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SPECIAL SECTION: HANFORD SITE

Inorganic Plume Delineation Using Surface High-Resolution Electrical Resistivity at the BC Cribs and Trenches Site, Hanford

hydroGEOPHYSICS, Inc., 2302 N. Forbes Blvd., Tucson, AZ 85745
^* Corresponding author (drucker{at}hgiworld.com).

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received 18 November 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 
A surface resistivity survey was conducted on the Hanford Site over a waste disposal trench that received a large volume of liquid inorganic waste. The objective of the survey was to map the extent of the plume that resulted from the disposal activities approximately 50 yr earlier. The survey included six resistivity transects of at least 200 m, where each transect provided two-dimensional profile information of subsurface electrical properties. The results of the survey indicated that a low resistivity plume resides at a depth of approximately 25 to 44 m below ground surface. The target depth was calibrated with borehole data of pore-water electrical conductivity. Due to the high correlation of the pore-water electrical conductivity to nitrate concentration and the high correlation of measured apparent resistivity to pore-water electrical conductivity, inferences were made that proposed the spatial distribution of the apparent resistivity was due to the distribution of nitrate. Therefore, apparent resistivities were related to nitrate, which was subsequently rendered in three dimensions to show that the nitrate likely did not reach the water table and the bounds of the highest concentrations are directly beneath the collection of waste sites.

Abbreviations: BCCT, BC cribs and trenches • bgs, below ground surface • DIC, depth of investigation characteristics • EC, electrical conductivity • EM, electromagnetic • ERT, electrical resistivity tomography

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 
The Hanford Site, located in eastern Washington State, is the center of one of most extensive cleanup operations in the nation (Gephart and Lundgren, 1998). The cleanup effort is the result of past processes dedicated to the production of weapons material during the Cold War. The processing of irradiated uranium produced large quantities of liquid waste that was disposed or stored in a number of ways, including single- and double-shelled underground storage tanks, cribs, trenches, French drains, reverse wells, and ponds. By one estimate, there was approximately 1 x 10⁶ ML (megaliters) of waste disposed directly to the ground, whereas 200 ML of liquid waste is currently stored in the 177 underground storage tanks (Corbin et al., 2005).

One of the chemical plants, U-Plant, was responsible for the processing of tank waste to extract uranium in a recovery process. The plant operated from 1952 to 1958, and much of the postrecovery waste was diverted to the BC cribs and trenches (BCCT) site, a waste disposal facility south of the 200 East Area in the central portion of the Hanford Site. The waste was generally neutral to basic in pH and contained high amounts of inorganic salts with significant amounts of radionuclides including uranium, plutonium, and fission products (USDOE, 2003). Additionally, the BCCT site received the largest inventory of technetium-99 (⁹⁹Tc) ever disposed to the ground at Hanford, which equated to approximately 410 Ci (Kincaid et al., 2006).

The design concept behind direct disposal of liquid to the unlined trenches and cribs at the BCCT site was referred to as specific retention. It was expected that the soil would retain the radiological constituents through ion exchange and prevent much of the waste from reaching the water table. The cribs were funnel shaped with sloping sides and open bottoms located a few meters below ground surface. The cribs received waste in 42 m³ (11,000 gallon) batches from a collection tank through piping that connected the cribs. Most of the crib plumbing is still in place, as verified by electromagnetic and magnetic surveys (Rucker and Sweeney, 2004). Trenches, on the other hand, were large open ditches approximately 154 m long, a few meters wide, and a few meters deep. An aboveground piping network delivered the liquid waste to the trenches and cribs. Through a separate site clearance geophysical survey with magnetic gradiometry and electromagnetic induction (Rucker and Sweeney, 2004), it appeared that most of the piping had been removed.

Since disposal, several borehole geophysical studies were completed at the BCCT site to ascertain the effectiveness of the waste sites to adhere to the provisions of specific retention (Brodeur et al., 1993; Fecht et al., 1977; Horton and Randall, 2000). The studies were aimed at quantifying the vertical migration of several radiological constituents of the waste, including cesium-137, cobalt-60, antimony-125, and europium-154. The studies showed that no significant vertical migration had occurred for these constituents from 1977 to 1999.

More recently, borehole core samples taken from the center of one of the disposal trenches have confirmed the presence of chlorides, nitrates, sulfates and other salts down to 44 m below ground surface (Serne and Mann, 2004). Based on the borehole results and modeling, it did not appear that inorganic contamination had reached the water table. Lateral migration of the contaminant plume is postulated as a likely explanation for the limited vertical distribution. For lateral migration to occur, alternating layers of coarse and fine grained (or wet and dry) sediments likely direct the contaminated liquid away from the source zone. Ward et al. (2004) described the importance of using saturation-dependent anisotropic properties to accurately model flow and transport at the Hanford Site.

Owing to the expense of borehole core sampling, a surface geophysical method was used at the BCCT site to map the subsurface electrical properties to understand the extent to which the waste has migrated. Electrical resistivity was chosen since the high concentration of nitrate in the vadose zone provided a suitable target for mapping. The resistivity method is also capable of imaging deeper than other electromagnetic (EM) methods at sufficient resolution. Paine (2003) presented an airborne EM study of a chloride plume, but the resolution was lower than typical resistivity surveys. For example, Watson et al. (2005) demonstrated that electrical resistivity could map a nitrate plume below the water table down to depths of approximately 20 m below ground surface using a dipole–dipole array. Adepelumi et al. (2005) and Titov et al. (2005) also used a dipole-dipole array to map shallow plumes beneath waste sites. Benson et al. (1997), Abu-Zeid et al. (2004), and Chambers et al. (2006) used a Wenner array to map the extent of an inorganic and organic (jet fuel) plume down to several meters. In each case, the plume is more highly resolved by virtue of higher data density relative to the Paine (2003) survey.

Although the above examples were successful at mapping shallow plumes, the plumes at the BCCT site are much deeper. Therefore, three objectives are outlined for this study: (i) to present a case study of electrical resistivity measurements at a nuclear waste disposal facility and demonstrate that the vadose zone can be mapped in excess of 50 m below ground surface at sufficient resolution to determine the extent of liquid waste migration, (ii) to demonstrate that apparent resistivity data from parallel lines can be used to qualitatively assess plume extent and depth in the vadose zone, and (iii) to demonstrate that tomographic inversion of electrical resistivity using commercial resistivity inversion codes can give a quantitative assessment of the plume through direct comparison with borehole data. The resistivity mapping is accomplished with modern resistivity equipment and using a less-common array type referred to as the pole–pole array. Like the other array types, the pole–pole array requires the use of four poles to complete a single measurement. However, one pole from each of the current source and potential measurement is placed effectively at infinity. With this arrangement, the pole–pole array configuration provides at least twice as many data measurements as the more common dipole–dipole array and three times that of a Wenner array. Lastly, the mapping technique and correlative methodology could potentially be upscaled to image electrical properties beneath the entire site to understand the large-scale distribution of nitrate beneath each of the trenches and cribs.

	Research Site

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 
Within the BCCT site (Fig. 1 ) are 20 trenches and 6 cribs over an area of approximately 20 ha. Of particular interest is trench 216-B-26 because of the relatively heavy loads of both highly mobile contaminant (nitrate and technetium) and those of low mobility (plutonium, strontium, and cesium), as characterized by their respective partition coefficients (Cantrell et al., 2003). The trench was operated from February to April 1957 and received approximately 5880 m³ of liquid with 9.45 x 10⁵ kg nitrate. The trench also received 4.3 x 10⁵ kg sodium, 3.2 x 10⁴ kg phosphate, 5.6 x 10⁴ kg sulfate, 1.9 x 10⁴ kg fluoride, and 1.3 x 10⁴ kg chloride (Corbin et al., 2005).

View larger version (62K): [in this window] [in a new window]   FIG. 1. Hanford Site base map showing the location of the BC cribs and trenches site.

In February 2004 a characterization borehole, labeled C4191, was drilled in the center of trench 216-B-26. Grab samples of soil (1 kg of soil) from this borehole were collected every 0.76 m from 5.3 to 104 m bgs. A total of 124 samples were taken, and 39 were characterized for various inorganic and radiological constituents. The soil samples were taken to the Pacific Northwest National Laboratory for sampling analyses, which included moisture content, gamma energy, and water dilution tests to extract pore water for pH, soil pore water electrical conductivity (EC), and major anions and cations. Moisture content included a thermogravimetric analysis. The soluble inorganic constituents were extracted with a 1:1 deionized water–to–soil sediment extraction method (Serne et al., 2002). The water extraction method was necessary due to the low volume of antecedent moisture content, which was typically less than 50 mL.

Figure 2 shows the data for a few of these measurements that are pertinent to this study, including lithologic-stratigraphic characterization, water content, EC of the pore water, and concentrations of nitrate, phosphate, and technetium (Serne and Mann, 2004). The data in Fig. 2 show some important elements that were key to initiating a geophysical electrical resistivity survey over the trench. First, the water content within the Hanford formation (upper 60 m) is generally less than 8% by weight with a variance that mimics the general lithology. Below the Hanford formation, and continuing to the water table, the water content decreases to about 3%, with the variability also decreasing. Second, the nitrate and technetium concentrations are very high, between 25 and 44 m bgs, which correlate well with the extremely high EC in the same depth section. There is a direct causal relationship between the nitrate and EC, where the ionic strength of pore water directly influences the EC. Griffin and Jurinak (1973) suggested that a linear relationship exists between ionic strength and EC from soil extracts, and Marion and Babcock (1976) found a better fit to ionic strength with log-transformed values of EC. A site-specific petrophysical relationship demonstrating the effect of moisture content and electrical resistivity (or conductivity) has yet to be developed for the BCCT site.

View larger version (40K): [in this window] [in a new window]   FIG. 2. Data from borehole C4191 showing lithology, water content, nitrate, phosphate, technetium, and pore water electrical conductivity (adapted from Serne and Mann, 2004).

Some ionic species contained in the waste do have significant sorptive capabilities, as shown by the phosphate concentration log in Fig. 2. The phosphate ion is retarded relative to the nitrate, with much of its mass locked in the top 15 m of the Hanford formation. Although not shown, the uranium-238 correlates well with the phosphate, demonstrating that this pair may also have similar fate and transport characteristics to one another. The EC log shows a slight increase in the near surface that may correlate with the increased phosphate concentration. However, the influence of nitrate on the electrical properties of the pore water is about seven times that of phosphate. Correlations of EC from borehole C4191 with key analytes are shown in Table 1.

	Theory

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 
Resistivity () is a volumetric property that describes the resistance of electrical current flow within a medium. Direct electrical current is propagated in rocks and minerals by electrolytic means. Electronic conduction only occurs in metallic-luster sulfide minerals, where free electrons are available. Rocks and nonmetallic minerals have extremely high resistivities (low conductivities), and direct current transmission through this material is difficult. Porous media, on the other hand, carry current through ions by way of electrolytic conduction. Electrolytic conduction relies on the dissociated ionic species within a pore space. Here, the conduction varies with the mobility, concentration, and degree of dissociation. Electrolytic conduction is relatively slow with respect to electronic conduction due to mass transfer rate-limiting processes and is strongly influenced by the structure of the medium.

Estimating resistivity is not a direct process. When current (I) is applied and voltage (V) measured, Ohms law is assumed and resistance is measured. Resistivity and resistance (R) are then related through a geometric factor over which the measurement is made. The simplest example is a solid cylinder with a cross-sectional area of A and length of L:

[1]

In such cases where the actual volume involved in the measurement is known, the result is called the "true" resistivity and is considered to be a physical property of that material. However, field measurements involve an unknown volume of earth. Consequently, resistivity calculations are based on the hypothetical response for the given electrode geometry over a homogeneous, isotropic, half-space. This results is what is termed apparent resistivity but which is more accurately called a half-space resistivity.

Field data are generally acquired using an established electrode array. A four-electrode array employs electric current injected into the earth through one pair of electrodes (transmitting dipole) and the resultant voltage potential is measured by the other pair (receiving dipole). The ratio of the transmitted current and observed potential is called the transfer resistance. Some common electrode configurations are dipole–dipole, Wenner, and Schlumberger arrays. Their use depends on site conditions and the information desired. Figure 3A , adapted from Telford et al. (1990), shows a schematic of the dipole–dipole configuration, where C1 and C2 are connected to the current source (i.e., transmitting electrodes) and P1 and P2 are connected to the voltmeter (receiving electrodes). For the four-electrode array, the geometric factor, K, is

[2]

where r₁ through r₄ are the distances between the electrodes defined in Fig. 3A. Equations [1] and [2] are used to estimate an apparent resistivity, which assumes that each measurement of transfer resistance was a result of point electrodes on the surface of a homogeneous, isotropic, half-space:

[3]

where the subscript "a" denotes the apparent resistivity. The apparent resistivity is not necessarily the true resistivity of the formation but a simplified resistivity that provides a starting point for subsurface evaluation. Other assumptions used in Eq. [3] are isotropy (i.e., no directional dependence of resistivity), no displacement currents (using a DC or low frequency current application), and that resistivity is constant throughout such that Laplace's equation can be assumed. Since the degree of heterogeneity is not known a priori, a true resistivity is not calculated from Eq. [3]. To obtain a true resistivity, electrical resistivity tomography (ERT) is required, which generates a model of true resistivity using an iterative inverse methodology given the measurements of apparent resistivity, electrode arrangement, and other boundary conditions. Discussions of ERT and the methods by which the true resistivity is calculated can be found in several sources, including Loke and Barker (1996), LaBrecque et al. (1996), and Oldenburg and Li (1999).

View larger version (12K): [in this window] [in a new window]   FIG. 3. Set-up of the resistivity four-pole array and pseudosection plotting methodology.

The calculation of apparent resistivity is simplified in the pole–pole array:

[4]

where a is the basic electrode spacing and n is the integer multiplier as the current and potential electrodes incrementally separate. Figure 3B demonstrates a linear transect of electrodes on the surface with the a spacing being the separation between each electrode and the n spacing increasing as the potential electrode moves away from the current electrode. The geophysical survey at the BCCT site included a fixed a spacing of 3 m, and n increased from 1 to 27. For a complete survey, each electrode has one turn at transmission, while potential measurements occur at all other electrodes in the array. Automated resistivity meters, such as the SuperSting R8 (Advanced Geosciences, Inc., Austin, TX), has the ability to conduct multichannel sweeps of potential measurements to significantly decrease measurement time.

Target Discrimination with Apparent Resistivity
The linear transect arrangement of electrodes produces a two-dimensional dataset of apparent resistivity as a function of x and z, where z is the dimension into the earth and x is along the surface. Although apparent resistivity is a function of the volume over which the measurement is made, its location is typically plotted as a point for ease of representation. The location of the point is a function of n and is loosely related to the depth of investigation. Hallof (1957) demonstrated that the intersection of two 45° lines extending downward from each of the current and voltage potential electrodes would produce a suitable pseudosection for interpretation. Others have used similar techniques to plot, for example, the depth to the maximum sensitivity in the electrode separation (e.g., Roy and Apparao, 1971). Using the Hallof approach, the pole–pole array has data plotted at a pseudo depth of

[5]

which is a linear plotting method.

Figure 4 shows an apparent resistivity demonstration of several array types, including the pole–pole array, with a resistive half-space earth (400 ·m) and a graded conductive target (20 ·m). The target dimensions are 21 x 10 m, and the top of the target is located at 10 m bgs. The target was modeled with a forward resistivity model in EarthImager2D software (Advanced Geosciences, Inc. Austin, TX) using the basic algorithm of Dey and Morrison (1979). Many electrical resistivity modeling codes use some elements of this algorithm, including RES2DINV (Loke and Barker, 1996) and DCIP2D (Li and Oldenburg, 1994).

View larger version (27K): [in this window] [in a new window]   FIG. 4. Apparent resistivity pseudosection comparisons for the Schlumberger, pole–dipole, dipole–dipole, and pole–pole array types for a discrete conductive target in a resistive homogeneous background.

Another view of the apparent resistivity data is shown in Fig. 5 , where vertical slices of data have been extracted at 81 m (center of the domain) and at 65 m. Figure 5A shows these slices as a function of the pseudo-depth for all but the pole–dipole array. In general, the pole-pole and dipole-dipole array show a decrease in resistivity at 81m (solid lines) that is loosely coincident with the target depth, while the Schlumberger array does not resemble the character of the target at all. Off-center at 65m (dashed lines), where the actual resistivity is a resistive homogeneous body, the pseudosection of the pole-pole shows less of an effect than the dipole-dipole. The Schlumberger array resembles the actual background better at the 65m slice.

View larger version (15K): [in this window] [in a new window]   FIG. 5. (A) Vertical slices through apparent resistivity data for Schlumberger, dipole–dipole, and pole–pole at 65 m (beside target) and 81 m (within target). (B) Schematic of linear and nonlinear pseudosection plotting.

The traditional linear pseudosection of Hallof (1957) has limitations with respect to a physical meaning of the earth. Many researchers, therefore, have taken a closer examination of the plotting method to allow for a more reasonable geological interpretation. The most widely accepted depth of investigation studies are those presented by Roy and Apparao (1971), Roy (1972), and Koefoed (1972), who defined a depth of investigation characteristics (DIC) model for determining the depth of a measurement. The DIC was determined by finding the depth at which a thin horizontal layer within a homogeneous background makes the maximum contribution to the total measured signal at the surface. The results were consistent in that the depth of investigation is a nearly logarithmic function of electrode spacing, regardless of how the depth of investigation is defined. This suggests a modification of the linear pseudosection (Edwards, 1977; Fink 1980). Figure 5B shows an example of a nonlinear pseudosection, based on a logarithmically based depth interpretation based on the electrode separation.

To facilitate the nonlinear depth plotting of apparent resistivity data, the logarithm of the n-spacing value was used in a second–order polynomial:

[6]

where z_log is the new interpreted depth location of the apparent resistivity value, and u₁...u₃ are coefficients to be determined by using collocated target resistivity values. For this analysis, we assume that target data comes from a borehole, and therefore, only the data from the vertical slice at 81 m, that is, the same data from Fig. 5A, are used in finding coefficients u₁...u₃. The consequences of using an equation like Eq. [6] are shown in Fig. 5B, where the resistivity values near the surface are pushed deeper relative to the linear pseudosection and the deeper resistivity is pulled up relative to the linear pseudosection. At one point, the two plotting strategies have the same depth location for a given electrode separation.

The coefficients in Eq. [6] can be determined using a nonlinear least-squares optimization procedure. To reduce the number of parameters, we equated u₁ and u_3; the results of the transformation are shown in Fig. 6A (with coefficients defined). Two other examples are shown in Fig. 6B and 6C, where the target depth has moved down incrementally 10 m. In each case, the parameters for the depth plotting of the nonlinear pseudosection have changed. Additionally, the minimum value of resistivity has increased as the target is placed deeper in the subsurface.

View larger version (17K): [in this window] [in a new window]   FIG. 6. Vertical slices for comparison of interpretation algorithms including pseudosection apparent resistivity and inverted true resistivity for a discrete conductive target at (A) 10 m below ground surface, (B) 20 m below ground surface, and (C) 30 m below ground surface; u1...u3 are coefficients determined by using collocated target resistivity values.

For the simple target problem identified in Fig. 4, the measured apparent resistivity data were inverted using EarthImager2D. Both robust and smooth models were evaluated, with the results of the inversion shown in Fig. 7 . The final goodness-of-fit statistic, as measured by the root mean square error, was 0.91 and 0.67 for the robust and smooth inversion results, respectively. For comparison, the linear and nonlinear pseudosection data are plotted to the left of the inversion results. All of the contoured resistivity and apparent resistivity data show a target in the general location of the actual target location. Additionally, all methods appear to smear the information laterally or vertically, referring to a smooth condition where boundaries may not be as well defined. For the apparent resistivity plots, the lateral boundaries of the interpreted target are smeared by pantleg effects. For the inverted resistivity plots, the vertical information below the target is smeared.

View larger version (29K): [in this window] [in a new window]   FIG. 7. Contours of resistivity for the different interpretation algorithms including linear pseudosection, nonlinear pseudosection, robust inversion (L1 normalization), and smooth inversion (L2 normalization).

	Geophysical Methodology and Results for Field Data

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 
During summer 2004, a pilot geophysical electrical resistivity survey was conducted at the BCCT site over trench 216-B-26. The survey included six two-dimensional lines; five of the lines were run in the general direction of the trench, and one was conducted perpendicular to the trench. Four of the five parallel lines were approximately 214 m long, with the fifth 316 m long. The offset between the parallel lines was 8 m. The single line run perpendicular to the trench was located approximately midway along the parallel transects.

Figure 8 shows the geophysical survey layout at the BCCT site. The electrical resistivity data were collected with the Sting R8 (Advanced Geosciences, Inc., Austin, TX) using the SuperSting Smart electrode cables with 72 take-outs. The cables allow for easy roll-along operation to quickly collect data, with each roll-along consisting of 12 take-outs for the survey. The basic a spacing was 2 m for lines 1 to 5 and 3 m for line 6.

View larger version (28K): [in this window] [in a new window]   FIG. 8. Geophysical survey map of BC cribs and trenches site showing transect locations relative to the trenches.

View larger version (64K): [in this window] [in a new window]   FIG. 9. Apparent resistivity plotted with nonlinear pseudosection collected over the waste sites at the BC cribs and trenches site.

View larger version (57K): [in this window] [in a new window]   FIG. 10. Inverted resistivity of the six lines presented in Fig. 9 using smooth inversion.

View larger version (11K): [in this window] [in a new window]   FIG. 11. (A) Apparent resistivity from Fig. 9 versus nitrate concentration from borehole C4191 for colocated data along line 1. (B) Inverted resistivity from Fig. 10 versus nitrate concentration.

[7]

where a and b are the slope and intercept for the regression data. The parameters for a and b are 505.2 and 1.207, respectively. The transformation equation and fitting parameters were used to convert the apparent resistivity data of lines 1 to 5 to nitrate concentration in milligrams per liter. The transformation revealed a minimum concentration of 187.2 mg L^–1 and a maximum concentration of 203,844 mg L^–1. The maximum concentration occurred in line 5, 80 m along the line at a depth of 31 m bgs.

The three-dimensional nitrate data set was rendered into a plume to show the spatial distribution of the data relative to the trenches. The rendering was possible due to the connectivity of the low resistivity values between transects, demonstrated in line 6. An inverse distance interpolation algorithm was used to interpolate the data to a regularized grid with cell dimensions of 2m x 2m x 2m. Figure 12 shows the results of rendering, where two nitrate values were chosen to demonstrate the transformation. The smaller plume represents the larger concentration of 75,000 mg L^–1, and the larger plume represents 35,000 mg L^–1. The figure also shows the relative locations of the trenches, wells, and resistivity transects, as well as the domain boundary of the rendered plume. Within this domain, the volume of soil that contains a nitrate concentration of 35,000 mg L^–1 or more is 49,000 m³, and the volume of soil containing 75,000 mg L^–1 or more is 12,670 m³. Outside the domain, no information is given and it is likely the low resistivity plume extends farther north and south based on the positions of the other adjacent waste sites.

View larger version (35K): [in this window] [in a new window]   FIG. 12. Three-dimensional rendering of the inferred nitrate plume beneath trench 216-B-26.

[8]

where the summation occurs over all cells, V_cell is the volume of the cell, C_NO3 is the concentration of nitrate, and is the change in volumetric water content between the time before disposal and at the time of the resistivity measurement. The main assumption for the mass calculations, which is validated by the sorptivity data of nitrate, is that the nitrate resides primarily in the pore water. The mass balance could be used to help confirm or refute the use of apparent resistivity in the direct conversion to concentration values. Alternatively, the inversion code could be accommodated to use the mass balance as a constraint.

	Conclusions

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 
A surface resistivity survey was conducted on the Hanford Site over a waste disposal trench that received a large volume of liquid inorganic waste. The objective of the survey was to map the extent of the plume that resulted from the disposal activities approximately 50 yr earlier. A single confirmational borehole showed that the plume resided 25 to 44 m bgs and that the electrical conductivity of the pore water provided a potentially good target for surface geophysical measurements. The survey included five lines of resistivity of at least 200m in length and were oriented roughly parallel to the trench. A sixth line was placed perpendicular to the five parallel lines and ran over several adjacent trenches.

The pole–pole resistivity array was used based on its ability to collect a large sampling data set, as well as on its ability to image deeper than the other arrays, such as the dipole–dipole or Wenner array. The resistivity data were presented as the apparent resistivity, which were calculated by an algorithm that assumes each measurement was collected within a homogeneous earth. This presentation style is referred to as a pseudosection, and is often the starting model for many ERT inversion algorithms. The advantage of the apparent resistivity pseudosection is that it displays actual measured data as opposed to an interpretation of the data obtained with ERT. Furthermore, the pseudosection of the pole–pole array provides the weakest edge effects, thereby facilitating the direct interpretation of resistivity data more reliably (Robain et al., 1999).

The disadvantage of the pseudosection interpretation methodology is that the depth location of the resistivity data point often lacks physical meaning. Hallof (1957) showed that a linear plotting algorithm was sufficient for the interpretation. Others have shown that the pseudosection can be more meaningful by calculating the depth of maximum sensitivity. Within our analysis, we showed that the pseudosection can be plotted meaningfully if direct sampling information is incorporated into the target depth estimation. For the BCCT site, a logarithmic function was used, and the fitting parameters for estimating depth were obtained through a least-squares methodology using the known borehole data. The results of this plotting methodology indicated that a low resistivity plume resides directly beneath all of the waste trenches that were imaged, but the edges of the plume are likely contained within the outer bounds of the footprint defining the trenches.

The relations developed to show the EC highly correlated to nitrate and the apparent resistivity highly (and indirectly) correlated to EC provide a mechanism by which an inference can be made to correlate resistivity to nitrate. This correlation was used to develop a transformation function to convert the resistivity to nitrate and render a three-dimensional depiction of the nitrate plume directly beneath trench 216-B-26. The major assumptions used for the transformation were that the resistivity anomaly is primarily caused by nitrate and that the nitrate data from the borehole sample is representative within the measurement area. One could take the step to estimate, using the same procedure, the spatial distribution of ⁹⁹Tc, given the same high correlations to EC. Lastly, if resistivity measurements could be conducted over the entire BCCT site, perhaps the fate of the nitrate could be better understood and the risk minimized to populations downgradient of the site. To accurately map the entirety of the BCCT site, however, it is recommended that more borehole samples be gathered to ensure high correlations to the analyte of interest.

	ACKNOWLEDGMENTS

 
Special thanks go to Mark Benecke of Fluor Hanford, Inc., and Mark Sweeney of Pacific Northwest National Laboratory. The work was performed for Fluor Hanford Group and the U.S. Department of Energy under Contract DE-AC05-76RL01830.

	REFERENCES

TOP ABSTRACT INTRODUCTION Research Site Theory Geophysical Methodology and... Conclusions REFERENCES

 

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