**4. Application of sequential metal(loid) extraction to evaluate the impacted area of the Sonora River acid spill**

### **4.1 Introduction**

A spill from a copper mine dam located in northern Sonora State, Mexico, occurred on August 6, 2014. Approximately 40,000 m<sup>3</sup> of a diluted acid solution containing iron, aluminum, copper, manganese, and minor amounts of arsenic, nickel, cadmium, zinc, lead, and chromium was released into the Sonora River basin. As part of contingency measures, the mining company built a natural gypsum dike and released 1800 tons of lime gravel along the river (190 km downstream from the source). The significant pH increase favored the precipitation of soluble ions and the sedimentation of suspended particles, mainly clays and iron oxy-hydroxides, likely with adsorbed metals [131]. These whitish sediments were removed and transported back to the mine. There was heavy rainfall the following September due to Hurricanes Norberto and Odile, which dispersed the remaining sediments along the river, possibly reaching the Rodolfo Félix Valdés Dam. However, precipitation was lower in the north, near the accident area, than in the southern portion of the basin. Additionally, pedogenic carbonates from river sediments helped raise the pH of the impacted water [132]. As a result of both natural and anthropogenic attenuation processes, in eight days the river water was already neutral in several areas [133]. The authorities consider the acid solution to have flowed 190 km downstream from the accident site to the Rodolfo Felix Valdés Dam [134]. Nevertheless, based on pH monitoring of the superficial water downstream, acidity (pH 2.6–3.7) reached no farther than 150 km downstream [135]. Total metal(loid) concentrations failed to reveal information concerning the size of the affected area, since it is naturally rich in metal(loids) and because there are waste deposits from current and historical mining in several places. Prior to the accident, significantly high metal values were found in natural compartments. The Mexican Geological Service [136] reported the geochemical composition of sediments with important metal(loid) concentrations [134]. Gonzales-Leon et al. [137] described the geological formation of the Arizpe sub-basin, reporting high natural values of several elements in the soil, including those classified as toxic to humans, such as arsenic. In 2006, the Technological Institute of Sonora reported that concentrations of arsenic, cadmium, and thallium in Sonora River basin groundwater were found to be higher than those specified in the water quality criteria for drinking water by Mexican regulations and other international institutions, such as the Environmental Protection Agency of the USA and the World Health Organization. All these data indicated high metal(loid) baselines. However, despite this fact and the important spill attenuation measures already mentioned, many residents believe that the metal(loid) concentrations found in water bodies and soils are solely due to the spill. This risk perception has caused concern and controversy among inhabitants, journalists, nongovernmental organizations, and the authorities [138, 139]. Consequently, spill impact evaluation is important, but discriminating the input from metal spill from that of other sources poses a formidable challenge, since this is an area in which all the metals could have come from the same ore deposits and may share the same isotopic footprints. Total metal(loid) concentrations in sediments, their sequential extraction and bioaccessibility, and other analyses were performed to enhance information on the consequences of the acid solution spill, both for the environment and for its human inhabitants [134]. Some of these results, together with sequential extractions of selected metals in mining soil and wastes, are presented to evaluate the usefulness of chemical fractionation schemes.
