Abstract
In arid and semi-arid regions, unified national water quality standards do not always ensure effective regulation of anthropogenic pressure on surface waters and may lead to inconsistencies in permissible discharge standards (PDS) calculations. This study substantiates the need for regional environmental quality standards using the Ilek River basin, one of the most impacted transboundary systems in Western Kazakhstan, as a case study. The research applies a mass-balance approach to PDS calculation and evaluates the influence of background pollutant concentrations and hydrological variability on regulatory outcomes. Boron and hexavalent chromium (Cr(VI)), typical for the natural and technogenic conditions of the basin, were selected as key indicator substances. Long-term hydrochemical and hydrological data were analyzed to assess spatial and temporal variability under different discharge scenarios. The results show that elevated background concentrations, especially of boron, significantly affect calculated PDS values. Under such conditions, unified standards may produce negative PDS values, formally indicating the impossibility of discharge regulation regardless of actual anthropogenic input. This reveals the limitations of uniform regulatory approaches in hydrologically variable and environmentally heterogeneous regions. The findings demonstrate that regionalized environmental quality standards, accounting for climatic conditions, background levels, hydrological regime, and ecosystem stability, are essential for scientifically sound discharge regulation and sustainable water resource management. The practical significance of the study lies in the applicability of the proposed approach for improving environmental regulation and water management practices.