Mobility and accessibility of Zn, Pb, and As in abandoned mine tailings of northwestern Mexico.

Generation, storage, and management of waste coming from industrial processes are a growing worldwide problem. One of the main contributors is the mining industry, in particular tailings generated by historical mining, which are barely maintained, especially in developing countries. Assessing the impact of a mining site to surrounding soils and ecosystems can be complex, especially when determining mobility and accessibility of the contaminants is required to perform ecological and human health risk assessment. As an effort to obtain information regarding mobility and accessibility of some potentially toxic elements (Zn, Pb, and As) from an historical mining site of northwestern Mexico, the abandoned mine tailings of San Felipe de Jesús in central Sonora and adjacent agricultural soils were investigated. Mobility and accessibility were assessed by means of sequential extraction procedures and using simulated physiological media. Additionally, an assessment of accidental oral intake was calculated considering the bioaccessible fractions. Results show that higher concentrations of contaminants were found in sulfide-rich tailings (Zn = 92,540; Pb = 21,288; As = 19,740 mg kg-1) compared with oxide-rich tailings (Zn = 43,240; Pb = 14,763; As = 13,401 mg kg-1). Concentrations in agricultural soils were on average Zn = 4755, Pb = 2840, and As = 103 mg kg-1. Zinc was mainly recovered from labile fractions in oxide-rich tailings (~ 60%) and in a lower amount from sulfide-rich tailings (~ 30%). Pb and As were mainly associated with residual fractions (80-95%) in both types of tailings. The percentage of mobile fractions (sum of water-soluble, exchangeable, and bound to carbonate fractions) in agricultural soils was as follows: Zn ~ 60%, Pb ~ 15%, and As ~ 70%. Regarding the phytoaccessible fraction, the studied elements in mine tailings and agricultural soil samples exceeded the threshold limits, except for As in agricultural soils. According to data obtained, toxic effects were also calculated. As for daily oral intake for non-carcinogenic effects in adults and children, only Pb and As exceeded reference dose values, especially in children exposed to sulfide-rich tailings and agricultural soils. Regarding carcinogenic effects of Pb and As, most of the samples were above acceptable risk values.

Environmental assessment and historic erosion calculation of abandoned mine tailings from a semi-arid zone of northwestern Mexico: insights from geochemistry and unmanned aerial vehicles.

Mining is known as one of the primary economic activities where exploitation of minerals and other materials have become essential for human development. However, this activity may represent a risk to the environment, starting from deforestation and ending with production of residues that might contain potentially toxic elements. Tailing deposits from historical mining are an example of waste that may represent an environmental concern when abandoned and exposed to environmental conditions. The town of Nacozari de Garcia, in northwestern Mexico, has three abandoned mine tailings (locally known as tailings I, II, and III) located around the urban area that represent important sources of dust and pollution. Images obtained using unmanned aerial vehicles (UAV) in conjunction with geochemical data are used to assess historic erosion calculation and pollution considering contamination and hazard indexes in tailings II and III. Digital elevation models of abandoned tailings were obtained using photogrammetry with UAV. A total of 37 surficial samples were collected from mine tailings to determine elemental concentrations (As, Cu, Pb, W, Zn) using portable X-ray fluorescence. Higher concentrations were found on samples from mine tailing II. Average concentrations followed the decreasing order of Cu > Zn > W > Pb > As for tailing II, whereas decreasing order of Cu > Zn > W > As > Pb was found for tailing III. Contamination Index (CI) values obtained from tailings II and III represent a low potential of pollution, whereas efflorescent crusts from these tailings represent a high potential of polluting soils and sediments by dust generation. Hazard Average Quotient (HAQ) values on both tailings suggest a very high potential of contamination if fluids infiltrate tailings and interact with surficial water and/or groundwater. Obtained surfaces of mine tailings II and III are 146,216 and 216,689 m2, respectively, which represent around 11% of the urbanized area. A loss mass of 321,675 tons was determined for mine tailing II, whereas 634,062 tons for tailing III, accounting for 0.96 million tons of total eroded mass. Since abandonment, calculated erosion rates of 493 t ha-1 year-1 (tailing II) and 232 t ha-1 year-1 (tailing III) are in agreement with those determined in other mining areas. CI and HAQ indexes provide good estimates of pollution associated with abandoned mine tailings from Nacozari de García. Historic erosion determined in these tailings is an environmental concern since eroded material and polluted water have been incorporated into the Moctezuma River, which feeds several villages, whose major activities include agriculture and livestock raising.

Dust-Metal Sources in an Urbanized Arid Zone: Implications for Health-Risk Assessments.

The available information concerning metal pollution in different dust sources and the health effects in children remains limited in Mexico. This study focuses on Hermosillo, which is an urbanized area located in the Sonoran Desert in which soil resuspension and dust emission processes are common. The metal content of arsenic (As), chromium (Cr), manganese (Mn), and lead (Pb) were determined in three dust sources (playgrounds, roofs, and roads), each representing different exposure media (EM) for these elements. The metal levels in dust were found in the order of Mn > Cr > Pb > As with the highest metal content found in road dust. Despite the similar average metal distributions, principal component analysis shows a clear separation of the three EM with playground dust related to Cr and Mn and road dust to As and Pb. However, the geoaccumulation index results indicate that dust samples are uncontaminated to moderately polluted, except for Pb in road dust, which is considerably high. In addition, the enrichment factor suggests an anthropogenic origin for all of the studied metals except for Mn. In this context, the hazard index (HI) for noncarcinogenic risk is >1 in this population and thus represents a potential health risk. The spatial distribution for each metal on EM and the HI related to the marginality index could represent a more accurate decision-making tool in risk assessment studies.

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

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.