Assessment of Geogenic and Anthropogenic Pollution Sources Using an Aquatic Plant Along the Sonora River Basin: Insights from Elemental Concentrations and Pb Isotope Signatures

Mining is an important activity in Mexico; however, despite its economic benefits, it carries potential environmental risks, including mine spills. On August 6, 2014, ~ 40,000 cubic meters of copper sulfate acid solution was spilled from the Tinajas 1 dam of the Buenavista del Cobre mine in Cananea, Sonora, northwestern Mexico. The solution was directly spilled into the Tinajas creek, which is a tributary of the Sonora River. This event had regional socioeconomic and environmental consequences. That is because the Sonora River provides water for agricultural and livestock activities in the region, as well as to human consumption of products derived from these activities. In an attempt to assess the influence of this spill along the Sonora River, samples of watercress (Nasturtium officinale), an edible aquatic plant known for its ability to uptake metals, were collected along: (a) the 2014 spill route (zones 1 and 2); (b) a zone that experienced a spill in the 1980s (zone 3); and (c) a reference zone (zone 4). The watercress samples were analyzed for concentrations of Cr, Sb, Ba, U, Cu, Cd, Zn, Ni, Fe and Pb to evaluate the effects of the spills along the riverbed. The study was supported with measurements of Pb isotopic ratios to evaluate watercress samples in a two-end-member mixing scenario. The results indicate that no significant statistical differences were detected when concentrations in watercress samples from the 2014 spill route were compared with those from reference zone. Significant statistical differences and relatively higher concentrations for Cu, Zn and Cd were found when comparing watercress samples from the 1980s spill route with those from reference zone. Concentrations of Ba, U and Sb were relatively higher along the 2014 spill route, possibly associated with the highly differentiated Laramide intrusive rocks of the studied area. Concentrations of Cu and Zn along zone 3 exceeded the FAO/WHO values, as well as the geochemical baseline levels of the Sonora River basin. Concentrations of Fe and Pb exceeded the maximum FAO/WHO values in both cases, but did not exceed the geochemical baseline levels of the Sonora River basin. Concentration of Cd in watercress samples exceeded the geochemical baseline, and it was five to nine times higher than the FAO/WHO value. Regarding Pb isotope ratios, a linear arrangement was observed between the two end-members, which were comprised by a geogenic component defined by the Pb isotope signatures of rocks representing the study area, and the other member (anthropogenic) was defined by isotope ratio obtained from a sample collected from the spilled copper sulfate solution in 2014. Watercress samples collected on the spill route yielded Pb isotopes signatures that suggest an influence from the spill of ~ 65 and ~ 42%, for zones 1 and 2, respectively. Pb isotope ratios in watercress samples from zone 3 were closer to the anthropogenic end-member. These Pb signatures reflect a mine spill that occurred in the 1980s, when Buenavista del Cobre mine was operated by a previous company. Finally, in watercress samples from zone 4, the Pb signatures were more likely acquired from the geogenic component.