.68 179 6.9582MD-F1L (adjacent to river) 198 1260 n.d. 58 86.9 2.64 174 9.2483MD-F2U (intermediate elev.) 34 1254 0.389 10 10.6 1.68 46.0 0.8784MD-F2L (intermediate elev.) 22 1032 n.d. 8.3 8.6 1.33 41.5 0.6385MD-F3U (highest field) 55 1058 0.721 11 19.2 1.79 54.7 1.1286MD-F3L (highest field) 50 1001 n.d. 11 19.1 1.67 54.0 1.7887MD-I1 (irrigation ditch) 8.2 718 n.d. 1.9 2.60 1.02 36.0 0.2488

892. Tasapampa90TP-F1U(adjacent to river) 11 46 0.353 11 1.0 -0.01 3.6 -0.0191TP-F1L (adjacent to river) 10 50 n.d. 10 1.4 0.30 5.4 0.1792TP-F2U(highest field) 20 112 0.260 30 0.6 0.24 7.8 0.3593TP-F2L (highest field) 17 88 n.d. 23 0.8 0.33 10 0.4494TPS1(highest field) 38 96 4.27 27 -0.25 0.15 7.8 n.d.95TPS2(intermediate elev.) 60 88 1.82 24 -0.25 -0.01 5.5 n.d.96TPS4(intermediate elev.) 82 934 1.85 10 -0.25 0.18 6.5 n.d.97

983. Tuero Chico99TC-F1U (adjacent to river) 34 195 0.362 15 8.0 1.10 54.4 1.22100TC-F1L (adjacent to river) 25 161 n.d. 14 7.5 0.83 52.5 0.93101TC-F2U (adjacent to river) 25 131 0.432 13 5.9 0.38 42.1 0.51102TC-F2L (adjacent to river) 23 123 n.d. 11 5.6 0.66 32.1 0.68103TC-F3U (high field) 23 105 0.369 26 2.6 0.53 15.6 0.48104TC-F3L (high field) 22 102 n.d. 15 3.1 0.53 14.7 0.55105TC-F4U (highest field) 22 97 0.44 22 0.9 0.31 3.7 0.20106TC-F4L (highest field) 20 67 n.d. 16 0.9 0.24 -0.1 0.14107TC1 (intermediate elev.) 33 33 n.d. 39 -0.25 0.32 71.9 n.d.108TC2 (intermediate elev.) 43 86 n.d. 41 -0.25 0.57 -2 n.d.109TC3 (intermediate elev.) 71 134 n.d. 30 4.2 1.03 30.4 n.d.110TC4 (intermediate elev.) 72 112 n.d. 21 4.6 0.72 21.7 n.d.111TC5 (intermediate elev.) 39 106 n.d. 30 2.4 0.64 15.5 n.d.112TC6 (intermediate elev.) 43 18 n.d. 29 5.0 0.15 24.9 n.d.113TC7 (intermediate elev.) 35 53 n.d. 22 2.1 0.51 29.4 n.d.114TC8 (adjacent to river) 43 101 n.d. 31 5.1 0.7 16.8 n.d.115TC9 (adjacent to river) 64 149 n.d. 33 7.7 1.13 19.4 n.d.116

1174. Sotomayor118SM-FCU (background) 18 84 0.437 26 -0.25 0.16 8.5 0.04119SM-F1U (high terrace, Jatun Khakha) 19 83 0.153 18 0.8 0.25 6.3 0.28120SM-F1M (high terrace, Jatun Khakha) 18 80 n.d. 17 0.9 0.20 6.1 0.25121SM-F1L (high terrace, Jatun Khakha) 19 86 n.d. 18 0.9 0.14 5.7 0.24122SotSed6(upper area, field 1) 67 55 n.d. 33 3.2 0.27 -2 n.d.123SM-F2U (high terrace, Jatun Khakha) 21 93 0.387 20 0.9 0.19 6.5 0.23124SM-F2M (high terrace, Jatun Khakha) 19 91 n.d. 21 0.5 0.09 4.5 0.17125SM-F2L (high terrace, Jatun Khakha) 16 78 n.d. 18 0.5 0.07 4.0 0.13126SotSed7(high terrace, lower area, field 2) 34 47 3 62 -0.25 0.23 -2 n.d.

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60Table 2(Continued)61

Pb Zn Hg Cu Sb Cd As Ag62127 SM-F3U (inter. terrace, Jatun Khakha) 45 239 0.432 27 4.7 0.87 24.8 0.60128 SM-F3M (inter. terrace, Jatun Khakha) 18 89 n.d. 17 1.2 0.30 11.3 0.30129 SM-F3L (inter. terrace, Jatun Khakha) 18 87 n.d. 18 1.1 0.29 11.5 0.32130 SM-F4U (low terrace, Jatun Khakha) 15 73 0.339 17 0.7 0.15 7.9 0.18131 SM-F4M (low terrace, Jatun Khakha) 15 74 n.d. 18 0.8 0.17 8.8 0.21132 SM-F4L (low terrace, Jatun Khakha) 18 93 n.d. 19 1.2 0.33 13.1 0.28133 SM-F5U (high terrace, Jatun Khakha) 24 116 0.371 22 1.6 0.32 13.6 0.30134 SM-F5L (high terrace, Jatun Khakha) 20 92 n.d. 21 1.0 0.19 9.7 0.17135 SM-F6U (high terrace, Jatun Khakha) 16 76 n.d. 17 0.7 0.14 7.6 0.15136 SM-F6L (high terrace, Jatun Khakha) 15 81 n.d. 17 0.9 0.16 7.6 0.14137 SM-F7U (low terrace, Jatun Khakha) 16 94 0.399 19 0.7 0.13 7.8 0.09138 SM-F7L (low terrace, Jatun Khakha) 17 83 n.d. 19 1.1 0.22 10.3 0.19139 SotSed12(inter. terrace, upper area, field 7) 43 45 n.d. 27 5.7 0.24 -2 n.d.140 SotSed13(inter. terrace, lower area, field 7) 37 39 n.d. 31 7.9 0.40 -2 n.d.141 SM-F8a(high terrace, Jatun Khakha) 20 97 n.d. 28 0.3 0.21 12.1 0.10142 SM-F8b(high terrace, Jatun Khakha) 18 89 n.d. 25 0.6 0.22 11.8 0.11143 SotSed8(low terrace, upper area, field 9) 43 21 n.d. 26 3.9 0.09 -2 n.d.144 SotSed9(low terrace, lower area, field 9) 45 42 n.d. 28 1.5 0.28 -2 n.d.145 SotSed10(high terrace, upper area, field 10) 46 84 n.d. 32 1.8 0.34 8.0 n.d.146 SotSed11(high terrace, lower area, field 10) 34 55 n.d. 30 2.9 0.35 5.7 n.d.147 SotSed15(high terrace, upper area, field 11) 65 138 n.d. 40 5.6 0.63 11.6 n.d.148 SotSed14(high terrace, lower area, field 11) 42 55 n.d. 33 1.7 0.55 29.2 n.d.149 SotSed5(terrace adjacent to R. Pilcomayo) 89 394 n.d. 77 15 1.93 101 n.d.150 SM-I1 (irrigation ditch) 71 404 n.d. 29.3 6.2 1.60 56.2 0.81151 SM-I2 (irrigation ditch) 69 467 n.d. 28.4 9.0 1.70 42.2 1.31152153 Fx–Field number. U–Upper area of field. M–Middle area of field. L–Lower area of field. Bold-values exceed minimum guideline concentrations154 for agricultural soils.155 n.d.–not determined.156 FromTurekian(1971) and Martin and Meybeck(1979).1

157 Data cited obtained fromKabata-Pendias(1995).2

444

approximately 80 km from Potosı(Fig. 1). The´445 medial and distal fan deposits are periodically446 inundated by floodwaters of the Rıo Pilcomayo.´447 Thus, the sediments underlying the lower agricul-448 tural fields(those close to the river) are composed449 of an interlayered sequence of fan and axial river450 deposits of undetermined age. The thickness of the451 axial sediments from the Rıo Pilcomayo increases´452 down-fan (toward the river). Cores extracted as453 much as 100 m from the river along the distal454 areas of the fan suggest that the upper 1–1.5 m of455 sediment are composed almost exclusively of456 sandy materials from the Rıo Pilcomayo.´457 The fields at Tasapampa are not irrigated with458 waters from the Rıo Pilcomayo, but rather with´459 waters derived from the tributary that feeds the460 alluvial fan complex. There are no known mining461 operations within the tributary basin, and, there-462 fore, the water is thought to be relatively ‘clean’.463 This is borne out by the heavy metal concentra-464 tions in the Tasapampa irrigation water(sample

465

TP2), which are within Canadian and Australian-466New Zealand guideline values(Table 1).467Soil samples were collected from five fields at468Tasapampa. Field F1 is located approximately 2.5469m above the high-water channel bed sediments470immediately adjacent to the river. Field F2 is471located approximately 30 m downfan from Tasa-472pampa well above the channel bed of the Rıo´473Pilcomayo(Fig. 3). Additional samples were col-474lected randomly from fields between these two475elevations(TPS-1, 2, 4). With the exception of476Hg and Pb, the fields do not appear to be exten-477sively contaminated by heavy metals(compared478to background levels;Table 3) and measured479concentrations for all of the analyzed elements are480well below Dutch, Canadian and German standards481for intervention(Table 2). Moreover, comparison482of metal concentrations measured in fields 1 and4832 demonstrate that differences in heavy metal484concentrations are minimal, in spite of their differ-485ences in elevation above the riverbed(Table 2).

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170Table 3

171Percent of soil samples exceeding regional background values172

Community n Pb Zn Cu Sb1 Cd Ag173178Mondragon´ 7 57 100 29 100 100 100179Tasapampa 7 43 14 29 14 14 502

180Tuero Chico 17 35 24 47 76 76 633

181Sotomayor 31 34 10 45 55 29 454

182Sotomayor 31 45 53 31 97 74 1004

183(using local184background data)185186Percentages based on maximum concentrations measured in187pre-mining alluvial deposits along Rıo Pilcomayo.´188Percentages based on global concentrations found in1

189shales.190ns4.2

191ns8.3

192ns20.4

161

162

163 Fig. 3. Photograph taken looking up the alluvial fan toward Tasapampa from sampling site on field 1. Sampling sites TPS-1, TPS-164 2, and TPS-4 are located between position of photographer and field 2. Tributary fan deposits are located to right of Tasapampa in165 background.

486

The highest field(closest to the homes in Tasa-487 pampa) exhibits slightly higher concentrations of488 Zn, Cu, Cd, As and Ag. Given the elevation of489 these fields, which are located near the apex of490 the alluvial fan, it seems unlikely that they have491 been recently flooded by the Rıo Pilcomayo. It is´492 possible, however, that the differences may be493 related to the eolian erosion of sediment from the494 bed of the Rıo Pilcomayo during the dry season,´495 and its subsequent deposition adjacent to the496 mountain front. The river valley in this area is497 much wider(occasionally)2 km in width) than498 in upstream areas, allowing for greater particle499 transport by wind. Significant and frequent col-500 umns of eolian sediment have been observed501 moving from the river toward adjacent hillslopes502 in July, August and September.

503 5.1.3. Tuero chico504 The agricultural fields at Tuero Chico are similar505 to those at Tasapampa in that they are located on506 a large alluvial fan complex(Fig. 4), and the507 distal and medial fan deposits interfinger with508 sediments from the Rıo Pilcomayo. Twelve fields´509 were sampled at Tuero Chico. Four of the fields

510

(F1, F2, TC8, TC9) are located at a relatively low511elevation, adjacent to the river(Fig. 5). These four512fields are underlain by a combination of distal fan513deposits and alluvial sediments from the Rıo Pil-´514comayo. Village leaders stated that they are typi-515cally flooded on an annual basis. Three of the516other fields(F3, F4, TC1) are located near the fan517apex, immediately downslope from housing at

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196

197

198 Fig. 4. Agricultural fields associated with Tuero Chico. Rıo Pilcomayo is located in the foreground.´

518

Tuero Chico, and are not flooded annually519 (although they may be flooded during extreme520 events). Thus, they are underlain primarily by521 alluvial fan deposits. The other fields are located522 between these extremes(Fig. 5).523 In contrast to Tasapampa, ‘clean’ water from524 the fan tributary is in short supply during the dry525 season at Tuero Chico and, thus, the agricultural526 plots are primarily irrigated by waters from the527 Rıo Pilcomayo that are brought in by an irrigation´528 system composed of an upper and lower canal.529 The irrigation waters exhibit a dark grey color530 characteristic of the contaminated milling effluent.531 A water sample collected for this study(during532 the dry season) possessed a Pb concentration(40533 mg l ) that exceeded the Canadian recommendedy1

534 maximum concentrations in irrigation water(10535 mg l ) (Table 1).y1

536 All the fields exhibit soil concentrations that537 exceed regional background values of Pb, Zn, Sb538 or Cd (Table 3). Combined with the deposition of539 contaminated particles during flood events, three540 possible scenarios can explain the elevated levels

541

of contaminants within the agricultural plots,542including(1) the contamination is primarily related543to the deposition of metal-enriched sediments dur-544ing flood events;(2) the high metal values are545derived from contaminated irrigation waters; and546(3) the contamination results from both the depo-547sition of contaminated flood deposits and polluted548irrigation waters. In addition, some of the metals549may be derived from the deposition of eolian550sediments, as mentioned above. In general, Pb, Zn,551Sb and Ag concentrations are higher in the lower552fields, close to the river, than they are in the upper553fields. This spatial trend suggests that soils receiv-554ing contaminated sediments during annual flooding555of the Rıo Pilcomayo are more heavily polluted.´556Heavy metal concentrations within all of the sam-557pled fields are below the Dutch, Canadian and558Germany standards for intervention(Table 2).

5595.1.4. Sotomayor560A total of 11 fields were sampled in Sotomayor.561All these fields are located on alluvial terraces562along the Rıo Jatun Khakha(Fig. 6). With the´

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203

204 Fig. 5. Schematic map of Tuero Chico showing the distribution of the sampled fields with respect to the Rıo Pilcomayo. Map is´205 not to scale.

563

exception of FC, all of the agricultural plots564 receive contaminated irrigation waters from the565 Rıo Pilcomayo(see below). An additional field´566 located on an alluvial terrace along the Rıo Pilco-´567 mayo was sampled in 2000. This particular field568 is periodically inundated by floodwaters of the Rıo´569 Pilcomayo and receives contaminated irrigation570 waters during the dry season.571 Field FC is located approximately 1.5 km572 upstream of Sotomayor and could not have

573

received contaminated sediments via flood flows574from the Rıo Pilcomayo. In addition, it is not´575known to have been irrigated with polluted waters.576Thus, its soils were sampled to provide insights577into local background concentrations of heavy578metals in the fields at Sotomayor. As expected,579Pb, Zn, Cu, Sb, Cd and Ag values cannot be580distinguished from regional background values. Hg581concentrations, however, are elevated well above582background. It is unclear at this time why the Hg

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209

210 Fig. 6. Schematic map of the agricultural fields at Sotomayor211 showing the distribution of sampled fields. Map is not to scale.583

concentrations are elevated in FCU, but it is584 possible that the high Hg values are related to an585 anthropogenic source farther upstream in the Jatun586 Khakha.587 With the exception of Zn, approximately 30–588 55% of the collected agricultural soil samples589 exceed the highest concentrations observed in pre-590 mining deposits(i.e. regional background) (Table591 3). A higher percentage(30–100%) of the samples592 exceed the background values of the analyzed593 metals observed in field FC(the background plot594 sampled at Sotomayor). Fields F3, F5 and F11

595

exhibit particularly high concentrations of all the596analyzed elements. While the soils in all of the597sampled fields exhibit heavy metal concentrations598above background levels, the metal concentrations599are below the Dutch, Canadian and German inter-600vention standards(Table 2).601The sampled agricultural plots at Sotomayor are602not periodically flooded by the Rıo Pilcomayo,´603and therefore, cannot be contaminated by the604deposition of metal-enriched alluvial sediments.605This is supported by the lack of a significant trend606in heavy metal concentrations with field(terrace)607height above the channel bed of the Rıo Jatun´608Khakaha, which would be expected if the metals609were primarily derived from polluted alluvial sed-610iments(Table 2). The primary source of contami-611nation is most likely to be irrigation waters.612At Sotomayor, water for irrigation is extracted613from the Rıo Pilcomayo via a canal, and is locally´614pumped to other irrigation channels located along615the Rıo Jatun Khakha. The perception of villagers´616is that problems arising from river water are least617significant near the end of the wet season when618the river is high. Samples for this study were taken619during the dry season, presumably when concen-620trations are at there highest levels.621Two sediment samples collected from the irri-622gation canals at Sotomayor(SM-I1, SM-I2; Table6232) exhibit significantly elevated concentrations of624the analyzed contaminants. The four collected625samples of irrigation water(Table 1) also exhibited626slightly elevated metal concentrations, although627all, with one exception(SOT5-6, Pb values of62881.4mg l ) are within acceptable limits. Most ofy1

629the metal concentrations in the irrigation waters630are comparable to those in the water from the Rıo´631Pilcomayo taken at Sotomayor(sample Sot1-2;632Table 3). This is not surprising given that the633irrigation water is taken directly from the river.634Some of the Pb, Sb and As concentrations, how-635ever, are higher than those in the river water636sample, suggesting that(1) there is another source637of these elements to the irrigation water;(2) the638composition of the river water itself is highly639variable or(3) an unknown process concentrates640these elements during transport through the irri-641gation canals.

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642

5.2. Downstream variations in metal concentra-643 tions within the agricultural soils

644 The agricultural soil data show that several of645 the fields at Mondragon, Tuero Chico and Soto-´646 mayor are contaminated with Ag, Cd, Cu, Hg, Pb,647 Sb and Zn compared to background levels found648 in pre-mining, Pilcomayo alluvium. The principal649 metal contaminant concentrations decrease in the650 order of Zn, Pb, Cu, Sb, Cd, Hg and Ag(Table651 2), similar to that found in other metal mining-652 contaminated river systems(e.g.Hudson-Edwards653 et al., 2003). Inspection ofTable 2illustrates that654 in general, there is a downstream decrease in the655 magnitude of contamination of the agricultural656 plots from the mines and mills at Potosı. At´657 Mondragon, Zn, Pb and Cd concentrations exceed-´658 ed recommended guideline values for agricultural659 soils within some of the sampled fields, particular-660 ly close to the river. Farther downstream at Tuero661 Chico and Sotomayor, none of the soil samples662 exceeded Dutch, Germany or Canadian action663 levels. However, a significant number of the sam-664 ples are clearly contaminated as demonstrated by665 comparison to regional background data(Table 3).666 The least contaminated community downstream667 of Mondragon is Tasapampa(Tables 1 and 2). It´668 is located upstream of both Tuero Chico and669 Sotomayor, and therefore, appears to represent an670 exception to the expected downstream trends in671 contaminant levels. In contrast to the immediately672 adjacent community of Tuero Chico(Fig. 1),673 however, its fields are irrigated using locally-674 derived (uncontaminated) tributary water, rather675 than waters from the Rıo Pilcomayo. The differ-´676 ence in contaminant magnitudes suggest that at677 least some of the metals observed within the soils678 are derived from irrigation practices. In fact, as679 mentioned above, most of the contamination at680 Sotomayor must be derived from irrigation canals681 and thus, the continued use of contaminated irri-682 gation waters could lead to a further buildup of683 metals within the agricultural fields.684 While irrigation waters appear to be an impor-685 tant source of contaminants within the soils,686 decreases in metal concentrations with increasing687 elevation above the river indicates that fields prone

688

to flooding, also receive contaminants by overbank689depositional processes.

6905.3. Metal concentrations in agricultural products

691Samples of carrots(including tops), onions,692lettuces, beans, maize, beetroots and potatoes were693collected from Mondragon, Tuero Chico, Tasapam-´694pa and Sotomayor along the Rıo Pilcomayo(Table´6954). The analysis was intended to provide a basic696understanding of the concentrations of metals with-697in the vegetables. Given the limited number of698samples that were obtained, the results should be699considered preliminary, and detailed relationships700between crop concentrations, metal content in701irrigation waters, and the degree of soil contami-702nation will necessarily require additional analyses.703Nonetheless, the existing data show that maize and704carrot tops exhibit the highest Zn concentrations,705and lettuce and onion the highest Cd concentra-706tions. Almost all of the Zn and Cd concentrations707in the analyzed crops fall within the minimum708recommended guideline values for commercially-709grown vegetables. The exceptions are a sample of710lettuce at Tuero Chico, which exceeds the Zn and711Cd guideline values, and an onion at Sotomayor,712which exceeds the Cd guideline for root vegetables713(Table 4).714Heavy metal uptake by plants grown in polluted715soils has been shown to be related to the total716amount of metal in the soil, the source and speci-717ation of the soil metals, soil pH, temperature,718cation exchange capacity of the soil, organic matter719content, plant species and the means through which720metals are distributed to plant parts during growth721(Haghiri, 1973, 1974; Abdel-Sabour et al., 1988;722Ward and Savage, 1994; Vousta et al., 1996;723Albering et al., 1999). The existing vegetable data724do not allow for a detailed analysis of the factors725controlling metal accumulation in the agricultural726products within the Rıo Pilcomayo basin. Howev-´727er, the lack of enrichment in Zn and Cd in the728vegetables is surprising given that many of the729soils in the villages are contaminated with these730metals. The minimal accumulation of these metals731may be related to their bioavailability within the732soils. Albering et al. (1999), for example, also733found that the concentrations of Zn and Cd in the

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216Table 4

217 Summary of heavy metals in vegetable samples(all values reported inmgyg)218

Health guidelines Pb Zn Hg Cu Cd Ag219224 Guidelines1 12 503 — 203 0.1 (root)4 —225 (for commercially sold 0.14 0.2 (leafy)4

226 vegetables) 0.254

227 (potatoes)228 0.30 (leafy)4

229 1. Mondragon´230 Maize (High Terrace; US W) -0.002 15.0 0.016 0.61 -0.005 -0.005231 Maize (High Terrace; US W) -0.002 9.67 0.010 0.25 -0.005 -0.005232 Maize (Low Terrace; US W) 0.004 16.5 0.011 0.29 -0.005 -0.005233 Maize (Low Terrace; US E) 0.011 48.2 0.009 1.02 0.010 -0.005234 Maize (Low Terrace; US E) 0.005 12.5 -0.005 0.72 0.006 -0.005235236 2. Tuero Chico237 Carrot 0.023 6.49 0.020 1.10 0.007 -0.005238 Carrot 0.014 6.17 0.024 1.05 0.008 -0.005239 Carrot 0.030 5.81 0.030 1.25 0.008 -0.005240 TF4 carrot 0.117 9.05 -0.010 0.82 0.060 0.010241 TF1 onion -0.005 5.53 0.081 1.46 0.067 0.053242 TF3 onion 0.029 4.65 0.046 0.95 0.056 0.015243 TF2 lettuce 12.1 63.8 0.169 8.05 0.330 0.265244 TF5 lettuce 0.623 8.81 0.050 0.80 0.100 0.018245246 3. Tasapampa247 TPC1 bean 0.026 3.16 0.034 0.72 0.044 0.011248 TPC2 onion -0.005 2.89 0.061 1.69 0.065 0.031249 TPC3 carrot 0.023 2.07 0.068 0.59 0.063 0.048250251 4. Sotomayor252 Carrot(Field a) 0.006 4.26 0.014 0.69 0.015 -0.005253 Carrot(Field a) 0.010 6.12 0.010 0.58 -0.005 -0.005254 Carrot(Field b) 0.217 15.8 0.046 3.21 0.030 -0.005255 Carrot(Field b) 0.172 29.6 0.055 3.49 0.036 -0.005256 Carrot(Sotcrop 2) 0.123 7.88 0.208 1.57 0.045 0.024257 Carrot(Sotcrop 4) 0.015 4.79 0.105 1.27 0.006 0.035258 Carrot top(Field a) 0.519 14.7 0.042 3.46 0.026 -0.005259 Carrot top(Field a) 0.217 15.8 0.046 3.21 0.030 -0.005260 Lettuce 0.295 9.55 0.013 1.81 0.023 -0.005261 Lettuce 0.652 11.9 0.028 2.15 0.037 -0.005262 Lettuce(Sotcrop 5) 0.810 10.4 0.110 1.36 0.091 0.050263 Onion 0.004 8.66 0.007 1.55 0.008 -0.005264 Onion -0.002 5.83 0.007 1.27 0.009 0.011265 Onion (Sotcrop 1) 0.663 20.5 0.082 1.17 0.370 -0.005266 Onion (Sotcrop 3) 0.239 6.40 0.173 1.62 0.041 0.018267 Beans(DP1) 0.005 8.97 0.086 2.50 0.071 0.033268 Beetroot 0.366 8.77 0.015 2.04 0.015 0.014269 Beetroot 0.010 4.05 0.005 1.23 -0.005 -0.005270 Potato 0.005 5.51 0.034 1.82 -0.005 -0.005271 Potato -0.002 5.52 -0.005 1.85 -0.005 -0.005272273 Values in bold are those analyses that exceed the minimum guideline values for crops.274 Data taken fromPless-Mulloli et al.(2001)1

275 UK lead in food(Amendment) regulations(1985).2

276 Food standards committee guideline 1950.3

277 EC regulation 466y2001; lead and cadmium values may change in March 2002.4

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734

crops of the Meuse River were within background735 ranges, in spite of the fact that they were grown736 on severely polluted floodplain soils. They sug-737 gested that the lack of crop accumulation of Cd738 and Zn was due to the relatively high pH of the739 floodplain soil (6.9"0.6) that reduced the availa-740 bility of the metals for plant uptake. Soil pH and741 other factors such as those listed above could also742 be responsible for the relatively low Cd and Zn743 concentrations seen in the Pilcomayo vegetables.744 The highest Cu concentrations are found in745 beetroots and beans, but for all the crop, Cu746 concentrations fall within the minimum recom-747 mended guideline values for these metals for748 commercially grown vegetables(Table 4). Carrots,749 onions and beans generally contain the most Hg,750 and onions and some of the carrots and beans the751 most Ag. No guideline values exist for Hg and Ag752 in vegetables. Thus, it is not possible at this stage753 to evaluate the potential risk that these crops pose754 with respect to these two metals.755 The metal in the Pilcomayo crops that poten-756 tially poses the most risk to consumers(using the757 guidelines inTable 4) is Pb. At Tuero Chico, three758 out of the eight samples analyzed(37.5%) exceed759 the minimum recommended guideline for Pb in760 commercially-grown vegetables, and at Sotomayor,761 11 out of 20 samples analyzed(55%) exceed the762 Pb guideline. Lettuces, onions and carrots(along763 with their tops) generally contain the most Pb.764 The high Pb values in lettuce(particularly sample765 TF2; Table 4) may be due to the incomplete766 removal of particulate matter from the samples767 prior to analysis(the vegetable skin that was in768 contact with the soils was removed for all except769 the leafy vegetables, insuring the complete remove770 of any particulate matter). It is important to note,771 however, that the concentrations of Pb in some of772 the beetroot and carrot samples(which were773 peeled) also exceed recommended values, sug-774 gesting that the Pb is accumulated in the tissues775 of these crops. Other authors have also found Pb776 concentrations that exceed recommended levels in777 carrots(e.g.Smigiel, 1994; Lacatusu et al., 1996;778 Malmauret et al., 2002) and lettuce(Lacatusu et779 al., 1996), suggesting that perhaps the growing of780 these vegetables on Pb-contaminated soils should781 be avoided.

782

5.4. Contaminant concentrations and sources with-783in drinking and river water

784Clearly, an important health consideration is the785concentration of metals within drinking water sup-786plies of the riverine communities. Metal concen-787trations within the water of the Rıo Pilcomayo is´788also important because of the inadvertent(or789occasionally the intentional) exposure of the local790populations to the water during river crossing,791bathing, etc. Metal analyses of drinking water at792Mondragon, Tasapampa and Tuero Chico are sum-´793marized inTable 5. This table also includes water794data from the Rıo Pilcomayo near each of the´795studied communities, and sample results from796Yocalla located upstream of the influx of effluent797from the mines and mills at Potosı.´798Hudson-Edwards et al.(2001) demonstrated that799the contemporary channel bed sediments of the800Rıo Pilcomayo upstream of Sotomayor contain´801appreciable amounts of pyrite and other sulfide802bearing mine tailings. Concentrations of SO4

803(Hudson-Edwards, unpublished data), Pb, Zn, Cu,804Sb and Cd in water from the Rıo Pilcomayo at´805Mondragon and Sotomayor are considerably higher´806than those at Yocalla, and may reflect the oxida-807tionyweathering of pyrite- and other sulfide-bear-808ing mine tailings in the Pilcomayo downstream of809Potosı(cf. Hudson-Edwards et al., 2001). Concen-´810trations of As in the river water at Mondragon and´811Sotomayor are lower than or similar to that in812river water at Yocalla, suggesting that sources other813than tailings weathering supply As to the river.814Heavy metal and As concentrations in the drinking815water supply at Mondragon, which is primarily´816derived from tributaries and springs, are within the817World Health Organisation(WHO) drinking water818guideline values(Table 5). The dissolved Zn819concentration(2500mg l ), however, is near they1

820WHO guideline value(3000 mg l ) for they1

821appearance and taste. The drinking water for Tas-822apampa comes exclusively from the Rıo Cachi´823Mayo, a tributary of the Pilcomayo. Dissolved824concentrations of metals and other compounds in825the drinking water at Tasapampa are all lower than826WHO guideline values(Table 5), except for Sb827(5.3 mg l ), which slightly exceeds the WHOy1

828provisional guideline concentration(5.0 mg l ).y1

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282Table 5

283 Drinking and river water analyses284

Trace metals Pb Zn Cu Sb Cd As285Health guidelines mg ly1 mg ly1 mg ly1 mg ly1 mg ly1 mg ly1

286292 WHO drinking water1 10 3000* 2000(P); 5 (P) 3 10 (P)293 guidelines and levels likely to give rise appearance, 1000*

294 to consumer complaints(shown as )* taste staining295296 1. Yocalla297 Rıo Pilcomayo at Yocalla(Y61–62)´ 0.3 32.9 2.2 -0.1 -0.1 13.4298299 2. Mondragon´300 Rıo Pilcomayo at Mondragon(MN1-2)´ 1.9 3587 19.6 30.4 27.1 21.0301 Drinking and irrigation water(MN5-6) -0.2 2500 1.9 -0.1 1.7 5.2302303 3. Tasapampa304 Drinking water(TP1) -0.2 -10 7.3 5.3 0.1 1.1305306 4. Tuero Chico307 Drinking water(TCW2) -0.2 62.4 6.1 -0.1 0.1 1.7308309 5. Sotomayor310 Rıo Pilcomayo at Sotomayor(SOT1-2)´ 23.9 40.7 16.0 5.5 0.3 13.9311312 (P)–Provisional guideline value. This term is used for constituents for which there is some evidence of a potential hazard but313 where the available information on health effects is limited.Values in bold are those drinking water analyses that exceed WHO314 guideline drinking water values.Values in italics are those river waters that exceed WHO guideline drinking water values, suggesting315 that the river waters should not be used for drinking purposes.316 Extracted fromWHO (1996, 1998).1

317 There is no health-based guideline level for these parameters. Explanations are given below for the likely customer complaint*

318 above the levels shown.

829

At Tuero Chico, the drinking water is obtained830 from springs located on the hillsides, some of831 which is stored in a tank located above the village.832 Dissolved concentrations of metals and other com-833 pounds are all lower than WHO guideline values834 (Table 5). Dissolved concentrations of metals and835 As in drinking water at Mondragon, Tasapampa´836 and Tuero Chico are for the most part, considerably837 lower than the concentrations in waters from the838 Rıo Pilcomayo (Table 5), suggesting that the´839 community strategies, such as using cleaner tribu-840 tary (non-Pilcomayo) water for drinking, reduce841 the levels of exposures to these metals.

842 5.5. Significances of metal exposure pathways in843 humans

844 Farmers refer to a range of natural hazards845 (principally floods, droughts, frosts and hail) as846 important causes for changing their production847 systems over recent decades and would not com-848 monly refer to water quality. In contrast, others

849

such as teachers, village nurses and community850leaders repeated the received wisdom from some851quarters that the quality of the water was much852worse than before and that various illnesses could853be attributed to it. While it is probable that the854quality of river water has deteriorated with the855onset in 1985 of froth flotation methods at the856mills in Potosı, it is more difficult to assess its´857effects on livestock and human health. Moreover,858the data needed to conduct detailed risk analyses859are currently unavailable. Nonetheless, the data860presented above provide insights into the signifi-861cance of the major metal exposure pathways for862the inhabitants of the four communities examined863in this investigation.864A potentially important pathway is the con-865sumption of contaminated produce grown on pol-866luted soils. Although short- and long-term867migration is important for household livelihoods,868farming in many communities produces agricultur-869al products that form an important component of870the local diet, and the deposition of sediments

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during floods is seen by farmers as a valuable872 method of soil renewal. In others, such as Soto-873 mayor, cultivated vegetables are not only con-874 sumed by the community, but are transported to875 urban markets(e.g. as Sucre) and sold to generate876 much needed income. Although the sale of agri-877 cultural produce to urban centres poses a potential878 health risk for urban consumers, it poses more of879 a risk to local villagers most of whose food comes880 from the community. There is also significant881 concern by local residents regarding the health882 impacts of the vegetables on their children.883 While the data are limited, concentrations of884 heavy metals in the analyzed crops from all of the885 communities were generally below the recom-886 mended guideline values putforth for commercial-887 ly-sold vegetables. The only significant exception888 was the concentration of Pb in produce from889 Sotomayor and Tuero Chico; approximately 55%890 and 37% of the carrot, lettuce and beetroot sam-891 ples, respectively, exceeded the recommended stan-892 dards, despite the fact that relatively low893 concentrations of Pb were found in the agricultural894 soils. With the possible exception of Pb, these data895 suggest that the consumption of vegetables from896 the studied communities is generally not a signif-897 icant exposure pathway, even when the soils are898 highly contaminated, such as they are in899 Mondragon.´900 The factors controlling the accumulation of Pb901 in some vegetables at Sotomayor and Tuero Chico902 are not clearly understood at this time and warrants903 further investigation. It is possible, however, that904 the accumulation is related to the use of contami-905 nated irrigation waters extracted from the Rıo´906 Pilcomayo. Thus, a possible means to reduce the907 risks associated with Pb in lettuce, carrots and908 beetroot would be to irrigate the fields with clean,909 rather than contaminated waters. Unfortunately, the910 availability of clean surface water supplies in these911 areas during the dry season is limited. Perhaps the912 most viable solution is the development of one or913 more groundwater wells for each of the914 communities.915 We only have a ‘snapshot’ of water geochem-916 istry within the Rıo Pilcomayo, the irrigation´917 canals, and the drinking water supplies of each of918 the four communities. The general perception is

919

that the degree of water contamination is most920severe during the dry season when effluent from921the mines and mills is not diluted by significant922quantities of water from hillslopes and tributaries.923Thus, the data presented here are likely to present924a worse-case scenario. Even so, dissolved concen-925trations of metals and As in drinking water at926Mondragon, Tasapampa and Tuero Chico are gen-´927erally lower than WHO guideline values. Thus, in928general, drinking water does not appear to be a929significant exposure pathway for the analyzed930elements using these guidelines. The exception to931this statement is the Sb concentration at932Tasapampa.933Given the concentrations of metals in agricul-934tural soils at all four of the investigated commu-935nities, one of the most important exposure936pathways for children may be the ingestion of937contaminated soil particles. Ingestion is likely to938be significant for very young children playing on939the soils while their parents work in the fields.940Perhaps, more importantly is the ingestion of soil941particles attached to vegetables that are eaten raw942without having been washed or otherwise prepared.943Risks from this potential exposure pathway could944be reduced by washing or peeling the vegetables945prior to consumption to ensure that the soil parti-946cles have been removed.947Dissolved concentrations of river waters from948the Rıo Pilcomayo were generally above back-´949ground values measured at Yocalla by up to several950fold, and some metals exceeded WHO guideline951values. Thus, the inadvertent and advertent con-952sumption of water from the river could pose a953risk. However, the use of Rıo Pilcomayo waters is´954thought to be rather limited for domestic purposes.955In contrast, livestock and poultry frequently drink956polluted waters directly from the river(and the957irrigation canals) and, therefore, these waters may958be an important exposure pathway for cows, pigs,959chickens, etc. Future exposure studies will include960an analysis of the consumption of potentially961contaminated livestock that frequently drink con-962taminated waters from the Rıo Pilcomayo or the´963irrigation canals. An additional pathway that will964be examined is the inadvertent ingestion of con-965taminated dust generated by eolian processes, par-966ticularly during the dry season. These future

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studies should also more fully explore the linkages968 between catchment management, biogeochemi-969 cal—physical processes, socio-economic structures970 and the availability of public health(e.g. Parkes971 and Panelli, 2001).

972 6. Conclusions

973 The heavy metal content of agricultural soils974 was examined in four communities along the Rıo´975 Pilcomayo located between approximately 25 and976 175 km from the mines and mills at Potosı. The´977 most significantly contaminated soils occurred978 upstream at Mondragon where Cd, Pb and Zn´979 concentrations exceeded recommended guideline980 values for agricultural use. Further downstream the981 degree of contamination semi-systematically982 decreased, and metal concentrations were below983 Dutch, German and Canadian guideline values.984 Contamination of the soils results from the depo-985 sition of contaminated soil particles on the fields986 during flood events, the application of contaminat-987 ed irrigation waters obtained from the Rıo Pilco-´988 mayo or a combination of these two processes. At989 Mondragon, contamination may also be related to´990 the historic deposition of contaminated alluvial991 sediments prior to channel incision and terrace992 formation.993 Metal concentration data for crops grown on994 contaminated soils are limited, but suggest that995 most vegetables did not accumulate significant996 quantities of heavy metals. With the exception of997 Pb in some produce from Sotomayor and Tuero998 Chico, vegetables from the four communities were999 well below the existing guidelines for heavy metal1000 content in commercially-sold vegetables. Lettuce1001 and carrots appear to concentrate metals(particu-1002 larly Pb) but additional research is needed to verify1003 the observed trend.1004 Dissolved concentrations of metals and As in1005 drinking water at Mondragon, Tasapampa and´1006 Tuero Chico are generally lower than WHO guide-1007 line values. The exception to this statement is the1008 Sb at Tasapampa.1009 The most significant exposure pathway appears1010 to be the ingestion of contaminated soil particles1011 attached to and consumed with vegetables. Risk1012 reduction associated with this pathway is possible

1013

by washing or peeling the vegetables(e.g. carrots,1014beetroots, etc.) prior to consumption. Drinking1015water does not appear to be a major exposure1016pathway, nor do vegetables. Other potential path-1017ways that have yet to be explored is the consump-1018tion of meat derived from locally produced1019livestock and poultry, and the ingestion of contam-1020inated, wind-blown dust.1021In this study, a wide range of guideline values1022were used as a screening tool to assess the risk1023associated with contaminated land(e.g.Edelgaard1024and Dahlstro, 1999; Bieber, 2000), none of which¨1025have been developed or accepted for Bolivia.1026There is a need to develop standards appropriate1027for Bolivian soils, waters, living conditions, and1028these should be applied according to site-specific1029conditions using proper risk assessment models1030(e.g. USEPA, 1991). Moreover, additional geo-1031chemical analyses and social surveys are required1032to develop effective metal and arsenic risk man-1033agement strategies.

10347. Uncited reference

1035CCM, 1999; MHSPE, 2000

1036Acknowledgments

1037We are grateful to D. Grow, J.S. Miners and1038J.N. Turner for field assistance, J. Archer for1039toxicity research, A. Osborn for analytical work1040using Wolfson Laboratory for Environmental Geo-1041chemistry facilities(Birkbeck-UCL, University of1042London) and the NERC ICP-AES Facility at1043RHUL (with the permission of its Director, Dr1044J.N. Walsh). We are particularly indebted to Lionel1045F. Villarroel Gonzales for his invaluable assistance1046in organizing logistical aspects of our fieldwork1047and for assisting in the collection of field data.1048Funding for this work was provided by the Nation-1049al Geographic Society(grant no. 6785-00 to all1050authors) and the US National Science Foundation1051(grant no. ERA-0207439 to JRM and PJL).

1052References

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