posited, and stored by suspended sediments.

Part of the sediment supplied to rivers arises from natural erosion, part from alterations to land from logging, farming, mining, and construction, and part from man-induced acceleration of river-channel erosion. Because of its importance, an analysis of trends in suspended sediment concentration has been made at those NASQAN stations that have sufficient long-term sediment data (R. A. Smith and R. B. Alexander, written communication, 1984). Nationwide, 225 such NASQAN stations have been established (fig. 5). Of these, the numbers of stations showing increasing and decreasing trends are nearly equal, but the trends at some important regional groupings of stations are predominatly in one direction or the other.

For example, there are a number of decreases in suspended-sediment concentrations in the Missouri Basin in Montana, North Dakota, and South Dakota. The specific stations showing decreases are located on the Missouri River mainstem as well as on tributaries including the Yellowstone, Knife, Cannonball, Grand, Belle Fourche, White, and James Rivers. Declining sediment concentrations previously have been reported for a number of locations in the Missouri Basin and have been attributed to the effects of reservoir construction throughout the basin during the 1950's and 1960's. Reservoirs

act as sediment traps, and the effects of a new reservoir on downstream sediment concentrations may be observed for an extended period of time as a new equilibrium is established between the river forces that carry and deposit sediment.

Figure 5 shows that increases in suspended sediment concentrations have occurred in the Columbia Basin, Oregon and Washington; the Arkansas and Red River basins, Oklahoma; and the Mississippi Basin near the junctions of the Missouri and Ohio Rivers with the Mississippi mainstem. In all three cases, changes in land use activity appear to be the most likely causes of the uptrends.

It is likely that the trends detected in suspended-sediment concentrations at NASQAN stations may be associated with specific types of land use. The Geological Survey has compared the number and direction of detected trends to soil erosion information for three land uses-cropland, forest land, and range and pasture land—as compiled in the Natural Resources Inventory by the Soil Conservation Service. Although the ultimate validity of the comparison may depend on some still unknown factors, such as temporary or longer term storage of sediment, the present results show that suspended-sediment concentrations are increasing more than decreasing in basins dominated by sheet and rill erosion from cropland. In

contrast, suspended-sediment concentrations seem to be decreasing more than increasing in basins dominated by sheet and rill erosion from forest land, and the same also may be true in basins dominated by erosion from range and pasture land. These findings, if borne out by more detailed investigations, would have important implications for the planning of national erosion-control efforts.

GROUND-WATER

CONTAMINATION

About one-half of the people in the United States rely on ground water for drinking water. In some places, such as Florida, New Mexico, and Long Island in New York, ground-water supplies more than 90 percent of the needs of the population. Because waste disposal is a common source of ground-water contamination, understanding the physical, chemical, and biological processes affecting the subterranean movement of contaminants is a key water-management issue. Physical processes that must be understood include the complex movement of ground water, the mixing of contaminants with the ground water, the filtration and sorption of contaminants by aquifer materials, and effects of buoyancy. The latter can result in contaminants concentrating at the top or the bottom of an aquifer, depending upon the density of the waste liquid. Chemical reactions can cause certain dissolved contaminants to join with other dissolved compounds to form new products that may be more toxic than their predecessors. In addition, bacteria and other microorganisms in ground water alter or decompose many organic contaminants; here again, new products of greater toxicity can be created.

Our understanding of the processes affecting the movement and fate of contaminants in ground water has increased significantly during the last several years. However, a great deal more work needs to be done. Although sophisticated mathematical models have been developed to simulate many aspects of contaminant movement and reaction in ground water, the methods to adequately measure all the parameters needed for these models have not

been developed. Other areas in which we need to improve our understanding are ground-water flow in fractured-rock systems, ground-water flow in the unsaturated zone, and clay mineralogy.

Research Investigations at Selected Ground-Water Contamination Sites

In 1982, teams of scientists from the Geological Survey and universities, representing all major earth science disciplines, began studies of selected ground-water contamination sites to advance our understanding of processes affecting movement of contaminants in ground water. Selected for study were ground-water contamination from a crude oil pipeline break near Bemidji, Minnesota, ground-water contamination from the infiltration of creosote and pentachlorophenol (PCP) from wastedisposal pits at Pensacola, Florida, and ground-water contamination from the infiltration of domestic wastewater effluent on Cape Cod, Massachusetts. The types of contaminants at the three sites are associated with many groundwater contamination problems throughout the Nation. The studies are designed to improve our understanding of basic processes, and the knowledge gained will have practical transfer value to other sites having similar hydrogeologic conditions and types of contamination. One of the research demonstration sites selected is an abandoned woodtreatment facility in northwest Florida. The 18-acre site, also a U.S. Environmental Protection Agency Superfund site, is located about 600 yards north of Pensacola Bay. During 80 years of continuous operation from 1902 to 1981, wastewaters generated from the use of creosote in the wood-treatment process were discharged into two unlined surface impoundments that are in direct contact with the sand and gravel aquifer, the principal source of water in northwest Florida. From 1902 to 1950, wood at the plant was treated exclusively with creosote. However, in 1950, PCP was introduced as a wood preservative at the plant, and its use steadily increased until the plant closed in 1981. Over the last few years of operation, the plant used about 25,000 gallons of creo

degradable (H. C. Mattraw, oral communication, 1984). Five different waterquality zones have been characterized beneath the site, including a "bioreaction zone" that seems to be the principal location where some of these organic compounds are degraded by microorganisms. Survey scientists are continuing specific research in each of these zones to better improve our understanding of the contaminant migration pathways, rates, controlling factors, and effects of the dominant chemical and biochemical processes.

NEW WATER-QUALITY STUDIES
BY THE U.S. GEOLOGICAL SURVEY

Regional Ground-Water Quality

Unlike surface waters that can be relatively easily monitored to provide information about regional changes in water quality, ground water cannot be monitored efficiently by a network of stations. Such a network would be a costly and ineffective method of assessing the quality of the Nation's ground water because it moves slowly. The volume of contaminated ground water is usually small, and it tends to occur in slowly developing pockets around point sources of contamination or in shallow, slowly expanding zones beneath areas of nonpoint pollution. Thus, a monitoring strategy in which samples are collected from observation wells chosen for areal coverage would fail to detect most developing contamination problems until a great deal of damage had been done.

As an alternative to fixed-station monitoring, the Geological Survey has begun a series of 14 regional groundwater quality studies to provide information on ambient ground-water chemistry with emphasis on organic substances and trace elements and to identify the causes, effects, and processes that are applicable to similar areas elsewhere. Study areas vary in size from a few tens of square miles to a few thousand square miles. Each area is characterized by relatively uniform climatic and geohydrologic conditions, and the land use, the ground-water flow system, and the inorganic chemical

quality of the ground water generally are well understood. Although results from these studies are not expected for several years, careful analyses to determine the factors affecting the quality of our ground water may allow extrapolation of these results to the national level.

Toxic Substances in Surface Waters

Water contamination by dioxin at Times Beach, Missouri, kepone in the lower James River, Virginia, and polychlorinated biphenyls in the Hudson River in New York State have heightened the public's awareness of toxic substances in surface waters. In fact, because ground-water and surfacewater systems are connected hydrologically, it is important that, in addition to understanding the processes affecting toxic substances in ground water, we understand the sources, distribution, transport, and fate of these substances in rivers, lakes, and estuaries.

During the past 25 years, the Geological Survey has conducted many individual studies of toxic substances in surface waters. Recently, a program has been started to increase our understanding of the processes affecting the movement and fate of different groups of toxic substances under different hydrologic conditions and to describe the occurrence, magnitude, and distribution of toxic substances in surface waters, bottom sediments, and aquatic biota and how these substances are changing with time. The new program consists of three integrated components-research, river basin investigations, and monitoring.

Much of the research is dedicated to the development of information on the migration pathways, rates, and controlling factors of the processes by which toxic substances move and undergo chemical and biochemical changes in rivers and to the development of improved study approaches, sampling methods, and analytical techniques.

The purpose of the river basin investigations is to develop an understanding of the movement and fate of specific classes of toxic substances under differ

ent hydrologic conditions. In addition, the river basin investigations will uncover locations and types of toxic contamination in rivers. This will dictate future monitoring needs concerning whether the concentrations of specific substances are increasing or decreasing and how long certain compounds remain in river systems after sources have been controlled.

Finally, the monitoring component of the program, beginning with sediments, will provide a broad view of the

occurrence and distribution of toxic substances in the Nation's rivers. Analyses of the data from the moni

toring efforts are expected to uncover additional needs for process-oriented research that will help identify candidate watersheds for future river basin investigations.

Sound technical information about the quality of the ground and surface water is essential in meeting the Nation's water-resources needs, in using the land wisely, and in preserving the quality of the environment. The Geological Survey will continue to provide the hydrologic data and understanding of hydrologic processes needed to make the fundamental decisions on the future water needs of the Nation.