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Metals and grain size distributionsin soil of the middle Rio Grandebasin, Texas USAV.K. Sharma 7 K.B. Rhudy 7 J.C. Cargill 7 M.E. Tacker 7 F.G. Vazquez

Received: 14 December 1998 7 Accepted: 19 Jaunuary 1999

V.K. Sharma (Y), K.B. Rhudy, J.C. Cargill, M.E. TackerDepartment of Chemistry, Texas A&M University-CorpusChristi, 6300 Ocean Drive, Corpus Christi, Texas 78412, USAFax: c512-994-2742

F.G. VazquezInstituto de Ciencias del Mar y Limnologia, UNAM,Universotaria, D.F., Apartado Postal 70–305, C.P. 04510,Mexico

Abstract This paper deals with the problem of in-creased heavy metal constituents in agriculturalsoils due to the expanded use of fertilizers and ele-vated atmospheric deposition. It discusses the ex-tent of contamination in soil and establishes an en-vironmental monitoring program in the chosenarea of concern in the southern coastal region ofTexas. Grain size, pH, and metals (Cu, Cd, Zn, Pb,Ni, Ba, As, Cr, Mn, and Fe) were determined insoils of the middle Rio Grande basin. The soilswere mainly of sand texture and alkaline in charac-ter. Fine sand constituted the major proportion ofthe soil, and clay and silt ranged from 8–30% ofthe soil. Correlations of metal concentrations tograin size and iron contents were performed. Me-tals, except Cd and Pb, gave positive to negative re-lationships with decreases in grain size. Silt gave norelationship with metal content while clay and silthad a positive relationship. All these metals had apositive correlation with iron in the soil. The re-sults indicate metals are associated with coarsesand, clay, and iron hydroxides surfaces of the soil.The comparison of metal content in soil of themiddle Rio Grande basin with metals from otherareas of the world suggests that it is relatively un-contaminated.

Key words Metals 7 Grain size 7 Rio Grandebasin 7 Contamination

Introduction

Heavy metal constituents in agricultural soils of indus-trialized countries have increased due to the expandeduse of fertilizers (inorganic, organic and sewage sludge),and elevated atmospheric deposition (Nriagu 1979; Nria-gu and Pacyna 1988). Many of these metals (e.g., Fe, Mn,Cu, and Zn) are vital to the nutritional budget of plantsand animals (Davis and Leckie 1978; Jones 1991). Howev-er, metals in high concentrations can be toxic and mayenter the food chain through leaching to water supplies.The extent of contamination in soil is usually measuredby determining the total concentration of metals andthen comparing with the metals in uncontaminated soil.The research to better understand the chemistry, toxico-logy, and food-chain transfer of metals in contaminatedareas has been extensive over the last two decades (Al-berici and others 1989; Kabata-Pendias and Pendias1992). In the last few years, an environmental monitoringprogram for understanding heavy metal concentration inthe populated coastal environments of the Gulf of Mexicohas been established (Summers and others 1996). Theprimary goal of this program is to monitor pollution in-dicator exposure, and biotic and habitat conditions. Onearea of concern is the Rio Grande basin located in thesouthern coastal region of Texas. Its ecological health isof interest because of anthropogenic inputs from agricul-tural harvesting, mineral production, petroleum produc-tion, and military activities.This paper provides results of grain size distributions andconcentrations of metals (Cu, Cd, Zn, Pb, Ni, Ba, As, Cr,Mn, and Fe) in soil samples collected in the middle RioGrande basin. The relationship of metal concentrations tograin size was determined to evaluate the influence thatthe surface area had on controlling the metal concentra-tions. In addition, metal concentrations were normalizedto iron to better assess the metal enrichment in soil sam-ples of the middle Rio Grande basin.

Experimental

Study AreaThe studied area of the Rio Grande basin is shown inFig. 1. The basin is bordered on the north and west by

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Fig. 1Geographic setting and location ofsampling sites

the Nueces River basin, and the Rio Grande River on thesouth. To the east lie bays, estuaries, the Padre IslandNational Seashore, and the Gulf of Mexico.

SamplingSoil samples at 42 sites (Fig. 1) were collected using anacid-cleaned Teflon-coated scoop. Prior to each use, thescoop was first cleaned with acid and then rinsed withMilli-Q water. The upper 5 cm of soil was removed bythe scoop and placed into acid-cleaned plastic bags. Theplastic bags were stored in the dark and then transportedto a Texas A&M University-Corpus Christi laboratory foranalysis.

Analytical techniquesGrain size analysis of the soil was carried out by dry-sieving using a Ro-Tap shaker and sieve sizes from 1.5 to14.5 F (Folk 1974). Each sieve fraction was weighed andthe weight percent of the fraction calculated. The pH ofthe soil was determined by adding 50 ml of Milli-Q waterto 5 g of soil and agitating for 30 min. The pH of the su-pernate was determined (Schofield and Taylor 1955).The acid digestion procedure was used for metal analysisof the soil. Briefly, approximately 1.0 g of the samplewhich had been dried at 60 7C, pulverized, and thorough-ly mixed was transferred to a beaker. Then, 10 ml of 1 : 1nitric acid was added to the soil sample. This sample was

heated at 95 7C and was reflexed for 10 min. The samplewas allowed to cool for 5 min and then 5 ml of concen-trated nitric acid was added. This mixture was further re-flexed for 30 min while maintaining the solution volumeabove 5 ml. The second reflux was followed by adding2 ml of Milli Q water and 3 ml of 30% hydrogen perox-ide. This solution was heated until the effervescence sub-sided and allowed to cool. After cooling, the sample wasfiltered and diluted to 100 ml with 2.5% (v/v) hydrochlor-ic acid and 5% (v/v) nitric acid prior to analysis.All metals except Fe were analyzed using ICP. A ThermoJerrel Ash Trace Scan ICP was used. Standard solutionsof SPEX, Certiprep were used to prepare calibration andstandard solutions. Iron in the samples was determinedby using Ferrozine technique (Stookey 1970). The accura-cy of determination and precision between replicateswere within B10%.

Results and discussion

Figure 2 shows the summary results of grain size distri-bution of soil samples collected in the middle Rio Grandebasin. Grain size is expressed in phi (F) units and thelarger the number, the smaller the grain size. Soil wasfound to be mainly of sand texture. Fine sand (3–4 F)constituted the major proportion of the sample. The clay

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Fig. 2Grain size analysis

Fig. 3Correlation of Cu concentrations against grain size

and silts ranged from 8–30% of the sample. Similargrain-size distributions have been found in other southTexas basins (White and others 1986).The pH values shown in Table 1 were between 6.28 and8.56. Most of the soil pH values were above 7.50 suggest-ing a slight alkaline nature of the middle Rio Grande ba-sin soil. The concentrations of metals are presented inTable 1 and are reported as milligram of metal per kilo-gram dry weight. Iron concentrations in soil are inweight percent and vary from 1.09–4.97% with a medianvalue of 2.48%. In comparison, Fe concentrations gener-ally vary widely in soil and range from ~1% to 120%with the median concentration of approximately 3% (Mu-rad and Fisher 1978).Some sites of the study area had anomalously high con-centrations of metals (Table 1). The concentrations of Bawere high at sites 23, 31, and 39 (608–856 mg kg–1) dueto drilling activities and the associated use of barite(BaSO4) in the drilling muds. Holmes (1986) has ob-served correlation between the location of oil and gaswells and high concentrations of Ba in south Texas Bays.Copper was high at site 29 (76.7 mg kg–1), while As washigh at site 30 (14.5 mg kg–1). The mineral deposits atthese sites were prominent and may explain the elevatedlevels. Concentrations of Ni were exceptionally high atsites 20, 21, and 29 (43.1–130 mg kg–1). Manganese wasalso high at site 21 (542 mg kg–1). The Mn-containing

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Table 1Concentration of metals (mg kg–1 dry weight) in soil of middle Rio Grande basin, Texas USA

Site pH Cu Cd Zn Ni Pb Ba As Cr Mn Fe (%)

1 8.56 3.01 0.31 26.5 5.87 9.87 63.0 2.01 5.12 22.3 1.672 6.84 2.79 ~0.01 21.5 3.54 3.77 23.2 1.47 2.43 42.2 1.163 7.53 4.46 ~0.01 38.7 8.53 6.54 115 4.07 9.06 76.2 3.734 6.28 30.5 0.02 31.4 5.80 4.75 61.2 3.84 8.44 45.9 3.125 7.87 2.46 0.06 40.8 5.33 5.34 235 7.58 7.04 88.5 4.036 8.33 7.61 0.79 40.9 10.9 8.91 141 4.84 5.90 74.9 2.287 8.21 8.90 0.15 38.9 15.6 4.77 305 2.67 21.7 296 2.738 7.69 1.69 0.16 177 4.05 3.24 133 4.34 4.28 45.8 3.019 7.18 3.43 ~0.01 43.5 6.61 3.28 179 1.76 8.62 36.9 2.38

10 8.01 16.7 0.11 43.4 9.09 2.45 235 4.74 10.9 245 2.8811 7.85 3.42 ~0.01 40.5 7.54 2.53 97.7 2.35 10.8 32.7 2.2012 7.96 3.47 ~0.01 36.2 6.29 4.07 83.5 1.34 3.72 61.0 2.5013 7.92 3.67 ~0.01 20.5 4.03 2.82 56.2 0.91 3.30 27.4 1.0914 6.52 5.51 0.38 30.4 6.06 3.05 130 1.06 7.34 36.6 1.1415 8.19 5.49 0.11 37.4 5.78 2.29 126 4.01 3.35 79.3 2.5316 7.03 5.70 ~0.01 20.3 7.06 2.41 51.1 0.47 2.35 11.1 0.9817 8.10 3.83 0.43 24.2 4.36 3.64 81.6 0.87 4.04 74.5 1.9018 8.24 7.34 0.91 39.5 7.30 2.98 110 2.68 4.96 101 1.7419 8.38 5.42 ~0.01 32.4 5.47 2.09 87.9 0.92 4.11 33.4 1.5719 A 7.50 6.16 ~0.01 55.3 6.70 1.57 138 0.92 5.05 51.5 1.8120 6.39 42.0 0.02 86.8 43.6 2.03 199 2.93 6.54 157 2.4621 8.16 9.26 0.74 42.2 43.1 3.58 282 2.84 5.33 542 2.4622 8.37 12.5 0.80 68.6 12.4 4.35 240 3.88 9.40 186 3.0923 8.13 11.7 0.04 42.5 9.82 2.80 608 4.01 8.80 239 3.4124 8.26 10.2 0.03 35.7 8.77 3.66 257 5.07 5.72 268 2.5725 7.84 4.98 2.07 24.3 4.31 1.09 71.5 0.53 4.81 43.1 1.9526 7.78 7.39 ~0.01 43.6 5.55 2.22 114 1.47 4.78 104 2.0127 8.09 14.4 0.61 50.9 8.85 3.61 287 4.37 5.38 263 3.1128 8.03 13.0 ~0.01 32.2 8.00 2.95 106 2.20 6.45 195 2.5629 8.00 76.7 0.14 318 130 1.35 450 2.94 9.85 217 3.0530 7.74 14.3 0.05 469 9.38 7.41 244 14.5 6.05 168 2.6531 7.06 16.9 ~0.01 72.6 12.1 2.44 856 3.08 8.82 161 2.9432 8.03 8.90 0.20 88.6 6.41 2.61 240 2.58 5.20 127 1.8533 8.04 10.8 ~0.01 54.3 8.29 1.13 207 1.99 7.96 199 2.8434 8.07 9.14 0.03 96.2 7.61 2.61 158 1.53 6.26 95.3 2.4735 8.03 4.08 ~0.01 24.5 4.57 0.64 135 1.71 3.98 53.7 1.4336 8.33 12.5 1.60 38.6 8.84 4.02 366 4.36 7.59 200 2.8337 8.33 6.59 ~0.01 25.3 5.44 2.37 250 1.79 4.29 149 1.7838 8.22 12.3 ~0.01 48.3 9.47 2.98 421 3.83 6.74 220 2.6039 8.31 10.9 0.07 38.8 7.80 2.99 707 4.14 6.99 222 2.9640 8.10 8.95 0.01 36.8 14.5 1.93 263 1.98 60.8 252 2.2142 8.00 15.5 0.16 48.5 11.0 5.33 557 8.91 8.02 399 4.97

Table 2Correlation coefficients for regression of soil metal content against grain size fractions

F Cu Cd Zn Ni Pb Ba As Cr Mn Fe

1.5 0.80 0.23 0.32 0.54 0.19 0.67 0.53 0.47 0.76 0.642.0 0.69 0.17 0.28 0.51 0.24 0.62 0.51 0.41 0.52 0.522.5 0.07 0.03 0.08 0.08 0.26 0.08 0.09 0.14 –0.04 0.083.0 –0.68 –0.10 –0.41 –0.44 0.05 –0.64 –0.45 –0.28 –0.55 –0.704.0 –0.75 –0.16 –0.34 –0.56 –0.23 –0.67 –0.51 –0.52 –0.57 –0.744.5 –0.10 –0.12 0.19 –0.08 –0.29 0.15 –0.12 –0.07 0.00 0.0514.5 0.61 –0.09 0.49 0.52 –0.07 0.62 0.40 0.41 0.49 0.64

minerals were noticed at the western part of the basin(sites 21–42) and it appears site 21 had high Mn-contain-ing minerals. Chromium concentrations were high at

sites 7 and 40 (21.7 and 60.8 mg kg–1, respectively) andman-introduced contamination may be the cause of highlevels of Cr. Sites where anomalies in metal concentra-

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Fig. 4Correlations of metal concentrations with Fe

tions were observed, as described above, were not consid-ered for correlation analysis.The results of correlation analysis against grain size frac-tions are presented in Table 2 and are shown in Fig. 3 forCu. The correlation of metals with Fe is shown in Fig. 4.The correlation of copper against grain size (F) in sandgave positive to negative relationships with increasing F(coarse to fine sand; Table 2, Fig. 3). No relationship be-tween Cu and silt was found. However, a positive rela-tionship of Cu concentrations with silt and clay contentwas found. This suggests that Cu is positively associatedwith clay in the study basin. A similar trend was seen forBa, Mn, and Fe (Table 2). Metals are generally correlatedwith the finest granulometric class of clay in soil (Leoniand Sartori 1997). Copper did not show a strong relation-ship with Fe (rp0.53). This indicates that Cu may also be

associated with the organic fraction of the soil. Organicmatter in soils can accumulate Cu by adsorption or com-plexation (Stevenson 1982). Therefore, Cu is considered amobile cation which interacts with soil mineral and or-ganic components (Kabata-Pendias and others 1992).Zinc, Ni, As, and Cr showed a similar pattern againstgrain size as was found with Cu but with smaller correla-tion coefficients (r^0.55) (Table 2). The correlations withFe were also not strong (r^0.60) except As which had arelatively stronger correlation (rp0.83). The results sug-gest that Zn, Ni, and Cr are accumulated in inorganicand organic fractions of the soil. The complexation of Znwith organics has been observed in soil at neutral and al-kaline pH range (Pitchtel and others 1997). The total or-ganic carbon (TOC) may therefore provide chelation forZn in soils of the basin. The chemistry of As is similar tophosphorous and interacts strongly with clays and ironoxyhydroxides.Cadmium and Pb concentrations were independent ofsand, silt, clay, and Fe content of the soil. This indicates

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Table 3Typical mean metal concentrations (mg kg–1 dry weight) in soils of the world

Soils Cu Cd Zn Ni Pb Cr Reference

Middle RioGrande basin,Texas USA

11.4 0.49 62.3 12.3 9.09 8.04 This study

Northeast USA 40.7 0.32 57.5 23.7 24.8 60.3 Frink (1996)2886 USASurface

29.6 0.26 56.9 23.9 12.3 P Holmgren andothers (1993)

Western Europe 19.6 0.78 68.1 27.1 39.1 57.1 Kabata-Pendiasand others(1992)

Central-EasternEurope

17.5 0.51 P 18.1 32.1 69.3 Kabata-Pendiasand others(1992)

World soils 20.0 P 50.0 40.0 10.0 200 Angelone andBini (1992)

that the occurrence of Cd and Pb in soil is controlled bysources other than aluminosilicates and iron hydroxides.Other sources may include mining activities and automo-bile atmospheric deposition (Engel and others 1996; Pageand others 1987).The comparison of metal concentrations in soil of thisstudy area with soils of the world is presented in Table 3.The concentration of metals in the middle Rio Grandebasin were equal or less than the metal contents in otherareas of the world. This suggests that middle Rio Grandebasin soil is relatively uncontaminated.The enrichment factors (EF) were calculated using aver-age soil values of Bowen (1979). The EF is the ratio be-tween metal/Fe in the sample and metal/Fe in averagesoil. The concentrations for Cu, Zn, Ni, Pb, Cr and Fe inaverage soil are 30, 90, 50, 19, 70, and 39,800 mg kg–1

(Bowen 1979). The EFs in the study area samples werenearly equal or less than 1.0. This implies that metals inthe middle Rio Grande basin are depleted relative to av-erage soil concentrations.

Conclusion

The middle Rio Grande basin soil ranges in size fromsand to clay. Sand constitutes approximately 75% of thesoil. The concentrations of metals, except Cd and Pb, arecontrolled by aluminosilicate, iron hydroxides, and or-ganic fractions of the soil. The probable sources of Cdand Pb include mining and automobile atmospheric dep-osition.

Acknowledgments The authors wish to thank the Ed RachalFoundation for support of this work. We wish to acknowledgeRichard Hay for stimulating discussion on this project.

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