Effects of transient surface water–groundwater exchange on the transport of organic micropollutants: temporary storage, retardation, and remobilization

Surface water–groundwater (SW–GW) interfaces are commonly evaluated as steady attenuation barriers, although field conditions are rarely steady. Storms, floods, seasonal stage fluctuations, tides, pumping, and managed recharge reconfigure exchange fluxes and flow paths. This critical review synthesizes how transient exchange alters organic micropollutant (OMP) transport and fate, distinguishing direct OMP observations from hydrological analogy, controlled experiments, models, and conceptual inference. Hydrologic forcing changes cross-interface loading and redistributes travel and residence times, producing rapid breakthrough, retardation, temporary storage, apparent aqueous attenuation, transformation, or remobilization. Within the literature corpus assessed here, transient water exchange and changing water ages are supported mainly by hydrological studies; direct event-scale field mass balances linking OMP storage to delayed release were not identified. We use hydrologically realized mobility, dynamic reaction area, and antecedent-state dependence as testable integrative concepts. Compound-specific fate depends on whether time-varying travel-time distributions overlap with favorable reaction conditions and inherited contaminant mass. Simultaneous boundary monitoring should be combined with travel-time-adjusted source–response pairing, conservative tracers, OMP measurements, transformation products, and spatially resolved reaction-state indicators. Models should constrain transient hydrodynamics before estimating retardation and reaction parameters, then test predictions against independent events. Event-resolved exposure assessment should report arrival time, peak concentration, cumulative breakthrough mass, exposure duration, and post-event recovery, with uncertainty and decision thresholds stated explicitly.