Sequential extraction of heavy metals from sediments of the Maipo River basin, Chile

• Metal fractionation patterns showed areas exceeding environmental safety limits. • Associated geochemical indices show the highest risks in urban/agricultural-impacted sites. • Critical finding with 35.78% Cr-organic matter in high agricultural activity zones. • Alarming mobility with 30% Cu in toxic exchangeable fraction near urban centers. • Environmental alert: high metal mobility in vital agricultural water sources. Heavy metal accumulation in river sediments represents a growing environmental concern due to potential toxicity and persistence. The Maipo River basin, which supplies 70% of drinking water and 90% of irrigation water to the Metropolitan Region of Chile, is exposed to various anthropogenic influences. This study evaluates the concentrations and fractionation of Cu, Cr, Mn, and Zn in sediments from four sites: San Francisco de Mostazal (SFM), Melipilla (Mel), Puente Pelvin (PP), and Isla de Maipo (IM). Total metal concentrations were determined via microwave-assisted acid digestion and atomic absorption spectrophotometry. Simultaneously, the BCR sequential extraction method was applied to assess the partitioning into four operationally fractions: soluble (F1), oxide-bound (F2), organic matter-bound (F3), and residual (F4). Results indicate that in the F1 fraction, percentage values ranged from 1.44% (Cu in PP) to 39.34% (Zn in SFM), while in the F2 fraction, values extended from 46.54% (Cu in Mel) to 12.70% (Mn in IM). The F3 showed values ranging from 35.78% (Cr in PP) to 1.87% (Mn in SFM). In the F4 fraction, values ranged from 78.00% (Cu in IM) to 20.64% (Cu in SFM). Total metal concentrations poorly predicted bioavailable fractions, demonstrating the inadequacy of concentration-based risk assessment approaches. The principal component analysis showed that sectors with the highest anthropogenic activity and sediments containing elevated OC% content and oxides exhibit greater metal accumulation.