Grantee Research Project Results
Final Report: A Stormwater Constructed Wetland Using Renewable and Recyclable Materials and Native Wetland Plants
EPA Grant Number: SU833556Title: A Stormwater Constructed Wetland Using Renewable and Recyclable Materials and Native Wetland Plants
Investigators: Idol, Travis , Real de Oliveira, Verawati Corte , Cho, Alyssa , McDowell, Brianna , Ferguson, Carol , Takara, Devin , Mihlbauer, Edward , Caraway, Kaori , Gautz, Loren , Saunter, Matthew
Institution: University of Hawaii at Manoa
EPA Project Officer: Page, Angela
Phase: I
Project Period: October 1, 2008 through September 30, 2009
Project Amount: $9,968
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2007) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Sustainable and Healthy Communities , P3 Awards , Sustainable and Healthy Communities
Objective:
The Ala Wai Canal in Honolulu drains the watershed encompassing Manoa and Palolo Valleys and empties into the Pacific Ocean in Waikiki, the center of tourism in Hawaii. The Manoa-Palolo watershed is heavily urbanized, and the streams have been highly channelized to promote rapid flow, prevent flooding, and accommodate urban development. As a result, the Ala Wai Canal is on the EPA 303(d) list of impaired streams on the Island of Oahu, with specific concerns for nutrients, pathogens, turbidity, lead, suspended solids, organochlorines and pesticides. The campus of the University of Hawaii-Manoa resides within Manoa Valley. A 1.37 m-diameter concrete storm drain carries stormwater from upslope. The majority of this water originates from parking lots, rooftops, roadways, and residential lawns of the Manoa community. The storm drain empties into an open ditch on the Manoa campus that flows into a concrete canal that drains into Manoa Stream and then into the Ala Wai Canal.
This open ditch provides an opportunity to temporarily capture water from the storm drain and treat it within a constructed wetland. Our objectives for Phase I of this project were to:
- measure the normal range of storm water discharge based on rainfall events;
- analyze the concentration and speciation of non-point source pollutants from the storm drain; and
- design a constructed wetland to capture and treat storm water from the storm drain.
Summary/Accomplishments (Outputs/Outcomes):
To complete the first objective, we installed a weather station within the storm water drainage area that measured air temperature, relative humidity, solar radiation, wind speed, and rainfall. Measurements were taken every 30 minutes and included the average temperature, relative humidity, and solar radiation; the average and maximum wind speed; and cumulative rainfall. Measurements were taken from October 26, 2007 through March 18, 2008. A summary of daily air temperature and rainfall is listed in Figure 1. Air temperatures varied from 15-30°C. The total rainfall was ~800 mm over the 5-month period, which is approx. half of the total annual rainfall for this area (Giambelluca et al. 1986). The maximum rainfall intensity was ~30 mm/hr, and the maximum daily rainfall was 110 mm. By way of comparison, the expected 10-year rainfall intensity is ~60 mm/hour. Over 90% of total rainfall events were < 3 mm.
Figure 1. Summary of Daily Air Temperature and Rainfall. Temperature in °C on the left axis; rainfall in mm on the right axis. Maximum temperature in red; minimum temperature in green; rainfall in blue.
We used a high resolution topographic GIS data layer and a city map of the storm drain network to determine the size and expected water discharge from the storm drain (Fig. 2). The drainage basin area was estimated at approx. 275,000 m2. This includes an elementary school, a shopping center, a public library, greenhouse facilities, homes, and several roads and parking lots. Based on the degree of urbanization in the drainage basin, we estimated approx. 80% of rainfall would run off into the storm drain network. For a 10-mm/hr rainfall event, that would result in approx. 1800 m3 of water moving through the storm drain. This would result in a peak flow of 0.5 m3/sec. Given the small area available for the constructed wetland, this presents a challenge for capturing and treating storm water during the peak portion of a rainfall event.
Figure 2. Drainage Area and Storm Drain Network.
The blue line delineates the total drainage area, based on topography breaks. The green lines delineate the storm drain network. Dots represent storm drain inlets. The white circle identifies the location of the proposed constructed wetland. Below the white circle, stormwater flows aboveground in a series of canals and concretized stream channels into the Ala Wai Canal.
We were able to make some progress on completing our second objective, to measure the pollutant concentrations of storm water. Because of the flashy nature of storm water flow, we were only able to successfully collect water samples from the storm drain during two separate rainfall events. The pH and electrical conductivity were measured in triplicate for each sample using a benchtop pH/conductivity meter. The first sample was taken during a brief, 0.75 mm event that came after a 2-week period with less than 1.5 mm of total rainfall. This represents the “first flush” of storm water that we can expect during most small rainfall events after a dry period. Average pH of this water sample was 8.2, and specific EC was 80 mS/cm. The second sample was taken during the latter stages of a more continuous rainfall event and represents the “average” water quality we can expect during wetter periods. Average pH of this water sample was 7.3, and specific EC was 43 mS/cm. This indicates that there is a high salt content in our water samples, perhaps a result of sea spray accumulation on roads or runoff of fertilizers from lawns. Analysis of these water samples for total nutrients and heavy metals is presently being conducted by the Agricultural Diagnostic Service Center at UH-Manoa.
Our third objective was to design a constructed wetland to capture and treat this storm water. We initially proposed a multiple-cell wetland with a detention basin to capture water and subsurface cells to treat the water as it moved through the system. We also proposed using native wetland plants as part of the system. To create our design, we measured the available area and found we had approx. 40 x 5 m of total area for the wetland. This did not include a 5 x 5 x 0.5 m concrete detention basin at the storm pipe outfall that is used to slow and direct water flow into the existing open ditch. After discussion of our options and constraints, we decided upon a wetland with 4 subsurface flow cells that utilizes renewable and recyclable materials as well as several native wetland sedges. A schematic of the design is illustrated in Fig. 3.
Figure 3. Constructed Wetland Design.
There are several innovative features of this wetland that enhance its sustainability. First, we plan to use recycled, graded concrete as a base material, rather than quarried stone and gravel. This will ensure rapid percolation of infiltrating water. The top layer will be a mixture of soil excavated during the construction of the cells and plant mulch generated by the grounds and landscaping department during maintenance of the campus landscape. The plant mulch will enhance water infiltration and serve as a low-nutrient organic source to help immobilize nutrients in the infiltrating water. This material can be replaced as needed. Coconut coir logs will be placed on top of the soil-mulch surface to filter storm water in the first two cells. A native sedge will be planted in the coir in order to anchor it to the substrate. A hydro-mulch sprigging technique will be used to rapidly establish two additional native sedge species over the remaining surface of the wetland cells.
We used a watershed model in order to determine stormwater runoff and wetland capacity. We used data from December of 2007, the wettest month of the measurement period, to simulate wetland performance. The maximum infiltration capacity of the wetland was estimated at ~200 m3/day. Under conditions of rainy weather over several days, the capacity drops to ~80 m3/day, meaning that most water flows through the system. The residence time of stormwater in the system ranged from 1-5 days, with an average time of ~1.5 days under continuously rainy conditions. The general recommended retention time is 2 days for effective treatment of wastewater. Although this is a significant limitation of our wetland design, the small available area for constructing our wetland is a general problem for highly urbanized areas. In a larger watershed context, the construction of a series of wetlands along a stream or stormwater system would allow for multiple opportunities to infiltrate and treat stormwater, decreasing storm flow and improving water quality overall.
Conclusions:
Our goal was to design a constructed wetland to capture and treat urban stormwater runoff in the Manoa-Palolo watershed in order to improve water quality in the Ala Wai Canal, an EPA 303(d) listed impaired stream. We used on-site weather data and GIS-based delineation of the drainage area and storm pipe network to determine the stormwater runoff in the area. Based on collection of stormwater samples, we found that the first flush of storm water has a high pH and electrical conductivity, meaning that this water is likely impaired for total suspended solids and nutrients. Additional analyses will reveal the degree of heavy metal contamination, as well. Using this information, we designed a multicell subsurface flow wetland that utilizes renewable and recyclable materials and native wetland plants to enhance its performance and sustainability. Based on a watershed model, we determined that this wetland has the capacity to provide reasonable treatment of stormwater, but the constraints of the available area for the wetland will limit the effectiveness during rainy periods or intense storm events. This is a general problem in urbanized areas and highlights the need for multiple treatment wetlands throughout an extensive stream and stormwater network to reduce storm water flows and maintain or enhance water quality.
Proposed Phase II Objectives and Strategies:
Our objectives for Phase II are to build the constructed wetland designed during Phase I, test its treatment performance, and develop an on-site educational display to raise awareness of the need for stormwater treatment and the possibilities of innovative and sustainable solutions such as constructed wetlands to improve water quality in urbanized watersheds. We will utilize local materials where possible to build the constructed wetland and work with local schools to design age-appropriate educational materials. Maintenance of the system will be accomplished through the UH Adopt-A-Landscape program, encouraging students, faculty, and staff on campus to volunteer their time to maintain this important project. Water samples will be sent periodically to the Agricultural Diagnostic Service Center on campus to test for nutrients and heavy metals to track the performance of the wetland.
Supplemental Keywords:
Constructed wetland, stormwater, runoff, infiltration, water pollution, water quality,Relevant Websites:
http://www2.hawaii.edu/~idol/P3.html Exit
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.