Indirect indicators of residual contamination in soils and sediments of the Sonora river basin, Mexico

Abstract

In Cananea, Sonora, the spill of 40 000 m3 of an acid solution rich in iron and copper reached the main water streams of the Sonora river basin, affecting soils and sediments and giving them an anomalous yellowish brown coloration. In the present study, residual contamination (RC) of soils and sediments in the basin was monitored, evaluating color, pH and electrical conductivity (EC) in 41 samples obtained throughout the basin at various depths in the vertical and horizontal directions in a soil profile (TIIZ1B) affected by RC. These properties were supposed to be indirect indicators of RC because of their modification by the influence of the poured solution. Additionally, total concentrations of potentially toxic elements present in the poured solution were determined in soils and sediments. There is an acceptable lineal correlation (0.75) between EC and the yellow color vector (b*), and between these and the concentrations of As, Cu, Fe and Zn (r = 0.52-0.91). The TIIZ1B soil profile analysis allowed relating color, pH and EC with mineralogical changes and increase in the concentrations of As, Cu and Fe. The increase in color vectors to red (a*) and yellow tones is related to the rise in the Fe concentration and to the increase of brightness (L*), the latter being linked to the precipitation of calcium sulfate. Concentration of As and Fe increases in the portion of the soil with the most acidic pH values, while Cu increases with the increase of pH and a negative point of zero charge. Color, pH and EC are useful parameters in the indirect determination of RC.