Sustainable Hydrochemical Reference Conditions in the Headwaters of Western Ukraine

Establishing reliable hydrochemical reference conditions is essential for water quality assessment and for the implementation of the European Union Water Framework Directive, particularly in regions where biological and hydromorphological data remain limited. This study aims to evaluate hydrochemical reference conditions in selected river headwaters of Western Ukraine and to examine the consistency between international and national water quality assessment approaches. Water samples were collected from four headwater and confluence sites and analysed for key physicochemical parameters, including nutrients, organic matter indicators, and major ions. Water quality was assessed using the Water Quality Index (WQI) and the Ukrainian Ecological Quality Index (IE), supported by correlation analysis and principal component analysis to identify dominant drivers of spatial variability. Most parameters complied with international and national standards, although elevated concentrations of ammonium, phosphates, biochemical oxygen demand, and nitrites were observed at specific sites. WQI differentiated headwaters with good and moderate water quality, whereas IE classified all sites as good, indicating methodological differences in sensitivity. Multivariate analysis showed that water quality variability was primarily controlled by biogenic and organic loading, while mineralization parameters reflected background geochemical conditions. The results demonstrate that hydrochemical indices can support the preliminary identification of reference conditions but also highlight systematic differences between assessment frameworks. These findings provide a methodological basis for harmonizing national water quality assessments with international standards and for improving reference site selection in data-limited regions.