THERMODYNAMIC CONTROL OF HEXAVALENT CHROMIUM AND BORON MOBILITY IN A RIVER SYSTEM: THE ILEK RIVER CASE STUDY

Abduali Bayeshov,  Bauyrzhan Kapsalyamov,  Gulzat Zholmuratova

doi:10.37238/2960-1371.2960-138X.2026.102(2).43

Abduali Bayeshov, Bauyrzhan Kapsalyamov, Gulzat Zholmuratova ЕНУ

DOI: https://doi.org/10.37238/2960-1371.2960-138X.2026.102(2).43

Keywords: hexavalent chromium (Cr(VI)); boron; Eh–pH thermodynamics; groundwater–surface water interaction; secondary contamination; alluvial aquifer; river pollution; source–pathway–receptor concept; natural attenuation limits.

Abstract

Persistent contamination of river systems by inorganic pollutants is often sustained by secondary sources and subsurface transport processes rather than by active point discharges. This study investigates the thermodynamic and hydrogeological controls on the mobility of hexavalent chromium (Cr(VI)) and boron (B) in a technogenically disturbed river system, using the Ilek River (Kazakhstan) as a case study.

Field measurements show that both surface water and hydraulically connected groundwater are characterized by neutral to alkaline pH (7.2–9.0) and persistently oxidizing conditions (Eh +300 to +600 mV). Thermodynamic analysis in Eh–pH space demonstrates that these conditions fully coincide with the stability field of dissolved Cr(VI), rendering its natural reduction to Cr(III) thermodynamically unfavorable in the absence of external reducing agents. In contrast, boron is shown to be redox-insensitive, with its aqueous speciation controlled solely by acid–base equilibrium between H₃BO₃ and B(OH)₄⁻, resulting in persistent dissolved behavior across the observed pH range.

Integration of thermodynamic calculations with hydrogeological data confirms that groundwater constitutes the dominant transport pathway for both contaminants from technogenic sludge repositories to the river, forming stable plume-type structures in permeable alluvial sediments. The absence of effective natural attenuation mechanisms explains the chronic nature of river contamination and the limited effectiveness of mitigation measures focused exclusively on surface waters.

Based on these findings, a thermodynamically justified management framework is proposed, emphasizing source stabilization, targeted Cr(VI) immobilization via reduction to Cr(III), and interception of groundwater flow. The results provide a transferable mechanistic basis for managing secondary contamination in other technogenically impacted river–groundwater systems.