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According to Sedell and others (2000), 80 percent of the freshwater resources in the United States originate in forests. Therefore, having healthy forests is critical to having clean water. The quality of water draining forested watersheds is typically the highest in the country (Binkley and Brown 1993, Clark and others 2000). Undisturbed forests or woodlands generally provide the best protection of land and water from sedimentation and other pollutants. The tree canopy and litter layer dissipate the energy contained in raindrops. Also, a continuous litter layer maintains a porous soil surface and high water infiltration rates; consequently overland flow is minimized in the forest. Forests slow stormwater runoff and provide watershed stability and critical habitat for fish and wildlife (Sedell and others 2000).
The body of literature that examines the role of forests, as compared to other land uses, in protecting water quality is significant. This section summarizes some of the key findings from various studies throughout the Nation and the South, related to sediment yield, nutrient yield, and biological conditions in forested versus nonforested watersheds. A number of these reports have been completed as part of the NAWQA program described in Section 5.2.3.1.
Patric and others (1984) and Yoho (1980) compiled the range of sediment yields from several small watershed studies throughout the Nation and the South, respectively. Both of these reviews concluded that forested lands produced a small fraction of the sediment yielded by more intensive land uses. Periodic timber harvesting activities occurred in many of the forested watersheds. Even with a wide diversity of forest types, geology, climate, and physiography, forested watersheds yielded far less sediment than areas where nonforest land uses occurred (Patric and others 1984). In the Upper Mississippi River Basin, sediment yield increased 150-fold from the forested headwaters to downstream areas dominated by other land uses, including agriculture (Mack 1967). Runoff and annual sediment yields were greatest from agricultural lands compared to pine plantations and mature pine-hardwoods in small watersheds in northern Mississippi (Ursic and Dendy 1963).
Faye and others (1980) compared erosion and suspended sediment yields in nine watersheds in the Upper Chattahoochee River Basin, Georgia, and reported the greatest suspended sediment yields from urban areas, compared with forested and agricultural lands. In a land-use study in Virginia, Jones and Holmes (1985) compared the effects of urban, agricultural, and forested land uses (silvicultural activities) on water resources. They concluded that forestry practices contributed little sediment; agriculture was an important source, and urban development contributed the most sediment (as well as other pollutants).
In a nationwide review of watershed characteristics and stream nutrient levels, Omernik (1977) found that streams draining agricultural watersheds had, on average, considerably higher nutrient concentrations than those draining forested watersheds. Nutrient concentrations were generally proportional to the percent of land in agriculture and inversely proportional to the percent of land in forest (Omernik 1977).
Spruill and others (1998) conducted a water-quality assessment of the four river basins in the Albemarle-Pamlico Drainage Basin - the Chowan, Roanoke, Tar, and Neuse. Highest nitrogen and phosphorous yields occurred in the highly agricultural and urbanized Neuse Basin, and lowest nutrient yields occurred in streams of the forested Chowan Basin. In a study of the Upper Tennessee River, Hampson and others (2000) found that sampling stations in forested watersheds had the lowest concentrations of total nitrogen, whereas stations in agricultural areas had the highest. Concentrations of nitrogen in urban and mixed land-use areas were significantly greater than in forested watersheds but were somewhat less than nitrogen concentrations in agricultural watersheds. As with total nitrogen, the lowest phosphorous concentrations were detected at sites in predominantly forested watersheds, whereas sites in urban and agricultural areas had the highest phosphorous concentrations (Hampson and others 2000).
In an assessment of biological indicators of the Apalachicola-Chattahoochee-Flint (ACF) River Basin, streams with forested land use had the best biological condition as shown by the Index of Biotic Integrity (Frick and others 1998). Lenat and Crawford (1994) conducted a study on the effects of land use on aquatic biota in three small catchment basins (forested, agricultural, and urban) in the Piedmont of North Carolina. Biological measurements showed large and consistent between-stream differences in the different watersheds. Invertebrate taxa richness criteria and biotic index criteria indicated good water quality, fair water quality, and poor water quality classifications in the forested, agricultural, and urban catchments, respectively (Lenat and Crawford 1994).For the purposes of this report, a GIS analysis was conducted to determine if a positive relationship could be demonstrated between water quality and forest cover for watersheds. The three general IWI categories, "better water quality", "less serious water-quality problems" and "more serious water-quality problems", were compared with percent forest cover for each of the southern watersheds. Percent forest cover was derived from the USFS Forest Inventory and Analysis data for each State and aggregated by watershed. However, because of the scale of the analysis (size of the watersheds) and other limitations in the use of the water-quality data at this scale, regional trends relating forest cover and water quality were not identified.
In addition, water-quality impairment based on IWI classification accounts for multiple factors and conditions that may have a greater impact on water quality than forest cover alone. Recent studies have concluded that the effects of human actions on nutrient loads may be disproportionately greater than the actual amount of anthropogenic cover in a watershed. Hession and others (1996) found that 80 percent of lake phosphorous load was attributable to agriculture, which accounted for only 25 percent of the watershed area. Nutrient export from agriculture was determined to be disproportionately greater than its area within a watershed. Although urban and suburban land use accounts for only 5 percent of the ACF River Basin, it has the most significant effect on streamwater quality (Frick and others 1998).
The scale of any watershed analysis is critical to determining specific relationships between land uses and water quality. Effects of land uses, including silvicultural practices, on water quality and aquatic biota are best studied and summarized at much smaller scales. This level of analysis was not possible for this report.
Based on a nationwide study of streams draining forested land, Patric and others (1984) concluded that land use has more influence on average sediment concentration in watersheds than does any other single factor. Land uses (practices) that are major sources of water-quality impairment in the South, and the pollutants that they may generate, are discussed in this Section. Figure 1 displays the leading point and nonpoint sources of pollution from 1988 to 1998. Figure 3 displays this information by State. Primary land-use practices (or types) affecting water-quality impairment in the South, can be grouped into five broad categories: agriculture, urbanization, resource extraction, hydromodification, and silviculture. At the local and regional level, land-use practices can dramatically affect soil condition and water quality, as well as water supply. Factors that affect land-use change include economic growth, population density, social development, political structure, attitudes and values, and technology (Turner and others 1993). Five major land-use categories are discussed in the subsections that follow.
From 1988 to 1998, agriculture was identified as the primary source of water quality impairment in the South. It accounted for a majority of the pollution impacting rivers and streams in the South (Figure 1). An annual average of approximately 44,326 miles of rivers and streams was impacted by agricultural activities during this period. Until the 1950's, the growth of agricultural land use generally kept pace with population increases (Novotny and Olem 1994). The majority of farming was conducted on small family farms without excessive use of chemicals. Since then, farming has shifted from family farms to larger corporate enterprises. Concurrently, farming began to rely on chemical fertilizers to increase plant yields, and on pesticides for insect and weed control. In general, as environmental awareness has increased, modern-day agricultural practices have begun to incorporate techniques that reduce potential environmental impacts.
Despite these advances, agricultural practices continue to be the primary source of water-quality impairment in the South (U.S. Environmental Protection Agency 2000a). For example, in North Carolina construction activities typically cause the highest erosion rates, but agriculture is the most common source of sediment problems because of the large amount of agricultural land use (Lenat and Crawford 1994). Agricultural activities such as field tillage, pesticide and fertilizer applications, drainage, irrigation, grazing, and feedlot operations are sources of significant nonpoint-source pollution (Neary and others 2000). Major pollutants associated with agriculture include sedimentation, nitrogen and phosphorous loading, changes in soil salinity, and introduction of pesticides, other toxins, bacteria, and pathogens. Agricultural practices are more likely to contribute certain pollutants than other land-use practices. Agricultural land cover is considered one of the principal sources of excess loads of nitrogen and phosphorous in receiving waters (Parry 1998).
Concentrated animal operations (CAOs) are a major agricultural practice that contributes significant amounts of pollutants to rivers and estuaries in the South (Burkholder and others 1997, Mallin 2000). For example, North Carolina experienced a rapid increase in CAOs between 1980 and 1990. The CAOs were exempt from land zoning laws and mandatory inspection programs. The waste lagoons were not required to have impermeable liners and some were even constructed below the water table (Burkholder and others 1997). During heavy rainfall events, the waste lagoons overflowed resulting in an increase in biochemical oxygen demand, fecal coliform and nutrients, and a decrease in dissolved oxygen possibly resulting in fish kills (Burkholder and others 1997). Typical agricultural practices and associated pollutants are summarized in Table 8 (Novotny and Olem 1994).
Urbanization is defined as land-use conversion caused by increased population density and activities associated with the creation of infrastructure to support populations, primarily within cities. Features of urbanization addressed in this Chapter include construction of homes and other buildings, infrastructure development such as municipal wastewater treatment plants and storm sewer systems, construction of industrial plants, urban sprawl, and creation of extended transportation routes, including mass transit.
Urban areas account for a small percentage of land in the South, but their effects on water resources have been severe. Urbanization represents the second overall leading source of impairment to water quality. Urbanization not only affects local rivers; it also contributes to water-quality impacts far downstream. According to National 305(b) Reports from 1988 to 1998, 5 of the 11 leading sources of water-quality impairment in the South were due to urbanization (Figure 1). These include both point and nonpoint sources of runoff in the categories of: (1) municipal (wastewater treatment plants), (2) storm-sewer/urban runoff, (3) industrial discharges, (4) land disposal (landfills), and (5) construction activities. These five sources impacted an annual average of approximately 36,248 miles of rivers and streams during this time.
In the first half of the 20th century, deterioration of water quality due to urbanization was primarily associated with point sources from industrial and commercial operations and treated and untreated domestic sewage. Point sources continue to contribute to water-quality impairment. It was not until 1970 that urban nonpoint sources of pollution were also recognized as contributing a significant portion of water-quality impacts. The following point and nonpoint sources of water-quality impairment are considered to be a result of urbanization:
Point Sources of Pollution
· Treated sewage discharges,
· Industrial discharges.
· Storm sewer outflows in urban centers, including pollutants such as car oil, detergents and other household and commercial solvents and chemicals.
· Spills or releases such as petroleum tankers and railcars.
· Unpermitted discharges from industrial or municipal sources.
Nonpoint Sources of Pollution
· Runoff (sedimentation and erosion) from construction activities.
· Runoff from roads and road construction.
· Sediment and contaminant transport from other impervious surfaces such as parking lots.
· Runoff from the application of pesticides and fertilizers.
· Runoff and leachate from landfills and septic tank systems.
· Leaking underground storage tanks and other improperly contained hazardous material storage tanks.
· Combined sewer overflows.
Other impacts to water quality due to urbanization include reduced flow in rivers and streams caused by increased demand for water resources such as drinking water and extensive land-use changes due to urban sprawl.
Hydromodification is the alteration of the flow of water, which changes water depth, stream velocity, and amount of discharge (U.S. Environmental Protection Agency 2000a). Throughout the history of modern civilization, sources of water have been modified to exploit available resources. As populations have increased, modification of nearby streams, rivers, wetland areas, and lakes has increased accordingly. Traditionally, activities such as the draining of wetlands for agricultural purposes and the development of urban centers along rivers and streams have been encouraged and considered to be signs of progress and economic growth (Mac and others 1998).
Hydromodification has been one of the leading causes of water-quality impairment in the South from 1988 to 1998. Hydromodification, including channelization, impacted an annual average of approximately 10,490 river miles during this time (Figure 1) and was the third leading source of water quality impairment, behind agriculture and urbanization. Hydromodification includes the following activities (specific literature citations for water quality impacts are included):
· Dredging: The excavation of bottom sediment to increase water depth and subsequent disposal of dredged material (Burke and Engler 1978, U.S. Army Corp of Engineers 1989).
· Channelization: The alteration of stream morphology for human beneficial uses, such as flood control and irrigation (Crance and Masser 1996, Mac and others 1998).
· Damming/Flow regulation: A barrier preventing and regulating the flow of water for the purpose of flood control, power generation, and water resources (Federal Interagency Restoration Working Group 1998, Mac and others 1998).
· Drainage of wetlands and swamps: The act of removing water from wetlands by altering the land for purposes such as conversion to farmland and urban development (see Chapter AQUA-2).
Resource extraction, as reported in the National 305(b) Reports, includes mining, petroleum drilling, and runoff from mine tailing sites (U.S. Environmental Protection Agency 2000a). It was one of the top five sources of water-quality impairment in the South from 1988 to 1998 (Figure 1). The most common minerals extracted by mining are coal and metallic ores (Novotny and Olem 1994). Nonpoint sources of pollution associated with resource extraction include mineral and sediment discharges from inactive mining operations, sedimentation and erosion runoff from roads, old tailings, and spoil pile leaching of contaminants. In addition, acid mine drainage can severely impact water quality by altering pH levels of rivers and streams.
A National Stream Survey by the USEPA reported that 10 percent of the streams in the Northern Appalachians were acidic due to acid mine drainage during spring baseflow (Mac and others 1998). Active mines are considered point sources of pollution, and a discharge permit is required for their operation. Nonpoint pollution sources such as erosion and sedimentation are associated with almost every abandoned surface mine (Novotny and Olem 1994). Although mining is not as widespread as agriculture, water-quality impairment is often severe.
Silviculture is "the art and science of controlling the establishment, growth, composition, health, and quality of forests and woodlands to meet the diverse needs and values of landowners and society on a sustainable basis" (Society of American Foresters 1994). It includes the application of scientific agricultural practices to grow trees for use as lumber or other products. The majority of forested land in the South has been subject to historical silvicultural activities of some type or extent. According to the National 305(b) Reports, silvicultural activities impacted annually an average of approximately 3,639 miles of rivers and streams in the South from 1988 to 1998 (Figure 1). Table 1 in Chapter AQUA-3 provides a breakout of this information by State.
Silviculture ranks low among water-impairing land-use activities in the South. Nevertheless, impacts from silvicultural activities can be considerable if BMPs are not applied. The major potential nonpoint-source pollutant resulting from silvicultural activities is sediment from roads and skid trails. Other minor nonpoint-source impacts on water quality include short-term increases in peak flows during storms, short-term increases in base flows, short-term increases in nutrient concentrations (primarily nitrogen and phosphorous), short-term increases in herbicides/fertilizers and derivative products, and thermal pollution (increased stream temperature). Elevated levels of organics and nutrients may result from leaching of disturbed or exposed soils. Fertilizer applications may alter stream chemistry in managed forests, depending on the type of fertilizer used and how it is applied (Society of American Foresters 2000). In comparison, pollutant loads from properly managed areas are considered negligible (Novotny and Olem 1994). Chapter AQUA-3 provides a complete discussion of the potential effects of silvicultural activities on water quality.
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content: Benjamin E. West |
created: 21-NOV-2001 |