Monsoon-driven nutrient pollution assessment and source tracking in tropical mountain headwaters using positive...

Tropical mountain ecosystems, driven by monsoonal hydrology and escalating land use, are highly vulnerable to nutrient enrichment, which threatens downstream water quality. This study investigates the spatiotemporal variability and quantitative source apportionment of dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silica (DSi) across surface water and groundwater in the Munnar Critical Zone Observatory (CZO), Western Ghats, India. Seasonal monitoring over three monsoon cycles revealed extremely elevated DIN/DIP ratios (up to 299:1), indicating severe phosphorus (P) limitation, reflecting rapid particulate P flushing combined with sustained anthropogenic nitrogen (N) inputs. This N enrichment contributes to a significant riverine DIN flux (3.79 × 10³ tons/year), dominating catchment-scale transport. Hydrological analysis confirmed agricultural leaching, with nitrate (NO₃-N) peaking in groundwater during the monsoon (6.98 ± 0.63 mg/L), while silicate weathering significantly enriched groundwater DSi (15.22 ± 2.81 mg/L). The Positive Matrix Factorisation (PMF) model apportioned 70.2% of NO₃-N and 81.3% of phosphate (PO₄^3-) to agricultural fertiliser inputs and 100% of NO₂-N plus 99.2% of ammonium (NH₄-N) to sewage waste; seasonally, fertiliser signals surged during the monsoon, while sewage contributions peaked in the post-monsoon baseflow. Distinct nitrogen cycling pathways were confirmed by the NO₃-N/NH₄-N ratios (16:1 in surface water vs. 10:1 in groundwater), signifying N loss via denitrification in septic-influenced anaerobic groundwater. These findings, underscoring significant N* excess (up to 107.35), quantify the high eutrophication potential being exported from this anthropogenically stressed headwater system. These results highlight urgent management needs, including optimised fertiliser application timing, restoring riparian buffers, and upgrading sanitation systems to curb nutrient pollution and safeguard downstream ecosystem services.