Urban creeks and lakes can provide important habitats for aquatic life, as well as aesthetic assets to communities. However, the primary function of many urban creeks is the conveyance of storm water to prevent flooding. Often, they have been channelized to assist in achieving rapid removal of storm water from an urban area. This channelization, coupled with development (paving) in the urban creek watershed, is at odds with providing a high-quality aquatic life habitat.
Urban creek flows can vary from a few cubic feet per second of groundwater-based flow to a thousand or more cubic feet per second during flood flow conditions associated with major runoff events. The high flows are detrimental to developing and maintaining desirable aquatic life habitat. At the same time, urban creeks and lakes can provide important aesthetic amenities and, in some cases, recreational fisheries and nursery areas for aquatic life.
Doing what’s best
In 1990, the U.S. Environmental Protection Agency (EPA) promulgated the national regulations that require cities with populations greater than 100,000 to develop storm water runoff programs to control pollution to the maximum extent practicable (MEP) using best management practices (BMPs). These requirements also are applicable to highway storm water runoff. The EPA did not define MEP or the BMPs that are to be used.
Urban storm water runoff pollution is to be regulated by National Pollution Discharge Elimination System (NPDES) permits that ultimately require compliance with water quality standards. While NPDES permits for domestic and industrial wastewater discharges are to prevent violations of water quality standards at the point of discharge or at the edge of a mixing zone if provided for, the NPDES permits issued to municipalities for controlling urban storm water pollution, while ultimately requiring compliance with water quality standards, did not specify a date by which compliance with water quality standards is to be achieved.
The current regulations require the use of a BMP ratcheting-down process to ultimately, but as of yet at an undefined date, control violations of water quality standards in the runoff. Justification for this difference in approach for regulating urban storm water runoff, compared with urban wastewater discharges, arises from the significantly different characteristics of urban storm water runoff.
The cost of retrofitting conventional BMPs, such as detention basins and grassy swales, to developed urban areas has been estimated to be on the order of $1 to $3 per person per day for the population served by the storm sewer collection system. These costs are primarily for the acquisition of land and effectively restrict implementation of these BMPs to new developments where their cost can be incorporated into the cost of the development.
While conventional BMPs are being installed, especially in new developments, the most popular conventional BMPs will not be adequate to treat urban or highway storm water runoff to achieve compliance with water quality standards. In order to achieve compliance with water quality standards for urban area and highway storm water runoff, it will be necessary to construct, operate and maintain treatment works of the type used in advanced wastewater treatment. The cost of such retrofitted treatment works in developed areas is projected to be on the order of $5 to $10 per person per day. These high costs will require a different approach to be developed for regulating urban area storm water runoff.
Evaluating the impacts
While there has been considerable work on the chemical characteristics of urban storm water runoff, little work has been devoted to evaluating the water quality and beneficial-use impacts of this runoff. Especially in light of the tremendous costs associated with providing for treatment and control of storm water runoff, it is important to properly assess whether a chemical constituent derived from storm water runoff that is present in an urban stream or lake is in a chemical form that is toxic or bioavailable, meaning it can cause pollution. Failure to make this evaluation can lead to expenditure of large amounts of public funds for the development and installation of so-called BMPs that affect little or no improvement of the beneficial uses of an urban stream, lake or other bodies of water receiving the storm water runoff.
Heavy metals—Copper, lead and zinc are found in almost all urban street and highway storm water runoff in concentrations that would violate EPA worst-case-based water quality criteria and state standards based on those criteria. Sometimes cadmium and mercury also are present above those criteria. These findings indicate that there is a potential for certain heavy metals in urban storm water runoff to be toxic to aquatic life in urban creeks.
Toxicity identification evaluation (TIE) studies have found that heavy metals in urban residential areas and highway storm water runoff are in nontoxic forms.
However, this is not necessarily the case for heavy metals in industrial storm water runoff. There are a number of examples in which heavy metals, such as zinc from galvanized roofs or copper from copper roofs, can be present in industrial storm water runoff in sufficient concentrations and available forms to be toxic to aquatic life.
Pesticide-caused aquatic life toxicity—Diazinon and chlorpyrifos have been phased out of urban use by the EPA due to their potential toxicity to children. These OP pesticides are being replaced by others, especially the pyrethroid pesticides, in urban areas. A number of these replacement pesticides are more toxic to fish and zooplankton than the OP pesticides. Furthermore, many of the pyrethroid pesticides tend to sorb strongly to soil particles and, therefore, will be transported in particulate form and accumulate in sediments.
D.P. Weston reported finding that some sediment-sorbed pyrethroid-based pesticides are toxic to some benthic (sediment-dwelling) organisms. This toxicity could represent significant adverse impacts on the aquatic food web in the urban stream or lake, which could lead to the need to control the use of pyrethroid-based pesticides used on residential and commercial properties.
Dissolved oxygen—Storm water runoff events can cause significant dissolved oxygen depletion in urban streams and other nearby water bodies, which is sometimes sufficient to cause fish kills. This situation is typically associated with an accumulation of oxygen demand in the sediments of urban storm sewers, which are flushed into urban streams during a runoff event.
Nutrients—Urban storm water runoff contains elevated concentrations of various nutrients—nitrogen and phosphorus compounds—that can lead to impaired water quality in urban creeks, lakes and downstream water bodies due to stimulation of algae and aquatic weed growth.
pH—There can be sufficient primary production (algal growth) in urban creeks and lakes to cause significant diel (over a 24-hour day) changes in pH and dissolved oxygen. This is especially true for those urban streams that have only limited areas where extensive canopy from trees along the bank shade the water.
Sanitary quality—Especially during dry weather flow, urban storm water runoff and, in some situations, drainage ways such as creeks in urban areas often have greatly elevated concentrations of total coliforms, fecal coliforms and E. coli.
In many communities, the design of the sanitary sewerage collection system is such that there can be discharges of raw sewage to urban waterways associated with pump station power failure, blockage of the sewer and other factors.
Furthermore, sanitary sewerage systems are sometimes poorly maintained, with the result that there can be discharges of raw sewage to nearby watercourses on an ongoing basis through leaks in the sewerage system. In addition, animals—including birds—can contribute significant amounts of fecal coliforms and E. coli to storm water runoff, which in turn can cause urban creeks to have poor sanitary quality.
TOC—Based on EPA regulations, domestic water supplies that have total organic carbon (TOC) concentrations above about 2 mg/L may be required to treat the water to remove the total organic carbon to that level, in order to reduce the potential for formation of trihalomethanes and other disinfection byproducts during the disinfection of the water supply. The concentrations of TOC in urban storm water runoff can be sufficient to adversely affect downstream water bodies that are used for domestic water supply purposes.
Excessive bioaccumulation of hazardous chemicals—Fish and other edible aquatic organisms taken from some urban streams have been found to contain excessive concentrations of legacy pesticides such as DDT, dieldrin and chlordane, derived from their former use in urban areas as well as from current runoff from urban areas that had been agricultural. In addition, fish and other aquatic life in urban streams can contain excessive concentrations of PCBs and dioxins/furans. Dioxins are known to be present in storm water runoff from urban areas and highways, and can therefore be present in urban streams and lakes, especially in the sediments. PCBs are sometimes found in urban stream fish due to spills of electrical transformer PCBs that have occurred in the urban stream watershed or illegal discharges of PCBs from industrial sources to the storm sewer system.
Some measurements of mercury in urban storm water runoff have shown that the concentrations are sufficient to potentially lead to excessive bioaccumulation of mercury in edible fish tissue. In urban streams or lakes in which bioaccumulation of mercury is a potential concern, fish should be examined to determine if they have excessive bioaccumulation of mercury.
PAHs, oil and grease and others—There are numerous organic compounds that are not pesticides or organochlorine bioaccumulatable chemicals, but are of potential concern in urban storm water runoff. These include oil and grease, PAHs and others included in the group of “total organic carbon.” In the oil and grease and TOC fractions in urban storm water runoff, there can be thousands of unregulated organic chemicals that pose a threat of toxicity to aquatic life and bioaccumulate in edible aquatic life, where they pose a threat to higher trophic-level organisms, including humans. Many of these have been in use and entering the environment for many years but have not been regulated.
C.G Daughton in 2004 indicated that, while there are more than 22 million organic and inorganic substances, with nearly 6 million commercially available, fewer than 200 are addressed by the current water quality regulations. He noted special concern that in general, pharmaceuticals and personal care products (PPCPs) are not regulated but can pose significant water quality concerns.
“Regulated pollutants compose but a very small piece of the universe of chemical stressors to which organisms can be exposed on a continual basis,” Daughton stated.
Suspended sediment and turbidity—If an urban creek watershed contains areas of new construction, or if the urban creek watershed and the creek have soils that readily erode, there can be significant increases in suspended solids and turbidity in the creek during runoff events. The increased turbidity makes the water turbid (muddy), which can affect aquatic life habitat.
Trash—Urban creeks are notorious for accumulating materials that people discard, including grocery carts, tires, paper, Christmas trees, shrubbery and lawn trimmings. While some of these items can inhibit flow, and thus lead to flooding, some of this material also provides habitat for aquatic organisms in the creek. The primary adverse impact of trash is on the aesthetic quality of the water body. Some creeks receive large amounts of trash. This is evidenced by the “creek days” that environmental and public groups conduct when debris of various types is removed from the creek.
Aquatic life habitat—As part of its Water Quality Criteria and Standards Plan, the U.S. EPA specifically delineated urban storm water runoff as a cause of deteriorated aquatic life habitat. The habitat degradation is a result of a variety of factors including channelization and increased urban stream flow due to paved development in the watershed.
The 2003 Center for Watershed Protection report contains an extensive discussion of the impact of urbanization with the associated increase in impervious cover (e.g., paving) in urban stream watersheds on the hydrological and morphological characteristics of urban streams. It reported that, when the percentage of impervious cover in an urban stream’s watershed exceeds about 10%, the stream’s characteristics are typically impacted. When the impervious cover exceeds about 25%, there tend to be severe impacts on the water body’s characteristics.
As part of the implementation of its Water Quality Criteria and Standards Plan, the EPA plans to pursue the use of bioassessment methodology to determine the degree of degradation caused by urban storm water runoff that would need to be corrected to develop desirable aquatic life habitat in urban streams and other water bodies that receive urban storm water runoff.
Defining the sources
It has become evident that there is a need for comprehensive water quality monitoring and evaluation programs to determine—for representative locations—the real, significant water quality-use impairments that are occurring in urban lakes and streams (and, for that matter, downstream waters) receiving urban area and highway storm water runoff. This monitoring and evaluation program should include defining the specific sources of the constituents that lead to the water quality use impairments.
Once the water quality problems have been defined and the sources of the responsible pollutants identified, then a reliable evaluation can be made of the management practices that can be implemented to control the pollution of urban streams and lakes by urban area storm water runoff associated constituents. In general, because of the high cost of treatment, it is likely that the management practices will focus on source control, as opposed to treatment of the storm water runoff.