The Fort Chaffee (Arkansas) P.O.W. Camp Landfill -- the Geophysical Surveys

by: Jeff Wynn

|| Introduction || Results ||

Introduction

A geophysical survey was conducted as an integral part of a larger USGS study of the Ft. Chaffee North P.O.W. Landfill site (also formally designated SWMU 32 by the U.S. Army Corps of Engineers) located on fort property just west of the main building complex, southeast of Ft. Smith, Arkansas (approximately 35^o 18'N, 94^o 19'W). The survey objective was to outline the World War II era landfill (for decades now covered with a soil cap) using geophysical methods.

Figure 1, portion of togographic map with pen pointing to the POW landfill at Ft. Chaffee.

Figure 1. Location map. Pen points to location of POW landfill at Ft. Chaffee.

A subsequent phase of the work used these geophysical data to optimize a soil gas survey which found a number of organic volatiles. The geophysical survey took place between 28 May and 6 June 1996, and utilized both magnetic and electromagnetic (EM) surveying instruments on a 25-foot (7.6-meter) grid. The total land area was about 16 acres.

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Figure 2, Photo of Faron Usrey holding the EM-31 conductivity-mapping instrument.

Figure 2 shows Faron Usrey, of the USGS Water Resources Division office in Little Rock, Arkansas, carrying the Geonics EM-31 conductivity-mapping instrument used to map the POW camp site. Note the briar patch behind him - we identified three different species of tick proliferating here, including the deer-mouse tick, the vector for the Lyme Disease spirochete.

Results

The combined magnetic and EM data show distinctive signatures that can be used to draw a closed boundary around the SWMU-32 landfill. The two different types of data (magnetic and EM) are in substantial agreement, adding confidence to where this boundary is drawn.

Figure 3, magnetic data superimposed on topography, Ft. Chaffee site.

Figure 3 shows the magnetic data draped over the topography of the site. The underlying rock is a greywacke sequence with occasional shale that is non-magnetic; the landfill clearly stands out in the central parts of the figure as "noisy" magnetic highs and lows of short wavelength and high amplitude.

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Figure 4, EM data (conductors) superimposed on topography, Ft. Chaffee site.

Figure 4 shows the EM data (with coils configured vertical in-line, measuring in-phase conductivity) draped over the topography. Most of the conductive anomalies lie within the magnetically "noisy" area of Figure 3. While there are several weak conductors outside this boundary, all but one are discounted as probably not being connected to the landfill. The exception is the strong anomaly (the red tear-drop in the lower-right center) centered between lines 10850N and 10950N, and between stations 9800E and 9950E. It is likely caused by conductive chemicals (a "plume") migrating along the surface or shallow subsurface from the landfill and pooling in the east-side coincident topographic low. This anomaly is too strong to be caused by ponding of rainwater only.

Figures 3 and 4 clearly outline not only the 50-year-old landfill, but also suggest that conductive liquids have migrated out of it and pooled in a topographic low caused by the long-ago placement of the north-south road that bounds the near side of both figures and marks the eastern edge of the site.

Further information about this site and the geochemical results can be obtained by contacting Dave Friewald, Supervisory Hydrologist, USGS Water Resources Division, Little Rock, AR, friewald@usgs.gov

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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
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Last modification: Friday, 25-Jul-2003 19:16:32 EDT (SRB)