spatial distribution of heavy metals in the soils of erath ...

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Leaded gasoline was banned in the United States in the 1980's. Today, methylcyclopentadienyl manganese tricarbonyl (MMT), an organometal, is used as an ...
STUDIA UNIVERSITATIS BABE -BOLYAI, GEOGRAPHIA, LIV, 2, 2009

SPATIAL DISTRIBUTION OF HEAVY METALS IN THE SOILS OF ERATH COUNTY, TEXAS M. DIA1, D. WEINDORF2, C. THOMPSON1, H. CUMMINGS1 H. CACOVEAN3, T. RUSU4 ABSTRACT. – Spatial Distribution of Heavy Metals in the Soils of Erath County, Texas. The presence of heavy metals in soils is a potential threat to plants, animals, humans and the environment. The soils of Erath County, Texas were examined to determine the spatial variability of heavy metals (Pb, Mn, Zn, and As) near the major highways (US-281, 377, 67, and State Highway 8 and 108) as well as unpaved county roads. It is hypothesized that heavy metals generated from combustion of motor fuel have accumulated near roadsides. However, their persistence in the soil varies with the distance from the road edge, direction of prevailing wind, traffic density, and type of road. Soil samples were collected along both sides of the road at variable distances: 25, 50 and 100 m from the road edge.The high concentration of Pb, Mn, and Zn in roadside soil was found to be associated with traffic related activities. The distribution of Pb, Mn, and Zn in roadside soil is related to traffic density, and distance from the road edge. The prevailing wind also had a significant effect on the accumulation pattern of Pb and Mn in roadside soil. Although As in roadside soil was higher than typical background levels, As distribution was not influenced by traffic density, distance from the road edge, or direction of prevailing wind. Rather, observed differences were attributed to the nature of the soil parent material. Keywords: Automobile; Gasoline; Heavy metals; Roadside soil; Erath County, TX.

1. INTRODUCTION According to the United Nations, the world’s population is expected to grow from 6.5 billion in 2005 to 9.2 billion in 2050. As the world’s population continuous to grow, it becomes increasingly important to understand the dynamic interaction between human activities, their immediate environment, and its quality for human health. Environmental contamination has become particularly important since it is gradually expanding from a local level to a regional level and will eventually become a global concern (Medvedev, 1999; Lopez et al., 2000). The rapid urbanization, growing transport intensity, and numerous human activities including municipal, industrial, commercial, and agricultural operations have created a problem of heavy metal contamination globally (Nirangu and Pacyna, 1988). The term “heavy metal” is a group of metallic chemical elements with a high density (>5.00 g cm-3) (Epstein, 2003). The most toxic heavy metals are arsenic (As), cadmium (Cd), chromium (Cr), nickel (Ni), lead (Pb) and Zinc (Zn) (Mazvila, 2001; Navas and Lindhofter, 2005). 1

Tarleton State University, Stephenville-76401, Texas, USA, e-mail: [email protected]; LSU AgCenter, Louisiana State University, Baton Rouge-70803, Louisiana. USA, e-mail: [email protected]; 1 Tarleton State University, Stephenville-76401, Texas USA, e-mail: [email protected]; 1 Tarleton State University, Stephenville-76401, Texas, USA, e-mail:[email protected]; 3 O.S.P.A. Cluj, Str. Fagului nr. 1, Cluj-Napoca, 400483, Romania, e-mail: [email protected]; 4 Universitatea de tiin e Agricole i Medicin Veterinar Cluj-Napoca, Romania, e-mail: [email protected] 2

M. DIA, D. WEINDORF, C. THOMPSON, H. CUMMINGS, H. CACOVEAN, T. RUSU

According to the Korte index, which indicates hazards to environmental quality, heavy metals are among the worst pollutants (Stravinskiene, 2005). Heavy metals are nonbiodegradable and long-term contaminants with the ability to accumulate in soils and plants (White and LeTard, 2002). Elevated emissions and their temporal deposition cause metal contamination, particularly in soils, that can significantly amplify the exposure of human, plant, and animal populations. The excessive exposure of humans to heavy metals via inhalation, ingestion, and dermal contact causes toxic effects (Mielke et al., 1999). It is generally accepted that children represent the most sensitive group (Shen et al., 1996). The exposure of children to heavy metals has toxicological effects on their physiology, development of vital organs, behavior, and nervous system (Hrudey et al., 1996). In terrestrial ecosystems, heavy metals come from different natural and/or anthropogenic sources. Natural processes include geological weathering of parent material, volcanic activities, and/or sedimentation. Anthropogenic activities include traffic related activities (fossil fuel combustion, wear of vehicular parts, and leakage of metal-containing motor oils), industry specific activities, the disposal of municipal waste (incineration and landfill), and the corrosion of construction/building material (Councell et al., 2004; Nadal et al., 2004). Heavy metals can accumulate in topsoil from atmospheric deposition by sedimentation, impaction, and interception. Therefore, soil serves as the most important sink for heavy metal contamination in the terrestrial ecosystem (Xiangdong et al., 2001). However, the distribution and persistence of heavy metals in soil is largely governed by factors including metal solubility, physicochemical soil properties, and other environmental factors (Hernandez et al., 2003). Several environmental studies have shown that heavy metal accumulation in soils and plants near the roadside is due to atmospheric deposition and traffic related activities (Uminska, 1988). Such contamination is generally attributed to the combustion of leaded gasoline and the consequent release of lead particles through automobile exhaust (Smith, 1976). Leaded gasoline was banned in the United States in the 1980’s. Today, methylcyclopentadienyl manganese tricarbonyl (MMT), an organometal, is used as an anti-knock agent in lieu of tetraethyl lead in once used in leaded gasoline. However, crude oil contains trace amounts of over 30 elements, including As, Cd, Co, Cu, Hg, Mn, Mo, Ni, Pb, V and Zn (Davydova, 2005). The content of these heavy metals in gasoline is quite stable and is capable of forming chelates and -complexes with petero-organics (Caroli, 2000). In addition to fuels, Cd and Zn are found in automobile tires which wear down while driving on roads. Therefore, the levels of these heavy metals are anticipated to rise in roadside environments, due to fossil fuel combustion and motor vehicle tire wear. According to the Texas Department of Transportation, the number of vehicles registered in the state of Texas has increased from 4,087279 in 1957 to 20,284,709 in 2007 (L. Buddie, Personal communication, 2008). In Erath County, TX, the number of vehicles has increased 396% in last five decades. Thus, the consumption of gasoline has also significantly increased. Through motor fuel combustion, tire wear, and leakage of auto lubricants, heavy metals accumulate in roadside soils and because of their nonbiodegradable nature, likely persist in soils for long periods of time. However, the influence of prevailing winds, distribution pattern of metals, influence of traffic density on metal accumulation, and length of heavy metal persistence in roadside soils remain largely unknown. The objective of this study was to assess the spatial variability of heavy metals in the soils of Erath County, TX by determining the variation of Pb, Mn, Zn and As in roadside soils with respect to traffic density, distance from the roadside, and direction of prevailing winds. 100

SPATIAL DISTRIBUTION OF HEAVY METALS IN THE SOILS OF ERATH COUNTY, TEXAS

2. MATERIALS AND METHODS 2. 1. Area description and sampling The study sites were randomly selected roadside soils located in the Erath County, TX. A total of 144 samples across 20 locations in Erath County were collected (Figure 1). Eleven sampling locations were adjacent to major highways (US Highway 281, US Highway 377, US highway 67, State Highway 6, State Highway 108) and another nine locations were adjacent to unpaved farm and county roads. Sampling points were located in areas where the soil was not disturbed by human activities (farming, etc.) or exposed to various chemicals used in commercial agriculture (fertilizer, pesticides, plant hormones, etc.). At each location, samples were collected at either 3 or 4 variable distances from both the sides of the road, depending on the location of the site. Sampling distances of variable, 25, 50 and 100 m from the edge of the road were collected. The variable distance at every point was always adjacent to the property fence line. Soil samples were collected using a Montana Sharpshooter to a depth of 10 cm, stored in plastic bags for transport and storage, then oven dried at 35º C for 3 days. The dried samples were sieved in a 2-mm sieve to remove gravel-sized material and large plant roots. Then soil samples were ground, homogenized, and packed into labeled bags for analysis. 2. 2. Analytical Methods Soil samples were analyzed for particle size distribution, soil reaction (pH), electrical conductivity (EC), Pb, Mn, Zn, and As content. Soil pH and EC readings were made according to Rhoades (1996) using a 1:2.5 soil to water mixture. Soil pH was measured with an Accumet Research AR20 pH/EC meter (Fisher Scientific International, Hampton, NH, USA). The instrument was calibrated between each replication, using certified buffer solutions at pH 4.00, 7.00, and 10.00, each with an accuracy of ± 0.01. Soil EC was measured from the mixture used for pH determination using a Traceable® Expanded-Range Conductivity Meter (Control Company Friendswood, TX). The sensitivity of the instrument is 0.01 to 200.00 dS m-1 with a resolution up to two decimals and an accuracy of ± 4%. A solution of 0.01 N KCl was used to standardize the EC meter. A modified hydrometer method was used to determine the relative percentage of sand, silt, and clay in all the soil samples (Gee and Or, 1996). Particle dispersion was accomplished via the use of soidium hexametaphosphate and mechanical agitation. Clay and sand percentages were derived from 24 h and 40 s hydrometer readings, respectively. The concentrations of Pb, Mn, Zn, and As were determined by acid digestion of soil samples followed by quantification using inductively coupled plasma atomic emission spectrometery (ICP-AES). For the digestion of soil samples, methods developed by Sparks (1996) and US EPA (2007) were employed. One gram of 2 mm sieved soil was mixed with 20 ml of 1:1 HNO3 + HClO4, and heated to 200º C for 1 h. Then the sample was cooled and a mixture of 5 ml HClO4 and 10 ml HF was added. The sample was reheated until a final volume of 2-3 ml was reached. The digested material was cooled and transferred to a 50 ml volumetric flask. Deionized water was added to make 50 ml of final volume and then filtered using Whatman No. 42 filter paper (US EPA, 2007). Heavy metal concentrations were determined using a Spectro-Ciros ICP–AES (Spectro Inc, Marlborough, MA, USA). The instrument was calibrated and standardized by using a 0.002, 0.02, 0.04, 0.1, 0.4, 1, 3, 4, 5, 10, 15, 20, 40, and 80 mg kg-1 multi-element standard. Reagent blanks, standard solution, and duplicate samples were used to assess contamination, precision and bias. The precision and bias in the analysis were generally