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standing of ground-water flow in other bedded-salt environments and to the commercial radioactive-waste disposal program. The research on variable-density ground-water flow also is applicable to the evaluation of localized hazardous-waste sites in which high concentrations of dissolved-waste constituents produce significant changes in fluid density.

Removing Organic Contaminants from the Subsurface by Inducing Air-Phase Transport

By Arthur L. Baehr

Toxic organic liquids such as gasoline and industrial solvents enter the subsurface in a variety of ways, including leakage from underground storage tanks and accidents involving pipelines and tank trucks.

Remedial action generally includes an effort to physically recover the spilled product; however, significant quantities of the spill can remain trapped between soil particles. Unfortunately, the threat posed to ground water may persist even if the organic liquid is immobilized in the unsaturated zone because the solubilized portion of the spill can later enter ground water during recharge or during high stages of a fluctuating water table. Thus, in environmentally sensitive areas, a secondary recovery method is required to reduce the possibility of ground-water contamination. Many organic substances exhibit significant volatility; thus they could be removed by inducing air movement in the unsaturated zone. An air-flow field can be established in the unsaturated zone by using paired boreholes through which air can be injected and withdrawn. Air laden with organic vapors moves along the induced flow path toward boreholes where it is withdrawn, analyzed, treated, and (or) released to the atmosphere, thus removing the contaminant from the subsurface. This fledgling technology has been successfully used to remove gasoline hydrocarbons and chlorinated solvents from unconsolidated unsaturated zones. Further research is

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Vapor transport

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Schematic of an air vent system.

required, however, to advance the understanding of processes that govern the transport of organic vapors, to predict cost effectiveness, and to design optimal venting systems.

The USGS is currently pursuing a collaborative research effort with the Department of Civil Engineering at the University of Connecticut. Sand columns residually saturated with gasoline were vented under steady flow conditions. A mathematical-transport model analysis of the hydrocarbon removal rates from these columns supports the conclusion that the success of venting technology will not be limited by local rates of phase transfer (from a liquid to a vapor phase) but rather will ultimately depend on the ability to establish an air-flow field that intersects the distributed contaminant. A mathematical model of compressible-fluid flow through unsaturated porous media has been developed and is currently being applied to evaluate unsaturated-zone permeability so that design of optimal borehole configurations can be determined.

Portion of a color composite of
Landsat Thematic Mapper bands
2, 4, and 7, showing area near
Lake Powell, Arizona-Utah

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National Mapping Program

Highlights

Digital Mapping
Supports the 1990
Census

By Patricia P. Dunham and
Bruce Y. McKenzie

When the Nation's census takers set out to gather information for the 1990 Decennial Census of Population and Housing, they will be assisted by the results of a cooperative effort between the U.S. Geological Survey and the U.S. Bureau of the Census.

In June 1987, the Survey completed a 4-year cooperative effort with the Bureau of the Census to establish a 1:100,000-scale digital cartographic data base of hydrography and transportation information. The Bureau of the Census will use these digital cartographic data files as the cartographic base for their automated geographic support system, the Topologically Integrated Geographic Encoding and Reference (TIGER) System, to support data collection for the 1990 Decennial Census. The TIGER System will also support future demographic and economic data collection, processing, and tabulation activities of the Bureau.

The Survey will make the 1:100,000scale hydrography and transportation digital data available to users through the National Digital Cartographic Data Base. The completion by the Survey of the 1:100,000-scale digital data provides a framework for use in other geographic information systems. The nationwide digital data coverage at this scale combines the qualities of sufficient detail and adequate content for use in regional applications, such as mineral resource analyses, hydrologic analyses, and management planning. When the census information becomes available in the 1990's, researchers will also have access to vast amounts of socioeconomic data, which may serve as a catalyst

for the widespread use of geographic information systems technology in the United States.

Preliminary discussions between the USGS and the Bureau of the Census began in late 1981, and, following a series of research projects, the Survey's 1:100,000scale cartographic data in digital form was selected as the most suitable base for the Census enumerator data-collection maps (fig. 1). In early 1983, a cooperative digital pilot project was initiated that enabled the two agencies to develop and test new software and equipment, expand production capacities, and enhance data processing techniques to collect and use the digital data. The success of the pilot project led to the commitment by both agencies to complete the collection of 1:100,000-scale hydrography and transportation digital data for the conterminous United States by mid-1987. Meeting this stringent deadline required both agencies to implement complementary digital production systems.

The base graphic product to be used for digital data collection was the Survey's 1:100,000-scale map series. When the cooperative digital effort began in 1983, this series consisted predominantly of topographic (contour) editions, and only 960 of the required 1,823 maps were available for digitizing. To provide the remaining base maps, the compilation emphasis was shifted to produce planimetric (without contours) editions. This change, coupled with increased personnel resources, enabled conterminous United States 1:100,000-scale planimetric coverage to be achieved in October 1986, 5 years ahead of the original goal for this map series.

To meet the digital production deadline, both agencies implemented highvolume digital production systems. The USGS had the responsibility for digitizing all hydrographic and transportation features from the 1:100,000-scale graphic maps and assigning feature classification codes to all data except roads. The Bureau of the Census was responsible for assigning feature classifications to roads. To handle

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Figure 1. A section of a U.S. Geological Survey l:100,000-scale base map and the corresponding area on a Bureau of the Census enumerator map. The enumerator maps will be used by Census field staffs to construct the address lists and geographic codes required for distribution and tabulation of the 1990 Decennial Census questionnaires.

the massive production workload, the Survey purchased, installed, and customized state-of-the-art automated digital cartographic data collection systems in four regional mapping centers. Additionally, quality-control procedures were established to ensure that the final digital product met both the standards of the Bureau of the Census and the standards of the USGS for entry into the National Digital Cartographic Data Base.

The cooperative effort was completed in mid-1987, and both agencies are using the digital data to meet individual shortand long-term national requirements. The successful completion of the effort, in which two agencies worked together to achieve an ambitious goal, represented Federal cooperation at its finest.

New Aerial

Photography Program
Underway

By Donald L. Light

The growing interest in highresolution, color-infrared photography for a wide variety of applications such as agriculture, forestry, soils, land and resource management, mapping, and numerous earth science studies has led to a reassessment of Federal and State agency needs for this type of photography. As a result, a new program, called the National Aerial Photography Program (NAPP), was begun in 1987 to provide higher resolution and

larger scale photographs of uniform quality for the 48 conterminous United States.

A predecessor program, the National High-Altitude Aerial Photography Program, was established in 1980 by Federal agencies to acquire 1:80,000-scale blackand-white and 1:58,000-scale color-infrared photography at a flying height of 40,000 feet. Aerial photography under this program has now been acquired, and complete uniform coverage of the conterminous United States will be available in 1988.

Photography under the new program will be acquired from a flying height of 20,000 feet with a single 6-inch-focal-length camera exposing color-infrared film at a scale of 1:40,000. The resulting photographs will be centered on quarter sections of standard USGS 7.5-minute quadrangles. In 1987, eight contractors began to acquire 249,207 square miles of aerial photography in Maryland, Ohio, Kentucky, Pennsylvania, Indiana, Nebraska, Utah, and Idaho. The first NAPP photographs became available in the fall of 1987.

A plan has been developed for acquiring photographic coverage of a State or sector of a State approximately every 5 years (fig. 2). The intent of this 5-year cycle is to enable Federal and State agencies to acquire photographs to meet and maintain individual management goals. Some revisions to the plan are anticipated depending on availability of funds and the priority expressed by State and Federal contributors to the program. The new program promises to meet the aerial photography needs of State and Federal organizations engaged in key projects of national interest.

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