Groundwater-driven particle mobilization reveals an underestimated pathway of phosphorus input to lakes

In wetland ecosystems, lacustrine groundwater discharge enriched in nitrogen and phosphorus concentration is an important component of lake nutrient budgets. However, in addition to groundwater fluxes, the transport of particulate matter accompanying groundwater discharge, especially the mobile fine particles, was largely neglected in existing frameworks. In this study, we identified typical particle mobilization associated with groundwater discharge at shoreline springs in an oxbow lake. Continuous monitoring of spring flow velocity and discharge pressure, combined with principal component analysis, reveals two distinct particle-mobilization regimes. A pressure-controlled mode dominates in settings with thick surficial clay layers, whereas a velocity-controlled mode prevails where the surficial clay cover is thin. The results show that the total phosphorus load carried by particles can exceed that of dissolved groundwater by two to three orders of magnitude, although a substantial fraction consists of coarse particles and relatively inert HCl-P. Importantly, fine particles (silt and clay) transport large amounts of reactive phosphorus (OP + NaOH-P), yielding fluxes comparable to or even several times higher than dissolved groundwater phosphorus under both strong and weak discharge sites. Coupled Sr isotope and rare earth element tracing indicates that these fine particles originate from pressure-induced detachment of the top clay aquitard and velocity-driven erosion of clay lenses within the aquifer, respectively. Physical simulations indicate that preferential loss of coarse particles during mobilization leads to significant enrichment of reactive P in the discharged fine fraction relative to its source. These findings indicate that solid-phase transport driven by groundwater discharge constitutes a previously overlooked but potentially important component of phosphorus cycling in wetland ecosystems. Since particulate inputs do not experience dilution in the same way as dissolved groundwater fluxes, fine-particle transport may represent a hidden and underestimated P source even in low-permeability zones traditionally considered to have weak groundwater influence on lake water quality.