Load- versus concentration-based water quality metrics: Field assessment of permeable pavement and filter drain under Mediterranean rainfall in Valencia, Spain

Abstract

Study Region Valencia, Spain. Study Focus Two full-scale Sustainable Urban Drainage Systems (SUDS), a permeable pavement (PP) and a filter drain (FD), were monitored over one year at a residential street in Valencia under Mediterranean rainfall conditions. Hydraulic and water-quality performance were evaluated using concentration-based (Event Mean Concentration, EMC) and mass-based (Pollutant Mass Loading, PML) indicators. Rainfall, flow, and water quality were monitored at a conventional gully (CG) as a reference point and at both SUDS outlets across 14 rainfall events. Parameters included conventional pollutants (TSS, COD, BOD, TN, TP), 12 trace metals and metalloids, hydrocarbon fractions, and 16 PAHs. New Hydrological Insights for the Region Both systems attenuated runoff consistently, with volumetric efficiencies of 24–100% (mean: PP 57%, FD 65%) and no measured overflow for events below 10 mm, extending to 30 mm for FD. Concentration-based efficiencies were moderate and event-dependent, with occasional negative values for TSS, Cr and Ag at PP. Mass-based indicators revealed consistently higher reductions for most pollutants (SOL: 65–95%), even when outlet EMCs exceeded those at CG, indicating that volume reduction drives pollutant export reduction in highly retentive systems. A general first-flush tendency was observed across the 20 analysed pollutants. These findings indicate that, in highly retentive systems, EMC-based assessments alone can understate SUDS performance in terms of pollutant export, and that both metrics provide complementary information.