Securing high-quality water resources in arid regions such as South Khorasan Province is a critical challenge. This study aims to provide a comprehensive assessment of groundwater quality for drinking and agricultural purposes, as well as to identify and delineate the sources of contamination. To this end, hydrochemical data from 279 groundwater samples were processed in a Python environment following rigorous screening based on the ionic balance error (IBE). Drinking water suitability was evaluated through a comparative analysis of three water quality indices (WHO-WQI, CCME-WQI and Weighted WQI), while agricultural suitability was appraised using key parameters including SAR, RSC, %Na, MH and KR. Furthermore, hydrogeochemical processes were investigated using Piper trilinear diagrams, Gibbs plots, and Principal Component Analysis (PCA). The results revealed that Na-K-Cl is the predominant hydrochemical facies, with evaporation–crystallization identified as the primary mechanism controlling water chemistry. According to the WHO model, over 70% (70.6%) of the samples fell within the permissible/desirable category for drinking; however, spatial analysis highlighted localized quality deterioration in southwestern and eastern hotspots. Regarding agricultural use, despite favorable classifications based on traditional "SAR" values, elevated levels of Magnesium Hazard (MH), Kelley’s Ratio (KR), and sodium percentage (Na% ) pose serious long-term threats to soil structure degradation and permeability reduction. Finally, PCA successfully distinguished two principal components: a geogenic factor (evaporite mineral dissolution) as the primary driver of overall salinity (explaining 55.9% of total variance), and an anthropogenic factor (NO3-and K+) originating from agricultural fertilizer application and wastewater seepage (explaining 11.2% of total variance).