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Students Investigate an Environmental Restoration Site in New Mexico
Twenty-six college and graduate school students spent 3 weeks digging disposal trenches that may contain World War II era radioactive waste.
by John F. Ferguson, W. Scott Baldridge, George R. Jiracek, Victor Gonzalez, and Paul A. Pope
Investigations conducted during the 1997 Summer of Applied Geophysical Experience (SAGE) field course at one site at Los Alamos National Laboratory (LANL) successfully delineated a waste disposal trench dug in the 1940s. The survey, which was popular with the students, provided them with important experience in "real world" geophysical problems and demonstrated that students can obtain useful and important results during a short field exercise.
The utility of the magnetic and seismic ground-penetrating radar methods will be demonstrated on a profile through one of the major waste trenches at the site. The magnetic and radar methods are sensitive to the presence of metallic objects buried within the trenches. The seismic refraction method uses the travel times of seismic waves to determine the wave speeds within the Earth. When the wave velocities increase with depth, the waves are bent or refracted back to the surface, where they are observed.
A low-velocity trench structure is defined by the seismic refraction data. Models of the trench structure are both accurate (linear dimensions are probably good to within a few meters) and somewhat different from prior expectations.
The SAGE faculty and students have investigated environmental restoration (ER) sites at LANL during the 1995, 1996, and 1997 seasons (see cover figure). The small-scale ER projects occupy only about 20% of the overall field effort, but are very popular with the students, many of whom express interest in careers in environmental or hydrologic geophysics.
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(Cover) A collage of seven photographs of SAGE students working at MDA-B on June 21 and 22, 1997. |
The SAGE field course has just completed its 15th year of operation in the Espa?ola basin area of northern New Mexico. Since the program began, 370 undergraduate and graduate students and professionals have investigated the structure of the Espa?ola basin segment of the Rio Grande rift with the faculty. SAGE is sponsored by the Institute of Geophysics and Planetary Physics (IGPP) at LANL. The faculty is drawn from Los Alamos, San Diego State University, University of California at Riverside, Purdue University, University of Texas at Dallas, and Golden West College. The program runs for 3 weeks (4 weeks for U.S. undergraduates) and includes background lectures, fieldwork, data processing, interpretation, and a final written and oral presentation by each student. Seismic, electromagnetic, and potential field methods are all applied. Every student participates in each of the field activities but specializes during the processing and reporting phases. The students are organized into teams for fieldwork, data processing, and interpretation. These efforts have resulted in four published scientific papers, all of which were coauthored by SAGE students. The SAGE class of 1997 was comprised of 14 undergraduate and 12 graduate students from diverse small and large schools throughout the United States as well as international graduate students from Mexico, Indonesia, Venezuela, and Belgium.
Beginning in 1995, a component of the activity has been directed toward the study of ER sites in the Los Alamos area. Two of seven field days are spent at the ER site and about 20% of the students specialize in the ER site analysis. This activity has been spurred by a growing interest among the SAGE students in environmental, engineering, and hydrologic careers. The program gives students close contact and participation in a LANL program and the opportunity to make real and timely contributions. The sites studied so far have typically involved trenches or pits dug into the local bedrock for waste disposal, although the 1996 site included a buried stream channel. All sites are thoroughly checked and approved by health and safety personnel and have required minimal or no personal protective equipment. SAGE has applied a variety of techniques to these projects including seismic refraction, ground penetrating radar (GPR), magnetic, DC resistivity, and the transient electromagnetic methods.
This experience has not only been valuable for the students; we have also learned how to effectively characterize trench sites in the Los Alamos area. The SAGE approach provides safe and cost-effective ways of obtaining critical information necessary to monitor or remediate the trench sites.
The trenches may contain either poisonous chemicals or radioactive substances or both. If these substances are mobilized by groundwater flow, they can possibly contaminate wells or surface water and become a hazard to people and the environment. Remediation might consist of complete removal of the waste or a stabilizing seal covering the trench. It is generally preferable to leave such sites as undisturbed as possible and monitor for leakage.
For instance, since it is possible to define the trench location and dimensions to within about 1 m using the seismic refraction method, one can then drill monitoring wells around and under the trench without fear of disturbing the possibly hazardous contents.
MDA-B is one of the oldest waste trenches at LANL. It is situated on a narrow mesa just east of the Los Alamos town site. It contains an area that served as a plutonium processing facility from 1945 until 1978. The mesa is a part of the Pajarito Plateau, a deeply dissected plateau underlain by Bandelier Tuff, which was ejected 1.22 million years ago from the Jemez volcanic center 25 km to the west. The tuff is either exposed at the surface or locally covered by soil, which in this arid climate is typically very thin. Borings made on the MDA-B fence line in 1966 suggest that there may be 1-2 m of "soil" overlying the tuff. A braided channel system was incised into the tuff soon after deposition. Soil and alluvium now bury the channel system.
During SAGE 1996, one of these channels was successfully mapped using the seismic refraction method. DP Mesa was isolated by the incision of DP and Los Alamos Canyons, which drain into the Rio Grande 12 kilometers to the east. More than 240 meters of volcanic rocks overlie Tertiary sediments. The water table lies 365 meters below the volcanic rocks. Although it is possible that hazardous waste might contaminate the deep aquifer, it is perhaps more likely that it would find its way into the surface water of the adjacent canyons.
During World War II, the Manhattan Project produced the first three nuclear weapons. One exploded on July 16, 1945, at the Trinity test site in southern New Mexico. The other two nuclear weapons devastated Hiroshima and Nagasaki, Japan, in August of 1945. The earliest references to disposal pits in MDA-B occur at about this time, but unfortunately, documentation was rather incomplete in those days. The site has been fenced for many years. It is confined to a narrow strip of land between a busy road on the north and the canyon to the south. A small pit existed in the southeastern part of the area as early as 1944. As many as three pits were dug in 1945--one described as 12 feet (3.7 meters) deep, 15 feet (4.6 meters) wide, and 300 feet (91.4 meters) long. It is possible that two parallel pits of this type were dug. There are at least five and perhaps more separate excavations in the area. The purpose of the SAGE geophysical study was to locate the now filled and buried trenches and measure their actual dimensions, since the historical documentation is unreliable.
An aerial photograph taken in January of 1948 shows a single long trench open. The site was closed that same year, following a fire in the disposal pit. A variety of debris was buried in the pit, including boxes of contaminated laboratory equipment such as gloves, glassware, paper, and small metal apparati. Radioactive plutonium, polonium, uranium, americium, curium, lanthanum, and actinium are thought to be present in the pit in very small quantities as well as hazardous chemicals (some of these may have started the 1948 fire). Larger objects were dumped as well: storage batteries, metal ductwork, and a contaminated truck from the Trinity test.
After closure, the site was used as a landfill for nonhazardous dirt and concrete from construction projects. The western two-thirds of the site were paved with asphalt and used as a trailer storage facility for Los Alamos County from 1966 to 1990. In 1982 and 1987, experimental capping procedures were studied in the unpaved eastern third. The cap consisted of 1-2 m of soil, crushed tuff and, in one area, an additional "biobarrier" of cobble and gravel. Various types of plant cover were cultivated and shallow moisture-monitoring holes were installed.
On June 21 and 22, 1997, the SAGE students and faculty conducted field experiments at the MDA-B site. Most of the work was done in the unpaved eastern portion (see figure 2), but some trials were done in the paved area. A complete total field magnetic map was made using a magnetometer. Four seismic refraction lines were shot transverse to the axis of the area and two longitudinally. Thirteen GPR profiles were surveyed at the western end of the unpaved area. Resistivity profiles were collocated with some of the radar profiles, but will not be presented here. All of the students participated in each survey and worked with each method. Groups of students rotated through the activities and reported on each.
| Figure 2 |
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Fig. 2. A map of the eastern, unpaved portion of MDA-B with 1-m topographic contours (white), fence outline (black), seismic refraction spreads, and shot points (red), magnetic/GPR lines 13 to 16 (yellow), and a residual total-field magnetic map with contours shaded in color. Notice the steep drop from the rather flat mesa top into the canyon south of the site. |
A chainlink fence, which interferes with both the magnetic and radar surveys, surrounds the site. Lines were laid out every 2 meters in a northeast-southwest direction and never closer than 5 m to the fence. Ninety-one lines covered the entire unpaved area for a total-field magnetic map, and 13 of the westernmost lines were profiled with the radar. About 350 measurements were recorded along each magnetic line. The data were smoothed, resampled, and gridded at a 1.2-meter interval for the map in the figure 2.
A pulsed radar system operating at 100 MHZ was used. The antenna spacing was 1 m and the soundings were 0.5 meters apart. The reflected airwave from the nearby fence is recorded on all of the profiles. Diffraction events are visible in the profiles in the area of the suspected trench. These events correspond to reflection of the radar waves from the tops of metallic objects. In the dry soils and tuff at this site, the radar signal has a 1.5-meter wavelength (assuming 0.15 meters per nanosecond), which is comparable to the trench cap thickness of 1-3 meters.
The seismic data were digitally recorded with up to 48 geophones at a 0.9-meter spacing. A blow by an "airless jackhammer," which has an impact similar to an ordinary sledgehammer, was used to create seismic waves at 11-meter intervals. Since the Bandelier Tuff is a highly attenuating medium, the signal-to-noise ratio declines significantly at maximum offsets. For this reason six hammer blows were added for each record. The five shot-per-line geometry provides enough redundancy to resolve laterally variable velocity structures, such as the trench.
The magnetic map (figure 2) provides an overview of the trenched areas. The anomalies are typically about 1500 nanoteslas peak-to-trough but occasionally reach 2000 nanoteslas. The anomalies are probably caused by large ferrous objects buried in the soil. The major anomalies are confined to a narrow zone centered about 10 m from the northern fence line. At the eastern end of the area, the anomalies appear to cover a wider zone a few meters farther to the south. This map may indicate two or three distinct excavations.
Simple depth estimation rules indicate that the sources of the magnetic anomalies are buried in the depth range of 2.7-5.1 meters with an average of 4.5 meters. One anomaly was modeled with a 2-meter wide, 2.5-meter long, 1-meter thick prism at a depth of 4.5 meters.
The seismic wave travel times (red symbols in part b of the figure 3) are interpreted by creating a model of the subsurface velocity structure (colored cross-section in part c). The travel times of the model are predicted by computer calculation (blue curves in part b). If the predictions match the observations reasonably well, the model is assumed to be a good approximation of the actual subsurface structure. The thick cap at MDA-B greatly complicates the modeling effort, as the topography of the pre-cap surface must be modeled as well as the trench. The horizontal position of the trench, which is fairly well constrained by the seismic data, corresponds to the location of the major magnetic anomaly patterns. The northern fence is located at distance 0 in the figure so that the magnetic and GPR lines begin within the trench, due to the 5-meter-wide fence exclusion zone. The trench appears to be about 13 meters wide, rather than 4.6 meters as documented. It was not possible to model the seismic data with two parallel trenches. The GPR profile is difficult to interpret. Shallow features are obscured, and the steeply dipping reflected airwave from the fence interferes on both ends of the profile. Reflections associated with the trench can be seen in the 30-70 nanosecond range, which corresponds to depths of 2.5-5 meters.
| Figure 3 |
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Fig. 3. An interpretation of collocated geophysical surveys across the MDA-B trench. a) Residual total field magnetic profiles for Lines 13 through 16. b) Seismic refraction traveltime curves for Spread 5; observed times are red symbols and computed times are blue lines. c) A P-wave velocity model of the trench and capping layers. d) The GPR reflection section for Line 13. |
The magnetic map obtained by the students at the MDA-B site at LANL provided a general guide to regions of particular interest, and the seismic refraction method delineated the trench with considerable precision. Both the GPR and magnetic data delimit the locations of large metallic objects in the trenches. The seismic data are too sparse to determine the three-dimensional structure of the site, but about 30 similar refraction spreads would enable such a model. Individual magnetic anomalies can be effectively modeled and indicate objects located on the trench floor.
NSF, DOE, and industry are continuing to support the SAGE field course for 1998. Another ER site investigation at Los Alamos is planned as well as basin-scale seismic, gravity, and electromagnetic surveys near Santa Fe. SAGE has boosted the careers of most of the 370 students who have experienced the course and will continue to provide a dynamic and exciting experience for future classes. For further information, visit Web site http://geont1.lanl.gov/SAGE/sage.htm.
Source: Eos, July 14, 1998, p. 329.
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