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Lessons from the national water-quality assessment: A decade of intensive water-quality studies indicates that reducing diffuse nonpoint water contamination requires new tools and a holistic management approach.

Water contamination from point sources such as discharges of sewage and industrial waste has been substantially reduced thanks to the Clean Water Act of 1972. Despite the gains, 40 percent of our waters are still too contaminated for basic uses such as swimming and fishing.

The overwhelming majority of water-quality problems we face today result from diffuse "nonpoint" sources of pollution from agricultural land, urban development, forest harvesting and the atmosphere. The pollution they deliver varies hour to hour and season to season making it difficult to quantify the sources. Moreover, contaminants now include hundreds of synthetic organic compounds like pesticides and volatile organics in solvents and gasoline, microbial and viral contamination as well as pharmaceuticals and hormones. The complex and demanding steps necessary to pinpoint and quantify these are just the first steps to effective management.

In 1991, the U.S. Geological Survey (USGS) launched a comprehensive water quality program called National Water-Quality Assessment (NAWQA). This program is a primary source for long term, nationwide information on the quality of streams, groundwater, and aquatic ecosystems. In more than 50 major river basins and aquifers across the nation (See map), USGS scientists collect and assess information on water chemistry hydrology land use, stream habitat, and aquatic life. Each NAWQA assessment adheres to a nationally consistent study design and methods of sampling and analysis so that water-quality conditions in a specific locality or watershed can be compared to those in other geographic regions. The consistent study design and methods also allow contaminants and aquatic ecology to be assessed on a comprehensive national basis.

However, only through collaboration with numerous government, research, and interest--group partners can we ensure that NAWQA information will meet the needs of local, state, regional, and national stakeholders. Input from these partners has been critical in defining the key water-quality resource issues on which to focus. The assessments thereby build local knowledge about water-quality issues and trends in each particular basin while also building an understanding of how and why water quality varies regionally and nationally.

Although a complete national picture of water-quality problems is not yet a reality, the last decade of studies by NAWQA has documented significant contaminant patterns in some of the most important river basins and aquifers. We can summarize the key findings in 36 major river basins and aquifer systems. These have significant implications for decision makers and planners at all levels that are critical to long term sustainability of communities and ecosystems.

Complex and multi factorial

NAWQA demonstrates nonpoint chemical contamination is an issue in the 120 agricultural and 35 urban watersheds assessed. ("Urban" primarily refers to residential and commercial development over the last 50 years.) The findings also show that water-quality conditions and aquatic health reflect a complex combination of land and chemical use, land-management practices, population density and watershed development, and natural features, such as soils, geology, hydrology, and climate. Contaminant concentrations vary from season to season and from watershed to watershed. Even among seemingly similar land uses and sources of contamination, different areas can have very different degrees of vulnerability.

Agricultural Findings

Streams and groundwater in agricultural basins almost always contain complex mixtures of nutrients and pesticides. Concentrations of nitrogen and phosphorus commonly exceeded levels that can contribute to excessive plant growth in streams. In nearly 80 percent of streams sampled average annual concentrations of phosphorus were greater than the U.S. Environmental Protection Agency (EPA) desired goal for preventing nuisance plant growth. Nitrate was most prevalent in shallow groundwater (less than 100 feet below land surface) beneath agricultural areas, where about 20 percent of samples exceeded the EPA drinking-water standard for nitrate. This finding may raise concerns where shallow wells are used for domestic supply

Pesticides are widespread. At least one pesticide was detected in more than 95 percent of stream samples and in more than 60 percent of shallow wells sampled in agricultural areas. A relatively small number of heavily-used chemicals accounted for most detections, specifically: herbicides atrazine, metolachlor, cyanazine, and alachlor.

Insecticides that were used in the past still persist in agricultural streams and sediment. DDT was the most commonly detected organochlorine compound, followed by dieldrin and chlordane. Their uses were restricted in the 1970s and 1980s and yet, more than 20 years later, one or more sediment-quality guidelines were exceeded at more than 20 percent of the agricultural sites.

Urban Findings

Water in urban areas have a characteristic chemical makeup or "signature" that is closely linked to the chemicals used in urban watersheds. Reflecting this, though somewhat surprising, insecticides occurred more frequently and usually at higher concentrations than in agricultural streams. All urban water bodies had concentrations of insecticides exceeding at least one guideline established to protect aquatic life. Another urban 'surprise' is that total phosphorus concentrations are as high as in agricultural streams, and exceeded guidelines in 70 percent of sampled streams. (See table above--Urban and Agricultural Pollutant Comparision)

More typical urban contaminants are volatile organic compounds, particularly commercial and industrial solvents, that occur widely in shallow ground water. DDT, chlordane, and dieldrin were found in urban stream sediments at higher levels than agricultural settings and exceeded guidelines at 36 percent of sampled urban sires. Moreover, these as well as PCBs were found in fish tissue from urban waterways, often at higher concentrations than in sediments.

Concern for human health and aquatic ecosystems

Concentrations of contaminants generally are at low levels--almost always below current EPA drinking-water standards. However, the risk to humans and ecological health remains unclear. Exposure is complicated because individual compounds seldom occur alone. Streams and ground water in areas with significant agricultural or urban development almost always contain complex mixtures of VOCs, nutrients, pesticides, and their breakdown products. Chemical breakdown products, which can have similar or even greater toxicities than parent compounds, are often as common in the environment as parent compounds.

Exposure is further complicated by lengthy periods of low concentrations punctuated by brief seasonal pulses of much higher concentrations. EPA standards are usually based on long term exposure to constant concentrations. Moreover, many contaminants and their breakdown products do not have current EPA standards, i.e., only half of the pesticides and VOCs measured have standards.

Current standards do not address exposure to contaminant mixtures and the possibility that the presence of multiple compounds, even at low concentrations, may have adverse cumulative effects. Finally, the potential impacts on the reproductive, nervous, and immune systems of aquatic organisms have not been tested. Many of the 20 most frequently detected pesticides are suspected endocrine-disrupting chemicals with the potential to affect reproduction or development of aquatic organisms or wildlife by interfering with natural hormones.

Vulnerability depends on lay of the land and the season

Natural features, such as geology, hydrology and soils, and land practices, such as tile drainage and irrigation, govern vulnerability to contamination because they affect the movement or transport of chemicals over land into aquifers. Concentrations of contaminants can thereby vary significantly in different regions of the United States, or even locally within a basin.

Groundwater, for example, is less vulnerable in poorly-drained areas where sediment is relatively impermeable and fine- grained. The groundwater underlying intensive agricultural areas in parts of the Upper Midwest are minimally contaminated for that reason. Conversely, well-drained areas where soils are permeable and underlain by sand and gravel, karst or other fractured bedrock readily transmit water. These are common in the Central Valley of California, parts of the Northwest, the Great Plains, and the Mid-Atlantic where elevated nitrate concentrations in aquifers are often present.

Hydrology and basin characteristics can also affect contaminant vulnerability. In the Mississippi Basin, the closer nitrogen sources are to large streams and rivers the more is transported to the Gulf of Mexico. Nitrogen is not removed as readily in the large streams and rivers by natural processes as in smaller tributaries. As a result, some watersheds contribute more, despite similar nitrogen sources or similar distances from the Gulf.

Seasonal patterns in water quality of streams emerged in most basins mainly due to the timing and amount of chemical use, the frequency and magnitude of runoff from rainstorms or snowmelt, and specific land-management practices, such as irrigation and tile drainage. (See Graphic--Seasonal Variation of Pesticides in Streams)

Atmospheric depositions can be significant sources of contaminants depending on the locale. Recent studies have shown that as much as 25 percent of the nitrogen entering the Chesapeake Bay comes from the atmosphere.

Water-Management Implications

We need to move beyond narrowly focused monitoring, fragmented planning and management to more holistic solutions for improving water quality. These holistic solutions recognize the interrelationship of air, water, land, and biota. They should also take into account scale effects in watershed planning and recognize the local regional and national importance of land and chemical use, natural features, and management practices on water quality in diverse settings.

This clearly means monitoring programs cannot rely on one or two samples per year. Instead, they should be designed to simultaneously measure multiple compounds in one sample. Measurements of stream flow during the different low-flow and high-flow conditions, in combination with water-quality samples, are needed to fully assess the amounts of contaminants transported by a stream throughout the year to a receiving body, such as an estuary.

Two effective ways to reduce contaminant levels in both urban and agricultural settings are to simply cut back on chemicals being used and applying these more efficiently. Unfortunately, current chemical-use information is generally insufficient for water-resource management and decision making. Improved tracking of chemical use is needed to definitively attribute specific pollutants to different sources in nonpoint runoff.

Controlling nonpoint source pollution also requires targeted and thoughtful actions based on local and regional vulnerability rather than uniform treatment of contaminant sources. This requires combined knowledge of chemical use, contaminant occurrence, and local and regional cause-and-effect controls by natural features. Then areas most vulnerable to contamination can be determined which increases the cost-effectiveness of policies designed to protect water resources in diverse settings.

The Holistic Reality: Everything is Connected to Everything Else

Rarely do decisions on the effects of land use or human actions in individual watersheds consider the cumulative or overall impact on the quality of the downstream resources and receiving coastal water. Instead, water-resource management and monitoring needs to recognize a hydrologic structure with multiple orders of nested, progressively larger watersheds, as opposed to one exclusively based on political boundaries. Furthermore, water monitoring can no longer ignore the connections between water quality and biological communities. In particular, quality of water resources should include measurement of containments in bottom sediment, in fish, or in the health of other aquatic species.

NAWQA findings over the last decade confirm the need for long term systematic and consistent monitoring, which is essential for distinguishing trends from short term fluctuations, evaluating how well management strategies are working, and choosing the most cost-effective resource strategies for the future. In addition, long term monitoring provides insights about unanticipated consequences such as long term persistence of selected organochlorine insecticides like DDT, dieldrin, and chlordane.

Water-resource management and monitoring strategies should be developed in accordance with the patterns and complexities of contaminant occurrence and influences on water quality. An understanding of these patterns will help water managers and policymakers in implementing their current environmental control and protection strategies, investing in monitoring and science, and developing futune environmental priorities.

[Graph omitted]

[Graph omitted]

Relative levels of contamination are closely linked to land use and to
the amounts and types of chemicals used in each setting. Thus, local and
regional management of chemical use can go a long way toward improving
water-quality conditions.


                      Urban     Agricultual  Undeveloped
                      Areas        areas        areas

Nitrogen             Medium     Medium-High      Low
Phosphorus         Medium-High  Medium-High      Low
Herbicides           Medium      Low-High      No data
Currently used     Medium-High  Low-Medium     No data
Historically used  Medium-High   Low-High        Low
                         Shallow Ground Water
                       Urban         Agricultural
                       areas            areas

Nitorgen              Medium             High
Phosphorus              Low              Low
Herbicides            Medium         Medium-High
Currently used      Low-Medium        Low-Medium
Historically used    Low-high          Low-High


The occurrence of nutrients and pesticides in streams is closely related to the time of year they are applied to Lands in the watershed. In the Lake Erie-Lake Saint Clair watershed, for example, elevated concentrations of the most heavily used agricultural herbicides, such as atrazine, were detected for 4 to 6 weeks after rainfall and runoff in the spring and early summer. Pesticides used mostly in urban areas, such as diazinon, are typically applied in late summer and the highest concentrations are detected in streams following those applications.


The National Water-Quality Assessment website reflects the USGS commitment to make its data and findings publicly available. There are hundreds of documents here from raw data tables to summary reports. The website's home page ( is organized into three main sections:


Simply click on one of the more than 50 major river basins and aquifer systems to obtain data about water chemistry, hydrology, land use, stream habitat, and aquatic life for each. Users can get specific contaminant data from any monitoring station in their region.


This section contains national synthesis reports on various containments as well many related reports and data sets.


Here users will find the key findings in short one-page summaries, usually with links.

There is also a Site Index Panel on the left side of the NAWQA home page. Click on the "Data" tab to find the aptly named "NAWQA Data Warehouse" link. Interactive maps with a variety of contaminant/geographic selection tools.

Pixie A. Hamilton is a staff hydrologist and communications specialist for the National Water-Quality Assessment Program.

Timothy L. Miller is chief of the National Water-Quality Assessment Program.
COPYRIGHT 2002 Soil & Water Conservation Society
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Author:Hamilton, Pixie A.; Miller, Timothy L.
Publication:Journal of Soil and Water Conservation
Date:Jan 1, 2002
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