A wide range of views on what MEP means exists. Some people advocate that MEP means achieving water quality standards at the edge of the mixing zone for the stormwater runoff. Others maintain that good housekeeping at industrial sites and proper street sweeping and litter pick-up practices in urban areas are adequate BMPs to achieve MEP.
Over the years, based primarily on hydraulic considerations, a number of structural BMPs have been developed for allegedly controlling water pollution from urban stormwater runoff. Detention basins, grassy swales and other vegetative areas and infiltration areas are often promoted as BMPs for this runoff. However, as discussed later, a critical review of the potential effects of stormwater runoff-associated chemical constituents raises significant questions about whether a detention basin is, in fact, a treatment system for removal of pollutants in urban stormwater runoff. A number of the issues associated with developing technically-valid, cost-effective approaches to the evaluation and management of urban and industrial stormwater runoff-caused water quality impairments are reviewed in this first article.
Stormwater Runoff Pollution
The USEPA's stormwater management regulations specifically delineate that urban stormwater runoff management programs control pollution of waters. Pollution is defined in these regulations as well as in the Clean Water Act and in many state regulations as the impairment of the designated beneficial uses of the waterbody receiving the stormwater runoff. In accord with the l972 amendments to the federal regulations governing water pollution control in the US (PL 92-500), all waterbodies in the United States were to be classified with respect to their designated beneficial uses. Uses such as domestic water supplies, propagation of fish and aquatic life, recreation, agricultural and industrial water supplies, navigation, and waste heat dissipation, etc., are typically considered. The l972 regulations established as a national goal that all waters of this country should be "fishable and swimmable." Normally, propagation of desirable fish and aquatic life and body contact recreation (wading and swimming), and of fresh water domestic supplies, require the highest quality.PL 92-500 also established as a national goal zero pollutant discharge. This was supposed to be achieved by the mid 1980s. It is important to emphasize that this goal was not zero chemical constituent discharge, i.e., the equivalent of distilled water. Instead it focused on controlling those chemical constituents which in fact cause pollution. As discussed below, it is important to distinguish clearly between pollutants and non-pollutants in developing stormwater runoff management programs.
The l972 federal regulations required the USEPA to develop water quality criteria which would serve as a basis for state water quality standards that when achieved will be protective of designated beneficial uses of water bodies.
Three types of constituents in urban and industrial stormwater runoff have the potential to cause water pollution, i.e., impairment of designated beneficial uses. One of these is the particulate matter present in stormwater runoff. Suspended and deposited sediments can affect water quality in ways not related to the chemical characteristics of the particulate matter, which can cause filling of the water body receiving the stormwater runoff. This, in turn, can interfere with navigation and change the characteristics of the water body. The settled particulate matter also can adversely impact fish and aquatic life through smothering of organisms and altering their habitat. Particulate can affect the optical properties of the waterbody by causing turbidity which can influence the aesthetic quality of a water body and the photosynthesis that may take place there.
The second major group of constituents of concern in urban stormwater runoff which may adversely affect designated beneficial uses is made up of pathogenic organisms, especially the enteric waterborne pathogens (bacteria, viruses or protozoans). These organisms are of concern because they can affect the quality of domestic water supplies, as well as the sanitary quality of water used for contact recreation. While typically in the past the sanitary quality of a water has focused on fecal coliforms, today increasing attention is given to the enteroviruses, especially the cyst-forming protozoans such as Cryptosporidium and Giardia. It is now well known that water meeting the coliform standard for contact recreation or domestic water supplies is not necessarily safe for consumption or contact recreation since the enteroviruses and the protozoan cysts are more difficult to control by chlorine disinfection than the coliforms.
Third on the list of concern in terms of potentially causing impaired use of receiving water bodies are the chemical constituents. Typically these receive the greatest attention in urban stormwater runoff quality management programs. Chemical constituents exist in aquatic systems in a variety of chemical forms, only some of which are toxic-available and therefore can be adverse to aquatic life and to other designated beneficial uses of a waterbody. With few exceptions, it is the dissolved forms that are toxic-available. While this has been known for over 25 years, the USEPA recently acknowledged this situation in its guidance for regulating heavy metals in ambient waters, where the Agency now recommends that the dissolved heavy metal concentration be used as an indicator rather than total heavy metals. The particulate forms such as those that may be removed in a stormwater detention basin normally are non-toxic and non-available. This same situation also applies to most other chemical constituents in stormwater runoff and other sources of chemical constituents. It is for this reason that stormwater detention basins are typically not effective in removing chemical pollutants. They, however, can be effective in removing suspended sediment. The impact of these particulates removed in a stormwater detention basin is not related to the chemical characteristics of the sediment.
Another factor to be considered in evaluating the potential water quality effects of chemical constituents in stormwater runoff is the duration of exposure that aquatic organisms can receive in the receiving waters for stormwater runoff. The shorter the duration of exposure, the greater the concentration of toxic-available forms that can be present without adversely impacting the designated beneficial uses of a waterbody. Because of the short-term, episodic nature of most stormwater runoff events, much higher concentrations of chemical constituents can be present than the worst-case or near worst-case USEPA criteria and state water quality standards, and still protect the designated beneficial uses of the water body.
Unreliable Reporting of Water Quality Significance
Unfortunately the USEPA and state regulatory agencies responsible for conducting the National Water Quality Inventory in which urban stormwater runoff is ranked as the second most important cause of water quality impairment in the country have been providing unreliable information to Congress and the public on this issue. A critical review of how this ranking was developed shows that it was assumed that any parameter value outside a water quality standard in the receiving waters for an urban stormwater discharge represented a water quality use impairment. As discussed here, and as is well known, significant excursions beyond water quality standards of the type available today can and do occur without any impairment of the designated beneficial uses of the water bodies in which the excess occurs.Chemical Constituents vs. Pollutants
Significant problems exist today in the stormwater runoff water quality evaluation and management field due to the fact that many of the individuals working in this field do not distinguish or properly distinguish between inert chemical constituents (non-pollutants) and pollutants. For a chemical constituent in stormwater runoff to be a pollutant, it must be present in the water body receiving that runoff in sufficient concentrations of available forms for a sufficient time to be adverse to the designated beneficial uses. For aquatic life-related beneficial uses, the chemical constituent, either alone or in combination with other chemical compounds, must adversely affect the numbers, types and/or characteristics of desirable aquatic life.Typically today, those working in the urban stormwater runoff water quality field inappropriately label as pollutants all chemical constituents in runoff that have been found to be pollutants in other situations. It is totally inappropriate to assert that a chemical constituent, such as copper present in highway or street runoff, adversely impacts the designated beneficial uses of the water body receiving this runoff because copper from a different source, such as in plating or mining wastes, is a pollutant in some other water body. This is technically invalid and can result in a massive waste of public and private funds assigned to control chemical constituents by various types of structural BMPs, e.g., stormwater detention basins, that will have little or no impact on the receiving water quality.
Beginning in the 1960s, several studies were conducted in various locations in the United States which demonstrated that urban stormwater runoff contained chemical constituents at significantly elevated concentrations compared to most ambient waters. In the late '70s and early '80s, the USEPA conducted a National Urban Runoff Program (NURP) in which studies were undertaken in several cities across the country that involved monitoring chemical constituent concentrations in stormwater runoff. It was known at the time the NURP studies were initiated (from the work done in the '60s), that chemical constituents in urban stormwater runoff typically were associated with particulate matter and were non-toxic and non-available. However, the Agency's NURP studies failed to determine the water quality effects of the elevated concentrations of chemical constituents present in the runoff samples investigated. This was a significant deficiency in the NURP studies which is still adversely influencing the cost effectiveness of stormwater runoff quality evaluation and management programs.
Based on the large amount of reliable information that has been developed, and the basic principles of aquatic chemistry, aquatic toxicology and water quality evaluation and management, it is more technically valid to assume that chemical constituents in urban stormwater runoff normally considered to be pollutants are, in fact, non-pollutants. While typical runoff from residential and commercial areas contains a wide variety of chemical constituents at concentrations above USEPA water quality criteria/state standards, it is rare that such examples result in significant impairment of the designated beneficial uses of the water bodies receiving the runoff. This situation arises from the fact that most of the chemical constituents in runoff from residential, street and highway, and commercial areas are in non-toxic, non-available forms. Further, because of the limited duration of exposure that desirable aquatic organisms can receive near points of runoff discharge, even the failure of such discharge to meet state standards for toxic-available forms in typical runoff will not result in a significant impairment.
Therefore, it is appropriate to regulate chemical constituents in urban stormwater runoff differently than the approach that has been used for municipal and industrial wastewaters. Failure to take the differences into account can result in large-scale waste of public and private funds applied to control chemicals in stormwater runoff that have little or no effect on the designated beneficial uses of the specific receiving waters.
While the focus here is on urban stormwater runoff, these same issues are equally applicable to rural and industrial stormwater runoff. To require, as is being done today, that runoff from industrial properties meet state water quality standards at the point where the runoff leaves the property represents gross over-regulation of its chemical constituents.
Taking a different approach for regulating urban, industrial and rural stormwater runoff chemical constituent control than has been used for end-of-pipe municipal and industrial wastewater discharges does not mean these discharges also are not in some instances being over-regulated today. The 1972 Amendments to the Water Pollution Control Act initially required that municipal and industrial dischargers achieve fixed degrees of treatment irrespective of the need to protect the designated beneficial uses of the receiving waters for the discharges, i.e., effluent standards. These discharges now are required to achieve water quality standards at the edge of a mixing zone in the receiving waters. The standards being applied to these discharges are designed to protect the designated beneficial uses under worst-case or near worst-case conditions. This means normally that municipal and industrial wastewaters treated to achieve water quality standards at the edge of a mixing zone in most instances receive more treatment than is necessary to protect designated beneficial uses, since the worst-case conditions the standards are designed to protect rarely occur in US waters.
Therefore, it is not that there is need to regulate urban industrial and rural stormwater runoff chemical constituents differently than the same constituents in municipal and industrial wastewater discharges. It is that in developing approaches for regulating urban stormwater runoff chemical constituents, the USEPA should not make the same mistake that it made in developing regulations for the classical point source discharges of municipal and industrial wastewaters.
Some regulatory agencies and environmental groups are attempting to define the MEP term to mean achieving state water quality standards at the edge of the mixing zone where the stormwater discharge enters the receiving water. While they acknowledge it is not possible to accomplish this today, they are attempting to develop regulations which establish these standards as goals defining MEP, and by which BMPs are to be evaluated. Such approaches are technically invalid and will grossly over-regulate stormwater runoff-associated chemical constituents.
Rather than trying to achieve inappropriately developed water quality standards for stormwater runoff discharge situations, the approach that should be followed is to first define on a site-specific basis what, if any, real water quality use impairment is occurring for a particular discharge. Where specific use impairments have been defined, then efforts should be made to determine their specific cause, i.e., the specific chemical constituents and forms that cause use impairment. When defined, efforts should be made to control these substances at the source. Only in situations where it is not possible to control at the source should structural BMPs be developed to treat the stormwater runoff for control purposes.
There is little doubt that the structural BMPs eventually needed to control real water quality problems associated with urban, street and highway, industrial, and rural stormwater runoff will be significantly different than the BMPs of the type being fostered today as appropriate for stormwater runoff pollution control.
The evaluation of the effectiveness of the BMPs in achieving MEP should be based on how well the BMP addresses/controls the water quality use impairment and not, as is typically done today, be based on the percent removal of a total chemical constituent across a structural BMP. Such an approach fails to recognize the aquatic chemistry and aquatic toxicology of chemical constituents in stormwater runoff as they may affect water quality.
