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.

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.