Issues in Managing Urban Stormwater Runoff Quality
Aquatic Plant Nutrients
The aquatic plant nutrients, nitrogen (N) and phosphorus (P) compounds, are of potential concern in urban stormwater runoff due to their ability to stimulate excessive growth of aquatic plants in receiving waters. The eutrophication (fertilization) of a waterbody can be significantly detrimental to water quality-related beneficial uses. It was found in the 1970s that urban stormwater runoff contains about 100 times the total concentrations of phosphorus that are typically derived from stormwater runoff from forested areas, and about 10 times the amounts contributed from many agricultural areas. It was also found then that substantial portions of the nitrogen and phosphorus components are in particulate forms that are not available to support aquatic plant growth.
As with most other chemical constituents in urban stormwater runoff, the total concentrations of a constituent, such as nitrogen or phosphorus, is an unreliable indicator of potential water quality problems. Sufficient work has been done, however, on the available forms of phosphorus found in this runoff to be able to estimate the quantities of algal-available P in a runoff water. Normally, this is equal to the soluble orthophosphate plus about 20 percent of the particulate phosphorus.
Some groups are calling for a ban on the use of lawn fertilizers in urban areas in an effort to try to reduce the phosphorus content of urban stormwater runoff. As in the case of other chemical constituents in such runoff, site-specific studies have to be conducted to determine whether controlling the phosphorus to a certain extent will have a significant effect on the water quality-related beneficial uses of the receiving water. It has been found that to change the degree of eutrophication of a waterbody to a perceptible amount, it is necessary to reduce the quantity of algal-available P entering the waterbody by about 25 percent. It is unlikely that curtailing the use of lawn fertilizers will have a significant impact on most waterbodies since such fertilizers represent a small part of the total phosphorus load in urban runoff. Further, except for some urban lakes which essentially receive only this type of runoff, it will be unlikely that reducing the amounts of nitrogen and phosphorus will significantly improve the eutrophication-related quality of waterbodies.
Stormwater Runoff Monitoring
Primary emphasis in stormwater runoff quality management programs today is being given to monitoring for selected parameters. A critical review of the typical program, however, shows that the extent and degree of monitoring being done provides essentially no useful information on the potential effects of the associated chemical constituents on receiving water quality. Grabbing a few samples of runoff from a few storms over a year and analyzing them for a few indicator parameters does not properly characterize the concentrations of total chemical constituents of potential concern, much less the concentrations of toxic-available chemical constituents that could be adverse to the designated beneficial uses of the receiving waters.
About all that can be said for the current urban stormwater runoff quality monitoring program is that it confirms what is already known. Based on total constituent analysis, there are chemicals in runoff from urban areas at concentrations that exceed the USEPA's water quality criteria, and state standards based on these. However, as discussed in Part I, exceeding a state water quality standard for a contained chemical in a runoff does not mean that a designated beneficial use impairment will occur in the receiving waters. To make that assessment, it is necessary to conduct site-specific evaluations of the effect of runoff-associated constituents on the beneficial uses.
The California Stormwater Quality Task Force has been working toward modifying the monitoring program requirements so that a number of stormwater management agencies could pool their monitoring resources to develop a fund that could be used to conduct site-specific evaluations. Rather than collecting additional stormwater quality data on the concentrations of selected chemicals, it is more technically appropriate and cost-effective to use the monitoring funds to define whether real water quality use impairments are, in fact, occurring in receiving waters.
Highway Runoff Effects
Several years ago, the Federal Highway Administration (FHA) sponsored a number of studies devoted to evaluating the water quality significance of chemical constituents in highway runoff. It has been known since the 1960s that runoff from urban streets and highways contains high concentrations of chemicals that, if in toxic-available forms, could have significant adverse effects on beneficial uses of receiving waterbodies. However, the work done in the 1960s showed that many of the chemicals from streets and highways were in non-toxic, non-available forms. This meant it was not possible to relate the analytically-measured concentrations of these compounds to water quality.
Unfortunately, those responsible for conducting the mid-1980s studies for the FHA did not properly evaluate whether the elevated concentrations of chemicals in highway runoff were in forms that could adversely affect the receiving water quality. The authors of these studies labeled all constituents as pollutants, without finding a case of water pollution (use impairment) in their studies.
The inappropriate labeling of these materials as pollutants is contributing to significant problems for federal and state highway departments. Environmental groups are filing suit against them to have the courts force them to control "pollution" from highway runoff arising from the elevated concentrations of alleged "pollutants." Experience shows it would be rare where highway and street runoff-associated chemicals would have any significant adverse impact on designated beneficial uses. The fact that heavy metals and other runoff chemicals are in non-toxic forms, coupled with the short-term episodic nature of runoff events, suggests it is rare that these compounds are real pollutants that should be controlled using best management practices (BMPs).
Highway litter, however, can cause significant impairment if it finds its way into receiving waterbodies. This litter also can contribute to flooding by blocking stormwater inlet structures. At this time emphasis in implementing BMPs for highways should be based on litter control and the control of erosion associated with new highway construction. There is no technical justification to assume that constructing detention basins, grassy swales, etc., for "treatment" of highway runoff is in fact controlling pollutants that are significantly detrimental to beneficial uses. Before any structural BMPs are constructed to treat runoff, site-specific investigations should be conducted that demonstrate that there is a real water quality use-impairment associated with the current runoff. Where such problems are found, then efforts should be made to try to control them through controlling the specific causes of the use impairment. It is unlikely that conventional structural BMPs will be effective in addressing these types of situations.
Stormwater Quality Modeling
A substantial literature has accumulated on the subject of stormwater quality modeling. Sophisticated computer models have been developed which are said to provide information pertinent to urban stormwater quality impact evaluation and management. However, a critical review of these models shows they are simply chemical constituent models that can describe to some extent the total concentrations of selected chemical constituents at some location in the stormwater runoff system. To be able to relate the concentrations predicted based on such models, it is necessary to conduct site-specific evaluations of the relationships between the total concentrations of the constituents of potential concern and the toxic-available forms in stormwater runoff from a particular area.
Further, there is need to relate the concentration of toxic-available forms in stormwater runoff to site-specific use impairments in the receiving waters. The current models do not provide this type of information. To be true stormwater quality models, they must incorporate basic information from aquatic chemistry and aquatic toxicology as they relate to the true water quality effects of stormwater-derived chemical constituents. It could be many years before such models will be available that can reliably assess stormwater quality effects.
Some have asserted that equilibrium-based water chemistry models such as the USEPA's MINTEQ model can be used to predict the concentrations of toxic-available forms of chemical constituents in urban stormwater runoff. Such assertions are technically invalid since many of the particulate forms of chemicals found in urban runoff have unknown chemical characteristics and for which there are no thermodynamic equilibrium data. At this time, the only reliable approach for assessing whether a particular runoff water will be toxic to aquatic life is through the direct measurement of toxicity. This cannot be accomplished with chemical measurements.
Because of the variable concentrations of chemical constituents in urban stormwater runoff, various investigators have attempted to characterize the concentrations found in a runoff event through the use of what is called an event mean concentration. While such an approach makes modeling of an event for total constituents easier to achieve, it fails to properly address how chemical constituents in urban, highway and other stormwater runoff sources influence aquatic life-related beneficial uses of waterbodies. It has been known since the 1960s that aquatic organisms respond to the concentration of available form/duration of exposure relationship that they experience. The event mean concentration for a stormwater runoff event is not a reliable approach for assessing the potential effects of chemical constituents on aquatic life, and should be abandoned.
Since it is not possible to reliably predict, using chemical measurements, whether a chemical constituent in stormwater runoff is toxic to aquatic life in receiving waters, the use of aquatic life toxicity tests is beginning to be more widely practiced. These tests can be used to determine whether the regulated as well as the unregulated chemicals in runoff present a potentially significant threat to aquatic life due to toxicity. Caution, however, must be exercised in the interpretation of results. The toxicity tests typically used significantly overestimate the actual toxicity since their duration provides longer exposure to aquatic organisms than they normally are exposed to in receiving waters. Ordinarily, the runoff is rapidly diluted, with an associated loss of toxicity. The aquatic life toxicity tests of the type available today should only be used as a screen for potential toxicity. They should not be used as a direct regulatory limit. If toxicity is found, then site specific investigations should be conducted to confirm the information.
Stormwater Runoff Sediments as Hazardous Waste
Increasing concern is evolving about the potential for stormwater runoff sediments that accumulate in detention basins, highway drop inlets, grassy swales, etc., being classified as a hazardous waste because of excessive concentrations of chemical constituents. Classification of a stormwater detention basin sediment as a hazardous waste can represent a significant increase in the cost of managing the sediments. Often managing a hazardous waste costs about 10 to 50 times more than using them as fill or placing them in municipal solid waste landfills. The USEPA, as part of implementing RCRA, has developed various procedures for classifying materials such as soils and sediments as hazardous waste. While there is potential concern about stormwater sediments from certain types of industrial properties being classified as a hazardous waste based on the origin of the sediment (the Derived-From Rule), the greatest concern for urban stormwater sediments collected in structural BMPs is the leaching characteristics under the Toxicity Characteristics Leaching Procedure (TCLP) test.
This test was developed as an administrative test to be used to determine whether a solid waste should be placed in a hazardous waste landfill or in a municipal landfill. The Agency was not trying to reliably delineate whether a material in a sediment or soil is hazardous. Rather, it was trying to limit the size of the hazardous waste stream that had to be managed as hazardous waste where the focus of the resources available would be on those wastes that represent the greatest hazard. Unfortunately, this test is being used for a variety of other purposes for which it is inappropriate and was not intended. A sediment or soil that passes the TCLP test can be hazardous to public health and the environment depending on how it is managed.
Another common mistake made in using the TCLP test is to assume that a material classified as hazardous according to TCLP also would be hazardous to aquatic life. TCLP classification is based on the leaching of selected chemicals from solid material under certain conditions which mimic to some extent the environment present in a municipal solid waste landfill, where the concern is that the leached constituents would become part of a groundwater-based domestic water supply. This approach has no relationship to whether the material is hazardous to aquatic or terrestrial life. The TCLP test focuses primarily on the potential for chemical constituents to cause cancer in people who are exposed over their lifetime through drinking water. These constituents are primarily Priority Pollutants. The critical concentrations for many of these regulated through the TCLP test have no relationship to the critical concentrations for the same constituents to aquatic life. In some cases aquatic life is more sensitive, and in others less sensitive than the TCLP values.
Detention basin sediments classified as hazardous waste is an even more complicated issue in some states. California has developed its own set of hazardous waste classification values. It uses a somewhat different leaching test and also has a set of total concentrations of chemical constituents in sediments or soil which define the sediment or soil as hazardous. A detention basin sediment that passes the TCLP test may fail California's Title 22 hazardous waste classification and would have to be managed as a hazardous waste. However, independent of the arbitrariness of these classification values, they are regulatory requirements that must be considered in the management of stormwater detention basin sediments.
Lead is of great concern if it is contained in detention basin sediments. Urban soils and soils near highways often contain lead at concentrations of at least 500 and frequently 1,000p;1,500 mg/kg. Ordinarily, this lead, originally derived from its additive use in leaded gasoline, does not leach sufficiently in the TCLP test to exceed the USEPA's arbitrary 5 mg/l hazardous waste classification limit. It does frequently cause sediments to exceed the California Title 22 limit of 1,000 mg/kg for classification as hazardous waste.
The approach that should be used to evaluate potential public health and environmental effects of chemical compounds present in sediments which collect in stormwater treatment structures installed as BMPs is to make a site-specific evaluation of the hazards that these chemicals represent at the various locations where the sediments accumulate. Those who are concerned with stormwater runoff quality and regulations on a daily basis should work with federal and state agencies to eliminate the use of the arbitrary approaches that are in effect today for classification of stormwater-derived sediments as hazardous waste.
An Overall View
The implementation of the 1987 Clean Water Act requirement for controlling pollution of the nation's waters by urban and industrial stormwater runoff is challenged by a number of complex technical issues. They need to be resolved before their can be cost-effective management of the real water quality problems associated with stormwater runoff. Much remains to be done to develop specific approaches that can be used to control stormwater runoff-caused pollution to the maximum extent practicable. The key issue in developing an effective management program is whether current stormwater runoff quality is in fact having significant adverse effects on designated beneficial uses of receiving bodies of water. Failing to properly define th real pollution problems of stormwater runoff could result in excessive waste of public and private funds in the regulatory effort.
About the Authors:
G. Fred Lee, Ph.D., P.E., D.E.E., and Anne Jones-Lee, Ph.D., are president and vice-president respectively of G. Fred Lee & Associates, a specialty environmental consulting firm located in El Macero, California.