Executive Summary 

Common practices for protecting water resources often fail to maintain either good water quality or healthy ecosystems. This failure is not due to a failure to control pollutant releases of sewage and industrial effluent so much as it is due to altered hydrology caused by the handling of stormwater runoff. Numerous studies link uncontrolled stormwater runoff from areas with impervious surface exceeding 10 percent to a rapid decline in water quality and stream health. Streams draining residential suburbs with typically high levels of impermeable surface experience two to five times the stream-channel enlargement of areas with less impermeable surface, endure increased flooding, are prone to low flow during droughts, and are biologically nonsupporting.

Traditionally, public entities have managed urban water resources using what might be characterized as a civil engineering, or technocratic approach that treats stormwater as a waste product of development. But the hydrologic cycle is too complex to respond predictably to such a rigid, narrow approach. Moreover, this approach reinforces, or even encourages, land-use practices that can substantially disrupt the hydrological cycle.

Responsibility for stormwater management is generally dispersed between various government agencies and departments. Stormwater systems are rarely built to handle runoff from anticipated future development. Inadequate design coupled with a lack of funds for maintenance often force managers to react to problems such as poor water quality and flooding with short-term, piecemeal solutions.

A number of political jurisdictions scattered across America are implementing innovative approaches to stormwater management. One such approach is the creation of user-fee based stormwater utilities to improve urban watershed and stormwater management. Case studies show that communities adopting this form of management can produce better water quality, healthier urban ecosystems, and improved quality of life. Such systems link the decisions of people who impact stormwater flows to the stormwater management system directly through a fee system linked to usage and impacts.

The cost-based, user-fee-funded stormwater utility encourages the recognition of stormwater as a resource, not simply as an event to be managed. The utility concept focuses management in a single organization, which can be public, private, or some combination thereof. Creation of a utility allows for dedicated infrastructure and management funding, with fees tied to impacts. The approach enables development of comprehensive preventative and enhancement programs.

New federal requirements for stormwater permits affecting smaller cities and court-mandated enforcement of the Clean Water Act on a watershed basis are spurring many municipalities to consider the user-fee concept for dedicated funding of improved stormwater management. Over 350 stormwater utilities have been formed nationwide, most in the last decade.

Stormwater utilities can provide an equitable means for many communities to fund improvements in water quality and reduce flood damage. However, achieving the goal of swimmable and fishable waters stated in the Federal Clean Water Act may eventually require additional steps such as comprehensive water resource management that combines water supply, sanitary sewage, stormwater drainage, and wildlife protection under a watershed-scale integrated water utility.

Three case studies illustrate the experiences of cities that have established stormwater utilities. Stormwater utilities can encourage development that uses natural hydrological cycles to maintain water quality and flourishing ecosystems. Bellevue, Washington, which established one of the first stormwater utilities in the nation, demonstrates that designing "with nature" can reduce the negative impact of impervious surfaces on aquatic systems, while creating highly desirable neighborhoods. Charlotte, North Carolina shows that stormwater controls can be retrofitted to already-developed neighborhoods through bioengineering of retention ponds and other steps such as stream-habitat improvements. Atlanta’s experience is more mixed, showing how the technocratic approach to water management is unsustainable, and a case history of mistakes to avoid in establishing a stormwater utility.

Rather than adopting growth boundaries or other regulatory approaches that put broad areas of private land off-limits to development, this study recommends that a market-based approach integrating economic and ecosystem needs could be implemented based on the following principles:

This study recommends that a market-based approach integrating economic and ecosystem needs could be implemented

Table of Contents

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Introduction*

Cities rely upon a clean supply of fresh water for their economic health, yet the expansion of urbanized areas can threaten this vital resource. Until recently, urban growth was seldom constrained by a shortage of clean water because high-quality supplies were relatively abundant in most areas of the United States. But "water wars," originally limited to the arid West, now erupt even in the humid Southeast, as municipalities and states wage legal battles over access to the remaining unspoiled water sources.

Traditionally, public entities have managed urban water resources, using what might be characterized as a civil engineering or technocratic approach. But the hydrologic cycle is too complex to respond predictably to such a rigid, narrow approach. Moreover, this approach reinforces, or even encourages, land-use development practices that can substantially disrupt the hydrological cycle.

Under the traditional approach:

A number of adverse impacts occur as the result of these traditional development practices:

Urban development practices that clear-cut land raise summer temperatures and lower dissolved oxygen in streams, stressing aquatic ecosystems. Removal of the tree canopy also increases runoff of stormwater and sediment, and the higher temperatures of urban heat islands can increase smog formation, stressing humans and natural systems.

The combination of these impacts undermines the hydrologic cycle required for ecosystem health and human well-being. Although specific conditions vary based on local development patterns and hydrology, most of the East and an increasing area of the West are affected. Dealing with these negative impacts creates huge costs as communities age. 

Atlanta provides an example of how the current technocratic approach to water management is unsustainable. The Chattahoochee River, by far the largest source of water to Metro Atlanta, was dammed north of the city during the 1950s, forming Lake Sidney Lanier. Water released by the dam supplies drinking water to more than three million Atlantans. In response to widespread concerns that development in the watershed surrounding the lake was leading to siltation and water quality problems, a $2-million study was commissioned. Released in 1997, the study estimated it would be only 15 years before silt and other pollution from unchecked growth would kill the fish in the lake.

A second reservoir northwest of the city is Lake Allatoona on the Etowah River. A pipeline was proposed to transfer water from Lake Allatoona to Lake Lanier in order to meet Atlanta’s growing needs. But due to rapid development and its smaller size, Lake Allatoona is filling with silt even faster than Lake Lanier. A study on Allatoona concluded the lake might be unable to support fish within 10 years. Although strong opposition makes it almost certain the project will never be built, it illustrates the type of planning characteristic of the technocratic approach that emphasizes large public works projects to store, transport, and treat water, rather than protection of water at its source.

Such failures cause significant environmental damage and can impose significant costs on municipalities. An example of these costs is provided by one of the 10 counties in the Metro Atlanta area. Cobb County, which draws water from Lake Allatoona, reported a $300,000 increase in the cost of treatment chemicals in 1999 out of a $4-million budget. Although increasing treatment costs for drinking water are only a tiny fraction of the full cost of damages that accrue from such failed management practices, the Allatoona example is a useful reference point.

Although poorly treated sewage and other types of pollution are major causes of urban water-quality impairment, the key condition limiting the recovery of urban streams in most locations is altered hydrology, including stormwater runoff.

A number of studies link uncontrolled stormwater runoff from areas with impervious surface exceeding 10 percent to a rapid decline in water quality and stream health. Streams draining areas with 25 to 30 percent impervious cover, typical of residential suburbs, experience stream channel enlargement from two to five times, increased flooding, and reduced biological-support functions.

For example, historic Druid Hills in metropolitan Atlanta serves as the site for a paired watershed study in which two nearby catchments, nearly identical except for different levels of impervious cover, were intensively studied over time to assess comparative conditions and impacts. Figure 1 shows a bird’s-eye view of the two catchments. The catchment in Fernbank Forest, an urban forest preserve, serves as the reference watershed. Impervious surface area in the Fernbank Forest watershed was measured at only five percent. The other watershed in the comparison, with 19 percent impervious cover, is a residential neighborhood surrounding Olmsted-designed Deepdene Park.

 

The two watersheds shown in Figure 1 are nearly identical except for the amount of impervious surface area associated with residential development. Fernbank, with 5 percent impervious area, was drained by a healthy stream that exhibited few signs of erosion. Deepdene, at 19 percent impervious area, had an unhealthy stream scoured by surges of stormwater from surrounding roads. During dry weather, the stream draining Deepdene had less base flow than the stream draining Fernbank.

Both watersheds are located four miles from the center of downtown Atlanta. Neither watershed had point-source discharges of pollution or sewer lines running near the streams. As Figure 2 shows, stream health in the forested Fernbank watershed was consistently in the excellent category.

"Water wars" now erupt even in the humid Southeast, as municipalities and states wage legal battles over access to the remaining unspoiled water sources.  

Note: Stream health measured by an index of biotic integrity based on standard collections of organisms known as benthic macroinvertebrates. These are stream insects and other small invertebrates consumed by fish. The index integrates impacts from factors that adversely affect water quality over a period of time. The sampling procedure is inexpensive and easy enough for use by schools and community volunteers. The results shown here were measured by the author using the Georgia Adopt-A-Stream Index. schools and community volunteers. Independent professional surveys conducted at the same sites in 1997 and 1999 using an index of biological integrity showed essentially the same results.

 

Source: Author 

 

Even though the headwaters showed some early signs of erosion from surface flow of stormwater off paved areas bordering the watershed, most of the stream looked healthy, with vegetated banks as shown in Figure 3. Note that in Fernbank, none of the paved areas drained directly into the stream.

A number of studies link uncontrolled stormwater runoff from areas with impervious surface exceeding 10 percent to a rapid decline in water quality and stream health.

By comparison, as Figure 4 shows, the stream draining the lightly developed residential neighborhood surrounding Deepdene Park was badly eroded by stormwater discharged from road-drainage culverts. Its biotic health was fair on average, varying from poor following a storm event to good after a long period of favorable weather.

The impaired condition of the Deepdene stream was best explained by changes in stream flow and sediment loading, as shown in Figure 5, caused by stormwater runoff from impervious surfaces.

In Figure 5, the increase in impermeable surfaces causes an increase in discharge following rainfall and a decrease in discharge (base flow) during dry weather. A dramatic increase in stream-borne sediment accompanies the surge of stormflow in the developed Deepdene Watershed, whereas in the forested Fernbank Watershed stream sediment increases only slightly. With no active clearing of land, most of the sediment comes from stream bank erosion. (These hydrographs were taken from stream gage recordings in the two watersheds).

The paired watershed study demonstrates that impervious cover is a good indicator of overall stream health.

Changes in stream flow following urbanization for a developed watershed (Deepdene) and a relatively undeveloped watershed (Fernbank). A hyetograph of rainfall (rain intensity over time) and hydrographs of the resulting stream runoff for a thunderstorm of 18 mm or 0.7 inches or rainfall on April 11, 1995 in Atlanta, GA (Data collected by the author from a rain gage, two simultaneously recording stream gages, and measures of suspended sediment). The increase in impermeable surfaces in the developed watershed causes an increase in stream discharge following rainfall and a decrease in discharge (base flow) during dry weather. A dramatic increase in stream-borne sediment accompanies the surge of stormflow in the developed watershed (Deepdene), whereas in the forested, mostly undeveloped watershed (Fernbank) stream sediment increases only slightly. With no active clearing of land, most of the sediment comes from stream bank erosion.

 

"The United States still has no adequate database on water quality. Nevertheless, water in the United States is clean and its quality has been improving over the last 20 years."

"There are two very conflicting messages. A lot of rivers are cleaned up—you can swim in them now. . . . But when you look deeper, many of the nation’s rivers are in worse shape than they have ever been."

The opinions expressed above represent two very different points of view, yet both may be correct depending upon the author’s analytic framework. Many of the most egregious cases of water pollution have been cleaned up—the Cuyahoga River in Cleveland that caught fire in 1969 will almost certainly never catch fire again. Yet scientific evidence supports the view that many of the nation’s surface waters are losing their ability to support life. As described later in this report, stream health is particularly poor within and downstream from urban areas.

Currently available data help illuminate the nature of this water-quality problem. Table 1 shows National water quality data compiled by the U.S. Environmental Protection Agency (EPA) from reports submitted by the states based on meeting so-called designated uses, as required by the 1972 federal Clean Water Act. Designated uses include fishing, swimming, and drinking.

A major problem with state data is that methods for collection and analysis are not subject to uniform quality-control procedures. This does not mean that quality control is completely lacking, but the procedures vary. The sites selected for testing may also bias the results since there is no nationwide randomized sampling procedure. Nonetheless, very different impressions of water quality can be conveyed using the same source.

Someone promoting the view that water quality is not a problem could accurately state that only 14 percent of America’s rivers surveyed in 1994 did not meet their designated uses. Although accurate, the statement is misleading. It does not mention that only a small portion (17 percent) of river miles were surveyed or that many of the river miles were only partially supporting (22 percent), which means that the waters met their designated use only part of the time and were therefore impaired. On the other hand, someone promoting the view that water quality is a problem could accurately state: "All that can be said with certainty is that 11 percent of our river miles were not impaired in 1994." This statement is accurate but misleading because it implies that 89 percent of rivers are unhealthy.

Because the human economy depends upon the hydrologic cycle, policy makers need science-based measures of the system’s health. The cycle consists of a combination of physical and biological processes. Due to the complexities of atmospheric circulation, measuring the physical component is fraught with as many difficulties as those facing climate-change researchers addressing the question of global warming. A more feasible and less controversial approach is to measure the biological component of the system as reflected by the health of aquatic organisms. This increasingly applied approach uses science-based indices of biotic integrity. During the last decade, 30 states plus the District of Columbia began surveying waters using measures of ecosystem health. Although completed surveys cover only a tenth of all surface waters, half the waters surveyed are biologically impaired.

More extensive surveys are available for water meeting designated-use categories (fishing, swimming, and drinking) and for aquatic species threatened with extinction. Designated-use surveys have been reported by states for half of all United States surface waters. According to the U.S. Environmental Protection Agency (EPA), the surveys show that 40 percent of surveyed waters are impaired and that 50 percent of impaired rivers are affected by urban and construction sources of stormwater runoff.

Although aquatic-use surveys are the most complete surveys available, they are incomplete indicators of aquatic health. The surveys are based primarily on a series of physical, chemical, and bacterial tests. Because the tests are generally spot checks, not continuous measurements, they frequently miss events (such as post-storm pollutant spikes, or sewage spills) that can kill entire communities of higher aquatic organisms (and sicken humans). Chemical surveys that are complete enough to screen for the thousands of possible contaminants are prohibitively expensive. Hence, more affordable, less complete surveys are the norm. By comparison, sensitive biological surveys are relatively inexpensive and are considered by many experts to better reflect the cumulative impacts of altered hydrology, habitat loss, and chemical toxins.

Figure 6 shows rivers and streams surveyed for meeting both designated use categories and biological health. Thirty states plus the District of Columbia and the Ohio River Valley Sanitation Commission have numeric data of sufficient quantity to be confident in the determination of biological integrity. The designated use category refers to water quality objectives, or uses, established by states under the Clean Water Act for the protection of fisheries (aquatic life designated uses). Since most states rely on chemical standards to represent conditions that protect aquatic life, the results can be quite different from those obtained by direct biological sampling. These data are for perennial rivers and streams flowing throughout the year and exclude nonperennial stream miles.

Figure 6 compares two surveys on the health of U.S. rivers and perennial streams. (Lakes, reservoirs, and estuaries are not included in this comparison.) The same group of 32 states and territories reported both surveys in the same year. The left chart displays the percentage of rivers and streams that supported, or failed to support their designated use category. The right chart displays the percentage of rivers and streams considered impaired or good based on an index of biological integrity.

 

Source: United States Environmental Protection Agency, Summary of State Biological Assessment Programs for Streams & Rivers (Washington, D.C, 1996); United States Environmental Protection Agency National Water Quality Inventory, Report to Congress (Washington, D.C., 1996).

 

Biological surveys are a direct measure of biological community, or ecosystem health, whereas chemical surveys are at best an indirect measure. Whereas biological integrity is one of the stated objectives of the Clean Water Act, most state water quality standards rely upon chemical tests to determine if water quality is high enough to support designated aquatic life uses such as warm water (e.g., bass) or cold water (e.g., trout) fisheries. Although only a small portion of America’s rivers and streams have been surveyed using a science-based index of biological integrity, available data show that ecosystem health may be a more serious problem than is indicated by use surveys reported under the Clean Water Act.

A second way to examine the ability of the nation’s waters to support life is to evaluate the number of aquatic species threatened with extinction. The Nature Conservancy (TNC) in cooperation with the Natural Heritage Network has compiled an extensive report on the state of U.S. plants and animals categorized by species group. In describing the purpose of the report, TNC states, "A national debate is now under way about the manner in which we as a society should protect our endangered living resources. All sides agree, however, that an essential ingredient in addressing this issue is reliable scientific information." The leading conclusion from the report is that animals that depend on freshwater habitats (mussels, crayfish, fishes, and amphibians) are in the worst condition overall. The shaded portions in Figure 7 shows that approximately half of freshwater aquatic species groups are extinct, imperiled, or vulnerable to extinction. (Though the reasons are unclear, amphibians, a group that inhabits both freshwater and terrestrial habitats, have 40 percent of species threatened with extinction.)

Why should we care about the possible loss of freshwater species when so many of them are seldom seen? In addition to their value as a genetic resource, their role in the food chain, and their aesthetic value, many of the aquatic organisms play an important role in keeping waterways healthy for human use. Freshwater mussels, one of the least noticed and most threatened species groups, are mollusks that live on the bottom and feed by filtering minute organisms from the surrounding water. Mussels remove suspended particles that bacteria attach to and keep the water clear. Mussels are vulnerable to extinction because they are long lived (some species up to 50 years or more), and susceptible to suffocation by sediment and poisoning by chemicals at relatively low concentrations.

Source: Bruce A. Stein and Stephanie R. Flack, 1997 Species Report Card: The State of U.S. Plants and Animals (Arlington, Virginia: The Nature Conservancy, 1997).

 

In recognition of the value of mollusks in cleaning up polluted coastal waters, Jacksonville, North Carolina announced a plan to clean up Wilson Bay, contaminated by the discharge of treated sewage effluent, by restocking it with more than a million clams, oysters, and mussels. The town’s spokesperson noted that a single oyster could filter 50 gallons of water in a day. The plan also includes a proposal to restock depleted populations of sturgeon, which formerly supported a commercial fishery. The goal of the project is to "kick start" the natural processes that once cleansed the now-sullied waters.

 

Urbanization can damage water quality out of proportion to the actual rate of development because impervious surface area in many regions now reaches or exceeds the biological threshold of 10 percent at which runoff begins to exceed natural "processing" capacity. A growing body of scientific evidence indicates a direct link between impervious cover and stream health. Thus, impervious cover in a watershed is a good indicator of the overall health of streams that feed rivers and lakes. Where impervious cover exceeds 5 percent, stream health begins to decline in some regions. As Figure 8 shows, with more than 10 percent impervious cover, stream health may be biologically impaired. At 25 percent cover, streams can be non-supporting. Nutrient loading and other types of pollution also increase in proportion to the impervious surface area in a watershed.

 

Source: Adapted from Deb Caraco, Rich Claytor, et al., Rapid Watershed Planning Handbook: A Comprehensive Guide for Managing Urbanizing Watersheds (Ellicott City Maryland: Center for Watershed Protection, October 1998).

In Figure 8, impervious cover in a watershed can be used to project the current and future quality of streams. Evidence suggests that larger bodies of water such as lakes, reservoirs, and estuaries are linked to the health of tributary streams. Healthy streams are in equilibrium and contain diverse communities of fish and aquatic insects. Impacted streams have unstable banks, increased levels of fecal bacteria and pollutants, are more prone to flooding, and have lower biodiversity. Nonsupporting streams are conduits for stormwater.

Urban development, which is often accompanied by marked increases in impervious surface, covers about five percent of the land area in the United States. East of the Mississippi, the median amount of development—the mid-point when states are ranked by their degree of development—is almost 10 percent. Figure 9 shows the percentage of land in the United States that is developed by Hydrologic Unit. Hydrologic units are used by the U. S. Geological Survey to catalogue watersheds.

 

 

But regional developmental densities are only part of the story. As Figure 10 shows, the type of development is also important. At the low end of the scale, residential development on estate lots covers 12 to 20 percent of the land with impervious surfaces. At the high end of the scale, shopping-center development results in over 90 percent imperviousness. At all levels of development, most of the impervious cover is in roads and parking lots.

 

Source: Adapted from Chester L. Arnold, Jr. and C. James Gibbons, "Impervious Surface Coverage: the Emergence of a Key Environmental Indicator," Journal of the American Planning Association, vol. 62, no. 2 (Spring, 1996), pp. 243-258.

 

Approximately 40 times more fresh water is stored in aquifers than is visible on the surface as streams, rivers, and lakes

As Figure 11 shows, a suburban subdivision and associated roads can dominate the landscape, leaving little room for natural ecosystems that are part of the hydrologic cycle.

Although no nationwide measurement of total imperviousness is available, its rate of increase warrants attention. Three trends underlie the increase in impervious surface area:

1. The U.S. population continues to grow at a rate of 20 million people per decade, a rate illustrated graphically in Figure 12. The increased population has been accommodated in housing with impervious rooftops. Assuming that housing has increased in proportion to the population since the turn of the century, this accounts for a 250 percent increase.

2. People have moved to cities where much of the surface area is paved, and concentrated runoff becomes a problem. As Figure 13 shows, the population was 40 percent urban at the start of the 20^th century, increasing to 75 percent by the close of the century. In the South, where urbanization lagged behind the rest of the United States, the trend is even more striking.

3. Meanwhile, as Figure 14 illustrates, surfaced road-miles have increased 2,300 percent since 1900. The total increase in impervious surface in the 20th century is probably much greater, because many roads built after World War II are multilane roads, and parking lots have expanded to accommodate rapid growth in vehicular traffic.

 

 

Notes:

1. Data covers rural and urban road miles from 1904 to 1990.

Includes soil-surfaced roads as well as slag, gravel, stone, bituminous, or concrete surfaces.

Background photos for Figures 13 and 14 are from Nova Development Corporation (818) 591-9600, clip art CD Art Mania 12,000.

The natural hydrologic system has evolved over billions of years to recycle water efficiently. In undeveloped circumstances, vegetated watersheds store and filter precipitation underground before it is gradually released to the surface through springs and seeps that feed streams. Approximately 40 times more fresh water is stored in aquifers than is visible on the surface as streams, rivers, and lakes. During periods of drought, water released from aquifers maintains the flow of streams that feeds rivers and lakes. But as Figure 15 illustrates, impervious surfaces and drainage networks that shunt runoff directly from road surfaces to streams inhibit the function of the natural hydrological cycle.

 

 

In Figure 15, roads seal the soil with an impermeable layer of pavement that blocks the recharge of groundwater. During periods of drought, stream flow is diminished, particularly in headwater streams. Rain flows quickly over pavement, flushing accumulated litter and pollution into storm drains that discharge directly into creeks and rivers. The increased volume of stormwater runoff, even after average rain showers, erodes and enlarges stream channels, causing siltation and destruction of adjacent habitat. Fortifying stream banks with rock and concrete can slow erosion at a particular site, but make the problem worse downstream.

During the middle of the 20^th century, the increasing density of roads and resulting problems with flooding led engineers to experiment with various approaches to dealing with increased stormwater runoff. Several approaches address these problems, depending on local conditions. However, the enclosed storm-drainage system that conveys surface runoff in underground culverts gained dominance, slowing further experimenta-tion for decades. Yet infrastructure managers discovered that speeding runoff from roads into creeks often caused flooding downstream. In an effort to reduce flooding, municipalities began requiring in the 1970s that developers install detention basins to hold back peak flow from major storms. Where the basin discharges through its outlet, the peak flow is reduced, but the downstream effect of detention depends upon how the basin’s discharge combines with the flow of other tributaries. In some watersheds, detention delays outflow from developed sites so that it overlaps onto the peak flow in the main stem, contributing to a higher combined flow. This problem with peak-flow detention was not foreseen, because the design standards were never tested for performance on a watershed basis, nor were the standards developed to protect streams or reduce pollution.

The U.S. Geological Survey conducted a landmark study on the effects of urbanization on stream channels in suburban Washington, D.C. beginning in the early 1950s. It showed that a surge of sediment enters streams when land is opened for construction. Less recognized is the long-term effect of increased runoff from paved roads, rooftops, and parking lots. The increase in impermeable area results in an increased volume of runoff, which is rapidly conveyed by storm drains into streams. The development-induced surge of runoff from each storm erodes stream channels, draining the area, gradually enlarging the channels, and depositing sediment downstream.

Over a period of 20 years the effect is dramatic. At the 25 to 30 percent impervious cover typical of many developed areas, stream channels experience two to five times stream channel enlargement. In addition to these effects of impervious surfaces, drainage networks dispose of stormwater as a waste product. Most of the pollutants that settle on the surface of roads wash off in a concentrated surge with the first flush of rainfall. With scouring, erosion, and pollution following every rain event, stream channels become biologically non-supporting extensions of the storm sewer system, requiring intensive treatment of water withdrawals for miles downstream. Since development is often concentrated along stream corridors, channel enlargement can result in extensive property damage. Cities face high costs as sanitary sewers and storm sewers are undermined by and leak into streams and rivers. Figure 16 graphically illustrates the destructive potential of development-induced stream channel enlargement.

The problem is much more severe in older cities where sewer lines leak into storm drainage systems and streams. An indication of the extent of this problem is the growth of an industry that contracts with municipalities to line the interior of leaking sewer pipes. According to the president of one such company, Evanco Underground Services, "If a sewer line is over 25 years old, it is likely to be leaking."

In the past, cities dealt with problems of bank erosion and channel enlargement by burying headwater streams in underground culverts and constraining larger tributaries in concrete or stone-lined channels. Some urban areas continue to bury and channelize streams despite widespread recognition that the practice worsens flooding and exports polluted water to communities downstream.

 

 

Pollution in urban streams is a very real problem. Point-source, or end-of-pipe discharges require a permit, but many illicit discharges from small pipes remain undetected. Surveys conducted through stream walks and storm sewer inspections can frequently uncover these illicit discharges and get them stopped through a program of enforcement. Numerous small or diffuse discharges are termed non-point pollution. Sources include particulate fallout from air pollution; fertilizers and pesticides applied to lawns; oil, antifreeze, and brake dust from vehicular traffic; pet feces on sidewalks and roadsides; and detergents and other chemicals from commercial activities. All of these substances are washed into streams by stormwater runoff. Chemical spills from industrial and commercial vehicle accidents can also discharge large quantities of pollution directly into storm drains that empty into streams. Effective stormwater management includes spill prevention and source-reduction programs that encourage proper disposal or recycling of potential pollutants.

Though pollution remains a problem, it is important to look at the underlying conditions leading to urban water-quality impairment. Many of the point-source discharges from industrial sites have been greatly reduced through market-driven technological changes and requirements of the federal Clean Water Act. What remains are the far more numerous and harder-to-control nonpoint-sources of pollution. The greatest source of urban nonpoint pollution is runoff from impervious surfaces. Typically, three-fourths of the pollution loading in urban streams is from stormwater runoff. According to the Nationwide Urban Runoff study conducted by the EPA, the decline in urban stream health is best explained by a combination of altered hydrology and pollutant loading.

In parts of some older cities, sewage and storm drains are combined; causing stormwater mixed with sewage to overflow as aging systems become overburdened. Figure 17 illustrates such a system.

 

In Figure 17, combined sewer systems mix stormwater and untreated sewage in the same system of collection pipes. During dry weather, sewage and small amounts of runoff entering the stormwater system (from car washing, for example) flow to the wastewater treatment plant. However, during wet weather, the capacity of the system is exceeded, and stormwater mixed with sewage overflows directly into creeks at points called Combined Sewer Overflows (CSOs). Combined sewers, leaking sewer lines, and illicit connections add to the problem of stormwater runoff.

The organisms that inhabit a healthy stream can tolerate a certain amount of stress while the stream purifies itself of pollutants. The ability of streams to absorb low levels of pollution and recover is known as assimilative capacity. Certain organisms that colonize streams can actually break down pollutants into less harmful substances, in effect making the streams natural water-pollution treatment areas. This concept is used as a legal rationale for issuing effluent-discharge permits. Some discharge is permitted as long as enough miles of free-flowing stream or river exist below discharge points for water quality to return to acceptable levels. However, when streams are stressed beyond their ability to adapt, either through excessive pollution loading, channelization, loss of riparian habitat, or changes in flow, assimilative capacity decreases, and the stream enters a spiral of decline.

 

A root cause of current water problems is land-use development patterns and water-management systems that disrupt the Earth’s natural hydrological cycle. Solar energy powers this vast cycle that recharges aquifers, streams, and lakes.

In Figure 18, solar energy powers the hydrologic cycle that recharges aquifers, streams, and lakes. The cycle operates through the interaction of physical and biological processes. Despite the abundance of water on Earth, very little is fresh. Water is distributed approximately as follows: oceans, 97.1 percent; ice, 2.25 percent; groundwater 0.6 percent; lakes (fresh and brackish) 0.015 percent; atmosphere, 0.001 percent; rivers, 0.0001 percent. The largest supply of fresh water is in the form of subsurface aquifers recharged through the process of infiltration of rainwater.

But as Figure 19 illustrates, urban development that results in substantial impervious surface can disrupt the cycle by replacing complex ecosystems adapted to the efficient recycling of water with impervious rooftops and pavement. Impervious surfaces block the recharge of groundwater, causing rainfall to rapidly run off into streams. Keeping rain (stormwater) on the surface instead of letting it soak into the ground depletes groundwater supplies upon which many communities depend for drinking water. When groundwater recharge is blocked, water tables drop, contributing to springs and streams going dry in the summer. Impervious surfaces, especially roads, also accumulate pollutants that are washed off in a concentrated surge with the first flush of rainfall. Replacement of natural vegetation with pavement also markedly raises the summer temperature of cities, reducing personal comfort, increasing the use of air conditioning (which increases air pollution), and further damaging the natural system of water recycling.

Pollution in urban streams is a very real problem. Point-source, or end-of-pipe discharges require a permit, but many illicit discharges from small pipes remain undetected.

In Figure 19, urbanization alters all parts of the hydrologic balance. Urban development begins disrupting the hydrologic cycle by replacing complex ecosystems that have evolved to infiltrate precipitation into the ground where it is stored and gradually released, with impervious surfaces that block groundwater recharge and increase runoff. The engineered system of surface storage, created by damming streams and rivers, is losing capacity as sediment eroded by increased runoff fills reservoirs, shortening their useful life.

The rapid runoff of stormwater into urban streams erodes their channels, causing the streams to deepen and widen. Stream-channel enlargement damages urban infrastructure by undermining bridges and sewer lines, collapsing structures, and eroding property. Increased runoff from impervious surfaces also results in an increase in damaging floods. Sediment eroded during the construction phase of new development, and later from enlarging stream channels, smothers aquatic life and accumulates downstream in water supply and flood-control reservoirs. The total cost of disruption to the hydrological cycle and resulting damage to urban infrastructure has not been calculated, but is likely to be very large. 

 

 

Three main approaches have emerged for addressing the negative impacts of development dominated by impervious land surfaces:

• The avoidance approach attempts to prevent development from happening by setting urban-growth boundaries, either through federal or state growth-management laws, or through local zoning codes. Outside the growth boundaries, development density is severely restricted; inside, additional develop-ment creates ever-higher densities.