New Interpretable Framework for Clustering Spatial Hydrogeochemical Data and Assessing Groundwater Quality and Chemical Evolution Factors: A Topological Synthesis Approach

This study proposes a new method for the topological synthesis of principal component projections and hydrogeochemical stoichiometric equality lines on self-organizing map (SOM) component planes to assess groundwater chemistry forming factors and quality in the Pleistocene unconfined aquifer of the Southern Bug and Sinyukha interfluve area, Ukraine. The hydrogeochemical characteristics clustered by the SOM were further examined using the graphical cross-validation method. The groundwater dataset used in the analysis consisted of 10 parameters (i.e., pH, total dissolved solids, Ca2+, Mg2+, Na+, K+, HCO3−, Cl−, SO42−, and NO3−) from 91 samples collected during the dry season. Subsequently, for SOM construction, we utilized six log-ratio relationships of milliequivalent ion concentrations. Based on the results, the hydrogeochemical groundwater data were classified into three clusters, which revealed three water types and processes controlling their chemistry: salinity driven by sulfate inputs (Cluster 1), highly salinity driven by nitrate-chloride and sulfate pollution (Cluster 2), and relatively fresh water governed by natural carbonate dissolution and silicate weathering (Cluster 3). The salinity types were identifiable in the northern part of the study area, characterized as the primary zone of initial intense pollution. High salinity types were identified in the eastern and southeastern parts of the territory (with delayed water exchange), whereas relatively fresh types were identified in the central part (with active water exchange) as well as in the western and southwestern parts. Modeling confirmed that extensive sulfate, nitrate, and chloride contamination led to anthropogenic degradation of the aquifer system.