Stormwater | Bioretention and Bioinfiltration BMPs


Photo: Rebecca C. Stack
Figure 1. Initial set of bioretention cells at the University of Maryland campus, spring 2003

Three researchers’ experience

By Robert G. Traver, Allen P. Davis, and William F. Hunt

Bioretention and bioinfiltration best management practices (BMPs) are becoming some of the most frequently used stormwater management tools in urbanized watersheds. Designed to address runoff volume, peak, and quality criteria, these BMPs can be used as an integral component of stormwater systems, augmenting and in many cases replacing pure detention-based methods. Designed to infiltrate, evaporate, and filter runoff, bioretention and bioinfiltration BMPs reduce stormwater runoff volumes and act as a filter to mitigate negative impacts from development and changing land use. A cornerstone of low-impact development (LID) design, these BMPs are at their most effective when distributed over a site and incorporated into the stormwater collection system. These control measures directly address recharge, water quality, volume, peak delay, and flow issues.

Despite research demonstrating the enormous and cost-effective benefits of these BMPs, perceived or real design issues have slowed their implementation and inhibit their use. Some of these issues stem from the fact that the bioretention and bioinfiltration concept is a new breed of BMP that integrates knowledge from a number of disciplines. Concerns over performance, groundwater contamination, longevity, and over-infiltration have limited their use. Past BMP design practices and guidelines were directed at smaller runoff volumes and did not take full advantage of volume and peak rate reduction as performance criteria.

To address these issues, professors from the University of Maryland, North Carolina State University, and Villanova University have teamed together to provide research-based recommendations to government and industry on LID stormwater practices, starting with bioretention and bioinfiltration BMPs. What follows is the experiences each of these universities has had in constructing and researching the benefits of bioretention and bioinfiltration BMPs.

University of Maryland
Two bioretention systems have been installed recently on the University of Maryland campus as part of a research, education, and demonstration program. The first is a pair of cells that were completed in the spring of 2003. The second is a set of approximately 10 cells that were spread throughout one area of the campus, installed in the spring of 2005. For a variety of reasons, difficulty was experienced in siting all of the cells, primarily because of concern over both underground and aboveground utilities. A university campus has few areas that do not have underground utilities.

The first pair of cells was constructed to capture runoff from an active student and athletic-event parking lot. Before the bioretention installation, the runoff sheet-flowed directly into Campus Creek, a small stream that runs through the university campus. With the construction of the cells, an asphalt curb was installed around part of the parking lot perimeter to allow an approximately 0.5-acre drainage area to be directed into the bioretention facilities. The flow is split evenly into the two cells (Figure 1).

The media in one cell is a standard Prince George’s County (MD) mix—that is, approximately 2.5 to 3 feet of 50% construction sand, 30% topsoil, and 20% organic matter (by volume). The other cell has an additional 1-foot depth with a mix of coarse sand and shredded newspaper. This layer, with an inverted underdrain to keep it continuously saturated, was installed to promote denitrification of runoff. This media mix was found to be effective in laboratory denitrification studies (Kim, Seagren, and Davis 2003). The cells were lined with polypropylene as a research tool to control the water balance.

A one-and-a-half-year hydrology and water-quality research and monitoring effort has been completed on these cells. Evaluation of 49 runoff events indicates that both are equally effective at reducing peak flows, delaying flow peaks, and reducing the outflow volume (Davis 2008). Eighteen percent of the monitored events were small enough that the bioretention media captured the entire inflow volume and no outflow was observed. During larger events where outflow occurred, it continued for many hours at very low flow rates. Water-quality improvements have also been documented. Suspended solids, phosphorus, copper, lead, and zinc concentrations have all been reduced. Limited data also indicate reductions in nitrate. Because of the limited amount of data and low influent nitrate concentrations, the effect of the additional sand and newspaper layer could not be discerned (Davis 2007). The attenuation of flow is important in the reduction of pollutant mass loadings.

An ongoing project is evaluating the development of flow patterns in the cells. Over time, the soil surface and vegetation characteristics have changed and have created a distinct flow pathway through the cells as the pool fills. This flow pattern will impact the retention time of the water in the cells. Accumulation of pollutants is being monitored, with an emphasis on heavy metals. This accumulation occurs predominantly in the surface media and is focused in the flow path, producing a three-dimensional variation of pollutant accumulation.

The cells have also been used for education. Numerous tours have included these bioretention facilities. The first Low Impact Development Conference, held at College Park, MD, incorporated several tours of these facilities. A number of engineering and environmental science classes at the University of Maryland visit and investigate the cells. A large sign has been posted near an active pedestrian walkway to describe the importance of stormwater management and the performance of bioretention to passersby.

Figure 2. Bioretention cell retrofit before existing storm drain from roadway, University of Maryland

Bioretention cells for the second round of installations were retrofitted around existing stormwater inlets (Figure 2). The same media mix, as described above, was used in all. They have been constructed into an existing parking lot island or between an existing roadway and Campus Creek. One cell is under active investigation to monitor both water quality and hydrology. Parameters under investigation include suspended solids; nitrogen and phosphorus compounds; and a suite of toxic metals, including copper, lead, zinc, cadmium, arsenic, mercury, and chromium. A few samples have also been investigated for polycyclic aromatic hydrocarbons. Preliminary data from the study suggest improvements in both hydrology and water quality.

A further environmental management issue that becomes clear with these bioretention facilities is that they are very effective in the collection of trash. Trash that flows in with runoff, as well as blowing trash, is effectively collected in the bioretention vegetation and media layer. In order to maintain a clean facility, collected trash must be removed relatively frequently. This visible environmental eyesore is prevented from entering our local water bodies.

Very little maintenance has been applied to the University of Maryland cells since their installation. The ecological transformation and evolution of the cells is being followed. Over time, they appear to be developing into unique ecosystems. Carbon cycles are developing, and changes are taking place as vegetation dies, organic matter is cycled, and new vegetation grows and volunteers into the facilities. How all of this impacts stormwater management issues is yet to be determined.
Additional initiatives are under way to incorporate more bioretention and other LID stormwater management technologies into the University of Maryland campus infrastructure. The success demonstrated by the existing facilities will facilitate these advancements.

Figure 3. Hal Marshall Bioretention Cell in the shadow of uptown Charlotte, NC

North Carolina State University
North Carolina State University; the City of Charlotte, NC; and Mecklenburg County, NC, have been collaborating on the study of 12 stormwater management practices in Charlotte since 2004. Charlotte is the largest city in North Carolina with an estimated population of greater than 660,000 people. The city is also center of one of the fastest-growing metropolitan areas in the United States. With this growth, Charlotte has experienced a boom in the usage of both traditional and innovative stormwater management practices. One of the practices studied by North Carolina State University was a retrofit bioretention cell located in uptown Charlotte, making it one of the most urban locations for bioretention in the state (Figure 3). The bioretention cell received runoff from a parking lot serving the Hal Marshall County Government Center and has since been named the Hal Marshall Bioretention Cell (HMBC).

Mecklenburg County designer Frank Hahne, P.E., designed the HMBC in 2002 and followed the conventional bioretention design format: The cell is 4 feet deep with a standard underdrain configuration. The cell’s ponding depth was approximately 6 inches, and it was designed to capture the 25-millimeter (1.0-inch) water-quality event. The HMBC surface area was 229 square meters (2,480 square feet) and drained 0.37 hectare (0.92 acre). The cell is on an embankment located adjacent to the city of Charlotte’s light rail line, with the bottom of the bioretention cell being approximately 20 feet above the railway line. This relatively dramatic difference in elevation would greatly impact the bioretention cell’s hydrology.

In part due to previous research conducted in Greensboro and Chapel Hill, NC, the fill media used at the site were predominantly mason sand mixed with an organic leaf amendment. The fill media had a very low phosphorus index (P index), which is a measure of soil-bound phosphorus (Hunt et al. 2006). This means that the cell was expected to be able to capture influent phosphorus in stormwater runoff.
The planting density was relatively high, with a stem count of approximately 1,100 stems per hectare (2,800 stems per acre), and vegetation species included blue flag iris (Iris virginica), hibiscus (Hibiscus spp.), sweet pepperbush (Clethra alnifolia), Virginia sweetspire (Itea virginica), and wild oat grass (Chasmanthium latifolium). All of these plants had previously been successfully used in other bioretention cells in North Carolina.

Figure 4. The HMBC with forebay in foreground

Another unique feature of the bioretention cell was a forebay that was retrofitted into the cell (Figure 4). Because the cell was built in part to be monitored, all the influent entered through a single concrete chute. This resulting concentrated flow caused erosion in the cell initially at the entrance point. The forebay was then installed to dissipate energy and provide initial settling of solids. The forebay was underlain by an impermeable membrane to prevent short-circuiting to the underdrains only 2 feet below the bottom of the forebay (as compared to 4 feet from most of the rest of the cell’s bowl).

The bioretention cell was monitored from February 2004 through March 2006 for many pollutants: total Kjeldahl nitrogen, ammonium-nitrogen, nitrate-nitrite-nitrogen, total phosphorus (TP), total suspended solids (TSS), biological oxygen demand, copper, zinc, iron, lead, fecal coliform, and Escherichia coli (E. coli). The first 10 pollutants listed were flow-weighted composites from 23 events. The latter two were grab samples and were collected 19 times for fecal coliform and from 14 events for E. coli. Collection averaged approximately once per month and were distributed seasonally. The rainfall events monitored ranged from 6.4 millimeters (0.25 inch) to 80.8 millimeters (3.08 inches), with a mean and median event size of 27.4 millimeters (1.08 inches) and 24.1 millimeters (0.95 inch), respectively.

Results from this two-year study were, for the most part, very promising. Efficiency ratios for most pollutants ranged from 0.30 to 0.70. In particular, TSS, total nitrogen (TN), and TP efficiency ratios were 0.60, 0.32, and 0.32, respectively. Efficiency ratios fail to compare, however, input and output pollutant loads. Because of the unique position of the HMBC, the volume of outflow was between 40% and 50% less than that of inflow. This roughly translates to pollutant load reductions for most pollutants—including TN and TP—of higher than 50%. The high TP removal (or accumulation of phosphorus in the media) was expected, as the fill medium was selected because it contained very little soil-bound phosphorus.

The only pollutant whose concentration and load increased was iron. This was attributed to the surrounding soil’s high iron content and an assumption that iron leached from these soils. The phenomenon has been observed at many locations throughout the North Carolina piedmont.

Perhaps the most promising results from the monitoring at the HMBC were those associated with pathogenic bacteria. Fecal coliform and E. coli effluent concentrations were both nearly 70% lower than influent concentrations. When comparing influent fecal coliform concentrations to the nationally predominant standard of 200 colony-forming units per 100 milliliters, 16 of 19 were above the 200-colony-forming-units-per-100-millileters limit. In contrast, of the 19 effluent concentrations, only five were above this same limit. This finding is reasonable, because two of the major means of pathogenic bacteria die-off are drying out and exposure to sunlight. A bioretention cell such as the HMBC is certainly expected to—and did—dry out between events. The cell was relatively well vegetated, meaning that bacteria were actually safer from ultraviolet exposure than they would have been in a more open bioretention cell.

The bioretention cell at the Hal Marshall County Government Center is a good example of an ultra-urban bioretention retrofit. It employed a few newer design techniques such as a low-phosphorus fill media and a forebay. The cell has performed admirably with respect to pollutant removal, including that of pathogenic bacteria indicator species. The HMBC is a model for the use of bioretention in ultra-urban environments.

For more information, including detailed reports of this and 11 other stormwater management practices monitored in Charlotte, NC, please visit the North Carolina State University Stormwater Group Web site: http://www.bae.ncsu.edu/stormwater.

Figure 5. Bioinfiltration Traffic Island during storm event

Villanova University
The Villanova Bioinfiltration Traffic Island (Figure 5) was constructed during the summer of 2001 as a stormwater retrofit best management practice to enable research, to act as a demonstration site, and to reduce runoff volume from the site. Designed to capture the first 2.5 centimeters (1 inch) of runoff from the 0.53-hectare (50,000-square-foot) watershed, the site is part of the Villanova Stormwater Best Management Practice (BMP) Research and Demonstration Park. Continuous monitoring of the site is ongoing, and the BMP is part of the USEPA 319 Nonpoint Source National Monitoring Program. The site is instrumented to monitor both water quality and quantity from both surface water and the vadose zone (Ermilio and Traver 2006). Over the last six years, the site is estimated to have captured approximately 85% of the annual precipitation and 70% of the runoff entering the site with little to no maintenance.

The BMP was built into an existing taffic island as part of a student parking area in Villanova’s west campus dormitory complex in 2001. The site was retrofitted by excavating a 6-foot-deep depression and backfilling with a sand-soil mixture. This created a bowl-shaped depression to hold the water for infiltration. The site was planted with salt-tolerant grasses and shrubs native to the New Jersey shore. What differentiates this from bioretention is that the site is designed to infiltrate, not retain, the stormwater after the water passes through the soil layer. There is no underdrain or geotextile preventing the water from seeping into the ground.

A calibrated and verified hydrologic model of the site was developed based on the monitored rainfall and depth data from several years of operation (Heasom, Traver, and Welker 2006). Application of this model using the recorded data allows for deeper understanding of both individual and long-term BMP performance. For example, in 2005, only six of the 77 recorded rainfall events recorded significant overflow, and those events required close to 2 inches of rainfall to cause overflow. That is specifically significant as the volume of the surface “bowl” of the BMP is approximately 0.6 inch over the watershed impervious surface.

A common question is the effectiveness of infiltration BMPs during extreme events. In October 2005, the site experienced an extreme late-peaking storm event of 6 inches over a day. As this was a late-peaking storm—the worst possible from a stormwater design basis—it was surprising to find that infiltration occurred throughout the entire storm event. There was no impact on the infiltration on the recession limb of the storm as compared to other recorded events. Even at the very end of the storm during the heaviest portion some reduction in peak flow was realized.

Water-quality aspects of this site are under study and are available through the USEPA 319 Nonpoint Source National Monitoring Program (www.epa.gov/OWOW/NPS/cwact.html). Water-quality questions include both surface-water and groundwater effectiveness and whether a first flush is observed. From a surface-water perspective, the removal is closely tied to volume. It is interesting to note that for overflow events, the nutrient levels leaving the BMP may be higher due to the plants in the bioinfiltration BMP. However, when coupling the hydrology and water quality, and when considering that out of 48 inches of recorded rainfall, only 5.5 inches left the site in 2005, the removal rate is impressive (Ermilio and Traver 2006).

It is clear from Heasom’s work (2006) that there first is a rapid infiltration at the beginning of the storm, which then changes to a constant infiltration rate. The rapid infiltration at the beginning of the storm greatly increases the performance of the BMP. Next, there is a cyclic seasonal change in infiltration rates, with the highest levels occurring in the early fall. The peak warm-season infiltration rate is about twice that of the winter. Comparing results from the bioinfiltration BMP with other sites suggests that temperature effects on viscosity may explain much of this effect. It is clear that the plant growth has a role in maintaining or reopening pathways. Finally, after six years of minimal maintenance, while it is clear that sediment has entered the BMP and that the surface layer is changing, no statistical degradation has been found.

To date the hydrologic and pollutant removal performance of this bioinfiltration BMP has proved to be exceptional over a multiyear period. The value of long-term continuous monitoring has been demonstrated, as well as the education value of the site to the continuous stream of visitors. Funding for developing the bioinfiltration traffic island and monitoring the site was provided by the Pennsylvania Department of Environmental Protection (PaDEP) through a Section 319 Nonpoint Source Implementation Grant (funded by the EPA), as well as PaDEP’s Growing Greener grant program. Further research funding sources include the EPA’s Water Quality Cooperative Agreement program and the Villanova Urban Stormwater Partnership (VUSP) partners (www.villanova.edu/VUSP). The authors wish to acknowledge the work of William Heasom (VUSP research fellow) and the many graduate students who have worked on this site.

Summary
As the practice of stormwater management moves toward addressing runoff volume and quality, new design practices and innovative technologies are being explored. While these devices may appear simple, many questions remain as to their function, performance, design, maintenance, longevity, and how to best utilize them as part of a watershed plan. Of great interest is how to model these and other BMPs from both a water-quality and -quantity perspective. Current guidance underestimates their hydrologic performance and thus limits their application.

The researchers’ objective is to explore and provide a fundamental understanding of the hydrologic and water-quality performance of bioretention as a stormwater management practice. Once this is achieved, it will allow the affirmation of design criteria correlated to expected performance, individually and cumulatively, throughout a larger watershed. This work is desperately needed by the stormwater community to remove barriers and to encourage the use of this attractive, cost-effective, and maintainable technology.

The authors wish to acknowledge their innumerable graduate and undergraduate students, as well as their many colleagues for help.

References
Davis, A.P. 2008. “Field Performance of Bioretention: Hydrology Impacts.” J. Hydrology, in press.

Davis, A.P. 2007. “Field Performance of Bioretention: Water Quality.” Environ. Engg. Sci., in press.

Ermilio, J.R., and R.G. Traver. May 2006. “Hydrologic and Pollutant Removal Performance of a Bio-Infiltration BMP.” Paper presented at the EWRI Water Congress in Omaha, NE.

Heasom, W., R. Traver, and A. Welker. 2006. “Hydrologic Modeling of a Bioinfiltration Best Management Practice.” J. Am. Water Res. Assn .42(5): 1329-1347.

Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. “Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina.” ASCE Journal of Irrigation and Drainage Engineering 132(6): 600-608.
Kim, H., E.A. Seagren, and A.P. Davis. 2003. “Engineered Bioretention for Removal of Nitrate from Stormwater Runoff.” Water Environ. Res. 75(4): 355-367.

Robert G. Traver is an associate professor in the Department of Civil and Environmental Engineering at Villanova University in Pennsylvania and is the director of the Villanova Urban Stormwater Partnership. Allen P. Davis is a professor in the Department of Civil and Environmental Engineering at the University of Maryland. William F. Hunt is an assistant professor and extension specialist in the Department of Biological and Agricultural Engineering at North Carolina State University in Raleigh.

SW October 2007

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