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concentrations in ground-water discharge, the process did not completely denitrify the ground water. As a result, ground-water discharge still affected the quality of water in the South Platte River.

Peter McMahon has over 10 years of experience as a USGS hydrologist. His work focuses on the impacts of biological processes on ground-water quality.

John Karl Böhlke is a research hydrologist whose recent work has focused on applications of chemical and isotopic methods for determining ground-water residence times and resolving sources, histories, and reactions of anthropogenic contaminants affecting aquifers.

David Litke has worked as a USGS hydrologist in Colorado for the last 10 years. His areas of interest include surface-water quality, water use, and the use of geographic information systems in water studies.

Environmental and Resource Studies in the U.S.–Mexico Border Region

atification of the North American Free

Trade Agreement (NAFTA) reinforces the need for geoscience data and information in the U.S.–Mexico border region. NAFTA will accelerate urban, agricultural, and industrial growth and trade along the border. Such growth and the current and anticipated potential for increased effects on the environment necessitate development of a large, coherent geoscience database. Such a database is vital to both nations in many disciplines, including land-use management, urban planning, civil engineering, exploration geology, environmental sciences, environmental regulation, resource management, waste treatment, and industrial mineral supply. The U.S. Geological Survey (USGS) has begun several cooperative projects to compile existing data and to provide new data related to these issues.

Data on the distribution and characteris

tics of mineral sites are essential for effectively planning industrial and urban development and for improving the quality of

life in the border region. A computerized mineral-site database containing information on more than 10,000 sites near the U.S.– Mexico border has been compiled in the USGS Mineral Resources Data System. Each site record provides the name of the site, its location, the commodity mined, the geology of the site, a description of the workings and deposits, and a history of exploration, development, and production at the site. The database includes both metallic and nonmetallic minerals and materials. Analysis of this information indicates that a wide variety of nonfuel mineral commodities are present in the border region. Mineral resources and mineral-related issues in the border region that will affect the economies of the United States and Mexico include the following. • Many nonmetallic minerals are used for improving water quality, isolating waste, and mitigating environmental hazards. Important resources in the border region include diatomite and zeolites for waste treatment, clays for sealing waste dumps, and limestone for removing sulfur from emissions produced in coal- or oil-fired power stations. Mineral occurrences and abandoned mine sites may be sources for acid-mine drainage and elevated levels of toxic trace elements in soils and ground Water. • Mineral materials such as limestone, sand, gravel, and gypsum are widely used in the border region as construction materials, especially in rapidly growing urban areas. • The border region produces about 18 percent of the world's copper as well as other metallic commodities, including zinc, lead, silver, and gold. Metals used in alloy manufacture, such as aluminum, cobalt, nickel,

Mineral-site data are available from the Minerals Information Offices of the U.S. Geological Survey in digital form or as listings, tables, and plots and are summarized in Circular 1098.

Area defined as the U.S.–Mexico border region.

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and iron, are produced in relatively small quantities or not at all; thus, for economic growth, these metals will need to be imported into the border region. • Minerals such as gypsum, clays, and sulfur are used to produce fertilizers, animal feeds, and pesticides. Another important layer of geoscience information for the U.S.–Mexico border region is a geologic map in digital format. The border region extends across six different geologic provinces from east to west—from the Gulf of Mexico Coastal Plain, across ancient rocks of the stable continental platform and remnants of past volcanic eruptions, to regions where the Earth's crust has been stretched and areas where rocks from ancient ocean environments have been stuck on to the continent. This complex geology complicates the task of producing a coherent map. Geologic maps of the States of Arizona and New Mexico are now available from the USGS in digital format at a scale of 1:1,000,000. The geology of the border region of Texas is being digitized at a scale of 1:500,000, as is the geology of California. Other current investigations pertinent to the border region include evaluating hydrologic basins that contain aquifers along the border, establishing baseline geochemical information in the larger drainage areas, and examining the dispersion of metals and the natural availability of potentially toxic substances in the region. The San Pedro River Valley in southern Arizona and northern Sonora, Mexico, is composed of several subbasins. A multidisciplinary study to determine the three-dimensional shapes of these subbasins, to describe the relations among the subbasins, and to assess the character and distribution of the sediments filling the basins is underway. This project is an essential first step in assessing the potential for ground-water contamination in subbasin aquifers. Existing geochemical databases are being analyzed and evaluated to provide a geochemical baseline. Some of the samples are being reanalyzed for elements particularly important in identifying potential sites of pollution. These data will also provide a regional hydrogeochemical framework for a large part of southern Arizona and for investigations related to the USGS National Water-Quality Assessment Program in central and southern Arizona. Several investigations are being conducted on the dispersion of metals from mined and unmined ore deposits. Such data will allow the effect of natural mineralization

on the natural distribution of potentially toxic systems to be compared with anthropogenic activities such as mining.

The establishment and continued construction of a geoscience database, including elements of geology, geochemistry, geophysics, and mineral sites, will allow for effective implementation of NAFTA. The geoscience database forms part of the necessary framework for development of land- and ecosystem-management plans and increased infrastructure, industry, and agriculture in the border region.

Norman J Page is Scientist-in-Charge of the USGS Center for Inter-American Mineral Resource Investigations, which conducts cooperative mineral resource investigations, technology transfer and training, mineral information exchange, and research.

Pesticides in the Atmosphere

ne of the first issues to be addressed by

the National Water-Quality Assessment (NAWQA) Program National Synthesis is the presence of pesticides in the environment. The goal of the National Synthesis is to use existing data and new data collected during NAWQA studies to assess the status, trends, and cause-and-effect relations for the Nation's highest priority national and regional waterquality issues. About 1.1 billion pounds of pesticides are used each year in the United States to control many different types of weeds, insects, and other pests in a variety of agricultural and nonagricultural settings.

Total agricultural use and the number of different chemicals applied to crops have more than tripled since the early 1960's. Increased use has resulted in increased crop production, lower maintenance costs, and control of public health hazards, but concerns about the potential adverse effects of pesticides on the environment and human health also have grown steadily. The Pesticide National Synthesis begins with detailed reviews of existing information on pesticides in the hydrologic system, including ground and surface waters and the atmosphere. Results for the atmosphere are summarized below.

Pesticides have been recognized as potential atmospheric pollutants since the

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Atrazine use in 1988 through the study area (A) and the precipitation-weighted concentrations of atrazine throughout the Midwestern and Northeastern United States from mid-April through mid-July 1990 (B) and 1991 (C).

organochlorine insecticides have been detected in the atmosphere of every State in which they were sought. Organophosphorous insecticides also have been heavily used for decades and are still in high use. As a class, they are not as environmentally persistent as the organochlorine compounds, but they have been detected in most States in which analyses have been made. Triazine herbicides have been in use since the 1960's, but studies in which these compounds are analyzed from the atmosphere did not begin until the late 1970's, when atrazine was found in rain in Maryland. Subsequent studies have detected high levels of triazine herbicides in rain in major corn-producing areas, such as the Midwestern United States. Acetanilide herbicides are frequently used in conjunction with triazine herbicides. Although they are not as environmentally stable as the triazine herbicides, they have been detected in rain at equivalent and even higher concentrations. Many other types of herbicides are used in agriculture, and many of them have been detected in the

air or precipitation throughout the United States.

Most Pesticides Studied Have Been Detected

E. from the reviewed literature shows that most of the pesticides for which analyses have been made have been detected in at least one atmospheric matrix. Compared with the hundreds of pesticides that have been and are being used, the number and variety of pesticides analyzed from and detected in air and rain are few. These figures do not mean, however, that the majority of pesticides used are not present in the atmosphere. There are several reasons why a particular pesticide has not been found—for example, low use, short atmospheric residence time (considering deposition and transformation), the timing of the sampling relative to the timing of use, the predominant atmospheric phase in which it will accumulate relative to the phase being sampled, and, perhaps most important, whether it has been analyzed for in the atmosphere.

High Atmospheric Concentrations of

Pesticides Show Seasonal Trends

Po occurrences in air, rain, and fog often show seasonal trends; the highest concentrations correspond to local use and planting seasons. In a recent U.S. Geological Survey (USGS) study, samples of rain collected throughout the Midwestern and Northeastern United States were analyzed for a variety of triazine and acetanilide herbicides used in corn and soybean production. The analyses show that the highest concentrations occurred where corn was most intensively grown and corresponded to the spring and summer planting seasons. Observed concentrations for August through March were considerably less. There is a very detailed and strong relation, both spatially and temporally, between atrazine use and concentrations in rain. Pesticides also have been detected at low levels during periods before and after the high-use seasons. These off-season occurrences could be the result of the

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volatilization and wind erosion of previously applied material or long-range transport from areas where the planting/growing season started earlier. The more persistent pesticides, such as organochlorine insecticides, have been detected in the atmosphere at low levels throughout the year even though they are no longer used in the United States. Another source of pesticides in our atmosphere is from long-range transport from areas outside the United States, such as Mexico, Eastern Europe, and Asia, where many organochlorine insecticides that have been banned in the United States are still being used in large quantities.

Effects of Pesticides on

Water Quality Not Well Documented

he potential contribution and relative

importance of pesticides from the atmosphere to a body of surface water depend on pesticide levels in atmospheric deposition and on how much of the water budget is derived from surface runoff and direct precipitation. However, very little research has been done on the deposition of pesticides into surface waters. The most clearly documented effects of atmospheric pesticides on human health and aquatic life, even at the low levels commonly found in air, rain, snow, and fog, are related to long-lived, environmentally stable organochlorine insecticides that concentrate in organisms through biomagnification (food chain accumulation), bioconcentration (environment/organism partitioning), or both. An example is the organochlorine insecticide toxaphene in the Great Lakes region. Toxaphene, which is carcinogenic to laboratory animals, was never used to any great extent in this area but has been detected in the air, rain, water, sediments, and fish. The most probable source for this contamination is long-range atmospheric transport from the high-use areas in the Southern United States and Mexico.

Determining the environmental significance of pesticides in air, rain, snow, and fog is difficult, and there are no existing national standards or guidelines for these matrices. The only available guidelines are for contaminants in terrestrial waters. In addition to human health concerns, aquatic organisms are often more sensitive to low-level pesticide

exposures than humans are, and the U.S. Environmental Protection Agency and the National Academy of Sciences have set maximum levels of several pesticides for the protection of aquatic life. The majority of pesticides in use today, however, do not have such established levels. Pesticide concentrations in rain usually are one order of magnitude or more below the human health standards or maximum contaminant levels for water. There have been several instances, though, where the concentrations of pesticides in rain and fog have exceeded the maximum contaminant level values for aquatic life in or near agricultural areas.

Improved Databases Are Needed

he extent of pesticides in our atmosphere

and their deposition into surface waters are not well known because there is no consistent nationwide monitoring of pesticides and their transformation products in atmospheric deposition. Existing data on pesticides in the atmosphere show that pesticides have been found in air, rain, snow, or fog throughout the Nation and that most pesticides studied have been found. The potential significance to water quality has not been extensively studied except in the Great Lakes area. The effects on the health of humans and aquatic organisms brought about by chronic exposure to low levels of a wide variety of insecticides, herbicides, and fungicides also are not well known. Water-quality investigations conducted as part of the NAWQA Program will consider atmospheric deposition as a potentially important source of pesticides, particularly during high-use seasons in high

use areas, but generally will not conduct For more information on pesticides in the

atmosphere, contact Michael Majewski at:

extensive sampling of atmospheric media. Telephone: (916) 979–2609, ext. 345

The NAWQA Program will work with other Internet: majewskiødcascr.wr. agencies and programs to encourage the usgs.gov development of more comprehensive

monitoring and the study of atmospheric

contaminants.

Michael S. Majewski is a research chemist with the Pesticide National Synthesis group of the National Water-Quality Assessment Program.

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