INTRODUCTION

The continued growth and vitality of the Nation is linked to the maintenance of or improvement in the quality of its water resources. Water quality is the physical, chemical, and biological characteristics of water with regard to its suitability for a specified purpose. Public awareness of the importance of water quality has increased greatly in the past two decades. The Congress has passed such major pieces of legislation as the Clean Water Act, the Safe Drinking Water Act, the Resource Conservation Recovery Act, and the Toxic Substances Control Act. State and local governments and industry have made significant commitments to water-pollution abatement. Through these combined efforts, the quality of the Nation's rivers and streams has improved significantly compared with the quality of 15 years ago. This is true even though industrial activity and population have increased during this period with corresponding increases in water use and in the volume of wastes discharged. For example, 15 years ago, low dissolvedoxygen levels were common in rivers and streams because of the discharge of large volumes of oxygen-demanding wastes. Today, as a result of the construction of new waste-treatment plants and the upgrading of existing plants, this is no longer true. Despite progress, several water-quality issues still remain. Among them are the contamination of surface and ground water from nonpoint-source pollution, acid precipitation, and the disposal of hazardous wastes.

The U.S. Geological Survey's role in water-resources management is to provide hydrologic data and understanding of hydrologic processes affecting the Nation's water resources. The Geological Survey monitors the quality of surface and ground water, conducts research to increase understanding of the processes that affect the quality of water, and conducts applied interpretive studies to determine the causes of specific observed changes in water

quality and to predict the nature of future changes that are likely to occur due to changes in land use or uses of surface- and ground-water resources. The three components-monitoring, research, and applied interpretative studies are highly interactive. For example, theories arising from research and interpretative studies provide the foundation for designing monitoring networks and for detecting waterquality trends. Further, today's research results may provide increased understanding or new techniques for next year's interpretive studies, and monitoring may uncover problems that require interpretive investigation or additional research.

The following examples of research, interpretive investigations, and monitoring describe some of the Geological Survey's efforts to provide the information needed for addressing the water-quality issues facing the Nation.

POTOMAC RIVER ESTUARY STUDY

The Potomac River Estuary Study, which began in October 1977, was one of seven Geological Survey river-quality assessments and the only one to concentrate on estuarine problems. As part of the Potomac River Estuary Study, the sources and fate of sediments and several major nutrients were examined. This information is important not only for developing water-quality management strategies for the Potomac River estuary, it is essential for developing management strategies to protect the Chesapeake Bay.

Estuaries are potentially the most productive as well as the most fragile and endangered areas of our Nation's coastal environment. Because they are the meeting place of saltwater and freshwater, estuaries are complex hydrodynamic, chemical, and biological environments. Sediments accumulate in estuaries along with the attached nutrients, metals, and trace organic compounds. The attached substances may become stored permanently or temporarily in the bottom sediments

thereby contributing to eutrophication and, in the case of metals and organics, sometimes accumulating in aquatic life forms.

From 1979 to 1981, water-quality data were collected twice weekly at six stations along the 116-mile tidal reach between Chain Bridge near Washington, D.C., and the Chesapeake Bay (fig. 1). Monthly amounts of sediment and nutrients passing the stations were estimated by a computer model and used, together with data from sewage-treatment plants, tributaries, and nonpoint sources, to determine sediment and nutrient budgets for the major segments of the tidal river and estuary (J. P. Bennett, written communication, 1984).

All of the suspended sediment contributed to the Potomac River estuary came from nonpoint sources. Of the total annual amount of suspended sediment; 56 percent came from sources upstream from Chain Bridge, 43 percent came from sources downstream from Chain Bridge, and 1 percent came from

Haines Pt.

Anacostia River

Piscataway Creek

Study Area

MARYLAND

Lower Cedar Point Wicomico River

Chaptico Bay

Cobb Island

Maryland Point

Bridge

Upper Machodoc~ Creek

POTOMAC

76°30'

ESAPEAKE BAY

St. Marys

River

VIRGINIA

Nomini Bay

Estuary

RIVER

PINEY

POINT

ATLANTIC OCEAN

CHESAPEAKE

BAY

Point Lookout

Smith Pointe

upstream-flowing saline water entering from the Chesapeake Bay. All of the suspended sediment was trapped in the estuary.

Nonpoint sources downstream from Chain Bridge contributed 17 percent of the nitrogen and 32 percent of the phosphorus. Sources upstream from Chain Bridge contributed 58 percent of the nitrogen and 43 percent of the phosphorus. The relative amounts of nitrogen and phosphorus contributed by point and nonpoint sources upstream from Chain Bridge are unknown. The balance of the nutrient loads, 25 percent nitrogen and 25 percent phosphorus, was contributed by point

sources.

Seventy-five percent of the nitrogen and phosphorus contributed to the Potomac was trapped in the estuary during the study period. Whether these materials are stored permanently in the estuary or will later be transported to the Chesapeake Bay under extreme hydrologic conditions, such as hurricanes, is unknown.

NATIONAL MONITORING
NETWORKS USED TO ASSESS
TRENDS IN RIVER QUALITY

Two nationwide water-quality monitoring networks (fig. 2), the National Hydrologic Bench-Mark Network and the National Stream Quality Accounting Network (NASQAN), are operated by the U.S. Geological Survey.

The Bench-Mark Network, which was begun in the mid 1960's and consists of collection sites in 52 basins, is designed to characterize the hydrology and water quality of small basins that are in a near-natural state. Study of streams draining these basins helps to explain how much of the variation in water

quality and quantity occurring in streams across the Nation is natural and how much is caused by human activity.

The NASQAN network, which was begun in 1973, provides a basis for continuously assessing the quality of major rivers with respect to natural and man-induced factors. The network currently consists of 501 stations that are located in 331 subregional drainage basins, collectively encompassing nearly the entire Nation.

Data routinely collected in the two networks include streamflow, concentrations of major inorganic and trace constituents (including heavy metals), bacterial indicators of pollution, and concentrations of several radiochemical constituents. A major purpose of both the Bench-Mark and NASQAN Networks is to detect trends in the concentrations of water-quality constituents. The problem of separating man-induced changes in water quality from the natural variability that results from seasonal change and differences between wet and dry years has complicated the analysis of data from these networks. However, Geological Survey scientists have recently developed new statistical techniques for detecting time trends in fixed-station water-quality data, and the techniques have proven successful for analyses of the constituents measured at the Bench-Mark and NASQAN stations. All trend analyses described in subsequent sections were conducted on water-quality data that were adjusted for the effects of flow and season.

Acid-Precipitation-Induced Trends at Bench-Mark Stations

The United States lacks long-term nationwide data on the chemistry of precipitation. Without these data, the next best possibility for detecting historic changes in precipitation chemistry is to evaluate trends in the

chemistry of sensitive streams and lakes that might be attributable to changes in precipitation quality.

Records from the Bench-Mark Network are particularly appropriate for investigating atmospheric influences on water quality during the last decade because consistent sampling and analytical methods have been applied for a 10- to 15-year period at each network station and because the basins have been relatively unaffected by changes in land use. Many of the Bench-Mark Network streams have a low capacity to buffer acids, and these are expected to be sensitive to small changes in acid deposition.

Industrial emissions of sulfur dioxide are a major source of acidity in precipitation. Once emitted, the sulfur dioxide forms dilute sulfuric acid and is transported in the atmosphere. Thus, one expected result of increases in acid precipitation would be increases in the sulfate concentrations of streams. In contrast, alkalinity is a measure of the capability of water to neutralize acid and is, itself, reduced by the neutralization process. Hence, acid deposition would be expected to decrease alkalinity, especially where the initial concentration of alkalinity is low, as is typical in regions devoid of certain rock types, such as limestone and marble.

Figures 3 and 4 show the results of applying the new statistical techniques to detect trends in sulfate and alkalinity data collected over the past 10 to 15

Figure 2. Locations of stations in the Bench-Mark and NASQAN water-quality monitoring systems in operation as of September 30, 1984.

years at Bench-Mark stations. Figure 3 also shows the direction of change in sulfur dioxide emissions (compiled by the U.S. Environmental Protection Agency) between 1965 and 1980. Figure 3 shows that sulfate concentrations in streams have tended to increase during the last 10 to 15 years over a broad area of the continental United States, extending from the Southeast to the Mountain States and the Northwest. By contrast, streams in the Northeast have tended to show slight declines in sulfate concentrations. The overall geographical pattern is similar to that shown by trends in sulfur dioxide emissions to the atmosphere during the same period.

Figure 4 shows that downward trends in alkalinity, suggesting increases in the acidity of precipitation, greatly outnumber upward trends within a broad area from the Southeast to the Northwest. In contrast, alkalinity trends at the Bench-Mark stations in the Northeast are consistently upward, suggesting a decrease in the acidity of precipitation. The geographical distribution of trends in sulfate and alkalinity are consistent with the reported sulfur dioxide emissions. Together, the results suggest a decline in the acidity of

precipitation in the Northeastern United States and an increase in acidity in most other regions during the last 10 to 15 years. The results also indicate that the effects of relatively small changes in atmospheric emissions are observable as changes in stream quality, a point that recently has been the focus of considerable national debate.

Trends in Suspended Sediment at National Stream Quality Accounting Network Stations

Although transport of sediment by flowing water is a natural process, excessive sediment is harmful to almost every beneficial use of water. Sediment fills reservoirs, clogs navigation channels, and drastically reduces the aesthetic and recreational values of rivers. Increased sedimentation, by raising the elevation of river-channel beds, can cause an increase in the frequency of floods. In the production of potable water, the cost of removing sediments generally exceeds the cost of all other treatment. Furthermore, other water contaminants, such as excess nutrients, toxic metals, and many of the pesticides, adhere readily to sediment particles and are transported, de