USGS Water Resources Information

graphic Stream Water Quality in the Conterminous United States -- Status and Trends of Selected Indicators During the 1980's

By Richard A. Smith, Richard B. Alexander, and Kenneth J. Lanfear
National Water Summary 1990-91 -- Stream Water Quality, U.S. Geological Survey Water-Supply Paper 2400)

Go to Table of Contents
or Go to previous section



Comprehensive and consistent data on nearly all aspects of environmental conditions at the national level currently are on the frontier of Federal statistical reporting. Therefore, some limitations are inherent in compiling a statistical description of national water-quality conditions and trends because reliable national data do not yet exist for many aspects of water quality. The primary reason for the lack of data is that water-quality monitoring is technically demanding and expensive, as are most types of environmental monitoring. Moreover, as monitoring programs have evolved over the past 20 years, ideas have changed about which water-quality indicators are important and how to measure them. As a result, long periods have elapsed between recognition of water-quality problems and the availability of national-level data that describe their occurrence, severity, and trends over time. Also, in conducting a statistically based analysis of water-quality trends, as many as 10 consecutive years of water-quality data commonly are needed to help distinguish short-term variability from long- term changes. (See article "Statistical Analysis of Water-Quality Data" in this volume for a discussion of statistical techniques of water-quality analysis.)

Currently (1992), two Federal water-quality monitoring programs are being implemented that will greatly enhance national water-quality data bases for future analyses -- the USGS National Water Quality Assessment Program (Hirsch and others, 1988; Leahy and others, 1990) and the U. S. Environmental Protection Agency (EPA) Environmental Monitoring and Assessment Program (U.S. Environmental Protection Agency, 1990a). Together, these national programs are designed to monitor a wide variety of biological, chemical, and physical indicators. However, nearly a decade will be required to amass the quantity of data needed to document environmentally and statistically significant trends. In the meantime, descriptions of national water-quality conditions and trends must be based on data from numerous separate, less comprehensive monitoring programs.


Within Federal, State, and local monitoring programs, many water-quality indicators are used to describe different aspects of water quality. Indicators range from those that are specific and narrowly focused, such as individual chemical or bacterial concentrations, to those that are integrative and broadly focused, such as indices of biological community health. In the middle of this spectrum lie water-quality indices combining two or more chemical concentrations and bioassays in which the toxicity of a water sample is tested on living organisms. Narrowly focused indicators tend to be more precisely defined and methodologically standardized and are more easily related to specific causes and sources of contamination. Integrative indicators, in contrast, are more broadly defined and tend to relate more directly to the consequences of water quality in terms of water use and human and ecological health.

Ideally, a national summary of stream water quality would include analysis of data from a mix of indicators chosen from this broad spectrum. For several reasons, however, the range in indicators from existing national data bases is severely limited.

Biological indicators. -- Perhaps the major limitation is a shortage of nationally representative biological data. In recent years, interest in the use of biological indicators rather than single chemical indicators has increased because they integrate many aspects of, and are more directly related to, human and ecological health. Most State governments monitor some biological aspects of water quality, and a few, such as Ohio, have large bioassessment programs (Ohio Environmental Protection Agency, 1987, 1992). However, because scientific consensus is lacking on the validity of any single biological indicator of streams, few national biological data bases exist.

Use support. -- Support of waterbody designated use is an integrative water-quality indicator that refers to the degree to which the quality of a waterbody is sufficient to support State-designated water uses, such as fishing and swimming. An EPA national data base exists for this indicator because section 305(b) of the Clean Water Act requires States to make a biennial, statewide assessment of designated-use support and to submit the findings to the EPA, which in turn submits a national water-quality report to the U.S. Congress. (See article "Nationwide Water Quality Reporting to the Congress as Required Under Section 305(b) of the Clean Water Act" in this volume for a more detailed discussion of State reporting of water quality.) State officials determine the degree of support of designated use on the basis of formally adopted State-specific standards and criteria and then describe the water quality of a waterbody as either fully supporting, partially supporting, or not supporting the designated use. However, two major difficulties preclude the analysis of use-support data for determining national water-quality conditions and trends. First, State-to-State differences in the standards and criteria used to determine use support make it difficult to interpret regional patterns in water quality; and, second, methodological changes over the history of the 305(b) program preclude any analysis of trends.

Chemical indicators.-- Despite considerable interest and some recent progress in collecting biological and integrative indicators of water quality, the majority of water-quality data that are available for analysis of conditions and trends of the NationŐs streams are relatively specific, predominantly chemical data. These include measures of sanitary conditions, plant nutrients, and the concentrations of other commonly occurring substances. Even within the category of specific chemical indicators, however, existing national data bases fall far short of covering the spectrum of current interest.

Toxic chemicals. -- A serious deficiency in chemical data is the nearly complete lack of national toxic- contaminant data bases. Organic and inorganic toxic compounds enter streams from many sources. A significant source of potentially toxic organic chemicals is pesticides. The number of agriculturally and domestically applied pesticides has increased over the past two decades and now includes hundreds of chemicals (U.S. Environmental Protection Agency, 1990b). From 1975 to 1980, regular sampling for 22 organochlorine and organophosphate pesticides occurred in a nationwide network of about 170 stations (Gilliom and others, 1985), but no nationally organized, long-term pesticide monitoring in streams has been undertaken since that time. As presented below in the section "Concentration and Transport of Herbicides in Agricultural Areas in the Midwest," however, data from recent short-term regional monitoring of selected pesticides are now available. Also, the FWS as part of its National Contaminant Biomonitoring Program (NCBP) has compiled a data base of analyses of trace- element and organic-compound concentrations in finfish that were collected periodically between 1970 and 1986. (Lowe and others, 1985; Schmitt and Brumbaugh, 1990; Schmitt, Zajicek, and Peterman, 1990; and Schmitt, Zajicke, and Ribick, 1985). Data through 1986 are summarized below in the section "National Trends in Pesticides and Toxic Trace Elements in Finfish Tissue." Although the NCBP is the only one of its kind, only a small fraction of the pesticides and other toxic substances currently released to streams is monitored.

Toxic organic substances used by industry also are known to enter streams. These compounds include volatile organic degreasing agents, polychlorinated biphenyls (PCB's), and polycyclic aromatic hydrocarbons from the chemical manufacturing and petroleum industries. Unfortunately, no nationally organized stream monitoring for any of these substances has ever occurred, and no national-level data are available.

Some stream monitoring for toxic inorganic substances has occurred, but reliable national data are lacking for arsenic, cadmium, chromium, copper, lead, mercury, and most other trace elements. Federal, State, and local governments routinely have monitored trace-element concentrations in streams for more than a decade, but several recent studies have concluded that traditional techniques of sample collection and analysis have seriously overestimated the concentration of dissolved trace elements in streams (Flegal and Coale, 1989; Shiller and Boyle, 1987; Windom and others, 1991). Based on the findings reported in these studies, the relatively abundant national trace-element data are not reliable for statistical summarization.

Go to next section.

National Water Summary on Wetland Resources
[an error occurred while processing this directive]
U.S. Department of the Interior, U.S. Geological Survey
Maintainer: Water Webserver Team
Last Update:12:58:00 Tue 30 Nov 2004
FirstGov, "Your First Click to the U.S. Government"