Trace Metals in Soils from La Paz Valley (B.C.S., Mexico), Are they Contaminated?

Abstract

Irrigation for agricultural purposes accounts for between 60 to 90% of the total water used for human activities, making it the largest consumer of freshwater on the planet. One of the most common practices for disposing of domestic wastewater has been its direct discharge, without treatment, into surface water bodies and the soil, resulting in public health issues and environmental problems, such as soil contamination. The aim of this study was to evaluate the contamination index of four soil samples in the La Paz Valley by analyzing the total concentration of trace element (cadmium, chromium, cooper, nickel, lead and zinc) to determinate contamination levels using the geoaccumulation index (Igeo). Additionally, four chemical variables were determined, organic matter (OM), total nitrogen, nitrates, and pH, to establish whether a relationship exists between the trace elements and the chemical variables. All trace element concentration ranges were below the total reference concentration according to the Canadian Of ficial Norm, and the Igeo values for the trace metals indicated uncontaminated concentrations. Furthermore, the average
OM percentages were categorized as low and to medium across the four sites (0.72 to 1.70%), two sites recorded very low mean nitrogen concentration (0.05-0.08%), high nitrate concentrations were observed at site A (62.7 to 146 mg kg−1) and B
(18.1 to 142.9 mg kg−1), and all the soils were alkaline (pH = 7.21 – 8.37). Significant positive correlations were found between OM vs Zn, Cu, and Ni, likely due to soil process that af fect the solubility and availability of these elements, or because these metals share common sources. Lastly, Cu and soil pH showed a significant positive correlation, influenced by several interaction factors. The varying irrigation characteristics at these four sites could af fect the chemical properties, but they do not appear to generate contamination, at least not in the elements analyzed in this study.