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standing of ground-water flow in other
bedded-salt environments and to the com-
mercial radioactive-waste disposal pro-
gram. 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 sig-
nificant 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 unsatu-
rated 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 environ-
mentally sensitive areas, a secondary
recovery method is required to reduce the
possibility of ground-water contamination.
Many organic substances exhibit sig-
nificant 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can be injected and withdrawn.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it is withdrawn, analyzed, treated, and (or)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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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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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.

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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

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engaged in key projects of national interest. -E vT N w nx o M. o pao A. o o J L. oln E —owv/ * VA MO NC TN [ ] 1988 AR SC [] 1989 Ms AL GA [] 1990 [ ] 1991 LA | 1992 FL 2 states sectors

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Figure 2. National Aerial Photography Program 5-year cycle. Col. ors indicate the proposed fiscal year in which photography will be obtained.

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