Hydrochemical regimes and nitrate-fluoride surveillance in groundwater of Odisha, Eastern India.

Groundwater-quality monitoring generates large multi-parameter datasets, but these data are often difficult to translate into clear evidence for surveillance and risk prioritization. This study analyses groundwater chemistry across Odisha, eastern India, using Central Ground Water Board monitoring records from 2019 to 2023. After geospatial deduplication and charge-balance screening, 1,268 site-level samples were retained for analysis. The study identifies reproducible hydrochemical regimes, interprets their major-ion characteristics, and examines how nitrate and fluoride exceedances are distributed across these regimes. Two stable hydrochemical regimes were identified along a dominant mineralization gradient. Regime 1 showed higher EC, TDS, and major-ion concentrations than Regime 0, indicating a more mineralized groundwater setting. Nitrate and fluoride exceedances were also more frequent in Regime 1. Nitrate exceedance increased from 4.45% in Regime 0 to 13.81% in Regime 1, while fluoride exceedance increased from 0.91% to 4.86%. District-adjusted models confirmed that these contrasts were not explained only by uneven district-level sampling. Regime-specific association patterns further indicated that the higher-mineralization regime was more strongly organized by salinity-related hydrochemical structure. The findings show that nitrate and fluoride screening burdens are concentrated in specific hydrochemical settings rather than being uniformly distributed across the monitoring network. The proposed regime-based interpretation can support targeted groundwater surveillance, improved allocation of monitoring effort, and priority screening in spatially heterogeneous groundwater systems.