134 I. ROSAS ET AL.Figure 1. Location of the counties (indicated by number) where dairy farms were sampled (indicatedby circle). On the other hand, the distribution and fate of this metalloid in the agricul-tural environment has not yet been evaluated. Therefore the present study wasundertaken with the following objectives: a) To determine arsenic concentration speciation in water, and to correlate these levels with the amount of extractable arsenic in soil. b) To determine the concentration of As in Alfalfa (leaves and roots) and corn’s silage, considering that these forages are consumed by dairy cattle. c) To evaluate the As biotransfer factor to milk. 2. Materials and Methods2.1. S AMPLINGDairy farms of seven counties located at Durango and Coahuila States, Mexico(Figure 1), were sampled during 1992, in both rainy and dry seasons. Well-water,soil, alfalfa (leaves and roots), corn’s silage, and cow’s milk were sampled in six-teen dairy farms during the ﬁrst sampling period (April) and in seventeen farmsduring the second and third sampling periods (July and October). The sampleswere obtained from each as follows:
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 1352.1.1. Well WaterTo determine arsenic, 1-L samples were collected in polyethylene bottles, previ-ously rinsed with 20% nitric acid and deionized water. To determine the physico-chemical parameters, 1-L samples were also collected in the same way, except thatbottles were not rinsed with nitric acid. Samples were placed into an ice box butnot frozen for transportation to the laboratory.2.1.2. ForageSamples of alfalfa (Medicago sativa) crops (roots and leaves) and corn’s silagewere put apart and placed into plastic bags. For the calculation of the As biotransferfactor to milk, 500 g (fresh weight) of the general dairy cow’s diet were collectedfrom 5 food containers at each farm (alfalfa, corn’s silage, soybean meal, calciumcarbonate, minerals and vitamins) and transported to the laboratory in plastic bags.2.1.3. SoilsSoils from alfalfa ﬁelds were sampled at 0–30 and 30–60 cm depth and 1 kg fromeach level was placed into plastic bags.2.1.4. Cow’s MilkCow’s milk was obtained during the cow’s milking and 1-L was placed into apolyethylene bottle previously rinsed with 20% nitric acid in deionized water.2.2. A RSENIC ANALYSIS2.2.1. Sample Preparation220.127.116.11. Water. The arsenic (III) was determined by extraction with ammoniumpyrrolidinedithiocarbamate (APDC) and methyl isobutyl ketone (MIBK) followingthe method described by Subramanian and Meranger (1981). The total arsenic wasdetermined in an aliquot of 10 mL acidiﬁed with 3 mL of concentrated HCl (PerkinElmer, 1979).18.104.22.168. Forage. In the laboratory, roots were rinsed with tap water and scrubbedwith a laboratory nylon brush until all soil was removed, after they were rinsed sev-eral times with distilled water and ﬁnally with deionized water. Leaves and corn’ssilage were carefully washed with distilled and deionized water. The samples weredried at 60 ◦ C. Afterwards, were grounded in a blender and 1 g was digested with5 mL of concentrated nitric and 2 mL of perchloric acids (Perkin-Elmer, 1982).After digestion, the samples were diluted to 50 mL with 1.5% hydrochloric acid.22.214.171.124. Soil. Soil samples were dried at 60 ◦ C and grounded in an agate mortarand passed through a 0.177 mm sieve. To determine total arsenic, 0.1 g of groundedsoil was digested with 20 mL of concentrated nitric acid and 10 mL of 15 Nsulfuric acid, after the oxidation of organic matter was completed and fumes of
136 I. ROSAS ET AL.sulfur trioxide were produced, the samples were cooled and diluted to 50 mL withdeionized water (Thompson and Thoresby, 1977). For extractable arsenic, 5 g ofsoil were extracted by shaking with 20 ml of a mixture of 0.05N HCl + 0.025NH2 SO4 acids (Kahn et al., 1972; Perkin-Elmer, 1982). The solutions were ﬁlteredand made up to 50 ml with the extracting solution.126.96.36.199. Milk. Samples of 10 g of raw milk were digested with concentratednitric and sulfuric acids and 30% hydrogen peroxide, and diluted to 50 mL with20 mL of concentrated hydrochloric acid and deionized water.2.2.2. AnalysisA Perkin-Elmer Model 460 atomic absorption spectrophotometer equipped with aMHS-10 Mercury/Hydride System and a HGA-2100 graphite furnace was used.A Perkin-Elmer arsenic electrodeless discharge lamp was used as a light source.Measurements were made with deuterium background correction. To determine total arsenic in water, forage, soil and milk, 1 mL of 10% potas-sium iodide solution was added to 10 mL of sample or an aliquot diluted to 10 mL,after 60 min, arsenic was determined by the hydride evolution method with sodiumborohydride and the MHS-10 Mercury/Hydride System (Perkin-Elmer, 1979). Cal-ibration was performed by preparing series of standards adding speciﬁc chemicalsfrom the various matrices. The As (III) was measured by Graphite Furnace AtomicAbsorption Spectrometry (GFAAS) by injecting 20 µL of the MIBK phase intothe graphite tube. The concentration of As (V) in water samples was obtained bysubtracting the values of As (III) from the total arsenic concentrations.2.2.3. Quality ControlFor quality control, the total inorganic arsenic was determined in a Water Pollu-tion Quality Control Sample supplied by US Environmental Protection Agency,Environmental Monitoring and Support Laboratory-Cincinnati (WP 476E). Themean value found was 22.4 ± 2.7 µg L−1 (average of 7 determinations), thevalue reported by USEPA was 24 µg L−1 . Forage, soils and milk samples werespiked with known amounts of arsenic, prepared and analyzed in the same waythat the samples. Recovery in spiked samples ranged from 100–103% for forage,96–100% for soils (total and extractable arsenic) and 95–106% for milk. For eachgroup of processed samples, blanks (deionized water and reagents) were includedthroughout the entire sample preparation and analytical process.2.3. P HYSICOCHEMICAL ANALYSIS2.3.1. WaterThe samples were analyzed for pH, SO2− , Cl− , NO− , Ca2+ , and Mg2+ . pH was 4 3measured using a Philips Model PW 9409 digital pH meter with a combinationglass electrode. Sulfates, Cl− and NO− were determined by ion chromatography 3
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 137with a Perkin-Elmer instrument equipped with a Model 250 Isocratic LC Pump,a Conducto Monitor III conductivity detector and a strong anion analytical col-umn Hamilton PRP-X100. Sodium, Ca2+ and Mg2+ were determined by atomicabsorption spectrophotometry.2.3.2. SoilTexture analysis was performed by the hydrometer method (Palmer and Troeh,1980). pH was measured in a slurry (shaking 5 parts of distilled water and 1 partof soil during 15 min). The organic matter was determined by oxidation of 1 gof sample with 10 ml of 1 N K2 Cr2 O7 and 20 ml of concentrated H2 SO4 andtitration with 0.5 N FeSO4 .7H2 O and o-phenanthroline as indicator (Palmer andTroeh, 1980).2.4. S TATISTICAL ANALYSISFor the statistical analysis, whenever a value of an element was below detec-tion limit, the half of the detection limit was used for computation purposes. Theone-way Kruskal Wallis test (one-tail test) was used to compare the arsenic con-centrations in groundwater, soil, forage, and cow’s milk samples measured at dif-ferent farms. The relationship between arsenic levels found in soil, forage, milk,and water were calculated by the Spearman coefﬁcient correlation. The Mann-Whitney test (two-tail test) was applied to compare the probability of differencesbetween means of total arsenic concentrations of two soils horizons and washedand unwashed plants. The As biotransfer factor (BTF) to milk was calculated using Steven’s equation(Stevens, 1991): Concentration of As in milk (mg L−1 ) BTF (day L−1 ) = Mean daily animal intake of As (mg day−1 ) ·Holstein breed: body weight, 600–700 KgAnimal intake = 10 kg of food (dry matter), 65L water 3. ResultsThe highest As concentrations were found in the following counties: Francisco I.Madero with 740 µg L−1 , Tlahualilo, 590 µg L−1 and San Pedro, 490 µ L−1 (TableI). Ninety percent of the total arsenic was detected as As (V). The Kruskal-Wallistest (H=23.6, p=0.05) indicated signiﬁcant differences among the water As levelsmeasured at the sampled counties. The results of the physicochemical analysesof the well water samples indicated that the pH ranged from 7.8 to 8.2, but muchhigher differences were found among the analyzed ions. Chloride ranged from 21.3to 87.2 mg L−1 , SO2− from 182 to 428 mg L−1 , Na+ from 34 to 410 mg L−1 , Mg2+ 4from 3.4 to 36.4 mg L−1 and Ca2+ from 29 to 120 mg L−1 (Table II).
138 I. ROSAS ET AL. Figure 2. Concentrations of total arsenic in 0–30 cm and 30–60 cm soil horizons.
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 139 TABLE I Statistical summary of the total inorganic arsenic, As(III) and As(V) concentrations (µg L−1 ) determined in 73 wa- ter samples from deep artesian wells collected at the dairy farms located at the seven counties of Comarca Lagunera Countiesa N Arithmetic Standard Maximum mean deviation Arsenic (III) 1 12 0.67 0.25 1 2 15 8.33 5.12 19 3 6 5.83 5.04 12 4 26 1.08 1.18 5 5 3 6.67 – 9 6 3 0.50 – <1 7 8 0.68 1.13 2 Arsenic (V) 1 12 42.33 14.57 73 2 15 349 156 581 3 6 406 315 730 4 26 57.54 74.45 262 5 3 415 – 484 6 3 7.00 – 7 7 8 29.00 3.12 35 Inorganic total arsenic 1 12 42.67 52.19 74 2 15 357 160 590 3 6 411 319 740 4 26 58.23 75.58 265 5 3 422 – 490 6 3 7.00 – 7 7 8 29.65 3.10 35 a The county numbers are indicated in Figure 1. Total arsenic soil levels ranged from 11 to 30 µg g−1 (Figure 2). The main typeof soil in the sampled farms was sandy clay loam (Table III). The extractable ar-senic was not higher than 15%, with the highest values found in farms at Tlahualiloand Francisco I. Madero counties (Figures 3 and 4). The Mann-Whitney test showednon-signiﬁcant differences in total As between soils 0–30 and 30–60 cm depthhorizons (Z=0.50, p=0.05).
140 I. ROSAS ET AL. TABLE II Chemical parameters in groundwater samples from farms located in different counties at the Comarca Lagunera Countiesa SO2− 4 Cl− PO3− -P 4 Na+ K+ Ca2+ Mg2+ pH mg L−1 1 213 24.4 0.02 72.5 52.4 70.9 7.2 7.9 2 335 29.5 0.04 410 16.3 120 15.1 8.0 3 245 21.3 0.04 198 7.8 29.0 3.4 8.2 4 344 68.0 0.02 95.0 39.3 108 20.4 7.9 5 278 87.2 0.01 66.0 22.6 39.5 5.3 8.2 6 182 19.3 0.01 34.0 48.5 57.3 5.9 7.9 7 428 70.1 0.01 185 24.8 116 36.4 7.8 a The county numbers are indicated in Figure 1.Figure 3. Arsenic concentrations in washed alfalfa and extractable arsenic in soils from dairy farmslocated at different counties.
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 141Figure 4. Proportion of extractable arsenic in soil from alfalfa ﬁelds in farms located at differentcounties. TABLE III Physical and chemical characteristics of soil samples from alfalfa ﬁelds at the Comarca Lagunera Countiesa Soil texture O.M. Sand Silt Clay pH % 1 Sandy clay loam 1.4 26 40 34 8.4 2 Sandy clay loam 0.6 33 40 27 8.3 3 Sandy clay loam 0.5 33 36 31 8.3 4 Sandy clay loam 0.7 35 36 29 8.5 5 Sandy clay loam 0.9 27 41 32 8.3 6 Clay loam 1.2 27 34 39 8.3 7 Sandy clay loam 1.05 36 36 28 8.6 a The county numbers are indicated in Figure 1.
142 I. ROSAS ET AL. TABLE IV Values of total arsenic in milk (ng g−1 ) collected from farms located at different counties Countiesa N Arithmetic Standard Minimum Maximum mean deviation 1 6 1.63 1.64 <0.90 3.90 2 6 1.04 0.50 <0.90 1.50 3 6 8.59 11.75 <0.90 27.40 4 18 2.42 3.49 <0.90 14.80 5 3 5.18 – 2.50 6.70 6 3 2.60 – <0.90 5.05 7 8 1.37 1.24 0.90 3.87 a The county numbers are indicated in Figure 1. A higher percent of total arsenic was observed in alfalfa at root level than atthe aerial structures (Figure 3), but a good correlation (r=0.70, p=0.01) exist be-tween the arsenic in roots and extractable arsenic in soil 0–30 and 30–60 cm depthhorizons. The median of the washed and unwashed alfalfa corresponded to 0.9 and 1.6 µgg , respectively and those corresponding to corn’s silage were 0.5 and 1 µg g−1 , −1respectively (Figure 5). About 50% of the arsenic was removed by washing. Sig-niﬁcant differences (Z=3.3, p=0.05) were observed between washed and unwashedplants. Total arsenic concentrations from cow’s milk ranged from 0.9 to 27.4 ng g−1with the highest values in farms at Francisco I. Madero and Matamoros counties(Table IV). Signiﬁcant correlations (p=0.01) between arsenic (III), (V) and total inorganicconcentrations in water with extractable As in soil (0–30 cm depth horizon), withAs in alfalfa (leaves and roots) and with As in milk were observed (Table V). The biotransfer factor calculated for arsenic in milk was 10−4 , except for Tlahualilocounty, with 10−5 . It was calculated based on the cow’s average daily intake ofarsenic that ranged from 4 to 38.8 mg day−1 (Table VI). 4. DiscussionHigh concentrations of arsenic in water, air and soil have been associated withindustrial wastes or natural processes (Borgoño et al., 1977; Whanger et al., 1977;Sims and Kline, 1991; Retana et al., 1993; Santander et al., 1994; Nakadaira et al.,1995; Hwang et al., 1997). Due to the toxicity of As to plants, animals and human,
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 143Figure 5. Box plot of arsenic concentrations in washed (1) and unwashed (2) of alfalfa and foragesamples. The triangle symbol marks the minimum and maximum values. The symbols in bottom andtop of the vertical line mark the 5th and 95th percentiles, respectively. The circle symbol, medianline, the bottom and the top in the box mark: the arithmetic mean, 50th, 25th and 75th percentiles,respectively.it is desirable to understand its behavior in the environment (Vahter and Morin,1980; Nissen and Benson 1982; USEPA, 1984; Wu et al., 1989; Sheppard, 1992). In the Comarca Lagunera, Mexico, high levels of arsenic have been detected inground water, used not only as drinking water for humans and dairy cattle, but alsofor agricultural irrigation. Forty percent of the water samples from dairy farms hadtotal arsenic contents above the USEPA (1980) and DOF (1996) National DrinkingWater Standards (50 µg L−1 )], but only 5 samples were below the WHO (1993)standard of 10 µg L−1 . Arsenic contents below the drinking water standard werefound only in one farm of Torreón. The arsenic concentrations measured in thestudied area were lower than those reported at other zones of Mexico, like Zimapan,a Village in Hidalgo (up to 1.09 mg L−1 ) (Armienta et al., 1997a). On the otherhand, As levels at Francisco I. Madero, Tlahualilo and San Pedro farms are withinthose values reported in Antofagasta, Chile, where skin cancers were described
144 I. ROSAS ET AL. TABLE V Linear correlation coefﬁcients between arsenic concentration in water and those from soil, alfalfa crops and milk Groundwater As (III) As (V) Inorganic total As Soil total As, 0–30 cm –0.289 –0.056 –0.046 total As, 30–60 cm –0.188 –0.145 –0.152 extractable As, 0–30 cm 0.494a –0.553a 0.539a extractable As, 30–60 cm 0.137 0.214 0.229 Alfalfa: stem 0.475a 0.509a 0.504a root 0.546a 0.559a 0.571a Milk 0.525a 0.510a 0.507a a Signiﬁcant at the 1% level (N=43). TABLE VI Estimated biotransfer factor to milk collected from dairy farms at the Comarca Lagunera Countiesa Forage Water Intake Milk BTF mg kg−1 mg L−1 mg day−1 µg L−1 1 0.60 0.042 8.7 1.7 1.9* 10−4 2 1.34 0.36 36.6 1.2 3.2* 10−5 3 1.21 0.41 38.8 8.9 2.2* 10−4 4 1.20 0.06 15.6 2.6 1.6* 10−4 5 0.88 0.42 36.1 5.3 1.4* 10−4 6 0.40 0.007 4.0 2.7 6.7* 10−4 7 1.03 0.30 12.1 1.4 1.1* 10−4 a The county numbers are indicated in Figure 1.among the residents, who had been drinking water containing from 0.05 to 0.96mg L−1 of As; and in southwestern Taiwan, where the As level ranged from 0.01to 1.82 mg L−1 , in artesian water (Morton and Dunnette, 1994; Tseng, 1977). Ithas been established a consensus of a threshold value of 100 µg L−1 (tropicalconsumption) for arsenic disease (Stöhrer, 1991).
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 145 Arsenic-concentration in the studied area, showed an increase of the As to-wards the north of the Comarca Lagunera, with lower levels at its center. Theobserved variations may be due to differences in the geology, the hydrogeologyand the abstraction regime of this area. The arsenic enrichment has been ascribedto magmatic processes giving rise to a hydrothermal system with high contentsof lithium, boron, arsenic and ﬂuoride (González-Hita et al., 1991). Differencesin arsenic concentrations have been found also at Zimapán, related to the variousarsenic sources (natural and anthropogenic) and to the hydrogeology of the valley(Armienta et al., 1997a). According to the results of pH and of major ions concentrations in the watersamples, the Souliné classiﬁcation was used (Custodio, 1983). Most of the watersamples pertain to the sulfate-sodium type and some of them to the bicarbonate-sodium type. The pH results, of 7.5 to 8.4, also indicate that precipitation re-actions including As are not likely to occur. It seems that this element is nottotally eliminated from water (Cherry et al., 1979). However, it is necessary tocomplete the data with more analyses of major and minor species to fully supportthis conclusion. None of the samples exceeded the allowable limit (1.0 mg L−1 ) of arsenic forwater used in irrigation (USEPA, 1973). Ground water is also consumed by dairy cattle at the Comarca Lagunera, whichis one of the most important dairy cows raising zone in Mexico. The arsenic con-centrations in the water could affect human health through milk intake, since theallowable limit for water used to feed cattle is 0.05 mg L−1 (USEPA, 1973). Total arsenic concentrations in soils from alfalfa ﬁelds at the dairy farms variedfrom 11 to 30 µg g−1 . These values are within the average normal concentrationsfor arsenic in agriculture soils in the USA (from 1 to 40 µg g−1 ) (Walsh et al.,1977), and lower than 50 µg g−1 , considered as guideline for soil pollution (Min-istry of Environment, 1990). Nevertheless, the As average concentrations are above6 µg g−1 reported for non-polluted soils (USEPA, 1985), and exceeded the 10 µgg−1 suggested as background (O’Neil, 1995; Sheppard, 1992). Arsenic in soils is normally bounded to clay containing amorphous aluminumand iron oxihydroxides. The mobility of arsenic depends on the kind of soil, pH,and on the iron, aluminum and phosphate contents (Pierce and Moore, 1982; Mokand Wai, 1994). It has been observed that a drop in pH and an increase in fulvicacid increases arsenic leaching (Bowell, 1994). At the Comarca Lagunera the soil ismostly clay, with a low content of organic matter and a alkaline pH (> 8.0). Thesecharacteristics explain the low percentage of extractable arsenic with respect tototal arsenic. It seems that arsenic is mostly retained on the silt and clay fractionsof the soils which constitute more than 50% of their granulometry. Nevertheless,some differences were observed among the sampled farms, the highest proportionsof extractable arsenic were found at Tlahualilo, F. I. Madero and San Pedro farmswith soil samples containing more than 30% of sand.
146 I. ROSAS ET AL. Arsenic pollution in soils may have effects on vegetation and easily affects theanimals that graze on grass. The observed concentrations of total arsenic in soils,although above the EPA values, are within those reported for agricultural soils inthe United States (Walsh and Keeney, 1975). The plant damage has been used as indicator to determine the critical concen-tration of soluble As in soils. Above 1 µg g−1 cause damage to crops (Huang,1994). The extractable arsenic at La Comarca Lagunera does not seem to representan important risk for the vegetation. However, some plants can tolerate higherconcentrations of arsenic. For plants to reach levels of 1 mg kg−1 of As on freshweight basis, soils levels must exceed 200 to 500 mg kg−1 (Aten et al., 1980). Onthe other hand, some crops can accumulate high levels of As even at much lowerlevels of arsenic in soil. For instance, alfalfa and grass accumulate up to 14 mgkg−1 of As when growing in soils containing 60 mg kg−1 of As (Bhumbla andKeefer, 1994). At the Comarca Lagunera, alfalfa and corn’s silage As concentrations weresimilar to the arsenic content in grass (as much as 3 mg kg−1 ) growing in soilscontaining up to 20 mg kg−1 of As (O’Neill, 1995). The As distribution in theplants was in general similar to that observed in other studies, with higher As levelsin roots than in stem and leaves. Variations in arsenic concentrations in plants were also observed for the variousfarms, related to As contents in soils. Higher As values were measured in plantsgrowing in soils with higher extractable arsenic. The samples from counties, 2, 3,4 and 5, corresponding to soils with higher peoportion of sand, had the highestAs levels. In general, lower levels have been found in plants growing on clays andsilts, with higher clay, minerals and Fe/Al oxide content, than in plants growing inlighter soils, e.g. sands or sandy-loam soils (O’Neill, 1995). This behavior reﬂectsthe fact that the composition of vegetation depends on the availability of an elementin the vicinity of the root system and the ability of the plant to absorb, transport,and accumulate the element (Lintern et al., 1997). The adsorption of metals fromthe liquid phase to the solid phase controls the concentrations of metal ions andcomplexes in the soil solution and thus exerts a major inﬂuence on their uptakeby plant roots (Alloway, 1995). The high proportion of sand in counties 2, 3, 4and 5, implies lower adsorption of arsenic and higher availability for the plants. Asoil-plant transfer coefﬁcient (metal concentration in the plant divided by the totalmetal content of the superﬁcial soil) from 0.02 to 0.09 was calculated, this valuesagree with the reported coefﬁcients at other places (Alloway et al., 1988; Alloway,1995). Alfalfa and corn’s silage were analyzed because they are important componentsof the diet of dairy cattle. The arsenic present in soil was absorbed by alfalfa cropand 37% of the samples were higher than 2.6 µg g−1 , considered as permissiblelimit for edible crops by US Public Health Service (Jones and Hatch, 1945). Ithas been reported an important accumulation of arsenic at root level (Sheppard,et al., 1985; Sheppard, 1992; Retana et al., 1993). The average root/stem-leaves
ARSENIC CONCENTRATIONS IN WATER, SOIL, MILK AND FORAGE 147ratio was of 1.7. However, almost the whole plant (with part of the roots) is usedas forage for dairy cattle including the absorbed arsenic associated to soil particles,this represented about 30% of the total arsenic in plants. The fact that the heavy metals have been capable of translocating into bovinemilk has been previously reported (Sharma et al., 1982; Stevens, 1991). The Co-marca Lagunera is one of the most important dairy producers in Mexico. Therefore,it is important to determine the contribution of arsenic from agricultural food chainto bovine milk. Ten percent of the milk samples had a concentration of As > 10 ngg−1 , suggested as permitted arsenic level (International Dairy Federation, 1986).The biotransfer factor has been calculated based in metal levels in food, but inthis study the arsenic present in drinking water was included, and factors up to 6.7× 10−4 , were higher than those reported in an experimental study (1.1 × 10−5 )(Stevens, 1991). It is important to consider that more than 30% of the arsenic measured in plantswas adsorbed by leaves and probably only a very low proportion of this is accessi-ble to cattle by ingestion (Hwang et al., 1997). Also, extractable data suggest thatthe inorganic arsenic absorbed by plants may be converted to organic arsenic com-pounds. Thus, methylation of arsenic in vegetables should substantially reduce theestimated risk associated with the food ingested dose (Nissen and Benson, 1982;Pyles and Woolson, 1982). In contrast, the solubility of arsenic in water, mainly asAs (V), determine that this could be the most important source of arsenic exposurein the studied area. AcknowledgmentsWe acknowledge to Hugo Padilla G. and Pilar Fernández for the review of themanuscript and his valuable comments. Also to CONACYT for the partial ﬁnancialsupport. ReferencesAlloway, B. J., Thornton, I., Smart, G. A. Sherlock, J. C. and Quinn, M. J.: 1988, Sci. Tot. Environ, 91, 41.Alloway, B. J.: 1995, in Alloway, B. J. (ed.), Heavy Metals in Soils, Blackie Academic and Professional, London.Armienta, M. A., Rodríguez, R., Aguayo, A., Ceniceros, N., Villaseñor, G. and Cruz, O.: 1997a, Hydrogeology J. 5, 39.Armienta, M. A., Rodríguez, R. and Cruz, O.: 1997b, Environ. Cont. Toxicol. (in press).Aten, C. F., Boourte, J. B., Martini, J. H. and Walton, J. C.: 1980, Environ. Toxicol. 24, 108.Borgoño, J. M., Vincent, P., Venturino, H. and Infante, A.: 1977, Environ. Health Perspect. 19, 103.Bowell, R. J.: 1994, Applied Geochemistry 9, 279.Bhumbla, D. and Keefer, R.: 1994, in Nriogu, J. (ed.), Arsenic in the Environment. Part I, John Wiley and Sons, New York.
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