Metals in residential soils and cumulative risk assessment in Yaqui and Mayo agricultural valleys, northern Mexico

Abstract

This investigation examines the extent of soil metal pollution associated with the Green Revolution, relative to agricultural activities and associated risks to health in the most important agricultural region of Mexico. Metal contents in bulk soil samples are commonly used to assess contamination, and metal accumulations in soils are usually assumed to increase with decreasing particle size. This study profiled the spatial distribution of metals (Ni, Cr, Pb, Cu, Fe, Cd, V, Hg, Co, P, Se, and Mn) in bulk soil and fine-grained fractions (soil-derived dust) from 22 towns and cities. The contamination of soil was assessed through the use of a geoaccumulation index (Igeo) and pollution index (PI). The results of this study indicated that a number of towns and cities are moderately to highly polluted by soil containing Be, Co, Hg, P, S, V, Zn, Se, Cr, and Pb in both size fractions (coarse and fine). Hazard index in fine fraction (HI_children = 2.1) shows that risk assessment based on Co, Mn, V, and Ni spatially related to power plants, have the potential to pose health risks to local residents, especially children. This study shows that risk assessment based on metal content in bulk soil could be overestimated when compared to fine-grained fraction. Our results provide important information that could be valuable in establishing risk assessment associated with residential soils within agricultural areas, where children can ingest and inhale dust.

Introduction

The potential public health risk associated with the intake of metals from dust and soil has been the subject of discussion in recent years (Wei and Yang, 2010). In urban areas, heavy metals in urban soils and dusts can be accumulated in the human body via direct inhalation, ingestion, and dermal contact absorption (De Miguel et al., 1998, Madrid et al., 2002, Marjorie et al., 2008). In agricultural areas, the human predominant exposure to metals is via the soil–crop system (Liu et al., 2007). The anthropogenic sources of metals in urban areas include traffic emission (vehicle exhaust particles, tire wear particles, brake lining wear particles), industrial and domestic emission, and atmospheric deposition (Morton-Bermea et al., 2009, Sezgin et al., 2003). Anthropogenic sources of metals in agricultural areas include mining, waste disposal, sewage, pesticides, fertilizers, and vehicle exhausts (Li et al., 2008, Montagne et al., 2007).

Elevated levels of metals in residential soils may pose risks to human health. The risk is especially high for children exposed to Pb, As, Cd, Cr, and Hg, because of their low tolerance to pollutants as well as the inadvertent ingestion of dusts or soils, through hand-to-mouth pathways (Acosta et al., 2009). Metal contamination in soil is commonly assessed by comparison against soil quality standards and regulations. Studies had shown that the concentration of metals in soils increases with decreasing particle size (Wei and Yang, 2010, Ljung et al., 2006, Wang et al., 2006). Smaller particles are of concern since they are more efficiently adsorbed (e.g. inhalation) than coarse fractions (Lin et al., 1998), and the fine-grained fraction of soil is easily suspended into the atmosphere by wind erosion. Stanek et al. (1999) suggested that risk assessment linked to dust ingestion is more reliable if based on fine particle size instead of bulk soil.

The Yaqui and Mayo valleys are the most important agricultural areas in Mexico. The valleys are located in southern Sonora, a border state with the United States of America (Fig. 1). The Green Revolution began in the Yaqui valley between the 1940s and the late 1970s. The valleys are situated on a coastal strip along the Gulf of California, and consist of an intensively managed agricultural region amidst a desert scrub forest bordered by estuarine ecosystems that provide critical habitat for migratory and resident water birds, marine mammals, fish, and shellfish populations (Flores-Verdugo et al., 1992). Both valleys host 566,000 ha of irrigated wheat-based agriculture and the development of the region is of vital economic importance to Mexico because it produces some of the highest wheat yields in the world (FAO, 1997).

The Yaqui and Mayo valleys are undergoing rapid socioeconomic and ecological changes due to the population growth, urbanization, agricultural intensification, and coastal aquaculture development. González et al. (1997) reported 399 industrial facilities in the Yaqui valley (247 small, 96 medium, 56 large). Obregon city is the second largest city in Sonora and contains most of the large industries. Many of these industrial plants produce refuse and sewage, which may affect the aquifer since this waste flows into the Gulf of California (González et al., 1997). Farming activities in the Yaqui valley generate a pollution load including manure and urine, amounting to around 3200 m³ day^− 1 for pork, 850 m³ day^− 1 for beef, and 650 m³ day^− 1 from poultry (González and Córdova, 1992). Most of this waste is applied to the ground and the rest is drained into the sewer (González et al., 1997). According to González and Córdova (1992) about 233 m³ day^− 1 of fertilizer is used in the Yaqui valley. Thus there are many sources of environmental contamination in the Yaqui and Mayo Valleys.

Metals are associated with many of the wastes produced in the Yaqui and Mayo Valleys, however only limited attention has been given to assessing these metal contaminants. Most studies from the Yaqui valley are focused on arsenic-exposure health effects (Adler, 2005, Cantú-Soto et al., 2009, Meza et al., 2004, Meza et al., 2005). Epidemiological studies performed at the Yaqui valley focused in the effects of arsenic exposure on nucleotide excision and provide evidence that supports the ability of arsenic to inhibit the DNA repair machinery, which is likely to enhance the genotoxicity and mutagenicity of other genotoxic compounds as part of a cocarcinogenic mechanism of action (Andrew et al., 2006). Guillete et al. (2006) conducted a study of breast development in a group of peripubertal girls from the Yaqui valley (pesticide exposure) and Yaqui foothills (non pesticide). The study shows that girls from valley towns displayed a poorly defined relationship between breast size and mammary gland development, whereas girls from the Yaqui foothills, where traditional ranching occurs, show a robust positive relationship between breast size and mammary size. Such differences were attributed to environmental influence, however, arsenic exposure through water consumption has been explored as the only exposure path in the Yaqui valley.

The main objective of this study was to determine the distribution of metals in bulk soils and fine-grained fraction (soil-derived dust) and to estimate the degree of contamination on the basis of geoaccumulation and pollution indexes. Attention will be given to the potential health risk to adults and children via fine-grained fraction and bulk soil ingestion. To our knowledge, this is the first study that characterizes the extent of soil pollution caused by the Green Revolution agricultural activities and associated risks to health in the Yaqui and Mayo valleys.