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URBAN RUNOFF WATER QUALITY: A SALMONID’S PERSPECTIVE
Joanne E. Richter
Watershed Professionals Network
Introduction
For the past 30 years there has been increasing attention and concern focused on the quality of stormwater that runs off impervious surfaces in urban environments, and what effect that runoff has on receiving water bodies and their associated biota. The Nationwide Urban Runoff Program (NURP) studies conducted in the early 80’s highlighted what many water quality experts had suspected for some time: urban runoff quality is extremely variable, there are many sources of urban runoff contamination, and urban stormwater is extremely difficult to manage and control (U.S. EPA, 1983). Water resource managers have made a lot of progress towards minimizing and mitigating for the effects of urban runoff, but with increasing attention focused on the plight of Pacific Coast salmonid populations, it is important to look at what effect urban runoff has on these threatened and endangered species. The following is a brief review of urban runoff water quality, and what effect it may be having on salmonid populations.
Typical Urban Runoff Pollutants
Urban runoff pollutants are many and varied depending on the land uses and pollutant sources present in an urban area. Typically loadings of urban pollutants are greatest from industrial and commercial areas, roads and freeways, and higher density residential areas. Although sources of specific pollutants may vary widely in urban areas, motor vehicles are recognized to be a major source of pollutants, contributing oils, greases, hydrocarbons, and toxic metals. The more cars and trucks we have, and the more streets and parking lots we build to accommodate these vehicles, the greater the concentration of urban runoff pollutants and the more money we have to spend managing these pollutants. In addition to the NURP studies (U.S. EPA, 1983), several other publications provide excellent reviews of urban stormwater quality (Makepeace et al., 1995; Pitt et al., 1995).
Major categories of urban pollutants include sediments, nutrients, microbes, and toxic metals and organics. Sediment concentrations in urban runoff are particularly problematic because of their ubiquitous nature, and the fact that many other pollutants occur in a solid state associated with sediment particles. Sediment loadings occur primarily from soil erosion and runoff from construction sites in urban areas. Road sanding can also be a major source of sediments. Increased sediment in urban runoff may cause significant biological, chemical, and physical changes in receiving waters including loss of water clarity, clogging of gills and filters of aquatic organisms, and aquatic habitat degradation including the smothering of spawning beds and benthic communities.
Sources of nutrients such as nitrates and phosphates include chemical fertilizers applied to landscaped areas, lawns, and gardens, failed septic systems, soil erosion, and atmospheric deposition. Excessive nutrients in urban runoff can stimulate algal growth and cause nuisance algal blooms. Urban runoff may also contain high levels of organic matter that can lead to depleted oxygen levels in water and sediment when it decomposes. This in turn may cause excessive odors and fish kills in receiving waters.
Microbes include hundreds of different kinds of bacteria, protozoa, and viruses that are ubiquitous in the natural environment. Many are beneficial, while others can cause diseases in aquatic biota, and illness or even death in humans. Some types of microbes are pathogenic (e.g., Giardia spp.), while others indicate a potential risk for water contamination (e.g., fecal coliform bacteria) and may limit swimming, boating, and consumption of fish and shellfish in receiving waters. Microbes are almost always found in high concentrations in urban stormwater, but are highly variable in nature and very difficult to eliminate. Primary sources of microbes include failed septic systems, and waste products from pets, birds, and wild mammals commonly found in urban areas.
Toxic pollutants commonly found in urban runoff include trace metals such as lead, copper, zinc, and organic compounds including oils, grease, phthalates, and chlorinated hydrocarbons. Copper, lead, and zinc were detected in more than 90% of stormwater samples from the NURP studies, and 14 toxic organic compounds were detected in more than 10% of the samples (U.S. EPA, 1983). Sources of toxics include the breakdown of metal products, vehicle fuels and fluids, vehicle wear, industrial processes, and the use of industrial and household chemicals such as paints, preservatives, and pesticides. Trace metals and organic compounds may be highly toxic to aquatic organisms and can bioaccumulate in fish and shellfish.
Effects on Salmonids
Probably the most immediate and obvious effect of urbanization is loss of natural vegetation, a resulting increase in impervious cover, and dramatic changes to the natural hydrology of urban streams. Urbanization nearly always results in land clearing, soil compaction, modification and/or loss of riparian buffers, and modifications to natural drainage features. Modified hydrology resulting from urbanization usually has the most deleterious effect on small urban streams.
Numerous studies over the past 10 years have shown that increased impervious cover in urban areas leads to increased volumes of runoff, increased peak flows and flow duration, and greater stream velocity during storm events. Other impacts include loss of large woody debris (LWD), increased bank erosion and bed scour, changes in sediment loadings, increased stream temperature, and decreased base flow. As a result of decreased quantity and quality of LWD and modified hydrology, salmonid rearing habitat is often significantly reduced and spawning habitat degraded by the cumulative effects of urbanization.
Urban runoff is also typically warmer than receiving waters and can significantly increase temperatures in small urban streams. Removal of riparian buffers, stream bank alteration, widening of channels, and decreased groundwater recharge during baseflow conditions also contribute to stream warming. Stream temperature is known to be very important for migrating adult salmonids and upstream migration can be delayed as a result of excessively warm stream temperatures. Excessive stream temperatures may also negatively affect incubating and rearing salmonids (Gregory and Bisson, 1997).
One of the most comprehensive studies of the effects of watershed urbanization on small streams was recently completed by a group of researchers from the University of Washington (May et al., 1997). These researchers focused on 22 watersheds in the Puget Sound lowland ecoregion that represented a range of urbanization defined by total impervious area (% TIA). They conducted extensive surveys of instream physical habitat and riparian structure, conducted macroinvertebrate sampling, and monitored chemical water quality for baseflow and storm events.
Results of this study showed that there was a dramatic decline in biological integrity and physical habitat conditions as watershed urbanization increased above 5% TIA. The frequency, volume, and quality of LWD decreased significantly as %TIA and riparian buffer encroachment increased, and coho rearing habitat was significantly reduced in low to moderately developed watersheds.
The University of Washington researchers also found that water quality degradation was low to moderate at moderate levels of urbanization, but dramatically increased in highly urbanized watersheds (TIA > 45%). Conductivity (a surrogate for total dissolved solids and alkalinity) was strongly correlated to the level of basin development in baseflow stream samples. Other chemical constituents that showed good correlations with degree of urbanization included total zinc, total phosphorous, and total suspended solids. However, this study found that water quality criteria were rarely violated except in the most highly urbanized watersheds. The stresses of water quality degradation are exacerbated by the increased suspended sediment and decreased intragravel dissolved oxygen, both factors associated with urbanization (May et al., 1997)
Management of Urban Stormwater
For the past 15 years, an ever increasing number of local governments and private industry have developed storm and surface water management programs to mitigate for the effects of urban runoff. Some of these programs were developed to manage the increased flows and frequent flooding associated with urban runoff, while others have taken a more comprehensive approach to dealing with both the quality as well as quantity of urban stormwater runoff. After spending millions and millions of dollars on these programs, utility rate payers and elected officials often demand to know if these programs are working? Are water quality criteria being met? Are native salmonid populations being protected?
Studies in the Pacific Northwest have shown that native vegetation and undisturbed ground cover can typically absorb 100% of a 1” storm with no resulting surface runoff. Typical urban development usually results in the near total removal or compaction of native ground cover, producing the dramatic increases in surface runoff and modified hydrology in receiving streams that was discussed earlier. Many urban stormwater programs attempt to mitigate for this increased runoff by constructing detention ponds or infiltration trenches, but these structural best management practices (BMPs) usually only recover about 25% of the original storage capacity of the undisturbed site.
Structural BMPs are also constructed to treat the quality of urban runoff. A recent summary of the pollutant removal efficiency of commonly used BMPs found that many structural BMPs such as wet ponds and sand filters can remove significant amounts of sediment and phosphorous in urban runoff, but have a much lower ability to remove nitrogen (Brown and Schueler, 1997). Unfortunately, BMP pollutant removal data are scarce for bacteria, hydrocarbons, and dissolved metals, as these parameters were only measured in about 20% of the BMP performance studies summarized in this report. Of the data summarized, most BMP groups showed a moderate to high ability to remove total lead, zinc, and petroleum hydrocarbons from urban runoff, but only a moderate ability to remove organic carbon.
Clearly urban stormwater runoff cannot be effectively managed by employing only structural BMPs.
Nonstructural BMPs such as drainage design and erosion control standards, enhanced maintenance and enforcement, wetland and riparian buffer protection ordinances, development controls, and educational programs are necessary to effectively manage urban stormwater. Also, the results of the University of Washington study (May et al., 1997) suggest that a tiered approach to urban watershed management and salmonid protection may be the most cost effective in the long run. Because
biological integrity and physical habitat conditions decline rapidly in urban watersheds with greater than 5% imperviousness (unless effective riparian buffers are in place), preservation and protection efforts may be better focused in undeveloped watersheds with high quality stream systems that currently support natural salmonid populations. Low to moderately developed watersheds (< 45% TIA) may be better candidates for rehabilitation and enhancement efforts that could restore the stream’s natural hydrologic regime. For highly urbanized watersheds (>45% TIA), water quality degradation becomes much more of a serious concern for aquatic biota, and should become more of the focus of watershed and salmonid management programs.
References
Brown, W. and T. Schueler. 1997. National pollutant removal performance database for stormwater BMPs. Center for Watershed Protection. Silver Spring, MD.
Gregory, S.V. and P.A. Bisson. 1997. Degradation and loss of anadromous salmonid habitat in the Pacific Northwest. In: D.J. Stouder et al. (eds). Pacific Salmon and Their Ecosystems, Status and Future Options. Chapman and Hall, New York.
Makepeace, D.K., D.W. Smith, and S.J. Stanley. 1995. Urban stormwater quality: summary of contaminant data. Critical Reviews in Environmental Science and Technology 25(2):93-139.
May, C.W., E.B. Welch, R.R. Horner, J.R. Karr, and B.W. Mar. 1997. Quality indices for urbanization effects in Puget Sound lowland streams. Washington State Department of Ecology Publication No. 98-04.
Pitt, R., R. Field, M. Lalor, and M. Brown. 1995. Urban stormwater toxic pollutants: assessment, sources, and treatability. Water Environment Research 67(3):260-275.
U.S. EPA. 1983. Results of the Nationwide Urban Runoff Program, Vol. 1, Final report. NTIS PB84-185552.
You can reach Joanne at:
jrichter@empnet.com
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