Delaware: Surface-Water Resources

Introduction

Delaware has abundant stream-water resources that are of a quality suitable for most uses including public, agricultural, and industrial supply; maintenance of aquatic life and wildlife; and recreation. In 1985, stream water supplied about 43 percent or 60 million gallons per day (Mgal/d) of the estimated 139 Mgal/d of freshwater used in the State. About 82 percent of stream water withdrawn was used for public supply, 13 percent for agriculture, and 5 percent for industrial purposes (U.S. Geological Survey, 1990, p. 204). Most stream water used for public supply in Delaware is withdrawn and distributed in New Castle County, the northernmost county of the State, where surface water supplies an estimated 275,000 users (Martin W. Wollaston, Water Resources Agency for New Castle County, oral commun., May 1991). Excluding the Delaware River, which is brackish in the reach along the eastern boundary of the State, Brandywine, Red Clay, and White Clay Creeks and the Christina, St. Jones, and Nanticoke Rivers are the principal streams in the State. Brandywine Creek, which originates in Pennsylvania, is the largest source of fresh stream water in Delaware and is the primary source of water supply for Wilmington. Hoopes Reservoir, on a tributary to Red Clay Creek, is used for water supply when diminished flow or unacceptable quality limits the use of Brandywine Creek. Streams have not been developed appreciably for water supply in the central and southern parts of the State. However, streams in these areas are potential sources of small freshwater supplies in their nontidal reaches.

Delaware is in the northeastern part of the Delmarva Peninsula, which is bounded on the west by the Chesapeake Bay and on the east by the Delaware River and Atlantic Ocean. Streams that flow into the Delaware River and Atlantic Ocean are affected by tides in their lower reaches. Saline water in the tidal reaches limits water use because the water is unsuitable for human consumption, irrigation of crops, and many industrial purposes. Streams in the western part of the State that drain into the Chesapeake Bay contain freshwater except for the Nanticoke River downstream from Seaford and Broad Creek downstream from Laurel, where dredged navigation channels permit tidal movement of saline water into the upper reaches of the deepened channels.

Figure 1. Land use, physiography, and population in Delaware. A, Major land uses. B, Physiographic divisions. C, Population distribution in 1990. (Sources: A, Major land uses modified from Anderson, 1967. 6, Physiographic divisions from Fenneman, 1946; landforms from Thelin and Pike, 1990. C, Data from U.S. Bureau of the Census 1990 decennial census files.)

Farming is a major industry in Delaware, and the central and southern parts of the State support the greatest concentration of agricultural activity (fig. lA). Most urban and suburban areas are in northern Delaware. Cypress Swamp, an area of about 50 mi² (square miles), straddles Delaware's southern boundary with Maryland.

Delaware is situated in two contrasting physiographic provinces--the Piedmont and the Coastal Plain (fig. 1.B), These provinces are separated by the Fall Line, which is marked by rapids and falls in stream channels and extends northeastward through Newark and Wilmington. The Piedmont province, which constitutes 6 percent of the State's area, lies north of the Fall Line and is characterized by a rolling terrain of hills, moderately deep valleys, and rapidly flowing streams. The Coastal Plain province, south of the Fall Line, is characterized by level terrain and streams with gentle slopes, meandering channels, and extensive contiguous wetland areas (U.S. Geological Survey, 1986, p. 181).

From 1980 to 1990, Delaware's population increased from 594,338 to 666,168 (U.S. Bureau of the Census 1990 decennial census files). Most Delawareans live in or near cities and towns having populations of 2,500 or more, and the majority reside in the Wilmington metropolitan area (figs. 1C and 2). As the population continues to increase, so too will the demand for safe, dependable water supplies.

Water-Quality Monitoring

The water-quality data on which this report is based resulted from analysis of samples collected at monitoring stations operated by the Delaware Department of Natural Resources and Environmental Control (DNREC). These stations are part of a statewide network operated since 1970 for the collection of stream water-quality data in an ongoing, systematic manner. The water-quality data are stored in STORET, the U.S. Environmental Protection Agency's (EPA) national data base. The streamflow data used to prepare parts of this report were collected by the U.S. Geological Survey (USGS). Water-quality and streamflow data are reported by water year--the 12-month period from October 1 through September 30. A water year is identified by the calendar year in which it ends. For example, water year 1991 comprises October 1, 1990, through September 30, 1991.

The data used in this summary of Delaware's stream water quality were obtained from water samples collected at nine monitoring stations at which data collection is systematic and continuing (fig. 2). Analyses of water samples collected at all nine stations are the basis for the discussion and graphic summary (fig. 3) of stream water-quality conditions during water years 1987-89, and data from seven stations are the basis for the discussion and graphic summary (fig. 4) of stream water-quality trends. Because the monitored streams generally are well mixed, total-constituent concentrations were derived from the analysis of water samples that were collected at a single point in the streams. The samples were analyzed by using standardized methods approved by the EPA or by using equivalent methods. If a method of sample collection or analysis changed over time, data were included in the evaluation of stream water-quality conditions or of stream water-quality trends only if the change in methodology did not affect the comparability of the data. Finally, water samples at all stations included in the evaluation were collected at relatively uniform time intervals, without consideration of streamflow. This sampling results in water-quality data that are representative of the full range of streamflow variability at the stations.

Water-Quality Conditions

Twenty-three percent of the stream miles assessed during 1988-89 by the DNREC (1990a, p. 2) supported all uses designated by the DNREC for purposes of water-quality monitoring and protection; 78 percent of the stream miles supported well-balanced and healthy populations of fish, shellfish, and wildlife; and 19 percent of the stream miles supported primary-contact recreation. The most widespread water-quality problem is excessive concentrations of enterococcal bacteria; these bacteria, which belong to the fecal streptococcal bacteria group, indicate the possible presence of pathogens introduced by fecal contamination. From the standpoint of human health, the most serious water-quality problem is that of toxic substances (organic compounds and trace elements), which are present in about 10 percent of the assessed stream lengths in the State (Delaware Department of Natural Resources and Environmental Control, 1990a, p. 3). Investigations by the DNREC have identified several potential sources of bacterial and chemical contamination including point-source discharges and runoff from agricultural and urban areas.

The following discussion of stream water quality in Delaware is organized by river basin (fig. 3). Where physiographic and landuse characteristics in different basins are similar, the discussion of those basins is combined. Graphs in figure 3 summarize certain aspects of stream water quality in the basins for water years 1987-89. The graphs show frequency distributions of data values that represent concentrations of selected constituents in stream water and measurements of selected physical properties of stream water. These constituents and properties are enterococcal bacteria, alkalinity (as calcium carbonate), total nitrate (as nitrogen), total ammonia plus organic nitrogen (as nitrogen), and total phosphorus (as phosphorus). The data are reported in colonies per 100 milliliters (col/100 mL) and milligrams per liter (mg/L). Sources and environmental significance of each constituent and property are described in table 1.

Water quality at each monitoring station is the result of geological, chemical, biological, and hydrologic processes that occur over a large area. Water-quality problems that affect aquatic life or public health only locally are not fully represented in this summary.

Brandywine, Red Clay, and White Clay Creeks and Christina River

A substantial quantity of freshwater in Delaware drains from the Piedmont province through streams in the northern part of the State. The principal streams are Brandywine, Red Clay, and White Clay Creeks and the Christina River. Together, these streams drain about 568 mi² in northern Delaware and southeastern Pennsylvania. Most of the basin is in Pennsylvania; only 25 percent of the total drainage area is in Delaware (Rasmussen and others, 1957, p. 35).

The nontidal reach of Brandywine Creek supports the maintenance and propagation of 26 species of fish. However, because enterococcal bacteria concentrations in the stream water sometimes exceed the State standard of 100 col/100 mL, the reach does not fully support primary contact recreation (Delaware Department of Natural Resources and Environmental Control, 1990b, p. 20-21). The quality of water in Red Clay Creek in Delaware also has been degraded by bacteria and by toxic substances, including zinc. At site 2 during water years 1987-89, the median concentration of enterococcal bacteria (210 col/100 mL) was among the highest for the nine water-quality conditions monitoring stations, and concentrations higher than 100 col/100 mL were common. Because of contamination in the water, sediment, and fish tissue, Red Clay Creek presently is not approved for swimming or fishing (Delaware Department of Natural Resources and Environmental Control, 1990c).

Figure 2. Selected water-quality monitoring stations, type of statistical analysis, and geographic features in Delaware. (Sources: Major land uses modified from Anderson, 1967; other data from U.S. Geological Survey files.)

Water from Brandywine Creek (site 1) and Red Clay Creek (site 2) is similar in chemical character and reflects the combined effects of basin geology and human activities on stream water quality. Discharges of municipal and industrial wastewater, and runoff from agricultural and urban areas have a significant effect on water quality at both sites. Median values of alkalinity at site 1 (54 mg/L) and site 2 (58 mg/L) were among the highest for the monitoring stations (fig. 3) and represent natural contributions from weathering of soils and rocks, along with contributions from wastewater discharges and agricultural activities. Median concentrations of nitrate in water from site 1 (2.3 mg/L) and site 2 (3.2 mg/L) were among the highest for the nine monitoring stations (fig. 3). Concentrations of total ammonia plus organic nitrogen at both sites were mostly less than 1 mg/L. The median concentration of phosphorus at site 2 (0.24 mg/L) was the highest for stations in the monitoring network, and the median concentration at site 1 was only slightly lower.

In the White Clay Creek basin, agriculture and, to a lesser extent, urban development are the principal human activities affecting stream water quality. Overall, water in White Clay Creek is of a quality suitable for most uses, although excessive concentrations of zinc have been detected occasionally at Newark and Stanton. Fishing is fully supported in the reach from the Pennsylvania-Delaware State line to the confluence with the Christina River. However, White Clay Creek does not fully support primary-contact recreation because enterococcal bacteria, the greatest water-quality problem, often are present at site 3 and other locations in concentrations exceeding the State standard (Delaware Department of Natural Resources and Environmental Control, 1990d).

The median alkalinity of stream water from site 3 (58 mg/L) was one of the highest for the monitoring stations (fig. 3) and reflected contributions from various natural sources and human activities. The median concentration of nitrate (3.2 mg/L) in water from site 3 was among the highest and reflected contributions from extensive agricultural operations in the basin. The median concentration of ammonia plus organic nitrogen (0.37 mg/L) was the lowest, and that of phosphorus (0.07 mg/L) was about average for stations in the monitoring network.

Because of consistently high concentrations of enterococcal bacteria, the Christina River does not fully support primary contact recreation (Delaware Department of Natural Resources and Environmental Control, 1990e). Median concentrations of enterococcal bacteria in water from site 4 (183 col/100 mL) and site 5 (210 col/100 mL) were among the highest for the network stations. The principal sources of enterococcal bacteria are discharges from a municipal wastewater-treatment plant, agricultural and urban runoff, and overflows from combined sewers carrying domestic wastewater and storm runoff. Except for the relatively high concentrations of enterococcal bacteria, constituent concentrations in water from the Christina River at sites 4 and 5 were in the middle range among those for the nine monitoring stations in the network. Compared to site 4, the smaller median alkalinity concentration (28 mg/L) at site 5 reflects increasing contributions to streamflow of less alkaline water from the Coastal Plain in the lower part of the Christina River basin. Median concentrations of nitrate decreased downstream from 1.5 mg/L at site 4 to 0.93 mg/L at site 5. Ammonia plus organic nitrogen concentrations were similar at both sites, as were phosphorus concentrations.

St. Jones and Nanticoke Rivers, Stockley Branch, and Broad Creek

Streams in the Coastal Plain are not used appreciably for water supply, mainly because abundant ground-water supplies are readily accessible. Water use also is limited by the relatively small discharges of Delaware's Coastal Plain streams, saline-water encroachment in the tidal reaches of the streams, and the often substantial distances between the streams and points of intended use. The St. Jones and Nanticoke Rivers are the principal streams draining the Coastal Plain of Delaware. Stockley Branch is typical of the many small streams draining to Rehoboth and Indian River Bays. Broad Creek is a major tributary of the Nanticoke River.

The St. Jones River basin in central Delaware contains numerous branches, tributaries, and drainage ditches. The river flows from the northwest to the southeast and passes through Dover, the largest municipality in the basin. Although the St. Jones River basin is the most urbanized and industrialized large drainage basin in central Delaware, nearly one-half of the land in the basin remains in agricultural use. In the Nanticoke River and Broad Creek basins in southwestern Delaware, land is covered primarily by crops and forest. Land use in the Stockley Branch basin in southeastern Delaware includes rural residential areas, scattered forests, agricultural operations, and extensive poultry and livestock operations.

The quality of water in the St. Jones and Nanticoke Rivers, Stockley Branch, and Broad Creek reflects the combined effects of natural factors in human activities. However, because natural concentrations of stream-water constituents are low in many Coastal Plain streams, input from human activities can have a substantial effect on the chemical characteristics of water in the streams. Specific water-quality problems in the St. Jones and Nanticoke River, Stockley Branch, and Broad Creek basins have restricted certain designated uses of these waters. Contamination by organic compounds (particularly polychlorinated biphenyls) and pesticides has rendered the St. Jones River from Dover to Bowers Beach unsuitable for fishing (Delaware Department of Natural Resources and Environmental Control, 1990f). Median concentrations of enterococcal bacteria at sites 7, 8, and 9 were among the lowest for the nine monitoring stations, yet primary-contact recreation in the Nanticoke River, Stockley Branch, and Broad Creek is not fully supported because of occasionally excessive concentrations of these bacteria at several locations (Delaware Department of Natural Resources and Environmental Control, 1990g,h,i).

At site 6 on the St. Jones River, the median concentration of nitrate (0.21 mg/L) was the lowest among those for the nine monitoring stations, whereas the median concentration of ammonia plus organic nitrogen (1,40 mg/L) was the highest. The distribution of these concentrations indicates the presence of ground-water inflow containing little or no dissolved oxygen mainly from poorly drained parts of the upper St. Jones River basin. The distribution also might signify that Silver Lake, a eutrophic water body immediately upstream from site 6, has a major effect on the form of nitrogen prevalent in the river by limiting the oxidation of ammonia and organic nitrogen to nitrate. Median concentrations of nitrate in water from the Nanticoke River at site 7 (2.6 mg/L), Stockley Branch at site 8 (3.2 mg/L), and Broad Creek at site 9 (2.7 mg/L) were among the highest for the monitoring stations. These concentrations reflect substantial contributions of shallow ground-water inflow containing nitrate from fertilizers and onsite septic systems and, to a lesser extent, nonpoint source runoff from agricultural areas. Nutrient concentrations in the Nanticoke River also are affected by point-source discharge of wastewater from municipal and industrial operations. At sites 7, 8, and 9 median concentrations of phosphorus were the lowest among the monitoring stations. Median alkalinity concentrations ranged from 14 mg/L at site 7 to 34 mg/L at site 6 and reflected small chemical contributions from the relatively insoluble sediments of the Coastal Plain.

Figure 3. Water quality of selected streams in Delaware, water years 1987-89. (Source: Data from U.S. Geological Survey, U.S. Environmental Protection Agency, and Delaware Department of Natural Resources and Environmental Control files.)

Table 1. Sources and environmental significance of selected water-quality constituents and properties (Source: Compiled by the U.S. Geological Survey, Office of Water Quality)

Water-Quality Trends

Trend analysis is a statistical procedure used to detect changes in stream water quality at a monitoring station over time. For this report, water-quality data from seven monitoring stations (fig. 2) were analyzed for trends by using the seasonal Kendall test (Hirsch and others, 1982), a method used extensively by the USGS. The graph (shown below) of the total nitrate concentration in Red Clay Creek at site 2 illustrates the trend inferred from the concentration data and demonstrates the variation in water quality that is common in streams.

When possible, constituent-concentration data were adjusted for changes in streamflow to preclude identifying a trend in concentration that was caused only by a trend in streamflow. The data were not adjusted when (1) more than 10 percent of the samples had concentrations lower than the minimum reporting limit for the analytical method used or (2) streamflow was controlled substantially by human activities. When the concentration data could not be adjusted for streamflow, trends were determined directly from the concentration data.

Statewide trends in measurements of selected physical properties of stream water and concentrations of selected constituents in stream water are shown on maps in figure 4. On each map, a trend is indicated at a monitoring station only if the data from that station were suitable for trend analysis. Trends in enterococcal bateria concentrations are not shown because the length of the data record was insufficient for trend analysis. For more information on the suitability critieria and on the trend-analysis procedure used for this report, see Lanfear and Alexander (1990).

Figure 4. Trends in water quality of selected streams in Delaware, by water years. (Source: Data from U.S. Geological Survey, U.S. Environmental Protection Agency, and Delaware Department of Natural Resources and Environmental Control files.)

Since 1946, information on the chemical quality of stream water in Delaware has been collected for various purposes. During that time, increased population and urbanization, along with changes in land use, agricultural practices, and wastewater-collection and treatment procedures, have affected the chemical character of the water. This report discusses changes in some aspects of water quality that have occurred from 1970 to recently (1990). McKenzie (1979) reported changes in water quality in Brandywine Creek at Wilmington during 1946-72, the St. Jones River at Dover during 1959-72, and the Nanticoke River near Bridgeville during 1957-72 and noted increasing concentrations of major inorganic chemical constituents over time in Brandywine Creek and the St. Jones River. No distinct trends in constituent concentrations were observed in water from the Nanticoke River.

Alkalinity

Alkalinity is a measure of the capacity of the substances dissolved in water to neutralize acid. In most natural waters, alkalinity is produced mainly by bicarbonate and carbonate (Hem, 1985, p. 106), which are ions formed when carbon dioxide or carbonate rock dissolves in water.

Upward trends in alkalinity were detected at sites 1, 2, and 3, which are in basins that drain areas of the Piedmont underlain chiefly by crystalline rocks and, to a lesser extent, carbonate rocks. Although the relative importance of different sources is unknown, the upward trends probably resulted from increasing alkalinity contributions from municipal wastewater discharges and runoff from urban areas. No trend was recorded at sites 6,7, and 9, which are in basins that drain the unconsolidated sediments of the Coastal Plain.

Total Nitrate

Nitrogen from point and nonpoint sources can be introduced into streams in several forms; nitrate is the end product of their oxidation. Upward trends in nitrate concentrations at sites 1,2, and 3 in northern Delaware (fig. 4) probably resulted from population growth and attendant increases in municipal wastewater discharge and runoff from agricultural and urban areas, mainly in parts of the basins in Pennsylvania. Point-source discharges in northern Delaware are small and intermittent and would not be expected to have a pronounced effect on trends. The increasing nitrate concentration in the Nanticoke River at site 7 probably resulted from increasing discharge of municipal and industrial wastewater and from increasing concentrations in surface runoff from fertilized cropland. Fertilizers also contribute nitrogen to shallow ground water that flows into the Nanticoke River. Site 9 on Broad Creek also is in an agricultural basin where fertilizer application has led to gradual increases in nitrate concentration in stream water. The decreasing nitrate concentration in the St. Jones River at site 6 might have resulted from increasing aquatic plant growth in Silver Lake and from reduction of nonpoint-source nitrogen contributions as land in the basin is converted from agricultural to nonagricultural uses.

Total Ammonia Plus Organic Nitrogen

Ammonia and organic nitrogen oxidize to nitrate in well-aerated streams. The presence of substantial quantities of either ammonia or organic nitrogen can indicate contamination from nearby urban or agricultural sources.

Downward trends in concentrations of ammonia plus organic nitrogen in Brandywine Creek at site 1, White Clay Creek at site 3, and the Nanticoke River at site 7 (fig. 4) probably reflect improvements in wastewater-treatment processes and reduction of nitrogen concentrations in ground-water inflow from areas where regional sewers have replaced septic tanks. The trend in concentrations at site 6 also was downward for water years 1970-88, although concentrations increased gradually in the later part of the trend period.

Total Phosphorus

The total phosphorus concentration of a water sample is a measure of the concentration of all forms of phosphorus present in the sample, dissolved and particulate. Human activities (table 1) can be important sources of phosphorus in streams.

In the Red Clay Creek, White Clay Creek, and Christina River basins, municipal wastewater discharges, fertilizers, and nonpoint-source urban runoff are the major sources of phosphorus in stream water. Although not presented in this report, the data on which the trend analyses were performed indicate significant decreases in phosphorus concentrations from the early 1980's to 1983, followed by a period of relatively low, stable concentrations. Specific causes for the decreasing concentrations at site 2 on Red Clay Creek, site 3 on White Clay Creek, and site 4 on the Christina River (fig. 4) have not been documented; however, the trends could be related to the replacement of high-phosphate detergents with low-phosphate or phosphate-free detergents and to improvements in wastewater-treatment processes. The decreasing concentration in Broad Creek at site 9 could have resulted in part from reduced concentrations in wastewater discharged to the shallow ground-water system through septic tanks. The ground water, which eventually discharges to streams, constitutes a major component of streamflow in the Coastal Plain.

Water-Quality Management

The DNREC, Division of Water Resources, is responsible for managing programs to protect water resources and control water pollution. The Delaware Environmental Protection Act of 1974 and Regulations Governing the Control of Water Pollution (1974) authorize the DNREC to administer the National Pollutant Discharge Elimination System program and to regulate wastewater-treatment facilities. The Division of Water Resources performs numerous regulatory, monitoring, and enforcement functions, including developing water-quality standards and criteria, assessing of stream water quality to determine compliance with effluent and water-quality standards, monitoring waste discharges and ambient water-quality conditions, and issuing permits to discharge wastewater. The Division also prepares a biennial water-quality assessment submitted to the U.S. Congress and the EPA, as required by section 305(b) of the Federal Clean Water Act (Delaware Department of Natural Resources and Environmental Control, 1990a-i).

Several water-resources management issues in Delaware require interagency cooperation. For example, the Division of Public Health, within the Delaware Department of Health and Social Services, is responsible for posting swimming advisories and regulating shellfish-harvesting areas. The Divisions of Water Resources and Public Health conduct water-quality monitoring activities in support of these programs. The Governor's Environmental Council assists the DNREC in identifying environmental matters of public concern, including water-resources issues, and addresses these concerns through appropriate policies and programs.

The State of Delaware is a charter member of the Delaware River Basin Commission, an interstate agency concerned with basinwide water-resources evaluation, planning, and management. Representatives from the DNREC participate in many of the special water-quality committees of the Commission.

Since 1970, the Division of Water Resources has operated a statewide stream water-quality monitoring program. Although the level of monitoring has remained relatively constant, the types of monitoring have changed to meet evolving needs. The most significant changes have been the addition of biological monitoring and toxics assessment to the regular stream water-quality program, and special surveys to evaluate specific water-quality problems.

To help improve the quality of surface-water resources in the State, volunteers from the Delaware Stream Watch Program, a surveillance conducted by concerned individuals, regularly visit selected streams and perform routine monitoring activities including visual surveys, physical and chemical water-quality measurements, and biological surveys. Stream Watch volunteers report their findings and any water-quality problems directly to the DNREC.

Selected References

Anderson, J.R., 1967, Major land uses in the United States, in U.S. Geological Survey, 1970, National atlas of the United States of America: Washington, D.C., U.S. Geological Survey, p. 158-159.

Delaware Department of Natural Resources and Environmental Control, 1990a, 1990 Delaware water quality inventory 305(b) report, volume 1-Summary: Dover, Delaware Department of Natural Resources and Environmental Control, 72 p.

_____1990b, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for Brandywine River: Dover, Delaware Department of Natural Resources and Environmental Control, p. 17-21.

_____1990c, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for Red Clay Creek: Dover, Delaware Department of Natural Resources and Environmental Control, 21 p.

_____1990d, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for White Clay Creek: Dover, Delaware Department of Natural Resources and Environmental Control, 32 p.

_____1990e, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for Christina River: Dover, Delaware Department of Natural Resources and Environmental Control, 54 p.

_____1990f, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for St. Jones River: Dover, Delaware Department of Natural Resources and Environmental Control, 38 p.

_____1990g, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for Nanticoke River: Dover, Delaware Department of Natural Resources and Environmental Control, 51 p.

_____1990h, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for Indian River Bay: Dover, Delaware Department of Natural Resources and Environmental Control, 81 p.

_____1990i, 1990 Delaware water quality inventory 305(b) report, volume II--Basin assessment information for Broad Creek: Dover, Delaware Department of Natural Resources and Environmental Control, 41 p.

Fenneman, N.M., 1946, Physical divisions of the United States: Washington, D.C., U.S. Geological Survey special map, scale 1:7,000,000.

Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p.

Hirsch, R.M., Slack, J.R., and Smith, R.A., 1982, Techniques of trend analysis for monthly water quality data: Water Resources Research, v. 18, no. 1, p. 107-121.

Lanfear, K.J., and Alexander, R.B., 1990, Methodology to derive water-quality trends for use by the National Water Summary Program of the U.S. Geological Survey: U.S. Geological Survey Open-File Report 90-359, 10 p.

McKenzie, S.W., 1979, Long-term chemical-quality changes in selected Delaware streams: Delaware Geological Survey Report of Investigations no. 34, 41 p.

Rasmussen, W.C., Groot, J.J, Martin, R.O.R., and others, 1957, The water resources of northern Delaware: Delaware Geological Survey Bulletin no. 6, v. 1, 223 p.

Thelin, G.P., and Pike, R.J., 1990, Digital shaded relief map of the conterminous United States: Menlo Park, Calif., U.S. Geological Survey digital image processing, scale 1:3,500,000.

U.S. Department of Commerce, Bureau of the Census, 1982, Census of population (1980), number of inhabitants: Washington, D.C., U.S. Government Printing Office, 84 p.

U.S. Geological Survey, 1986, National water summary 1985-Hydrologic events and surface-water resources: U.S. Geological Survey Water-Supply Paper 2300,506 p.

_____1990, National water summary 1987-Hydrologic events and water supply and use: U.S. Geological Survey Water-Supply Paper 2350, 553 p.

Prepared by Gary N. Paulachok and Joel D. Blomquist, U.S. Geological Survey; "Water-Quality Management" section by John F. Davis, Delaware Department of Natural Resources and Environmental Control

For Additional Information

District Chief
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore MD, 21237