Beyond Conventional Indices: A Novel Fluorescence Intensity Ratio to Decipher Nitrate Signatures in Anthropogenic Groundwater Recharge Systems

Groundwater nitrate contamination necessitates reliable and efficient methods for source tracking and process understanding. Although dual nitrate isotopes (δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻) are powerful tracers, their application is limited by their high cost and analytical complexity. Optical spectroscopy of dissolved organic matter (DOM) offers a rapid and cost-effective alternative. Here, we evaluated and advanced the use of DOM fluorescence as an integrative proxy for predicting groundwater nitrate dynamics. Although significant, individual optical indices showed weak correlations with nitrate indicators (R² = 0.14-0.25, p < 0.05). Integrated DOM indices improved the significant correlations, yet the performance remained moderate (R² = 0.27-0.62, p < 0.01). We introduced a novel specific fluorescence intensity ratio (FIR), which demonstrated strong and significant linear relationships with all nitrate indicators (p < 0.01). Optimal FIR offered substantially improved prediction (R² = 0.60-0.76, p < 0.01), while region-based FIR yielded comparable robustness (R² = 0.56-0.68, p < 0.01) and greater stability. Anthropogenic groundwater recharge markedly strengthened FIR-nitrate relationships, highlighting its role as a key hydrological driver. Critically, the FIR approach generalized well across five groundwater and three surface water systems in China (groundwater: R² = 0.45-0.99, p < 0.01; surface water: R² = 0.40-0.86, p < 0.01). These findings established FIR as a viable, cost-effective, and rapid proxy for nitrate and isotopic screening prior to large-scale water quality assessment.