The mining activity in Mexico is intense, particularly in the northwest, where the production of Au and Cu is 33 and 84 % of the total national. The Arizona-Sonora region is characterized by the presence of three metallogenetic belts of Au, Cu, and Au-Ag deposits. Therefore, the presence of historic and low volume artisanal mine tailings is commonly found along this area, near to human settlements and river basins. Mexican regulations for mining waste are becoming more severe but the abandoned mine waste like tailings are almost ignored. It is well known that, mine tailings are a source of PTEs, and acts spreading this element constantly to the environment in some cases for long time scales. According to the EPA, in USA and the INEC in Mexico, the most important contaminants includes: Sb, As, Be, Cd, Cr, Cu, Hg, Ni, Ag, Pb, Se, Tl and Zn.

Since the availability and toxicity of PTEs are strongly correlated to their chemical speciation, synchrotron techniques have become key tools for studying the biogeochemistry of PTEs in environmental matrixes such as plants, water and soils at atomic and molecular level. The principal techniques are μ-XRF (imaging & tomography), μ-XAS (chemical speciation and structure) and μ-FTIR, (interaction with organic molecules). In this work we use a combination of wet chemistry and synchrotron techniques in two sites polluted by mining activities, with the aim to improve the knowledge on the biogeochemistry of the most relevant PTEs present in the study site and determine if they represent a potential threat to the environment and human health. One of the oldest polluted sites affected by As and Pb, presents a naturally attenuation, since soluble fractions of As are low (<10%), and also its presence in plants tissues (0.4%) and soils (2.6%) around, compared with mine tailings content (>12,500 mg Kg^-1). Our results showed that the presence of arsenates like kankite (FeAsO₄∙3.5H₂O), can moderate As availability, by adsorption and lixiviation resistance. Lead spreading at the same site, is controlled by dissolution of minor phases presents at the mine tailings like beudantite (PbFe₃(AsO₄(SO₄) (OH)₆) structure. A previously analysis at the second site, showed that Mn content is high at mine tailings (>54100 mg kg^-1) and changes its oxidation state (+2 to +3) trough soil profile and distance from the source at agricultural soils. Mn biological function and toxicity depends on its oxidation state (II and III), the comprehension of its biogeochemical behaviour is necessary to improve the risk assessment and remediation technologies to be used at this and other sites. Further analysis will include their spatial variation as a function of chemical speciation and availability to improve the risk assessment and the use of soil for agricultural and human activities.