In Situ Partial Exfiltration of Rainfall Runoff. I: Quality and Quantity Attenuation

Rainfall runoff impacted by anthropogenic activities transports significant quantities of particulate, aqueous, and complexed constituents. These diffuse, unsteady, and stochastic event-based loadings are unique challenges for water quality (concentration, mass) and quantity control (volume, peak flow). While many infiltration/exfiltration structural best management practices (BMPs) or low impact development practices are implemented, few in situ data sets are examined for actual events and temporal-based BMP behavior, in part due to costs of such examinations. Fewer studies provided a statistical and mechanistic interpretation for event-based BMP performance. The design, water quality, and quantity functions of a partial exfiltration reactor (PER) utilizing Fe-coated sand is examined specifically across three water quality type rainfall-runoff events over a 10-month period. Reduction of total concentrations for metals (Zn, Pb, Cu, and Cd), ranged from 24 to 93%, while total mass reductions ranged from 57 to 98% due to exfiltration. Reduction in total suspended solids concentrations ranged from 23 to 86% while reduction in total mass ranged from 69 to 96%. Chemical oxygen demand concentrations reductions ranged from 37 to 70%. Storm water volume reductions ranged from 55 to 70% through variably saturated exfiltration to surrounding clayey glacial till soils $(K_{\mathrm{sat}}={10}^{-6}\mathrm{cm}/s)\text{,}$ while peak flow reductions ranged from 36 to 85%. Results of statistical analysis indicate that a passive downflow PER is capable of functioning as an in situ water quality and quantity control BMP for rainfall runoff. Results indicate that as structural and nonstructural controls are implemented, monitoring, examination, and understanding of event-based and life-cycle performance are critical to achieve both quantity and quality goals.