Rain water that runs off from sites that are being developed (during construction) or have been developed (after landscaping is complete) is called stormwater. Until recently stormwater management focused largely on reducing the peak rate of runoff from large, rare rainfall events. This led to the creation of numerous small, post-development, individual-parcel, stormwater basins, each designed to attenuate the peak stormwater outflow rates for 2- to 100-year return period storms. Over the past decade, much of the effort relating to stormwater management has focused on improving the quality of the runoff. Since stormwater is not routed to off-site wastewater treatment facilities, but is usually piped directly into local streams, improving stormwater quality usually occurs by directing the site runoff to low impact development (LID) practices or units designed to improve stormwater quality.
The largest volume pollutant is, and will most likely always be, sediment, which is most prominent in construction site runoff. Other pollutants of interest are grouped into plant nutrients, such as nitrogen and phosphorus, pathogens, metals, such as lead, zinc, cadmium, and organics, which include volatiles and decomposing waste and plant and animal materials.
Processes and Principles used to Remove Pollutants
Processes used to remove stormwater pollutants utilize physical, chemical, and biological mechanisms. Specific removal mechanisms include chemical adsorption, microbial transformation, plant uptake, sedimentation, and filtration. Each of these mechanisms is shown below in Table 1 and briefly explained.
Table 1. Pollutant removal mechanisms.
|Pollutant Removal Mechanism||Pollutants|
|Adsorption to soil particles||Dissolved metals and soluble phosphorus|
|Adsorption to organic matter||Organic compounds|
|Precipitation to less soluable species||Phosphorus; metals|
|Plant uptake||Nutrients; nitrogen and phosphorus|
|Microbial decomposition||Organics, pathogens|
|Exposure to sunlight and dryness||Pathogens|
|Sedimentation and filtration||Total suspended solids, floating debris, trash, soil-bound phosphorus, some soil-bound pathogens.|
Before we look at mechanisms and processes that can be employed to remove pollutants from stormwater, it is instructive to examine the time and volume distribution of stormwater pollutants. It has been shown conclusively that the first water running off a site is the dirtiest, most polluted water. This first runoff water has been named the "first flush". Based on research at many sites, the first flush is usually the runoff emanating from the initial 0.5 to 1.0 inches of rain from any storm. Most ordinances and stormwater design manuals simply give a value to be used, such as the runoff from the first-inch of rain, or the first inch of runoff. This means that it is most important that stormwa- ter low impact development (LIDs) successfully capture and treat the first flush of runoff. Capturing and applying a treatment process to the entire stormwater runoff volume is a desirable goal, but one that is difficult and expensive to accomplish, especially for large, rare runoff events (floods).
The most effective pollutant removal processes pass the stormwater runoff through the soil or bring it into contact with the soil or a soil-like media, such as mulch or compost. Collecting the stormwater into water-based systems, such as wetlands and wet basins where biological treatment can be facilitated is less effective because substantial time is required for the biological processes to improve the stormwater quality. The least effective methods of stormwater pollutant removal do not incorporate soil or ponded water in the treatment process.
Infiltrating stormwater into the soil or passing stormwater through a soil/media mix such as mulch or compost is the most effective and successful approach to improving stormwater quality. This means every effort must be made to infiltrate the stormwater into the soil. Fact Sheet F-260 provides more information about how to facilitate infiltration of stormwater into the soil. When stormwater passes through the soil, nearly all of the processes listed in Table 1 become active and are applied to the stormwater. The stormwater becomes part of the soil-water matrix where microbes can break down organic forms of carbon and nitrogen, and nitrogen and phosphorus can be adsorbed onto the soil particles. The soil also acts as an effective filter that removes pathogens, sediment, and other particulates from the stormwater. Now let's look at each pollutant removal process in some detail.
Adsorption is a chemical process that removes most metals, many organics, and phosphorus. The process takes place when stormwater comes in contact with organic matter and soil particles. Soil particles have electrical charges as do dissolved metals and soluble phosphorus. When these charges are complementary, dissolved metals and phosphorus are attracted to the soil particles. Organic matter also attracts and captures other organic compounds.
Chemical precipitation is the process of a pollutant in stormwater reacting with another chemical in the soil to form a relatively insoluble compound. An example is that phosphate in the stormwater will react with aluminum or iron in the soil to form relatively insoluble iron or aluminum phosphate. Some other pollutants (e.g., metals) will have similar reactions, thus removing the pollutants from the stormwater.
Plant uptake occurs when plant nutrients, found in stormwater runoff, find their way into the root zones of growing plants. Since plants need these nutrients to meet their physiological needs, the plants will take up the nitrogen and phosphorus to meet their nutrient needs, thus removing a portion of these plant nutrients from the stormwater. It must be remembered that when plant leaves fall off in the Autumn, most of the nutrients taken up by the plants (now residing in the plant leafs) are recycled back to the soil. For nutrient removal by plant uptake to be successful the plant leaves must be removed from the uptake area.
Microorganisms live in the stormwater and/or the soil. Organic matter, especially carbon and nitrogen, found in stormwater runoff will break down (or decompose) from complex long-chained molecules to more simple shorter-chained molecules by becoming part of the food chain for microorganisms. For carbon, this is usually referred to as reducing the biological oxygen demand (BOD) of the water. For organic-nitrogen, this is usually referred to as mineralization where organic-nitrogen is decomposed into ammonium-nitrogen using the microbes as instruments of conversion. Microbes also consume harmful pathogens.
Exposure to sunlight and dryness helps kill off pathogens, which typically prefer wet conditions. Pathogens captured on the soil or on plants during stormwater runoff events will quickly dry out after the storm. The sunlight and the desiccation process will usually kill most pathogens. It is important that pets and wildlife are prevented from depositing their wastes in stormwater sensitive areas.
Sedimentation and filtration are physical processes that remove soil particles, litter and other debris. Sedimentation occurs because water slows down, allowing suspended soil particles to settle out of the water. Vegetation provides a limited amount of filtration and aids in sedimentation. Because the inflow water must pass through vegetation, some pollutants can be "snagged" by the plant mass. This is the process of filtration. Sedimentation and filtration are primary removal mechanisms for total suspended solids (sediment), litter and debris, nutrients attached to sediment particles-such as some forms of phosphorus, and bacteria and other pathogens that are also attached to sediment.
Non-Soil Stormwater Pollutant Removal Processes
Seeing the need for stormwater pollutant removal, various groups have developed devices designed to address one or more of the major pollutants found in stormwater. These devices often fit in manholes or other below-grade chambers. They attempt to remove selected pollutants by applying one or more of the processes discussed above. Some of these devices have been quite successful, others beg for improvement. Any current stormwater related magazine will have many advertisements relating to these devices.
Effectiveness of Pollutant Removal
For several years, researchers at the University of New Hampshire Stormwater Center have evaluated a wide variety of storm water quality treatment practices. They collect stormwater runoff from the University of New Hampshire campus commuter parking lot and direct this stormwater into various stormwater treatment practices. The influent and effluent water quality are sampled and evaluated over extended periods of time. Below are several summary plots showing how many stormwater treatment practices have removed various pollutants. These box and whisker plots summarize the water quality results well. Each vertical bar represents one LID evaluated and contains a rectangular box, which contains a horizontal red line. The horizontal "red line" is the mean of the measured values. The top and bottom of the box represent the values that were one standard deviation from the mean. The dots above and below the box show the range of values measured. The first box in each figure represents the influent stormwater quality and is the initial value to which the various LID values should be compared.
Figure 1 shows the effectiveness of several stormwater LIDs in removing zinc. Zinc was chosen as a surrogate for all heavy metals. Most other heavy metals found in stormwater have similar chemistry and properties, thus similar removal rates. Those LIDs that caused the stormwater to contact soil or media removed a very large portion of the heavy metals, followed by those LIDs that provided limited contact with soil or media. Wetlands also removed zinc very effectively.
Figure 1. Box and whisker plot showing the effectiveness of several stormwater LID in removing zinc. (Taken from T. Ballestero, 2006).
Figure 2. Box and whisker plot showing the effectiveness of several stormwater LID in removing total suspended solids (TSS) or sediment. (Taken from T. Ballestero, 2006).
Figure 2 shows the effectiveness of several stormwater LIDs in removing total suspended solids (TSS) or suspended sediment. Those LIDs that actually captured and stored the stormwater removed the largest portion of the suspended sediment, followed by those LIDs that provided limited storage of the stormwater runoff. The swales were least effective in removing the suspended sediment.
Figure 3 shows the effectiveness of several stormwater LIDs in removing phosphate. Other than the infiltration chamber, none of the LIDs tested effectively removed phosphate. Several LIDs actually added phosphate to the stormwater.
Figure 3. Box and whisker plot showing the effectiveness of several stormwater LID in removing phosphate. (Taken from T. Ballestero, 2006).
Figure 4 shows the effectiveness of several stormwater LIDs in removing diesel range petroleum hydrocarbons (DRPH). DRPH was chosen as a surrogate for all volatile hydrocarbons. Most other volatile hydrocarbons found in stormwater are removed in a similar way. Those LIDs that brought the stormwater to contact soil/media, mulch, vegetation, or another organic material (Asphalt) were reasonably effective in removing very large portions of the hydrocarbons. The swales were the least effective in removing hydrocarbons.
Figure 4. Box and whisker plot showing the effectiveness of several stormwater LIDs in removing diesel range petroleum hydrocarbons. (Taken from T. Ballestero, 2006).
Figure 5 shows the effectiveness of several stormwater LIDs in removing dissolved inorganic nitrogen. None of the LIDs evaluated were effective in removing inorganic nitrogen from the stormwater flows although the LIDs that were wet or contained water did improve the water quality to some degree, with the gravel wetland removing approximately 90% of the inorganic nitrogen. It should be noted that the experiments that produced the data reported here in did not generally provide sufficient time for plant uptake.
Figure 5. Box and whisker plot showing the effectiveness of several stormwater LIDs in removing dissolved inorganic nitrogen. (Taken from T. Ballestero, 2006).
Figure 6 shows the effectiveness of several stormwater LIDs in removing bacteria. Enterococci are a genus of lactic acid bacteria and represent all other bacteria found in stormwater. Those LIDs evaluated that brought the stormwater into contact with the soil were more effective in removing the bacteria from the stormwater flows with the sand filter and bioretention removing bacteria most effectively. Several LIDs increased the bacteria in the stormwater.
Figure 6. Box and whisker plot showing the effectiveness of several stormwater LIDs in removing bacteria. (Adapted from T. Ballestero, 2006).
Though a great deal of variability is shown in the New Hampshire Stormwater center data reported herein, it is clear that those LIDs that bring the stormwater in contact with the soil were most effective in removing pollutants from the stormwater runoff. The improvements in water quality were somewhat less when the stormwater came in contact with soil-like media or mulch. Collecting the stormwater into water-based systems, such as wetlands and wet basins were less effective, often because of limited contact time with the microorganisms present. The least effective methods of stormwater pollutant removal were those treatment LIDs that did do not expose the stormwater to soil, media, or a wet environment.
Ballestero, T. 2006. Powerpoint presentation delivered to BE 467 stormwater class. Tom Ballestero is director of the New Hamphsire Stormwater Center.