Constructed Wetlands for Stormwater Management

Abstract

Wetland systems have long been used for treating municipal and industrial wastewater, and are often more cost-effective than advanced wastewater-treatment systems. The available literature on the performance of wastewater-wetland systems is fairly extensive; however, information on the use of natural or constructed wetlands for controlling stormwater pollution, a newer application, is relatively scarce. In addition, the data available on wastewater wetlands may not apply directly to stormwater wetlands because of the unique characteristics of stormwater runoff, including: highly variable flow rates that are usually intermittent and seasonal; two-to-three order of magnitude between-storm differences in the chemical and suspended solids composition of stormwater runoff; and, site-specific nutrient ratios and concentrations. Consequently, in stormwater wetland systems, plant species' performance and removal efficiency is often tied to the biota's ability to tolerate these extremely variable conditions. Additional factors which effect pollutant removals include: the comparative contributions of settling, sorption, plant uptake under various hydrologic (e.g., detention time) conditions, and a pollutant's partitioning between the substratum, water column, and plant tissue. To address these unknowns and design concerns, a bench-scale vegetative-treatment-cell study is being conducted for the purpose of examining nutrient and metal dynamics and removal efficiencies of three individual plant species under various stormwater pollutant loadings and detention times. The plant species being evaluated are: cattails (Typha latifolia), reeds (Phragmites sp.), and bulrushes (Scirpus sp.). For each of these species, two detention times (7 day and 14 day) and two pollutant loading levels (e.g., COD and SS >100 mg/l and <75 mg/l, respectively) are being investigated to determine wetland-process dynamics; relative removal efficiencies of stormwater-associated pollutants (e.g., total phosphorus and copper) as a function of detention time, water depths, and pollutant loadings, and expected solids removal and effluent-particle-size distributions.