BMP Trains

Keywords

stormwater, littoral, detention, FDEP

Description

This report describes an investigation sponsored by the Florida Department of Environmental Protection (FDEP), in which DB Environmental, Inc., with assistance from Community Watershed Fund, evaluated the potential of littoral zone vegetation to enhance contaminant removal performance of a wet detention pond.

Abstract

This report describes an investigation sponsored by the Florida Department of Environmental Protection (FDEP), in which DB Environmental, Inc., with assistance from Community Watershed Fund, evaluated the potential of littoral zone vegetation to enhance contaminant removal performance of a wet detention pond.

For this effort, we constructed an experimental facility at a 1 hectare wet detention pond in Brevard Co, Florida that contained pre-existing littoral vegetation (distinct stands of cattail (Typha sp.) and pickerelweed (Pontederia cordata)) in addition to an unvegetated shoreline. Ten compartments, 3.7 m by 9.0 m, were deployed within the facility, effectively isolating sections of the pond. Each compartment contained both littoral and deepwater areas, with the littoral zone typically comprising 20% of the surface area of each compartment.

We utilized these compartments to assess contaminant removal during nine simulated storm events from May through December 2004. For each event, we fed pond waters into all compartments simultaneously over a 9-hour period. Native contaminant concentrations in the pond waters were low, so during each simulated storm event we utilized a spiking solution to amend compartment inflows with the following constituents: chemical oxygen demand (COD), 20 mg/L; ammonia-N (NH3-N), 2 mg/L; nitrate-N (NO3-N), 2 mg/L; soluble reactive P (SRP), 0.4 mg/L; copper (Cu), 0.1 mg/L; and lead (Pb), 0.1 mg/L.

Inflow and outflow constituent concentrations were measured during each simulated event. The parameters measured were COD, total suspended solids (TSS), total P (TP), SRP, total kjeldahl nitrogen (TKN), nitrite + nitrate-N (NO2 + NO3-N), NH3-N, Cu and Pb. During interevent periods, which typically lasted two weeks, we also collected littoral zone and open water samples within each compartment to characterize temporal and spatial variations in contaminant concentrations.

We examined the contaminant removal effectiveness of several treatments in this study: compartments with unvegetated vs. vegetated (cattail or pickerelweed) littoral zones; compartments with unvegetated littoral zones that subsequently were planted with cattail or DB Environmental, Inc. Page 2 pickerelweed; and compartments with littoral zones containing cattail where the macrophytes were killed with an herbicide mid-way through the study.

Those contaminants exhibiting highest removal rates during the study, based on mean concentration reductions during the inter-event periods in the littoral region of unvegetated compartments, were NO2 + NO3-N (98%), Pb (93%), SRP (89%), Cu (88%) and NH3-N (87%). Moderate removal rates were observed for TP (63%) and TKN (37%), while relatively poor removal was documented for TSS (27%) and COD (10%). Percentage contaminant removal rates in compartments with vegetated littoral zones were comparable to those in unvegetated compartments.

During inter-event periods, water quality often improved more rapidly and to a greater extent in the shallow littoral region than in the deeper open water region of the compartments. This difference was statistically significant for TP and NH3-N in unvegetated and pickerelweed compartments. Contaminant removal effectiveness within littoral and open water regions, however, was not consistently influenced by presence of either cattail or pickerelweed, whether in existing stands or newly planted.

Presence of vegetation had little long-term effect on contaminant removal rates, although we did observe some short-term differences between treatments. Herbiciding of cattails resulted in a short-term increase in littoral and open water TP and SRP concentrations, but little or no effect on TSS, COD, N or metals concentrations. Additionally, while few water chemistry differences were noted, we did observe in the final months of the study that unvegetated compartments developed a higher standing crop of filamentous algae than vegetated compartments. Similarly, at this time the herbicided cattail exhibited the highest cover of floating duckweed among treatments.

Contaminant removal effectiveness probably was related to the chemical form and concentration of the constituent in the inflow waters. Native COD and organic N in the pond waters were relatively recalcitrant, whereas the spiked aliquots of COD (fructose) and N (NO2 + NO3-N, NH3-N) were readily removed within the compartments. Inflow TSS concentrations to DB Environmental, Inc. Page 3 the compartments were typically 10 mg/L or less, much lower than the average TSS levels found in central Florida urban runoff. These low inflow TSS levels probably explain the low percentage removal rates for this constituent. Additionally, due to low TSS levels, most of the contaminants were provided to the compartments in a dissolved form. This study therefore provides an extensive data record on removal of dissolved nutrients and metals under low TSS conditions, information that should prove useful for wet detention pond performance modeling and design purposes.

Data from this study do not support the hypothesis that littoral zone emergent vegetation, either existing or newly-planted, enhances pollutant reduction in a wet detention pond. However, it should be noted that due to the low TSS levels in the simulated runoff, this study does not represent a definitive evaluation of effects of vegetation on littoral zone pollutant removal effectiveness. Pollutant removal performance of the various treatments (e.g. littoral vs. open water; vegetated vs. non-vegetated compartments) might differ with high inflow particulate concentrations, a situation where sedimentation, rather than biological treatment, would be the dominant removal process for the bulk of the contaminants.

Our experimental facility proved flexible and effective for testing different vegetation treatments that received comparable pollutant loads under replicated conditions. In a final section of this report, we recommend several investigations, such as replicating this effort in ponds with different soil conditions, and evaluating contaminant removal performance under high TSS inflow loads, that should further define littoral zone and macrophyte vegetation effects on detention pond water quality.

Contributors

DB Environmental, Inc.

Date Created

9-2005

Type

Publication

Number of Pages

258

College

College of Engineering & Computer Science

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