Areas with low pesticide use usually have low detection frequencies (Barbash and Resek, 1995). Conversely, areas where a pesticide is detected frequently are often those of high use. However, low frequencies of pesticide detection are often encountered in areas of high use, indicating that other factors influence pesticide movement.

Most studies of pesticides in surface water focus on the midcontinent region where large amounts of pesticides are used. Goolsby and others (1993) found that herbicides are detected at low levels throughout the year in the rivers of the Midwest, including the Mississippi River. The amounts transported by streams and rivers in the Midwest are generally less than 3 percent of the amount applied, but can still result in concentrations above the MCL. Atrazine (and its metabolites), alachlor, cyanazine, and metolachlor, used principally for weed control in corn and soybeans, were the principal contaminants detected, and are also the most widely used pesticides in the region. Such chemicals, once in drinking water supplies, are not controlled by conventional treatment technologies (Miltner and others, 1989). About 21 million people in the Midwest rely on drinking water from surface sources, about 42 percent of the population.

High concentrations of atrazine in some water supplies in the Midwest have prompted concerns that public water utilities will have to install expensive water treatment systems in order to meet Safe Drinking Water Act requirements. In 1990, about 29 percent of public utilities dumped powdered activated carbon into their systems to spot-treat for organic chemicals, primarily pesticides (American Water Works Association, 1992). If all the treatment plants withdrawing from surface sources upgrade their treatment systems to remove pesticides, annual treatment costs would increase by $400 million per year (Ribaudo and Bouzaher, 1994). Because of these concerns, EPA has placed the triazine herbicides (atrazine, cyanazine, and simazine) under special review due to potential health and ecological concerns. DuPont has already announced that it will phase out its cyanazine production.

Some pesticides leach into underlying aquifers. EPA's survey of drinking water wells found that 10 percent of the CWS's and 4 percent of rural domestic wells contained at least one pesticide (1990). Pesticides or their transformation products have been detected in the ground waters of 43 States (Barbash and Resek, 1995). However, the EPA estimated that less than 1 percent of the CWS's and rural domestic wells had

concentrations above MCL's or Lifetime Health Advisory Levels (the maximum concentration of a water contaminant that may be consumed safely over an average lifetime). Problems were found more frequently in shallow wells in agricultural areas. A sampling of wells in corn- and soybean-growing areas in the Midwest found 28 percent of wells had detectable levels of selected pesticides and metabolites, but none exceeded the MCL (Kolpin, Burkart, and Thurman, 1993). Atrazine was the most frequently detected compound.

Groundwater vulnerability to pesticides varies geographically, depending on soil characteristics, pesticide application rates, and the persistence and toxicity of the pesticides used (fig. 2.2.4) (see chapter 3.2, Pesticides, for more discussion of persistence and toxicity). Areas with sandy, highly leachable soils, such as central Nebraska and the blueberry barrens of Maine, have high vulnerability ratings. Highly vulnerable areas characterized by heavy applications of generally toxic materials on fruit and vegetable crops include the San Joaquin Valley in California, Florida, and southern Arizona. In contrast, the Corn Belt, despite the widespread use of chemicals, particularly herbicides, has a lower rating than other areas because the predominant soils are not prone to leaching.

Animal Waste

Animal operations can generate large amounts of waste which, if improperly handled or disposed of, can affect the quality of surface- and groundwater resources. Improperly constructed storage pits or lagoons at confined facilities can break or leak, releasing large amounts of concentrated waste directly into surface water. Dissolved material can leach into groundwater if lagoons or pits are improperly lined. Pastured animals allowed to graze near or to water in streams can contaminate water. Improper application of animal waste on fields, such as spreading on frozen ground, can result in large amounts being flushed into water bodies after rain or a thaw.

An issue of increasing importance to water quality is the management of manure from confined animal operations. This stems from increasing concentration in the animal industry, a number of incidents where manure has contaminated local water bodies (see box "Animal Waste Storage Failures"), and a greater awareness of the potential for contamination of drinking water supplies by waste-borne parasites. Larger operations, particularly for hogs and dairy cows, now characterize the industry. As animal production units grow increasingly large and

groundwater. Dissolved salts and other minerals can have significant impacts on surface- and groundwater quality. Increased concentrations of naturally occurring toxic minerals, such as selenium and boron, can harm aquatic wildlife and degrade recreation opportunities. Increased levels of dissolved solids in public drinking water supplies can increase water treatment costs, force the development of alternative water supplies, and reduce the lifespans of water-using household appliances. Increased salinity levels in irrigation water can reduce crop yields or damage soils so that some crops can no longer be grown.

Dissolved salts and other minerals are an important cause of pollution in the Southern Plains, arid Southwest, and southern California. Total damages from salinity in the Colorado River range from $310 million to $831 million annually, based on the 1976-85 average levels of river salinity. These include damages to agriculture ($113-$122 million), households ($156-$638 million), utilities ($32 million), and industry ($6-$15 million) (Lohman, Milliken, and Dorn, 1988).

The USGS reports mixed trends of salinity in surface water (Smith, Alexander, and Lanfear, 1993). Measures of dissolved solids (mostly ions of calcium, magnesium, sodium, potassium, bicarbonate, sulfate, and chloride) indicate that water quality improved at more stations than it worsened. However, while salinity trends in water for domestic and industrial purposes generally improved during the 1980's, salinity worsened for irrigation purposes. Among USGS cataloguing units (watersheds) having significant irrigation surface-water withdrawals, the percentage of stations having annual average dissolved solids concentrations greater than 500 mg/L increased during 1980-89 from 30 to 33 percent.

Reducing Loadings from Agriculture Farmers can take many steps to reduce loadings of agricultural pollutants to water resources. Both structural and management practices are available to farmers. In a study of 16 of USDA's 242 Water Quality Program projects, 134 different practices were installed, nearly half of which were labeled "new and innovative" (USDA, NRCS, 1996). Water quality practices work by managing water and chemical inputs more efficiently, or by controlling runoff. Practices include pest management, nutrient management, irrigation water management, animal waste management, tillage management, and runoff control (for more on practices, see chapters 4.1-4.6).

Groundwater Vulnerability Indexes

The groundwater vulnerability index for nitrates (GWVIN) was developed by Kellogg and others (1992). It is a function of soil leaching potential, nitrate leaching potential, precipitation, and nitrogen fertilizer use. Excess nitrogen per acre is the difference between the amount of nitrogen from commercial fertilizer and animal manure applied, including credit for nitrogen fixed by previous leguminous crops, and the amount taken up by the

crop.

The groundwater vulnerability index for pesticides (GWVIP), also developed by Kellogg and others (1992), is a function of soil leaching potential, pesticide leaching potential, precipitation, and chemical use. It is an extension of the national-level Soil-Pesticide Interaction Screening Procedure (SPISP) developed by the Natural Resources Conservation Service (Goss and Wauchope, 1990). GWVIP does not depend on the amount of chemical applied, but the type of chemical, its leaching potential, and the leaching potential of the soil to which the chemical is applied. The GWVIP can be weighted by persistence and toxicity to further account for potential harm to the environment. Persistence is defined as the soil half-life. Toxicity is defined as the acute oral toxicity to rats. Chronic toxicity or toxicity to fish would have been preferred, but these data are not available for most pesticides. For further discussion of weighting for persistence and toxicity, see chapter 3.2, Pesticides.

USDA has had several programs that provide farmers the means to adopt water quality practices, including the Agricultural Conservation Program, Water Quality Incentive Projects, and the Water Quality Program. Most current programs focus on providing education, technical, and financial assistance to farmers to get them to adopt alternative management systems that protect water quality. Education raises farmer awareness not only of the potential financial and environmental benefits of alternative practices, but also of the link between the practices they implement and local water quality. Technical and financial assistance provide the means for a farmer to actually try a new practice and to acquire the skill to apply it effectively. Failure of voluntary programs to achieve needed changes in farming practices may increasingly result in regulations, already occurring in a number of States (see chapter 6.2, Water Quality Programs, for more on Federal and State programs).

Improvements in water quality from farmers' efforts to reduce pollutant loadings often take years to detect and document. The links between improved management and observed changes in water quality are complex. As many as 10 consecutive years of water quality data are needed before long-term changes can be distinguished from short-term fluctuations (Smith, Alexander, and Lanfear, 1993). Phosphorus accumulated in bottom sediments will affect water quality long after conservation practices have dramatically reduced phosphorus loadings in runoff. Similarly, fish, insects, and other biological indicators of a healthy stream may not reach acceptable levels until many years after water quality improves and riparian habitat is restored. Aquifers may take decades to show improvements in quality after chemical management is improved. In most project areas, agriculture is not the only source of pollution.

In addition, many projects do not establish or maintain adequate water quality monitoring for detecting changes in water quality. National water quality monitoring systems already in place are inadequate for detecting changes in small watersheds where water quality programs have generally been targeted. For these reasons, improvements in water quality may in fact be taking place undetected.

Costs and Benefits of Pollution Control

The assessment of policies to reduce pollution from agricultural production requires a complete knowledge of benefits and costs to water users of changes in water quality. Benefits and costs are measured in terms of changes in economic welfare, represented by consumer and producer surpluses. Estimating the economic effects of changes in water quality is complicated by the lack of organized markets for environmental quality. There are no observed prices with which to measure economic value. A number of methods exist for deriving these measures. One method for estimating consumer surplus is to study an individual's behavior in averting the consequences of poor environmental quality, such as expenditures made to prevent household damages from salinity. A second approach is to exploit the relationship between private goods and environmental quality (when it exists) to draw inferences about the demand for environmental quality. A third approach is to ask individuals to reveal directly their willingness to pay for changes in environmental quality.

When water quality is a factor in the production of market good, the benefits of changes in quality can be

inferred from changes in variables associated with the production of the market good. There are two avenues through which benefits can be obtained. The first is through changes in the price of the marketable good to consumers. The second is through changes in incomes received by owners of factor inputs. The choice of approaches for estimating consumer and produce welfare effects depends largely on the availability of data and the nature of demand for water quality.

Economists have conducted numerous studies of the value of water quality over the years. Most of these studies have focused on specific sites or "local" water quality issues (Crutchfield, Feather, and Hellerstein, 1995). Relatively few studies have looked at the costs of water pollution and the benefits of pollution reduction on a nationwide scale, and none have included costs to all classes of water users (table 2.2.3). However, the results of these studies indicate that the annual benefits from improving water quality could total tens of billions of dollars. Water quality benefits from erosion control on cropland alone could total over $4 billion per year (Hrubovcak, LeBlanc, and Eakin, 1995).

Although increasing, public funds spent on nonpoint source pollution are small compared with the expenditures on point sources. Between $80 and $100 billion of public funding was spent on water pollution control during the 1980's (Ervin, 1995), mainly to control pollutants from municipal sources. In contrast, only $1 to $2 billion has been spent on agricultural water quality initiatives over the last two decades (Ervin, 1995). This spending is much less than the potential benefits from improved water quality. However, an increasing amount of financial and other resources is being directed to agricultural nonpoint source pollution. USDA spent $194 million on water quality-related research, education, technical assistance, financial assistance, and data activities in 1995. Such expenditures have doubled since 1989, despite an overall decrease in USDA expenditures for conservation. Farmers themselves have spent an unknown amount on water quality practices, although in many cases changes were made to enhance profitability. In addition, EPA made over $65 million in regional grant awards to States for agricultural nonpoint source programs in 1994-95, an increase of 50 percent over the previous 2-year period. These funds are frequently contracted to cooperating agencies such as local conservation districts to support project implementation. (For more information on water quality programs, see chapter 6.2.)