This chapter covers conditions and trends through calendar year 1998 using data and information available as of December 31, 2000.
The United States is a water-rich nation, blessed with abundant and varied aquatic resources:
Although there are significant geographic and seasonal differences in these aquatic resources, many are share a common attribute -- they are under stress, threatened either by development, resource exploitation, or impaired water quality.
HYDROMETEOROLOGY
Climate plays a major role in the timing, availability, and magnitude of water resources, and in some cases, affects water quality, such as through acidic deposition and stormwater runoff. The contiguous United States usually receives about 28-30 inches of precipitation annually, has an average annual temperature of 53oF, and is impacted by about 8 hurricanes and tropical storms and 837 tornadoes annually. The year 1998 broke many records in terms of climatic extremes.
The United States had the fifth wettest year on record in 1998 with a national average of 32.61 inches of precipitation (Figure 6.1). (The wettest year on record is 1973 at 33.99 inches.) Considerable regional and seasonal variation in precipitation occurred throughout the year. About 22 percent of the country was much wetter than normal while about 2 percent was much drier (Figure 6.2).
A record dry April-June resulted in drought conditions from the Southern Plains to the Gulf Coast states, and spring and summer heat and drought led to massive wildfire outbreaks in Florida. Late summer and autumn rains from tropical systems helped abate the dry conditions in the South, while the drought intensified in the eastern United States. The region from the central Atlantic Coast to New York experienced the second driest July-November on record with local water restrictions implemented in many areas.
Warmer-than-normal temperatures exacerbated drought conditions in many parts of the country. The U.S. average temperature in 1998 was 54.9oF (12.57oC), the warmest on record (Figure 6.3). About 75 percent of the country averaged much warmer than normal for 1998, while less than 1 percent was much cooler than normal. A mild winter from the Northeast to the Great Lakes saved on heating and snow removal costs while a protracted summer drought/heat wave from Texas/Oklahoma eastward to the Carolinas resulted in extraordinary runs of daily temperatures 90oF or hotter, $6.0 to $9.0 billion weather-related damage/costs to agriculture and ranching, and at least 200 deaths. (See Chapter 11. Global Environment for a discussion on global temperature in 1998, the warmest in the past 119 years.)
The climate of 1998 was also punctuated by extreme storm events. A total of 14 hurricanes and tropical storms developed in the North Atlantic basin during 1998, almost two times the long-term mean of about 8 hurricanes and tropical storms (Figure 6.4). And a record 1,417 tornadoes were reported -- the largest annual total in the 1950-1998 record (Figures 6.5). (Note: The increasing trend may be an artifact related the use of better observing practices and instrumentation, especially weather radar and satellites.) The year 1998 also set a record with seven weather-related disasters each causing $1 billion or more in damages/costs, although several other years had higher total annual damages/costs (Figure 6.6). Floods alone claimed 102 lives and caused $2.5 billion in damages in Fiscal Year (FY) 1998 (October 1997-September 1998), while U.S. Army Corps of Engineers flood control projects and activities prevented an estimated $13.4 billion in additional damages (Figure 6.7).
WATER USE
Surface waters provide about three fourths of overall U.S. freshwater requirements and groundwater one fourth. Groundwater is the source of drinking water for about half the general population and nearly all the rural population. The renewable water supply is more than 4 times the amount withdrawn and almost 15 times the amount consumed. But some parts of the country, especially the West and Southwest, are beginning to approach the physical limits of their water resources.
Prior to 1980, U.S. water use grew by more than a factor of 10, rising from 40 billion gallons per day (bgd) in 1900 to 445 bgd in 1980. Since then, water withdrawals have declined and remained fairly constant (Figure 6.8). The decline in water withdrawals is particularly significant in light of population growth of 16 percent during the same period.
Among sectors, the largest declines were in irrigation and thermoelectric utilities, while the "public supply" and "rural domestic and livestock" categories showed continual increases from 1950 to 1995. The 4-percent increase in public-supply withdrawals from 1990 to 1995, compared to a 7-percent increase in population served by public supply, indicates that conservation programs have been effective in lowering public supply per capita use. The 13-percent increase in rural domestic and livestock withdrawals is attributable to an increase in livestock withdrawals, especially animal specialities withdrawals, which were 43 percent higher during 1995 than during 1990.
More water (fresh and saline) continues to be withdrawn for thermoelectric power generation than for any other category, peaking in 1980 at 210 bgd and fluctuating around 190 bgd since then. Instream use (hydroelectric power) increased steadily from 1950 to 1975 and has fluctuated above 3,000 bgd since then. Changes in hydroelectric power water use are closely related to the availability of surface water.
Industrial withdrawals declined from 1980 to 1995 after remaining about the same for the years reported between 1965 and 1980. Lower industrial withdrawals are the result of new industries and technologies that require less water, improved plant efficiencies, increased water recycling, changes in laws and regulations to reduce the discharge of pollutants, and conservation measures.
Irrigation withdrawals during 1995 were 2 percent less than during 1990 and 1985. The 1995 irrigation application rat -- 2.1 acre-feet per acre -- was slightly less than the 1985 average of 2.2 acre-feet and well below the 1975 and 1980 average of 2.5 acre-feet per acre. The decline in irrigation rates is the result of improved irrigation techniques, implementation of more efficient irrigation systems, and because application rates in the eastern United States tend to be less than in the western United States.
DRINKING WATER
The United States enjoys one of the best, safest supplies of drinking water in the world. The federal-state approach to drinking water protection emphasizes protection of groundwater and surface water supplies and improved drinking water monitoring, treatment, and distribution.
Under the Safe Drinking Water Act (SDWA), EPA has developed maximum contaminant level (MCL) and treatment technique requirements for more than 80 contaminants in drinking water. (MCLs are contaminant concentrations set at or near levels at which there are no known human health effects. Treatment techniques are developed instead of MCLs for contaminants that are difficult to measure.) Public water systems must use appropriate treatment and conduct routine monitoring to ensure that the water provided to consumers consistently meets health-based standards at the tap.
Public Water Systems
Public water systems (PWSs) are classified according to the number of people they serve, the source of their water, and whether they serve the same customers year-round or only on an occasional basis (Text Table 6.1). In FY1998, there were 170,376 PWSs -- 54,367 community water systems serving 252.5 million people, or 91 percent of Americans, in their primary residences; 20,255 non-transient, non-community water systems (for example, schools, factories, office buildings) serving 6.3 million people; and 95,754 transient, non-community water systems (for example, gas station, campground) serving 16.8 million people. (Note: The actual number of individuals served by PWSs is smaller than the sum of components listed above because millions of Americans drink water from, and are counted as users of, more than one PWS during the course of the year.)
Community Water Systems
Most of the information on trends in drinking water quality comes from community water systems (CWSs). In FY1998, about 5,000 CWSs serving 28.9 million people in states, tribal nations, and U.S. commonwealths and territories reported health-based violations of drinking water standards. This represents a reduction since peak year 1994 when over 7,000 CWSs serving 56.2 million people reported health-based violations. In FY1998, as in previous years, the MCL for the Total Coliform Rule (TCR), which must be met by all types and sizes of PWSs, was the health-based standard most frequently violated (Figure 6.9). An additional 10,002 CWSs serving 28.3 million people violated SDWA provisions by failing to monitor and report drinking water quality, thus making it impossible to know whether health-based standards were violated or not.
Drinking Water Infrastructure Needs Survey
In its first report to Congress from the 1996 Drinking Water Infrastructure Needs Survey, EPA concludes that much of the nation's drinking water infrastructure suffers from long-term neglect and serious deterioration and that significant investments are needed to protect public health and ensure the availability of safe drinking water. Results of the Needs Survey are used to develop a formula to allot funds for the Drinking Water State Revolving Fund grants to states. The next Needs Survey will be released in 2001.
Waterborne Disease Outbreaks
Data from a collaborative surveillance system on the occurrence and causes of waterborne-disease outbreaks associated with drinking water have been available since 1971. The number of outbreaks reported during 1997 -1998 (17) is lower than those reported for any 2-year period since 1971. Nonetheless, 2,138 people became ill. Of the 10 outbreaks with known infectious etiology, 6 were caused by parasites (4 by Giardia and 2 by Cryptosporidium) and 4 by bacteria (3 were attributed to E. coli and one to Shigella sonnei). Two other outbreaks were caused by copper chemical poisoning and 5 more were of unknown etiology.
Waterborne-disease outbreaks peaked during 1979-1983 (Figure 6.10) while the relative proportion of outbreaks associated with various types of water systems (e.g., community, non-community, or individual) has remained fairly constant. The recent decrease in the number of outbreaks reported could be caused by improved implementation of water treatment regulations, increased efforts by water utilities to produce drinking water better than EPA standards require, and efforts by public health officials to improve drinking water quality, but it also may reflect, at least in part, changes in surveillance activities or be a reporting artifact.
The number outbreaks associated with drinking water systems supplied by groundwater represented the largest proportion (88.2 percent) of such outbreaks since 1978, when outbreak source water was first recorded (i.e., surface water versus ground water). Of the 15 outbreaks associated with ground water during 1997-1998, a total of 10 were associated with a system that had a treatment problem or distribution deficiency; 4 were associated with systems that had untreated water; and 1 had an unknown problem. Since ground water is often not routinely disinfected and almost never filtered, wells and springs must be protected from sources of contamination (e.g., surface runoff, septic tank drainage, and sewage discharges).
Of the 2 outbreaks associated with surface water supplies during 1997-1998, one was an outbreak of giardiasis from chlorinated, but not filtered, drinking water and the other was of unknown etiology. Outbreaks associated with surface water demonstrate the importance of requiring water systems to provide an adequate chlorine concentration and contact time to inactivate Giardia and other organisms that are relatively chlorine-resistant, especially if the surface water is unfiltered.
AMBIENT WATER QUALITY
During the past 30 years, many scientific studies -- some national in scope -- have been launched to assess the quality of the nation's surface and ground water. Because these studies often differ in study design, sampling dates, and spatial coverage, it is not possible to combine the results into a single national assessment. However, information from individual studies can be used to assess the general health of aquatic resources. Highlights from some of these national studies are presented in this section.
National Water Quality Inventory
The Environmental Protection Agency's (EPA) National Water Quality Inventory provides useful insights into the quality of the nation's surface waters. In EPA's twelfth biennial report, covering the year 1998, states, territories, and tribes evaluated water quality in 23 percent of the nation's river and stream miles, 42 percent of lake acres, and 32 percent of estuary square miles. The results showed that, for the assessed waters, 55 percent of river and stream miles, 46 percent of lake acres, 47 percent of estuarine square miles, and 80 percent of ocean shoreline miles fully support the water quality standards evaluated. The remaining assessed waters are threatened or impaired to varying degrees (Figure 6.11). (These estimates should be treated with some caution since different states use different standards for evaluating the support of designated uses, and the sampling approaches also vary across states).
For a subset of assessed waters identified as impaired, the 1998 National Water Quality Inventory presents the leading pollutants, and sources of pollution reported by states, territories, and tribes. Across all waterbody types, states and juridictions reported that:
National Water-Quality Assessment (NAWQA) Program
The U.S. Geological Survey (USGS) initiated the NAWQA Program in 1991to describe the quality of the nation's surface and ground water resources, using nationally consistent methods. Because of widespread environmental and public-health concerns, nutrients, pesticides, and trace elements were the first water quality issues addressed by the NAWQA Program. Some regional and national insights on these pollutants are as follows:
National Estuarine Eutrophication Survey
This survey, conducted by the National Oceanic and Atmospheric Administration from 1992 to 1997, covers 138 estuaries, representing over 90 percent of estuarine surface area of the conterminous United States. The results of the survey along with information on nutrient inputs, population projections, and land use were analyzed at an August 1998 National Assessment Workshop to address key questions concerning the severity and extent of eutrophication in estuaries. Workshop participants concluded the following:
Beach Closings and Advisories
Since 1988, the National Resources Defense Council (NRDC) has undertaken a annual survey of beach closings and beachwater-monitoring and public notification programs in coastal and Great Lakes states. In 1997, EPA picked up the survey as part of its Beaches Environmental Assessment, Closure and Health (BEACH) Program. Data from the EPA survey, supplemented by information from NRDC's survey, show that during 1998, at U.S. ocean, bay, Great Lakes, and some freshwater beaches, there were at least 7,236 days of closings and advisories (more than in any other year), 41 extended closings and advisories for six to 12 weeks (also a record number), and 36 permanent closings and advisories for more than 12 weeks (Figure 6.13). Sixty-three percent of 1998 beach closings and advisories were based on monitoring that detected bacteria levels exceeding beachwater-quality standards, 27 percent were in response to known pollution events (such as sewage treatment plant failure or breaks in sewage pipes), 8 percent were precautionary due to rainfall known to carry pollution into swimming waters, and 2 percent were either of unknown origin or from other sources such as algae or chemical pollution. Reported sources of pollution causing closings and advisories in 1998 include sewage spills and overflows (34 percent), polluted runoff and stormwater (22 percent), rain or preemptive (16 percent), and source unknown or other reason (28 percent).
National Fish and Wildlife Consumption Advisories
The states, territories, and native tribes have primary responsibility for protecting residents from the health risks of consuming contaminated non-commercially caught fish and wildlife. They do this by issuing consumption advisories for the general population, including recreational and subsistence fishers, as well as for sensitive subpopulations (such as pregnant women, nursing mothers, and children). These advisories inform the public that high concentrations of chemical contaminants (e.g., mercury, PCBs, dioxin) have been found in local fish and wildlife and recommend limiting or avoiding the consumption of certain fish and wildlife species from specific waterbodies or specific waterbody types (e.g., all lakes). Canadian provinces and territories issue similar advisories.
In 1998, 2,506 U.S. waterbodies were under advisories, a 9-percent increase from the number reported in 1997 and a 98-percent increase from the number reported in 1993 (Figure 6.14). The increase in advisories generally reflects an increase in the number of assessments of contaminant levels prompted by increased awareness of health risks associated with the consumption of chemically-contaminated fish and wildlfie. Although advisories exist for a total of 38 contaminants, most advisories issued in 1998 (as in previous years) involved five primary contaminants: mercury (1,931 advisories), PCBs (679), chlordane (104), dioxin (55), and DDT (34).
Oil Spills
The number of oil polluting incidents in and around U.S. waters has fluctuated somewhat since 1970, while the total volume of spills has generally declined (Figure 6.15). This decline represents the combined effects of prevention and preparedness to protect U.S. coastal waters from oil pollution and is particularly significant in light of continued growth in U.S. waterborne commerce, particularly imported oil.
The majority of spills since the 1970s have involved small discharges between 1 and 100 gallons. Exceptions are the notorious EXXON VALDEZ oil spill in 1989 of 10.5 million gallons and other significant spills in recent years, as shown in Text Table 6.3. Crude oil and heavy oil in combination are the types of oil spilled most frequently, and tank ships and barges are responsible for the majority of these spills. Most spills occur within 3 nautical miles of the U.S. coastline; the Gulf of Mexico coastal waters and internal rivers such as the Mississippi, Ohio, and Arkansas Rivers are where the greater majority of spills occur.
Wetlands
When Europeans settled what is now the conterminous United States, there were an estimated 221 million acres of wetlands. Since then, more than half of the original wetlands acreage has been destroyed by dredging, draining, filling, leveling, and flooding for other uses, primarily rural and urban development, agriculture, and silviculture. The latest U.S. Fish and Wildlife Service report on wetland status and trends estimates that 105.5 million acres of wetlands remained in the conterminous United States at the end of 1997.
Although wetlands continue to be converted to other uses, much as been done to slow the annual rate of loss, including implementation and enforcement of wetlands protection measures, elimination of incentives for wetland drainage, and public education and outreach about the ecological, social, and economic values of wetlands. Between 1986 and 1997, the average annual rate of wetland loss was 58,500 acres, down from 458,000 acres per year between the mid 1950s and mid 1970s (Figure 6.16).
There has also been a shift in what causes wetland loss. In earlier studies, agriculture accounted for 80 percent of wetland loss. Today, the rate of wetland loss to agriculture is substantially reduced and now urban development is the leading cause of continued wetland loss (Figure 6.17).
Ninety-five percent of the remaining wetlands are found in freshwater regimes. This includes 50.7 million acres of freshwater forested wetlands, 25.2 million acres of freshwater emergent marshes, and 18.4 million acres of freshwater shrub wetlands. There are also an estimated 5.5 million acres of freshwater ponds. Between 1986 and 1997, 98 percent of all wetland losses involved freshwater wetlands. Causes of these losses mirror the national trend.
The long-term trends in freshwater wetlands since the 1950s show that freshwater marshes have declined by the greatest percentage of all wetland types with nearly 24 percent (8 million acres) lost, while freshwater forested wetlands have sustained the greatest overall loss in area, declining by 10.4 million acres. Freshwater shrub wetlands and open water ponds, in contrast, have increased in area since the 1950s (Figure 6.18).
Estuarine and marine wetlands, while comprising only 5 percent (5.3 million acres) of total wetland area in the conterminous United States, are critically important as nursery, feeding, breeding, staging, and resting areas for many fishes, shellfish, mammals, and birds. Salt marshes and shrub wetlands make up an estimated 87 percent of estuarine wetlands while non-vegetated wetlands -- beaches, flats, shoals -- account for the rest. Between 1986 and 1997, there was a net loss of 10,400 acres of estuarine and marine wetlands for an estimated annual loss of about 1,000 acres. The major factor in estuarine and marine wetland loss was filling or draining for urban and rural development, which accounted for 43 percent of the losses in this wetland category.
The long-term trends indicate that estuarine vegetated wetlands declined in area at a much reduced rate from earlier decades (Figure 6.19). Restoration, protection, and monitoring of coastal areas contribute to the reduced rate of loss. Non-vegetated wetland types have remained fairly constant over time.
Wastewater Treatment
The latest EPA Clean Water Needs Survey (1996) includes detailed estimates of the capital costs eligible for funding under the State Revolving Fund (SRF) provisions of the 1987 Amendments to the Clean Water Act (CWA). The survey covers publicly owned, municipal wastewater collection and treatment facilities, facilities for the control of combined sewer overflows (CSOs), activities designed to control storm water (SW) runoff and nonpoint source (NPS) pollution, and programs designed to protect the nation's estuaries. The primary objective of the survey is to update and expand the documented costs for all program categories eligible for SRF funding.
Nationally, 16,024 wastewater treatment facilities are identified in the 1996 survey. These facilities provide service to 190 million people, representing 73 percent of the total population (258 million). When all needs are met in 2016, there will be an estimated 18,303 publicly owned wastewater treatment facilities serving 275 million people, or 90 percent of the projected population of 305 million.
According to the 1996 survey, the level of treatment has changed significantly over the last eight years. The number of facilities providing less than secondary treatment has declined by 90 percent since 1988. In 1988, 1,789 (11 percent) of the 15,591operational facilities were providing less than secondary treatment. This declined to 868 (6 percent) in 1992 and to 176 (1 percent) in 1996.
At the same time, there has been a steady increase in the proportion of facilities providing secondary and greater than secondary treatment. The number of facilities providing secondary treatment has increased by 10 percent, while those providing advanced wastewater treatment increased by almost 30 percent since 1988. In 1996, 28 percent (4,428 out of 16,024) of the operational treatment facilities are providing greater than secondary treatment (Text Table 6.4).
From 1992 to 1996, total needs decreased by $15.5 billion. This reflects, in part, progress made in meeting the nation's water quality infrastructure needs. For a given facility, a reduction in need may signify completion of project construction, reduction in the original project scale, or elimination of the need for projects included in previous surveys. In contrast, an increase in need signifies entirely new facilities being required or new projects to upgrade or expand existing facilities. Underlying factors that influence these changes include continued population growth, deterioration of existing facilities, and increasingly stringent water quality requirements. Changes in needs also reflect efforts to improve the quality of the data in the needs survey database through a substantial redocumentation effort.
The largest needs occur in New York, Illinois, and California. New York has $16 billion in needs, while California and Illinois have needs in excess of $11 billion. Sixteen additional states have needs in excess of $2 billion. Needs continue to be generally concentrated in the highly populated northeastern states (New Jersey, New York, and Pennsylvania) and in the Great Lakes states (Illinois, Michigan, and Ohio) as well as in Florida and Texas. The less populated states, generally located in the Rocky Mountains and the Plains, have lower levels of needs.
Most combined sewer overflow (CSO) needs are concentrated in the northeastern states (Massachusetts, New Jersey, New York, and Pennsylvania) and in the Great Lakes states (Illinois, Indiana, Michigan, and Ohio). Illinois has the largest documented CSO needs ($9.4 billion), indicating that considerable effort has gone into documenting this state's CSO problems and into developing municipal CSO program plans. Indiana, Massachusetts, Michigan, New Jersey, New York, Ohio, and Pennsylvania all have CSO needs in excess of $2 billion. This geographical concentration of CSO needs reflects the age of the infrastructure in these areas and the fact that combined sewers were considered acceptable practice at the time many older sewer systems were built.
Water quality program needs for small communities are significant, reflecting, in part, the continuing efforts to extend wastewater collection and treatment to small communities. The total documented needs for communities with populations less than 10,000 are $13.8 billion, representing 11 percent of the total documented needs for the nation.
References
Barbash, J.E., G.P. Thelin, D.W. Kolpin, and R.J. Gilliom, Distribution of Major Herbicides in Ground Water of the United States, National Water-Quality Assessment Program, Water-Resources Investigation Report 98-4245 (U.S. Geological Survey, Sacramento, CA, 1999). (http://water.wr.usgs.gov/pnsp/rep/wrir984245/text.html)
Barwick, R.S., D.A. Levy, G.F. Craun, M.J. Beach, and R.L. Calderon, "Surveillance for Waterborne-Disease Outbreaks -- United States, 1997-1998." Centers for Disease Control and Prevention, Morbidity and Mortality Weekly Report, May 26, 2000/49(SS04), and earlier reports in this series. (http://www.cdc.gov/mmwr/preview/mmwrhtml/ss4904a1.htm)
Bricker, S.B., C.G. Clement, D.E Pirhalla, S.P. Orlanda, and D.R.G. Farrow, National Estuarine Eutrophication Assessment: Effects of Nutrient Enrichment in the Nation's Estuaries (U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, Washington, DC, 1999).
Dahl, T. Status and Trends of Wetlands in the Conterminous United States 1986 to 1997 (U.S. Department of the Interior, Fish and Wildlife Service, Washington, DC, 2000). (http://wetlands.fws.gov/bha/SandT/SandTReport.html)
Larson, S.J., R.J. Gilliom, and P.D. Capel, Pesticides in Streams of the United States -- Initial Results from the National Water-Quality Assessment Program, Water-Resources Investigation Report 98-4222 (U.S. Geological Survey, Sacramento, CA, 1999). (http://water.wr.usgs.gov/pnsp/rep/wrir984222/text.html)
Litke, D.W., Review of Phosphorus Control Measures in the United States and Their Effects on Water Quality, National Water-Quality Assessment Program, Water-Resources Investigation Report 99-4007 (U.S. Geological Survey, Denver, CO, 1999).
Natural Resources Defense Council, Testing the Waters - 2000: A Guide to Water Quality at Vacation Beaches (NRDC, New York, 2000). (http://www.nrdc.org/water/oceans/ttw/titinx.asp)
Nolan, B.T., B.C. Ruddy, K.J. Hitt, and D.R. Helsel, "A National Look at Nitrate Contamination of Groundwater," Water Conditioning and Purification Magazine (1999).
Rice, K.C., "Trace-Element Concentrations in Streambed Sediment Across the United States." Environmental Science Technology, 33, 2499-2504, 1999.
Ross, T.F. and J.N. Lott, A Brief Climatology of Extreme Weather and Climate Events in the U.S. and Around the World (National Climate Data Center, Asheville, NC, 2000). (http://www.ncdc.noaa.gov/ol/reports/billionz.html)
Solley, W. B., R.R Pierce and H.A. Perlman, Estimated Use of Water in the United States in 1995, USGS Circular 1200 (GPO, Washington, DC, 1998). (http://water.usgs.gov/watuse/pdf1995/html/)
U.S. Army Corps of Engineers, Annual Flood Damage Report to Congress for Fiscal Year 1999 (USACE, Washington, DC, 2000). (http://www.usace.army.mil/inet/functions/cw/cecwe/flood99/)
U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Climate Data Center, Climate of 1998 Annual Review: Annual U.S. National Overview (NCDC, Asheville, NC, 1999). (http://www.ncdc.noaa.gov/ol/climate/research/1998/ann/us_annual.html)
--, Tropical Cyclones of the North Atlantic Ocean, 1871-1986 (Storm track maps) (NCDC, Asheville, NC, 1992, with updates through 2000).
U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Storm Prediction Center, Monthly Tornado Statistics (Internet accessible data files). (http://www.spc.noaa.gov/climo/torn/monthlytornstats.html)
U.S. Department of the Interior, United States Geological Survey, The Quality of Our Nation's Waters -- Nutrients and Pesticides, Circular 1225 (DOI, USGS, Reston, VA, 1999). (http://water.usgs.gov/nawqa/)
U.S. Department of Transportation, United States Coast Guard, Pollution Incidents In and Around U.S. Waters: A Spill/Release Compendium, 1969-1999 (DOT, USCG, Washington, DC, 2000). (http://www.uscg.mil/hq/g%2Dm/nmc/response/stats/aa.htm)
U.S. Environmental Protection Agency, Office of Water, National Water Quality Inventory: 1998 Report to Congress (EPA, OW, Washington, DC, 2000). (http://www.epa.gov/305b/)
--, 1998 National Summary of Public Water Systems Compliance (EPA, OW, Washington, DC, 2000). (http://www.epa.gov/safewater/annual/)
--, IMB SDWIS/FED Factoids, FY1998 Inventory Data (Internet accessible data from SDWIS 98Q4 frozen inventory table). (http://www.epa.gov/safewater/data/getdata.html)
--, Update: National Listing of Fish and Wildlife Advisories, Fact Sheet EPA-823-F-00-016 (EPA, Washington, DC, 2000). (http://www.epa.gov/ost/fish/)
