Hydrogeochemical controls on chromium speciation and potential human health risks in surface waters of an ultramafic mining watershed.

Accurate assessment of chromium contamination in mining-influenced watersheds requires distinguishing between the environmentally contrasting Cr(III) and Cr(VI) species, yet environmental monitoring and risk assessments still rely predominantly on total chromium concentrations. This study evaluated whether chromium speciation provides additional information beyond operational total chromium for assessing chromium occurrence and potential human health risks in an ultramafic, mining-influenced watershed. Surface-water samples were collected from 28 sampling stations across the Pülümür and Ovacık regions of the Eastern Anatolia Basin (Türkiye) between March 2025 and February 2026. Chromium speciation was determined using species-specific UV-Visible spectrophotometric methods and integrated with hydrochemical characterization, multivariate statistical analyses, mineralogical investigations (XRD, SEM-EDS, and FTIR), and deterministic and empirical-bootstrap human health risk assessments. Operational total chromium, defined as the sum of separately measured Cr(III) and Cr(VI), ranged from below the detection limit to 578.13 µg L-1. For contextual screening, this calculated value exceeded World Health Organization (WHO) drinking-water guideline for total chromium at 14 stations; however, it was not an independently measured total-Cr concentration. The highest Cr(VI) concentration occurred at a localized hotspot, whereas Cr(III) predominated throughout most of the watershed. Nevertheless, Cr(VI)-based screening thresholds were exceeded at multiple stations under the assumed untreated drinking-water ingestion scenario, indicating that operational total chromium alone did not consistently identify the spatial distribution of Cr(VI)-related potential risk. Screening-level hazard quotient (HQ) estimates were higher for children than for adults; however, direct untreated consumption of the sampled surface waters was not verified. Mining-influenced stations generally exhibited higher chromium concentrations, although the relative contributions of geogenic weathering and mining-related mobilization could not be quantitatively apportioned. Overall, chromium speciation provided additional environmentally and toxicologically relevant information beyond operational total chromium for evaluating chromium mobility and screening-level potential human health risks in this ultramafic watershed.