Heavy metals, radionuclides, and rare earth elements in drinking water treatment sludge: Source apportionment and health risk implications

Abstract

Drinking water treatment sludge (WTS) is increasingly considered for beneficial reuse, yet its role as a sink for geogenic contaminants and the associated chemical-radiological health risks remain insufficiently characterized. This study provides an integrated assessment of WTS from a full-scale conventional surface-water treatment plant in Astana, Kazakhstan, linking contaminant occurrence, source processes, and probabilistic human health risk. WTS samples collected over eight months in 2024 were analyzed for trace metals and uranium isotopes. Multivariate analysis (PCA/PMF) resolved three dominant geochemical controls explaining 73.5% of total variance: (1) Fe-Mn (oxyhydr)oxide-mediated trace-metal scavenging, (2) lithogenic inputs from zircon-bearing granitoids, and (3) hydrogeochemical mobilization of oxyanion-forming elements (As-U-V). These results indicate that WTS composition is governed by interacting process-driven and geogenic mechanisms rather than discrete anthropogenic sources. Several contaminants exceeded regulatory-relevant thresholds. Pb (1063 mg/kg) and As (52 mg/kg) surpassed land-application benchmarks, while total natural uranium activity (1258-1725 Bq/kg) exceeded Kazakhstan's screening level (1000 Bq/kg). Probabilistic modeling (2D Monte Carlo) showed risk exceedances under all conditions. Non-carcinogenic risk was severe (median HI: 54.4 for children; 5.8 for adults), driven almost entirely by ingestion (approximate to 95-98%) and controlled by Pb and As. Carcinogenic risk surpassed acceptable thresholds across the full distribution (median TCR on the order of 10(-)& sup3;), with arsenic alone contributing > 98%. Uranium isotopes added a secondary but measurable radiological burden. By coupling mechanistic source apportionment with probabilistic chemical-radiological risk assessment, this study shows that WTS can concentrate geogenic contaminants and that a small subset of elements controls most of the human health risk. These findings support the need for risk-informed management frameworks for WTS reuse.