Advancing groundwater flowpath modeling to quantify lag times and legacy nutrient dynamics for improved diffuse source pollution mitigation

Groundwater flowpath dynamics are fundamental to understanding the transport of nutrients from terrestrial landscapes to receiving surface waters. This review systematically examines the mechanisms, modeling approaches, and water quality implications of groundwater-mediated nutrient transfer. We focus on nitrogen (N) and phosphorus (P) dynamics across the soil-groundwater-river continuum, which are driven by coupled hydrological and biogeochemical processes and by anthropogenic activities (e.g., fertilization and artificial drainage). Conventional watershed models are constrained by inherent structural frameworks and algorithmic formulations (e.g., a single recession coefficient and lumped generalization of spatial units), resulting in limited capability to accurately simulate groundwater flowpath processes. The improved and recently developed watershed models have enhanced simulation performance via internal optimization or reconstruction (e.g., SWAT-2R, ELEMeNT) and external coupling techniques (e.g., SWAT-MODFLOW, Agro-IBIS-THMB). However, major challenges remain, including quantification of subsurface P losses, representation of biogeochemical reactions, and capture spatio-temporal heterogeneity. Overall, the fundamental mechanisms governing groundwater flowpath processes remain incompletely elucidated, and current watershed models face considerable challenges in accurately characterizing the complex dynamics of subsurface N and P losses. Future efforts should prioritize the establishment of high-resolution, multi-indicator, and multi-scale integrated monitoring systems; deeper investigation into coupling mechanisms understanding of hydrological and biogeochemical processes at soil-groundwater interfaces; and develop advanced hybrid models that integrate mechanistic, statistical, and microbial process representations. These efforts will provide a critical scientific foundation for addressing the persistent challenges of diffuse source N and P pollution.