ES&T
FEATURE Assessing hazardous waste problems Geophysical techniques are becoming more useful for locating hazardous wastes and estimating groundwater contamination
Roy B. Evans a Glenn E. Schweitzer Environmental Monitoring Systems Laboratory U. S. Environmental Protection Agency Las Vegas, Nev. 89114-5027
Groundwater contamination has become one of this nation's most pressing environmental problems. It stems from improperly buried wastes, poor effluent disposal practices, improper use of ag-
"Present affiliation: University of Nevada at Las Vegas, Las Vegas, Nev. 89154 330A
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ricultural chemicals, and accidental or careless spillages. The growing concern over groundwater contamination has led to the current emphasis on the safe disposal of hazardous wastes and on the cleanup of improperly buried wastes. To accomplish these objectives, it is necessary to locate the buried wastes, determine the hydrological features that influence leachate migration from a waste site, and identify contaminant plume characteristics. Reliable and inexpensive methods for delineating subsurface features near hazardous waste sites are therefore needed. Investigation of hazardous waste sites has emphasized the use of drilling to obtain information about the geologic setting; monitoring wells to obtain
samples of groundwater; and laboratory analyses of water, soil, and waste samples. During the past 10 years, the extensive development of remote-sensing geophysical equipment, including field methods, analytical techniques, and associated computer processing, has greatly improved the ability to characterize hazardous waste sites. Essentially, geophysical methods can be used to detect buried materials and contaminant plumes. They are also used to determine groundwater flow characteristics, to find contaminants in the unsaturated (vadose) zone above the water table, and to provide detailed information about subsurface geology. This ability to characterize the subsurface rapidly without disturbing a site This article not subject to U.S. copyright Published 1984 American Chemical Society
often can result in better site assessment at less cost and risk. The information normally required for determining hazardous waste remedial action includes the location and volume of buried waste and the distribution and level of subsurface contaminants. An important aim of geophysical sensing is to assist in acquiring such information more rapidly and at less cost. Characterizing the site spatially by geophysical means permits the efficient location of monitoring wells and the reduction of risks involved in exploratory drilling. These geophysical methods, which were originally developed for the defense, mining, and petroleum industries, also can be of help in estimating conditions between monitoring wells.
Geophysical techniques There are several geophysical techniques currently being used in the evaluation of hazardous waste sites and groundwater contamination: • Electromagnetic and resistivity methods define groundwater contaminant plumes. • Resistivity and seismic techniques determine local stratigraphy. • Metal detectors and magnetometers locate drums and other buried metal. • Ground-penetrating radar defines the boundaries of buried trenches and other subsurface installations.
Subsurface investigation The use of seismic refraction and resistivity to investigate a subsurface geologic structure is illustrated in the following example: A waste disposal facility was proposed for a site where local geology indicated that the underlying rock was limestone overlain by layers of quartz sand and clay. The limestone was an important aquifer and was assumed to contain solution channels that could create conduits for leachates from the proposed landfill. Regional geology suggested that the limestone was over-
lain by a massive clay layer of sufficient thickness to provide an effective barrier to landfill leachates that would otherwise contaminate the aquifer. Before construction, data on the lateral extent, continuity, and thickness of the clay layer at the site were needed to ensure that the clay would adequately protect the aquifer. Borings through the clay layer were not appropriate because of the potential for improper sealing, which could have created pathways for the migration of contaminants. Thus,
seismic refraction and resistivity were used to produce a faster, more reliable survey at lower cost and risk than could have been achieved with monitoring wells alone. The approach called for 10 seismic refraction lines over the 25-acre site. The data indicated a three-layer system, tentatively identified as sand, sandy clay, and massive clay about twice as thick as the combined thickness of the sand and sandy clay layers. The seismic instrument's hammer was not large enough or powerful enough to detect the top of the limestone. However, calculations provided the minimum depth to the top of the limestone, and a minimum thickness for the overlying clay layer could be ascertained. Five resistivity sounding stations overlapped the seismic stations. Their data indicated a four-layer system. The first three layers were the sand, sandy clay, and massive clay layers identified by the seismic method. The fourth was identified as limestone. The depth to the top of the limestone was calculated to be 12 m; the thickness of the massive clay layer was found to be 8 m. These results are summarized in Figure 1. The geophysical survey results were confirmed through five shallow auger borings, which also provided samples for subsequent mineral and permeability analyses in the laboratory. The samples revealed seven layers of sediment
FIGURE 1
Comparison of data obtained by various geophysical methods"b
a
By resistivity, seismic refraction, and auger methods interpretation of these data yields a generalized geologic section
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FIGURE 2
EM data showing spatial extent and magnitude of conductivity anomaly3'"
a
Three-dimensional perspective view "Six borings have missed the burial site
FIGURE 3
Contour plot of EM conductivity anomaly showing extent of buried contaminants 3
a
Anomaly shown in Figure 2
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varying from dry sand to clayey sand, sandy clay, and massive plastic clay. Major soil changes found through the borings correlated well with the seismic and resistivity results. The top of the massive clay layer was determined by the seismic method to be at a depth of 7.5 m. This was within 0.33 m of the depth found through the auger borings. By means of resistivity, clay material was identified at a depth of 3 m, with massive clay at 7.5 m. The homogeneity and flatness of the subsurface strata permitted excellent correlation between the seismic, resistivity, and auger boring data. Characterization of buried wastes At another site ground-penetrating radar, electromagnetic induction, and magnetic surveys were used to locate and estimate the volume of buried wastes. An abandoned disposal site was thought to be located in an open, grassy area, 300 m wide by 400 m long, which had been used as a playground. Stream sediment analysis had shown high concentrations of organic compounds near the park, and a subsurface investigation using six monitoring wells had revealed trace levels of contaminants at the site. However, the actual locations of the burial sites and the type and extent of contamination were unknown. Therefore, geophysical techniques were used to locate and map any buried disposal areas, to provide an estimate of the depth and volume of their contaminants, and to locate better positions for the six monitoring wells with respect to the site. Parallel survey lines for electromagnetic and radar measurements were established at an interval of 15 m. Data from both types of measurements revealed the boundaries of the disposal area. Radar was unable to penetrate to the base of the contaminants because of the conductive nature of the contaminant material, but it did indicate that the top of the waste was within 1 m of the surface. Electromagnetic induction (EM) soundings showed that the maximum depth of the buried material was probably not more than 5 m. The EM data provided an estimate of the quantity of buried wastes. The burial area was then surveyed with a magnetometer to check for the presence of steel drums. The high-sensitivity magnetometer was ineffective because of the extremely high variation in magnetic susceptibility of the soil or waste material. Because the objective of the magnetometer survey was to locate steel drums buried at shallow depths, a fluxgate magnetic gradiometer was used at reduced sensitivity to minimize the effects of the wide varia-
tions in the magnetic response of the soil or waste material. Only two distinctive magnetometer anomalies were found that might have indicated the presence of steel drums. The magnetometer survey showed, rather, that the buried materials consisted primarily of nonmetallic wastes with possibly a few scattered drums. Radar, EM, and magnetic surveys were completed on three additional days and revealed the boundaries of the burial zone. Detailed maps were produced in less than one week. Figure 2 shows the extent and magnitude of subsurface conductivity. The contour map (Figure 3) provides an accurate means of locating contaminant boundaries. Figure 2 shows that the six monitoring wells were not positioned in the best locations. If the conductivity information depicted in Figure 2 had been available before the wells were drilled, the project hydrogeologist would have been able to choose better sites for the wells. Leachate plume delineation Resistivity surveys were conducted to map a landfill leachate plume. Groundwater monitoring had identified contamination at a 30-year-old landfill situated over an unconfined limestone aquifer with a shallow water table. It was located near a well field that provided drinking water for a large city. Chemical analyses of groundwater samples showed that the contaminant plume contained several inorganic and organic compounds. The plume was electrically conductive because of the presence of inorganic ions—principally sodium, potassium, magnesium, and chloride. Resistivity soundings, used to estimate conductivity as a function of depth, were obtained by varying electrode spacings and plotting the observed resistivities against the corresponding electrode spacings. Initially, several resistivity soundings were made in the area of the landfill to determine the approximate depth of the leachate plume within the aquifer. Electrode spacings of 5 m and 15 m were selected to map lateral changes in resistivity around the landfill. The 5-m spacing was chosen to map contamination at relatively shallow depths; the 15-m spacing was selected to measure conductive contaminants within the core of the plume. Because the area had been developed, it was difficult to run long profile lines, so it became necessary to place resistivity stations where space was available. The plume was found to lie between the land surface and a depth of 20 m. To define the plume, the data for each station were plotted and con-
Site assessment. A geophysical field test for the presence of hazardous wastes.
FIGURE 4
Map of leachate plume 3 " c
"Mapped by resistivity methods "Shows change in plume over a four-year period c Shaded area represents 200 ohm-ft contour
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FIGURE 5
3
Map showing EM conductivity data at a hazardous waste site '
a b c
c
lsopleth map Shaded area indicates plume leaving the site Note considerable variation in surrounding conditions
FIGURE 6
Three-dimensional perspective view of EM data shown in Figure 5"
a
Note plume in center of plot and variability in conductivity caused by natural geohydrologic conditions
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toured on a map (Figure 4a). A few years after the initial resistivity survey was completed, additional monitoring wells were installed closer to the landfill, at a distance of about 1.5 miles downgradient and in the direction of groundwater flow. After this well field had been pumped intermittently for about two years, increasing levels of ammonia were detected. A newly installed early-warning monitoring well, however, had failed to indicate this contamination. Before the siting of additional wells, a second resistivity survey was carried out—four years after the first one— with the same equipment and at the same station locations and time of year as were used in the previous study. However, the new survey was extended beyond that of the earlier one in order to cover more area downgradient. The second survey provided the following information (Figure 4b): • The plume had shifted direction and migrated to the northeast in response to a change in the local groundwater gradient. This was caused by a newly active auxiliary well field. The plume had extended into the new well field's cone of influence, thus bringing about the observed excesses in ammonia. • The early-warning well was inadvertently located in a small zone of lower permeability, characterized by fine sand and clay, rather than in the limestone that dominated the area. • Comparison of the two sets of geophysical data revealed that the lateral extent of the plume had increased by approximately 1 km, which corresponds to a migration rate of about 0.5 m/d, roughly one-half the velocity of the regional groundwater flow. • The areal extent and vertical extent of the plume were determined by resistivity measurements, which permitted a calculation of the total volume of aquifer contamination. The survey also identified a number of other sources that contributed to the contamination of the aquifer. The resistivity survey allowed a more accurate understanding of the site's conditions by providing a map of the leachate plume, a measure of the plume's velocity of migration, an indication of the factors influencing the plume's path, and an evaluation of previous water quality data. In addition, new monitoring wells could be installed with a high degree of confidence that their locations were better suited to prevailing groundwater conditions. Locating monitoring wells Electromagnetic induction aided in the choice of locations of monitoring
wells that were used to map another contaminant plume. The wells were needed for a groundwater investigation that would determine the direction and extent of the migration of contaminants originating from an uncontrolled hazardous waste site. Geologic logs revealed a highly variable setting composed of sand and gravel lenses with a clay matrix. The occurrence of these more permeable sand and gravel deposits could influence significantly the path of contaminant movement and the placement of monitoring wells. A detailed EM survey was conducted to map contaminant migration, to determine the locations of monitoring wells, and to guide soil sampling. The field data consisted of about 30 parallel profile lines that were 1000 m long and spaced 30 m apart around the perimeter of the site. In less than one week, two EM systems collected high-density data to 6-m and 16-m depths. The data were digitized and plotted both in contour (Figure 5) and in threedimensional (Figure 6) formats. The contoured data were used to position the monitoring wells accurately. The three-dimensional view aided in the assessment of overall contamination. The EM data showed a high degree of natural variability at the site and helped to identify clearly the existence and extent of two plumes. Available geological information, along with data from well-placed borings, revealed that these natural conductivity fluctuations were associated with river-deposited lenses of sand, gravel, and clay. The two sets of EM data (6-m and 16-m depths) revealed that most of the variations in the sand and clay deposits lay within 6 m of the surface. Below that depth, conditions were much more homogeneous. Water flow and contaminant migration may be expected to follow the most permeable routes—usually paths with low clay content, corresponding to low conductivities—in this shallow, unconfined system. Therefore, high levels of contaminants might be anticipated in these more permeable zones, and the EM low readings could be used to place monitoring wells beyond the limits of the conductive plume. Four wells were positioned within the identified plume. For background determination, a fifth well was placed upgradient in a low-conductivity sand lens. Augering confirmed the deposits of sand and gravel, as suggested by the EM-derived maps and geologic literature. The plume boundary delineated by the EM method represented the extent of transport of the conservative ionic parameters. These are chemical constit-
uents that are not easily sorbed by geologic sediments and that do not undergo extensive chemical or bacterial decay. Quantitative data obtained from the wells indicated that the boundary of the conductive plume appeared to coincide approximately with the 1-ppm contour of the priority pollutants. It should be emphasized that the EM method only senses conductive contaminants; it does not measure pollutant concentrations. The situations described above demonstrate some of the geophysical method applications in the assessment of hazardous waste sites and groundwater contaminant plumes. In such applications, geophysical surveys can save tens of thousands of dollars that otherwise would be spent on exploratory drilling. However, such surveys have their limitations, so available information about site geology should be reviewed carefully before geophysical measurements begin. Expanded applications To date, most geophysical methods for plume detection have been applied to inorganic contaminants, such as chlorides and sulfates, in groundwater. These compounds have high electrical conductivities that permit relatively easy detection. Now, considerable attention is being directed to techniques for detecting organics. There also is great interest in finding the contami-
nants, when they are in the unsaturated zone, before they reach the water table. During the past two years, EPA's Environmental Monitoring Systems Laboratory in Las Vegas, Ne v., has been conducting field investigations of the capabilities of different geophysical techniques. In addition to the techniques discussed above, attention has been devoted to the relative cost and ability of complex resistivity (CR) for detecting inorganic and organic plumes. The feasibility of subsurface organic vapor sampling as a technique to complement electrical geophysical methods and groundwater sampling is also being studied. Initial investigations have been made at a field site where a shallow aquifer was contaminated by effluents and spillages from an industrial site about two miles upgradient. The contaminant plume contains benzene from an accidental release that occurred several years ago, as well as high concentrations of inorganic salts originating from leaking holding ponds. Geophysical methods have been applied along transects across the plume where the water table is at a depth of 10-20 ft and the top of the clay confining layer (aquiclude) is 13-75 ft below the surface. Methods used at the site included EM, CR, conventional resistivity, and measurements of organic vapor in soil. Soil and water were sampled at 23
FIGURE 7
Comparison of results from several geophysical techniques3
a
Shows concentrations of total dissolved solids and total organic carbon (TOC) in groundwater
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wells drilled into the aquiclude along one transect. The soil was analyzed for salinity and grain size. Laboratory CR measurements were made on some soil samples. The wate·· was analyzed for pH, temperature, specific conductance, total and purgeable organic carbon, total dissolved solids, benzene, chlorobenzene, certain inorganic ions (CI", N O 3 - , S 0 4 2 " , H C O 3 - , Ca 2 + , K + , Mg 2 + , and Na + ), and Si0 2 . The EM clearly showed a conductivity peak, apparently caused by the combination of salts deposited in the vadose zone and conductive substances in the groundwater. Conventional resistivity measurements confirmed that same conductive peak; this method was slower because electrodes had to be placed in soil that had been saturated
with water to reduce contact resistance. Apparent conductivity and phase data from CR also showed this peak. The phase angle decreased with increasing total organic carbon (TOC) concentration in the groundwater. The phase correlated positively with frequency over the contaminated portion of the transect, decreasing to zero at about 0.125 Hz. It changed little with frequency over the uncontaminated part of the transect. The relationship between phase and TOC may be coincidence, however, because the TOC plume was identical to the conductive plume. In the future, at a different site, these methods will be tested on an organic plume containing no conductive inorganics. Organic vapor concentrations were
How well do they work? The usefulness of geophysical techniques depends upon several factors: • the depths of target contaminant plumes and objects; • the contrast of contaminant plume conductivity with the background conductivity of nearby uncontaminated groundwater; • the intrinsic conductivity of the soil matrix; • the presence of overlying or underlying highly conductive zones, such as saline aquifers and clay lenses; and • the presence of cultural interferences, such as power lines, buried cables, sewers, and water mains. Table 1 lists ranges of important variables for six geophysical methods. On the basis of such information, it may be possible to estimate the likelihood of success when using these methods to locate buried containers or to map contaminant plumes. With a knowledge of subsurface stratigraphy, it may be possible to select measurement parameters for a particular geophysical technique, such as electrode or coil spacings. Choosing the best parameter enhances the ability of a given technique to locate the target while avoiding the influence of adjacent subsurface features and cultural interferences. TABLE 1
Typical ranges of geophysical variables Method
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Output variables
Unit
Typical range
1-10" 0-100
Resistivity Sounding depth
ohm-m meters
Ground-penetrating radar
Depth of reflectors
meters
Electromagnetic induction
Conductivity Penetration depth
millimho/m meters
0.10-1000 0.75-60
Metal detection
Penetration depth
meters
0-3
Seismic refraction
Depth to refracting interface
meters
1-30
Magnetometry
Magnetic field intensity gradient Magnetic field intensity anomaly Ferromagnetic target depth (single drum)
gamma/rn
0-300
gamma
0-3000
meters
0-5
Resistivity
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