Urban stormwater runoff is a growing contributor to the impairment of surface waters. Nature-based technologies, including green roofs, vegetated swales, grassed filter strips, bioretention, and pervious pavements have been demonstrated to be effective in mitigating detrimental runoff characteristics. Lacking, however, has been a fundamental engineering analysis approach to these technologies. Current design guidance is based on empirical recommendations and most performance data are based on limited, localized observations. Flow balances, including infiltration, evapotranspiration and surface discharge, based on fundamental fluid dynamics principles can be employed through analysis to understand flow management. Water quality improvements will occur through a specific unit processes or mechanisms, including sedimentation, filtration, adsorption, biotransformation, bio-uptake, and heat transfer. The performance of a specific technology will depend on the facility configuration and makeup, climate, surrounding soil characteristics, topography, and the site hydrology. Applying fundamental flow and water quality processes to storm water management technologies will allow quantitative design and predictable performance characterization.
Our legacy infrastructure relies heavily on impervious surface. Stormwater runoff generated from this surface continues to cause myriad problems within our watersheds, including flooding, erosion, lack of ground water and base flow recharge, and aquatic habitat destruction (Wang et al. 2003, Walsh et al. 2005). Within the past decade, a number of novel, nature-based stormwater control measures (SCMs) have been developed and are being implemented to help address the growing urban stormwater problem. Specific technologies include bioretention, vegetated swales and filter strips, green roofs, and permeable pavements. Nature-based SCMs commonly include some type of soil-based media to promote water storage and infiltration. Water quality improvements, due to pollutant removal mechanisms such as sedimentation, filtration, and biological transformation, are expected as well and have been documented in several studies.
While implementation continues at an increasing rate, our fundamental understanding of the performance of these facilities is still forthcoming. Designs are highly empirical and performance results are broadly applied across different situations. Performance is assumed to be independent of many highly dynamic conditions to which these facilities are constructed and exposed, including different land uses, climates, rainfall patterns, geometry, sizes, and soil and vegetation characteristics.
Fundamental flow and mass balance analyses, however, can shed new light on the capabilities (and limitations) of such facilities, and can lead to advancements and improvements in performance. A simple diagram representation of these balances is presented in Figure 1.
Input flows are dependent on the corresponding land use in the watershed and the various rainfall patterns of exposure. Upon entering the SCM, the flow has a limited number of pathways. Storage can occur in a surface ponding area and also within the pores of the soil/media/pavement/gravel of the facility. Subsurface storage may be encouraged in some SCMs by the use of an underdrain that is upturned (Davis et al. 2009).
The desired primary flow pathway for these SCMs is infiltration into the surrounding soils. High permeability, sandy media are usually specified for nature-based SCMs to encourage infiltration. Other SCMs such as permeable pavements utilize underground storage areas. In either case, net flow into the surrounding soils (exfiltration) will depend on the hydraulic characteristics of the soils, as well as the interfacial area between SCM media and native soils, leading to variable performance with design. Infiltration rates vary seasonally, in part due to the temperature dependence of water viscosity (Emerson and Traver 2008). Thick, leafy vegetation also produces some storage, and maintains flow pathways via their root structure. The infiltration pathway will recharge ground water and contributes to local stream baseflow.
During a rainfall event, the evapotranspiration (ET) from an SCM is likely negligible. However, stored water will undergo ET during the drying times between rainfall events, restoring the ability of the SCM to capture runoff. Due to the collection of runoff from the contributing impervious area, it should be noted that more water is available (up to 10 to 30 times) to a SCM then a “normal’ vegetated site. Therefore, on an annual basis, evapotranspiration (ET) may be a significant pathway in the SCM water balance. One North Carolina bioretention study has estimated the ET contribution as 18 percent (Li et al., 2009). ET is the primary water pathway for green roofs, with studies showing as much as 60 percent loss due to ET (Hathaway et al. 2008).
Excess water volume, beyond these pathways, will be discharged as surface flow. Design enhancements to the SCM can target reduction of surface discharge through increased infiltration/exfiltration, storage, and ET.
Stormwater quality is compromised in several ways during impervious overland flow. Deposited soils and wear particles from buildings, vehicles, and pavements are mobilized. Similarly, heavy metals such as copper, lead, and zinc are mobilized as they are corroded and worn from building materials; automobile also wear components such as brake pads. Hydrocarbons are contributed by fuel and other vehicle leaks and atmospheric deposition. Excessive fertilizer use on lawns and garden areas and atmospheric deposition will contribute nitrogen and phosphorus compounds. Taken cumulatively, the chemical water quality makeup of urban stormwater runoff can be quite poor.
Focusing on specific physicochemical or biological processes that are operative in nature-based SCMs can allow removal mechanisms and efficiencies to be examined. Of these processes, most are known and successfully exploited in water and/or wastewater treatment. Fundamental science and engineering information is available and employed in design and operation of these systems. These same principles can be applied to SCMs under the proper operational and environmental conditions.
Primary mechanisms for removal of particulate matter from water include sedimentation and filtration. Several SCMs provide sedimentation, where ponding and other pooling areas are encouraged. Effective sedimentation requires that the water flow velocity be slowed so that time is available for the particulate matter to settle to the water bottom within the designated sedimentation area. Larger, higher density particles are more effectively removed via sedimentation.
Filtration/infiltration technologies will filter fine particles. Porous media filtration is very effective for particulate removal. The use of relatively fine media and the resulting relatively low flow rates compared to rapid sand filtration common in drinking water systems can provide excellent removal of particulate matter. Large particles should be strained out at the media surface. Smaller particles are captured by the media via sedimentation, interception, and diffusion transport mechanisms. Consequently, the large bulk of the captured solid load is expected at the surface of the filter SCM. Supporting this background theory, excellent removal of suspended solids in field bioretention systems has been noted (Davis 2007, Li and Davis 2009). Laboratory studies also indicate excellent particulate removal and accumulation at the media surface (Li and Davis 2008a, 2008b).
Capture of particulate matter is important in managing urban stormwater quality because a number of important pollutants tend to adsorb or partition to some extent onto the urban particle load. These include a number of toxics, such as many heavy metals (lead, copper, zinc, cadmium) and hydrophobic organics, such as polycyclic aromatic hydrocarbons (PAH). Some fraction of the phosphorus load is also in particulate form. Therefore, efficient particle removal leads to effective removal of several other pollutants of importance.
Treatment of bacteria and other pathogenic microorganisms should follow mechanisms similar to those for particulate matter. Laboratory studies suggest that bacteria are efficiently removed by bioretention media (Rusciano and Obropta 2007, Zhang et al. 2010). Field studies support the concept of filtration and ultimate die off of pathogen indicator species as well (Passeport et al. 2009, Hathaway et al. 2009). Understanding microorganism removal and fate are complicated by the complexity of the microorganism itself and the cell wall. Also important is the growth or decay of the microorganisms once captured by the media.
The soil-based media common in filtration/infiltration facilities provides numerous opportunities for the adsorption of a variety of pollutants. Hydrophobic organic compounds such as PAH and other fuel-based hydrocarbons will partition into soil organic matter. Since many media designs require high or supplemented organic material, the opportunity for adsorption is high.
Heavy metals also tend to bind strongly to soil media. Both the organic and inorganic fractions, especially hydrous oxides (iron and aluminum oxides) provide complexation sites for the binding of metals. Metal adsorption is generally a strong function of pH, with increasing metal adsorption at higher pH. Because of the low metal concentrations typical of urban runoff (101–102µg/L), effective adsorption tends to occur within the pH range of the soil media (6–7).
Orthophosphate will also adsorb to soils. Significant work in the agriculture sector has found that in acidic soils the adsorption of phosphorus is primarily controlled by the content of amorphous iron and aluminum oxides in the soil. These minerals provide surface area and binding for the phosphorus. SCM media can have highly variable amounts of phosphorus adsorption capacity and previously-loaded phosphorus. Since lakes and rivers are very sensitive to even low phosphorus levels, phosphorus surface discharges will have to be small to provide excellent water quality. Much SCM media will not have adequate phosphorus capacity and ways to supplement this capacity should be explored. Understanding phosphorus dynamics in nature-based SCMs will involve greater understanding of the chemical makeup of the media.
Other pollutants of interest in urban runoff may also be adsorbed. These will include ammonium, which should be adsorbed onto negatively-charged soil particles, but may be subsequently biotransformed.
The adsorption process in a media-based SCM should behave as an adsorption column, as depicted in Figure 2. As the media is loaded with runoff, the pollutants will be adsorbed by the surface media. A local equilibrium may even be reached. As the sites become equilibrated with the input (not saturated), the active loading zone of the column (SCM media) will work downward. The capacity of the media will be utilized as the working zone approaches the bottom of the available media. Therefore, as with filtration, the majority of the pollutant accumulation will occur at the media surface as noted for metals (Li and Davis 2008c) and PAH (DiBlasi et al. 2009). The lifetime of the media will depend on the strength of the adsorption and the concentration of the incoming pollutants. The media may be expected to have a long lifetime for metals and hydrocarbons (many years), but less for phosphorus (few years or less).
Biological transformation of a pollutant usually involves the pollutant acting as an electron donor or acceptor during a metabolic process of the organism. In aerobic environments, microorganisms will utilize oxygen as an electron acceptor, with the pollutant being the electron donor. Biological environmental processes are generally slow as compared to physicochemical processes and are not generally expected to be operative during the timeframe of a runoff event. However, adsorbed or otherwise trapped pollutants can be biotransformed during the inter-event duration. Therefore, the impacts of hydrology and flows are not necessarily based on the storm event, as for most pollutants, but maybe more on inter-event conditions. The efficiencies of the biological processes will depend on the media environment. Transformations will be most rapid with the proper mix of water and oxygen (water tends to limit oxygen availability), substrate availability, and temperature. The rates will likely decrease during times of water saturation, periods of very dry conditions, and low temperatures (Hunt et al. 2006).
Hydrocarbons can be biodegraded aerobically under optimized environmental conditions (Rittmann and McCarty 2001). Therefore, it is reasonable to expect hydrocarbons in runoff to be biologically degraded in SCMs, albeit slowly. As discussed above, typical fuel hydrocarbons can be strongly adsorbed by the organic matter in the media. During the time between rainfall events, on the order of days, these hydrocarbons can be degraded. The environmental conditions are appropriate for this process: adequate microorganisms (many studies have shown that native populations can biodegrade hydrocarbons), oxygen near the surface, appropriate pH and temperature, and availability of micronutrients.
This process has been examined by Hong et al. (2006). In these studies, toluene, naphthalene, and used motor oil were loaded onto a bioretention compost material during a stormwater runoff scenario. Mass balance and microbial evidence suggests that the sorbed hydrocarbons were significantly biodegraded (greater than 90 percent) after 5–15 days. The compost was able to remove the hydrocarbons from the stormwater and the microbial processes transpired subsequently.
Transformation of ammonium to nitrate through nitrification can also be expected in SCM media. This is an aerobic process that proceeds typically via Nitrosomonas and Nitrobacter species.
This biological process has been noted to be important in bioretention because the ammonium can first be readily adsorbed onto the media during a runoff event. As with the hydrocarbons, sorbed ammonium is available for biotransformation continuously and transformations will occur between storm events. Subsequent rains will wash this accumulated nitrate from the media. Excess nitrate leaching from laboratory bioretention studies has been noted in several cases. Usually these are un-vegetated systems with highly aerobic environments that will promote nitrification (Davis et al. 2006, Hsieh et al. 2007).
Creating anoxic conditions may play a critical role in exploiting SCMs for nitrogen management. Denitrification is an anoxic process, where nitrate acts as the electron acceptor and the electron donor is typically some type of organic material, which should be abundant in bioretention and other similar SCMs. Nitrogen gas, N2, is the product of denitrification, a benign nitrogen form that can be released to the atmosphere.
Creating anoxic zones in SCMs can be problematic because the media and configuration are generally established with a goal of high flow rates, and therefore the media tend to be aerobic. Denitrification can be expected in areas of water saturation where common aerobic processes have readily consumed the available oxygen.
Anoxic conditions can be expected under several circumstances. First, a specific engineered zone that will remain saturated can be created in a bioretention cell or some other SCM with subsurface storage. This may result from a raised underdrain (Kim et al. 2003, Hunt et al. 2006) or a less impervious media layer over a more pervious one (Hsieh et al. 2007, Cho et al. 2009). Denitrification may be expected only within trapped water and low efficiency may be expected for water that free-flows through these zones.
Otherwise, areas of micro-anoxic conditions are possible dispersed throughout the media of a SCM. Pockets of high organic matter content, low flow media, and high microbial levels may lead to areas in which denitrification can occur. Nitrate performance can vary greatly among SCMs such as bioretention (Davis et al. 2009; Hatt et al. 2009; Li and Davis 2009).
The role of vegetation in SCM performance has not been firmly established or quantified. The vegetation can impact several important water and pollutant fate pathways. Physically, strategic placement of vegetation can assist in addressing important flow issues, such as slowing or directing flow. Thick stands of vegetation contribute to high ET and are expected to contribute to the uptake of nitrogen, specifically, but also phosphorus and possibly other trace pollutants. Several recent studies have demonstrated the importance of vegetation in bioretention nutrient management (Bratieres et al. 2008, Lucas and Greenway 2008). Of course, from a mass balance perspective, removal of plant-accumulated pollutants from the SCM necessitates instituting a cutting or harvesting program. Vegetation may also promote subsurface microbiological and/or physicochemical processes within and around the rhizosphere.
Thermal pollution is a concern in most Northern states, but even impacts cool trout waters of states as far south as North Carolina and Tennessee. SCMs that expose runoff to cooler temperatures at the bottom of the media column (bioretention) or gravel layer (permeable pavement) are better able to mitigate thermal effects. Designers can reduce thermal loads by using somewhat lower permeability fill media or by employing storage zones such as those described previously for bioretention. Field studies have demonstrated the benefit of increased hydraulic retention time for bioretention (Jones and Hunt 2009) and permeable pavement (TRCA 2006). Perhaps the greatest thermal load benefit employed by SCMs is exfiltration. Even if temperatures are not well mitigated within the SCM, water that infiltrates will eventually cool to ambient ground temperature (Jones and Hunt 2009).
Some SCMs can add to thermal loads by exposing ponded water to sunlight. Field studies have shown that outflow temperatures leaving wet ponds and created wetlands is often greater than that of influent runoff (Jones and Hunt 2010, Kieser et al. 2004). Shallow media systems can also add thermal load to runoff due to direct sunlight exposure to the fill media, which eventually warms runoff prior to discharge. Understanding flow rates, heat transfer rates, hydraulic retention time, and potential infiltration are critical for optimizing thermal load reductions.
While novel nature-based SCMs are employed to improve the flow and quality characteristics of urban runoff, metrics to quantify SCM performance remain too focused on event-based traditional end-of-pipe discharges and inhibit advances in mechanistic understanding of performance. Generally, current practice uses some fixed performance metric value, typically a percent removal, to describe the water quality or flow improvement for SCMs. This is overly simplistic for two reasons. First, as discussed above, lumping all designs and applications into a single performance does not account for the wide variability in design and expected performance. Second, because of the great variation in stormwater flows and concentrations from event to event, percent pollutant or volume removals are highly variable and are not a proper metric for water quality protection. A SCM with a low percent removal performance receiving a low concentration pollutant may produce better water quality than a high percent removal receiving a high concentration (Strecker et al. 2001; Wild and Davis 2009). Proper evaluation of SCM performance at both the site and ultimately the watershed scales requires understanding the fundamental processes of water balances and pollutant removals under the appropriate design, stormwater flow, and environmental conditions. Since the input to SCMs follow a probability distribution, distributed outputs are expected and performance metrics should be based around this type of output.
Nature based SCMs are not black box practices that operate via unknown empirical treatment processes. The operations and treatments occur via specific flow and environmental process pathways that are known to engineers and scientists in other applications. The stormwater application is unique because of its high variability of input, operating, and design conditions. Carefully applying fundamental principles for the various operative processes will allow better design and performance. Continued research in this area promises more efficient management of urban water and reduced impact on the surrounding water environment.
This work was supported by the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET).
Author Bio and Contact Information
Allen P. Davis is Professor in the Department of Civil and Environmental Engineering at the University of Maryland, College Park, MD 20742–3021. He can be contacted at firstname.lastname@example.org.
Robert G. Traver is Professor in the Department of Civil and Environmental Engineering, Villanova University, Villanova, 19085. He can be contacted at Robert.email@example.com.
William F. Hunt is Associate Professor and Extension Specialist, Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC 27695–7625. Contact email: firstname.lastname@example.org.