Background levels and reference values for metals and metalloids in agricultural soils in an inter-Andean valley in Colombia

Background levels and reference values for metals and metalloids (Cd, Ni, Pb, Cr, Co, As, and Hg) were determined for agricultural soils in the flat and piedmont areas of an inter-Andean valley of the Cauca River, Colombia. Simple random sampling (SRS) was conducted at 489 sites, including Mollisols, Vertisols, and Inceptisols. PERMANOVA analyses were performed to evaluate the significance of metal and metalloid concentrations in relation to watershed and soil order. Statistical methods were used to determine background levels of metals and metalloids. Reference values for metals and metalloids in agricultural soils were proposed using the 75th percentile of the dataset after removing anomalous values. Background levels and reference values for As and Hg were below the detection limit (0.10 mg kg-1). Cd, Cr, Ni, Pb, and Co showed a heterogeneous distribution, with moderate to high variability and low concentrations. The background levels found for Cd, Cr, Ni, Pb, and Co were 0.13 mg kg-1, 46.57 mg kg-1, 28.57 mg kg-1, 2.52 mg kg-1, and 14.00 mg kg-¹, respectively. These values are considered low. The proposed reference values for metals and metalloids were 0.23 mg kg-¹ for Cd, 85.00 mg kg-¹ for Cr, 45.00 mg kg-¹ for Ni, 5.08 mg kg-¹ for Pb, and 21.00 mg kg-¹ for Co, thus being less conservative than those reported in other studies conducted in Latin America. This underscores the need to propose local reference values for metals and metalloids, which can serve as tools for safeguarding and maintaining soil quality.