Springs as sentinels: hydrogeological mediation and anthropogenic forcing of nutrient dynamics in tropical aquifers.

Springs, as critical groundwater discharge points, are highly sensitive to coupled hydrogeological and anthropogenic perturbations in rapidly transforming tropical landscapes. This six-year study (2018-2023) investigates nutrient dynamics in springs across distinct hydrogeological settings of the Western Ghats, India. Significant spatial variability in dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved silica (DSi), and physicochemical parameters (EC, TDS, DO) were observed (one-way ANOVA, p < 0.05), reflecting heterogeneous recharge pathways and land-use influences. Springs in Southern Kerala (SK), especially SK1, SK2 near Varkala Cliff, showed elevated DIN, while DIP remained low with limited interannual variability but clear spatial differences. In Southern Karnataka (DK), DK5 and DK6, exhibited comparatively higher DIP and DIN, with greater temporal variability than other springs. Elevated DIN:DIP ratios (> 26:1) indicate nitrogen enrichment and potential phosphorus limitation, although this threshold was derived from surface-water systems. Higher DIN:DSi ratios in SK contrasted with silicate weathering signatures in DK, indicating differences in subsurface flow paths, soil CO2 dynamics, and rock-water interaction. Na-normalized Cl/NO3 and NO3/Cl-Cl relationships suggest agricultural and sewage inputs, although these interpretations remain indicative without isotopic tracers. Land-use analysis showed substantial increase in built-up area (~ 36% in DK and ~ 4% in SK), supporting anthropogenic nitrogen loading. DIN patterns were spatially heterogeneous, with elevated concentrations at SK1 and SK2, and pronounced temporal variability at DK5 and DK6, rather than a uniform increasing trend. These findings demonstrate that tropical spring nutrient dynamics reflect interactions among hydrogeological structure, biogeochemical processes, and land-use transitions, highlighting their vulnerability to environmental change.