Surface Water: Rivers and Lakes - Water Quality Of The Nation's Riversand Streams
Causes of Pollution in Rivers and Streams
The states reported that pathogens (bacteria), siltation (the smothering of river and streambeds by sediment, usually from soil erosion), habitat alterations, and oxygen-depleting substances were the four most common causes of pollution in our nation's rivers and streams. (See Figure 3.4.) In 2000 pathogens from point and nonpoint sources affected about 34% of all polluted river miles. Silt was the second most common cause of pollution in the assessed rivers and streams, affecting 31% of those considered impaired. Habitat alterations were listed as the cause for impairment of 8% of the assessed river and stream miles, or 22% of the impaired miles. Oxygen-depleting substances were responsible for approximately 20% of the polluted
FIGURE 3.4
Leading pollutants and stressors in impaired rivers and
streams, 2000
FIGURE 3.5
Effects of siltation in rivers and streams
PATHOGENS.
In the 2000 National Water Quality Inventory, pathogens (bacteria) were identified as the leading cause of water-quality impairment, responsible for polluting 93,431 river and stream miles. Bacteria provide evidence of possible fecal contamination that may cause illness. States use bacterial indicators to determine if rivers are safe for drinking or swimming. The most common sources of bacteria are urban runoff, inadequately treated human sewage, and runoff from pastures and feedlots.
SILTATION.
Silt was the second most important source of pollutants to rivers and streams. In an earlier report, National Water Quality Inventory—1998 Report to Congress (June 2000), silt had ranked as the top cause of pollution entering the waterways of the United States. In the later report, siltation had dropped to second place. Silt, composed of tiny soil particles, impaired 12% of the assessed rivers and streams, which is 31% of the impaired river and stream miles reported in the 2000 EPA Inventory. (See Figure 3.4.)
Silt alters aquatic habitats, suffocates bottom-dwelling organisms and fish eggs, interferes with light transmission to underwater plants, and clogs the gills of fish. The habitat of aquatic insects that live in the spaces between pebbles and rocks is destroyed when these spaces are filled with silt. Loss of aquatic organisms can radically affect the health of certain fish species and other wildlife that eat them. (See Figure 3.5.) Excessive silt can also interfere with recreational use and drinking-water treatment. The primary sources of silt are agriculture, urban runoff, forestry, logging, and construction.
OXYGEN-DEPLETING SUBSTANCES.
Oxygen is vital for the animal life in waterways. Lack of oxygen occurs as a result of the oxygen-consuming processes by which organic matter decays. The risk of oxygen depletion is therefore greatest in waters affected by high discharge of organic matter, and also where substantial production of algae and other plants occur. Oxygen-depleting substances were the third leading pollutant type listed in the 2000 National Water Quality Inventory, affecting
FIGURE 3.6
Leading sources of river and stream impairment, 2000
Sources of Pollution in Rivers and Streams
The three leading sources of pathogens, siltation, and oxygen-depletion in U.S. rivers and streams in 2000 were agriculture, hydrologic modifications, and habitat modifications. (See Figure 3.6.)
AGRICULTURE.
Agriculture was the leading source of pollution, responsible for about 48% of the reported water-quality problems in impaired rivers and streams. (See Figure 3.6.) The term "agriculture" captures a number of activities, from large-scale factory farming to landscape plant nurseries and fish farming. The agricultural uses that are most frequently responsible for contributing pollutants to water were:
- Nonirrigated crop production (Rain is the sole water source.)
- Irrigated crop production
- Range grazing
- Pasture grazing (land where a specific crop is grown to feed animals either by grazing animals among the crops or harvesting the crops)
- Animal-feeding operations
The EPA's 2000 report shows that the three agricultural activities that had the most degrading impact on rivers and streams together were responsible for 53.7% of the impaired river and stream miles reported; nonirrigated crop production (26,830 miles); animal feeding operations (24,616 miles), and irrigated crop production (17,667 miles).
HYDROLOGIC MODIFICATIONS.
Hydrologic modifications were responsible for 20% of the impaired miles of rivers and streams (53,850 miles). (See Figure 3.6.) These modifications include such things as flow regulation, channelization, dredging, and the construction of dams. Modifications of these kinds alter the habitats of rivers and in so doing can cause them to become far less suitable for aquatic life.
HABITAT MODIFICATIONS.
The modification of river and stream habitats can have a similar and equally destabilizing impact on aquatic life. The EPA report defines habitat modifications as all those changes to habitat that do not directly affect water flow. That would include such things as the removal of woody debris, logging activities, and/or land-clearing practices. Habitat modifications were ranked third in terms of sources of pollutants impairing rivers and streams in the 2000 National Water Quality Inventory. A reported 37,654 river and stream miles were degraded due to habitat modifications, accounting for 14% of the impaired river and stream miles. (See Figure 3.6.)
Municipal Sewerage Systems
Large cities, suburban areas, and many small towns are served by municipal sewerage systems. Wastewater treatment facilities vary in capacity from treating as little as 10,000 gallons per day (gpd) to treating 400 million gpd. The degree of treatment provided to the sewage also varies and may be primary, secondary, or tertiary treatment. Primary sewage treatment uses screens to remove debris, a grit chamber to settle out grit and sand, and solids settling. Following this process, the liquid waste is generally disinfected with chlorine and discharged to a water body. The settled solids are transported to a sludge digester for microbial digestion and conversion to bio-solids. Primary treatment achieves a 40% to 60% reduction in bacteria and total solids (nutrients and oxygen-demanding substances).
Secondary sewage treatment uses primary treatment, adds another settling step, and aerates the effluent to accelerate microbial digestion, resulting in a 70% to 90% reduction in bacteria and suspended solids. The liquid waste is disinfected before discharge. Tertiary treatment removes nutrients and additional suspended solids by adding chemical or microbial treatments, or filtration to remove additional suspended solids and reduce nutrients. Sometimes wetlands are also used to provide additional treatment. The effluents are generally disinfected before discharge. Tertiary treatment is generally used in nutrient reduction strategies to reduce algal blooms, trophic levels, and hypoxia (oxygen deficiency). The success of each category of wastewater treatment is directly dependent on the treatment method selected and on whether the treatment plant is properly sized for the volume of wastes handled. If the plant is overloaded, treatment success declines. Since 1974 progress has been made in upgrading sewage treatment plants, but much remains to be done.
Sewage sludge is the solid, semisolid, or liquid untreated residue generated during the treatment of domestic sewage in a treatment facility. The method of biosolids disposal depends on the level of treatment the sludge receives and on its contaminants. The most common methods are land application as fertilizer or soil conditioner, transport to landfills, incineration, and composting for sale as a lawn and garden additive. Overboard disposal of sludge in oceans and estuaries is no longer permitted. When properly treated and processed, sewage sludge becomes biosolids, a nutrient-rich material that can be safely recycled and applied as fertilizer. Biosolids production is strictly controlled by federal and state regulations.
Many older towns and cities have combined sewer systems; that is, sewer systems that transport sewage in dry weather and sewage and rainwater in wet weather. Combined sewers during heavy rainfall events can quickly overload a wastewater treatment plant, resulting in the need to "bypass" or divert portions of the combined flow overboard. Bypass releases a mixture of rainwater, raw sewage, oil, and gasoline from runoff, along with fertilizers and pesticides from lawns and gardens, into waterways.
Combined sewer overflows are one of the major sources of bacteria, nutrients, and silt in waterways. The solution to combined sewer overflow is separation of the sewer and storm water systems. This is prohibitively expensive, as it frequently requires tearing up urban streets and sometimes buildings to replace the existing combined sewer with separate lines for sewage and storm water runoff. Alternatives include expanding the waste-water treatment plant capacity, building separate treatment facilities for the storm water flow, capturing and storing storm water flow and feeding it into the treatment plant during periods of low sewage flow, and constructing special wetlands to treat the effluent. Each of these alternatives is land-intensive and costly. One estimate has placed the cost of correcting the combined sewer problem as greater than the annual gross national product of the United States.
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