A Practical Method to Evaluate Ground Water Contaminant Plume Stability

Transcription

1 A Practical Method to Evaluate Ground Water Contaminant Plume Stability by Joseph A. Ricker Abstract Evaluating plume stability is important for the evaluation of natural attenuation of dissolved chemicals in ground water. When characterizing ground water contaminant plumes, there are numerous methods for evaluating concentration data. Typically, the data are tabulated and ground water concentrations presented on a site figure. Contaminant concentration isopleth maps are typically developed to evaluate temporal changes in the plume boundaries, and plume stability is often assessed by conducting trend analyses for individual monitoring wells. However, it is becoming more important to understand and effectively communicate the nature of the entire plume in terms of its stability (i.e., is the plume growing, shrinking, or stable?). This article presents a method for evaluating plume stability using innovative techniques to calculate and assess historical trends in various plume characteristics, including area, average concentration, contaminant mass, and center of mass. Contaminant distribution isopleths are developed for several sampling events, and the characteristics mentioned previously are calculated for each event using numerical methods and engineering principles. A statistical trend analysis is then performed on the calculated values to assess the plume stability. The methodology presented here has been used at various contaminated sites to effectively evaluate the stability of contaminant plumes comprising tetrachloroethene, carbon tetrachloride, pentachlorophenol, creosote, naphthalene, benzene, and chlordane. Although other methods for assessing contaminant plume stability exist, this method has been shown to be efficient, reliable, and applicable to any site with an established monitoring well network and multiple years of analytical data. Introduction Evaluating plume stability is important for the evaluation of natural attenuation of dissolved chemicals in ground water. U.S. EPA (1998) states that the primary line of evidence in evaluating natural attenuation is historical ground water chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points. When characterizing ground water contaminant plumes, there are numerous methods for evaluating concentration data. Wiedemeier et al. (2) discussed common approaches for evaluating plume stability using both graphical and statistical techniques. Graphical methods include the following: (1) the preparation of contaminant concentration isopleth maps; (2) plotting concentration data vs. time for individual monitoring wells; and (3) plotting concentration data vs. distance downgradient for several monitoring wells. Common statistical methods for evaluation of Copyright ª 28 The Author(s) Journal compilation ª 28 National Ground Water Association. temporal and spatial trends include regression analysis (U.S. EPA 26), the Mann-Whitney U-test (Mann and Whitney 1947), and the Mann-Kendall test (U.S. EPA 26; Gilbert 1987). Graphical plume stability analysis by comparing isopleth maps over time can provide compelling visual evidence for natural attenuation. However, a comparison of apparent plume size over time does not always provide a complete analysis. Consider, for example, the case of a plume that discharges to a surface water body, or a plume geometry that is persistent over time. In this case, the plume area would remain relatively unchanged, whereas the overall plume average concentration and mass may be decreasing. The change in plume mass would not be necessarily reflected in the visual analysis of isopleth maps. However, a quantitative analysis of changes in overall plume concentration and mass would provide a better understanding of the plume stability. A common approach for evaluating plume stability is the use of statistical analysis techniques for single-well data. However, chemical concentration trends at individual monitoring wells may show different trends. For example, at a given site, there may be wells exhibiting decreasing Ground Water Monitoring & Remediation 28, no. 4/ Fall 28/pages

2 trends in some wells while exhibiting indeterminate or even increasing trends in other wells. Furthermore, single-well trend analyses are somewhat dependent on monitoring well location and the number of wells. For example, a site may only have one or two wells exhibiting decreasing trends, with many wells exhibiting indeterminate or increasing trends or vice versa. One could ostensibly increase the number of wells exhibiting decreasing trends simply by installing more wells in the area of the plume exhibiting decreasing trends. However, evaluating trends in the overall plume area, average concentration, and mass provides a more thorough understanding of the stability of the entire plume as opposed to discrete locations within the plume. This article presents practical techniques for calculating and evaluating trends in overall plume area, average concentration, mass, and center of mass. Rather than relying on complex computer models based on numerous assumptions, the techniques presented are relatively simple and based on the use of grid files generated from commonly used contouring and spreadsheet software. The examples presented in this article were generated using Surfer Ò (Golden, Colorado) by Golden Software and Microsoft Excel (Redmond, Washington). The major advantage of using computer software to perform all calculations is that trends in the individual metrics (e.g., plume mass) are not subject to variability resulting from human bias. For example, it is common practice to generate hand-drawn isopleth maps. In this case, there is bias in the interpretation of individual isopleths as well as bias between monitoring events. This would occur when one operator interprets isopleth frequency and location differently than another operator who may come on to a project several years later. However, when isopleths are generated based on a grid file generated from a computer-based gridding algorithm, data are interpreted consistently from event to event, thereby eliminating both spatial and temporal bias. The plume stability analysis described in this article is an effective tool to clearly demonstrate the occurrence (or nonoccurrence) of natural attenuation of a contaminant plume. This method is efficient because it requires only the analytical data supplied by a laboratory to perform the assessment, and it is reliable because all calculations and interpretations are conducted by the software, thereby eliminating any operator bias and subjectivity. This method is relatively easy to implement and would be applicable to most contaminated sites with an established monitoring well network and multiple years of analytical data. The method presented in this article is a combination of graphical and statistical analysis techniques. Contaminant concentration isopleth maps are prepared for various sampling events. The overall plume area, average concentration, and mass are then calculated for each sampling event using innovative techniques. In order to demonstrate that a plume is shrinking, temporal trends in these calculated values should result in statistically significant decreasing trends. An increasing trend in any of these values would indicate that the plume is expanding. In addition to temporal trend analysis of plume characteristics, the center of plume mass is calculated. To further demonstrate plume stability, the location of center of plume mass should not show continual downgradient migration over time. As contaminant mass is removed via pumping or natural processes, the location of the center of plume mass should be relatively stable or exhibit a general receding trend. In order to demonstrate the use of these plume-wide natural attenuation parameters, this article describes a plume stability analysis that was conducted for a former wood-treating site in central Louisiana. The plume stability analysis was conducted for a naphthalene plume over the period 1987 through 25. DescriptionofSite The site is located in central Louisiana and is the former location of a wood-treating facility that treated utility poles from 1937 until it was closed in September Dimensional lumber products such as cross-ties and guard posts were also treated at the site. Creosote and pentachlorophenol were used for treating. Although the site is impacted by many polynuclear aromatic hydrocarbons (PAHs), the plume stability analysis was focused on naphthalene concentrations in ground water. Naphthalene was selected as the indicator of creosote in ground water for the site because it is the PAH constituent found in the highest concentration in creosote, is the most mobile PAH, and constitutes the majority of the contaminant mass present in the ground water plume at the site. The uppermost water-bearing unit at the site is the Bentley sand. Monitoring wells at the site are installed in both the upper part (Upper Zone) and the lower part (Lower Zone) of the Bentley sand. The naphthalene plume occurring in both aquifer zones was assessed as part of the plume stability analysis. There are 57 monitoring wells at the site, of which 53 are screened in the Bentley sand. Twenty-eight of these wells have been monitored on a quarterly basis as part of the ongoing monitoring and corrective action programs at the site, which began in A site diagram is shown in Figure 1. Methodology In order to conduct the plume stability analysis, a primary contaminant of interest must first be selected. Once an indicator contaminant is selected, then concentration isopleth maps are developed for each year of the monitoring record. If many years of data exist, then the analyst may use every other year in order to reduce the overall effort. The underlying grid files used to develop the isopleth maps are then used to calculate plume area, average concentration, mass, and the geographic location of the center of mass. Trends in each of these metrics may then be evaluated using standard statistical analysis techniques. Detailed descriptions of each step are discussed subsequently. Generation of Contaminant Concentration Isopleth Maps Naphthalene concentration isopleth maps representing the naphthalene plumes in both the Upper and the Lower aquifer zones were developed for selected sampling events. 86 J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4: 85 94

3 Figure 1. Site diagram. Analytical data reported for each monitoring well varied seasonally, with some wells reporting quarterly results and others reporting less consistently. To remove seasonal variations, an annual average concentration of each year was calculated starting with 1987 through 25. Due to the volume of data, only the odd years between 1987 and 23 were used in the analysis. Naphthalene concentration isopleth maps were developed for each of the Upper and Lower aquifer zones for each of the years presented previously. The isopleth maps were each delineated to a naphthalene concentration of 1 lg/l. The raw monitoring well data were log transformed prior to generating the grid file in Surfer. When using contouring software such as Surfer, a grid file is generated simply importing raw data from a spreadsheet in, Y, Z format, where and Y refer to the well coordinates and Z refers to the log-transformed concentration. The grid file may be created using any number of gridding methods available within the software; however, kriging was used to generate the grid files presented in this article. Although it is beyond the scope of this article, care should be taken in the selection of grid mesh size. In general, the finer the grid mesh, the more accurate the gridbased calculations will be. Once the grid file is created, the grid is then inverse log transformed back to real world concentrations using a grid operation utility within Surfer. The isopleth maps are then generated by Surfer by direct interpolation of the grid file. The analytical data used to generate the Lower Zone naphthalene isopleth map for 25 are summarized in Table 1. Figure 2 shows an example of analytical data represented by a Surfer grid file, and Figure 3 shows an example of the data represented by an isopleth map based on the underlying grid file. Calculation of Plume Area, Average Concentration, and Mass The plume area, average concentration, and mass are calculated based on the grid file generated by Surfer. In order to calculate these parameters, the plume boundary concentration must first be defined. In many cases, the individual contaminant cleanup level is used, and in the current example, the naphthalene plume boundary is defined by its respective site-specific cleanup level of 1 lg/l. Once the plume boundary is defined, the plume area is determined using the grid volume calculation utility within Surfer. This utility generates a grid volume report that includes, among other parameters, the positive volume and positive planar area of the grid file within the specified boundary concentration. The planar area is based on units used to generate the grid file. In the current example, the area of the plume, which is determined directly from the grid volume report, is 117,915 m 2 (11.8 ha). J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4:

4 Table 1 25 Lower Zone Naphthalene Data DB-44 Well Y Concentration (lg/l) Log Concentration 12 DB-29 [3] DB ,256 99,655 NS DB ,191 99, DB ,826 99, DB ,61 99, DB ,213 99,31 NS DB ,686 98,83 NS DB ,127 98, DB ,934 98,867 ND 2. DB ,152 99,275 ND 2. DB ,472 99,668 ND 2. DB ,83 1, DB-33 5,12 98,384 NS 2. DB ,931 99,369 NS 2. DB ,72 98,54 ND 2. DB ,54 97,495 ND 2. DB ,586 98,191 ND 2. DB ,815 98,17 ND 2. DB ,279 98,192 ND 2. DB-4 498,54 98,34 ND 2. DB ,382 97,282 ND 2. DB ,942 98,579 ND 2. DB ,756 97,26 ND 2. DB ,22 1,539 ND 2. Notes: ND ¼ naphthalene not detected; NS ¼ well not sampled. Y Coordinate DB-34 [NS] DB-26 DB-27 DB-25 [19] DB-28 DB-21 [56] DB-22 [7,6] DB-24 [NS] Coordinate DB-18 [9,4] Center of Mass DB-15 [NS] Naphthalene Concentration ( g/l) DB-23 [NS] 1, 5, 1, DB DB-39 1 DB SCALE IN FEET Figure 3. Isopleth map of 25 Lower Zone naphthalene data. The average plume concentration is calculated using grid operations based on principles used in cut-and-fill computations commonly applied in civil engineering applications. The positive volume of the grid file (within the specified boundary concentration) is included in the grid volume report. The units of the grid volume in the current example are lg/lm 2. The units of volume of the grid in this case are meaningless by themselves; however, when the grid Figure 2. Grid representation of 25 Lower Zone naphthalene data. 88 J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4: 85 94

5 volume is divided by the planar area, the result is the average Z level of the grid file, or in the current case, the average concentration in units of lg/l. Following is a simple analogy that demonstrates this concept. Imagine a pile of soil roughly in the shape of a cone with a height of 6 m and a circular base with a diameter of 6 m. The volume (V) of the cone is given by the following equation: V = p 3 D2 4 3 H = p =18pm3 4 The planar area (A) of the cone is given by the following equation: A = p 3 D2 4 = p 3 62 =9pm2 4 The area and average concentration values were used to calculate the plume mass for each sampling event evaluated. In order to calculate the plume mass, assumed values for aquifer thickness and effective porosity of 4.6 m and 3%, respectively, were used for the plumes in both the Upper and the Lower zones. It is noted that the mass calculation may not necessarily be an indication of the actual mass of the plume; rather it is a means to combine the variables of area and concentration into one meaningful indicator variable. As the crux of the plume stability analysis method is to observe relative changes from year to year, applying constants to the mass calculation has no bearing on the output of the analysis. However, because the assumed values for aquifer thickness and effective porosity were based on site-specific data, the mass calculations are considered to be representative estimates of the actual dissolved contaminant mass. The mass of the naphthalene plume in the Lower Zone for 25 is calculated as follows: i h1:179e þ 5m 2 3 Plume mass = h i h i h i i 172 lg=l 3 4:6 m 3 :3 3 h1 L=m 3 1E þ 9 lg=kg = 173 kg In order to determine the average Z level (i.e., average height) of the soil pile, it makes intuitive sense that the average level would be 2 m (one-third of the height of a cylinder of the same dimensions). This may be calculated mathematically by dividing the volume of the cone by the planar area, or 18p m 3 divided by 9p m 2, or 2 m. The same convention applies to a three-dimensional surface represented by a grid file. In the case of the example of soil pile, the Z level, or height, is in units of meters; whereas in the plume volume calculation, the Z level is in units of lg/l. In both cases, when the volume is divided by the planar area, the result is the average Z level in the units of the Z dimension. For the 25 naphthalene Lower Zone plume, following are the grid volume and planar area values taken directly from the grid volume report generated by Surfer. Grid volume : 1:252E þ 8 lg=lm 2 Grid planar area : 1:179E þ 5m 2 The average concentration is then easily calculated by dividing the grid volume by the planar area: Concentration = 1:252E þ 9 lg=l m 2 1:179E þ 5m 2 = 162 lg=l Because the planar area is based on the plume boundary concentration (i.e., 1 lg/l), the average concentration calculation provides the average concentration above the specified plume boundary (i.e., 1 lg/l). The plume boundary concentration must be added to the average concentration calculation to determine the actual plume average concentration. Therefore, the 25 naphthalene Lower Zone plume average concentration is determined as follows: Final average concentration = 162 lg=l þ 1 lg=l = 172 lg=l As stated previously, the values for area, average concentration, and mass were calculated for the odd numbered years from 1987 through 25 for well in both the Upper and the Lower zones. A summary of the plume stability characteristics for the naphthalene plume in both aquifer zones is shown in Table 2. By observation of the data in Table 2, a general decreasing trend is apparent in each of the naphthalene plume characteristics for plumes in both the Upper and the Lower zones. The plume characteristics of area, average concentration, and mass for each plume were initially plotted to observe changes in each parameter from year to year. A trend analysis of the data using linear regression techniques was conducted to determine if significant underlying trends in the data were present. Note that there are many statistical tools for evaluating trends in environmental data, as provided in U.S. EPA (1992, 26). However, estimating trends using the slope of the regression line is a common approach that is adequate for most data sets. The trend analysis was conducted in accordance with procedures outlined in U.S. EPA (1992). The decision rule for identifying significant trends, simply stated, is the following: d If the calculated confidence interval of the regression line slope contains the value, there is insufficient evidence to conclude that there is a trend. d If the confidence interval of the regression line slope includes only negative or positive values, it can be concluded that there is a significant decreasing or increasing trend, respectively. The trend analysis was conducted using the regression analysis utility in Excel. Trend analysis summaries for the naphthalene plumes in Upper and Lower zones are provided in Figures 4 and 5, respectively. J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4:

6 Table 2 Summary of Naphthalene Plume Stability Characteristics Upper Zone Plume Lower Zone Plume Sampling Area (ha) Average Concentration (lg/l) Mass (kg) Area (ha) Average Concentration (lg/l) Mass (kg) Based on observation of the trend analysis summaries, the trend in plume area for plumes in both the Upper and the Lower zones is neither increasing nor decreasing, indicating that the trend in plume area is stable. However, the trend analysis for mass and concentration resulted in statistically significant decreasing trends (at the 95% confidence level). In addition to trend analysis results of stable or decreasing trends, both plumes have undergone considerable overall s in each of the plume characteristics evaluated. For example, the following s were identified based on data for the naphthalene plume for the period 1987 through 25: Upper Plume Lower Plume 25% area 1% area 63% concentration 47% concentration 72% mass 53% mass For visual comparison, the 1987 and 25 isopleth maps for naphthalene in Upper and Lower zones are shown in Figures 6 and 7, respectively. Calculation of Plume Center of Mass The center of mass, when plotted over many years, provides a good understanding of the overall general stability of the plume. For example, an expanding plume would show a general downgradient migration along the plume centerline; a receding plume would likewise show a receding center of mass along the plume centerline, and a stable plume would show a generally stable center of mass location through time. The plume center of mass is simply the centroid of the grid file and is easily calculated using Excel. In strict terms, the center of mass is actually calculated as the center of concentration because the plume isopleth map is based on an underlying grid file with the Z coordinate in units of lg/l. However, because constants are used to convert the average concentration to mass (as discussed previously), the locations of the center of mass and concentration coincide with each other. The coordinate of the center of mass is calculated as the weighted average coordinate using the respective Z value at each grid node as the weighting factor. In a similar manner, the Y coordinate of the center of mass is calculated as the weighted average Y coordinate using the respective Z value at each grid node as the weighting factor. For each sampling year evaluated, the naphthalene plume center of mass was calculated using a grid file consisting of 4, grid nodes (2 3 2 grid). The grid file may be opened in any spreadsheet program; Excel was used for the current example. As stated previously, the grid file contains 4, rows of YZ data. The file should first be filtered to include only those records with a Z value greater than the specified plume boundary (e.g., 1 lg/l). This ensures that the center of mass calculation is based solely on grid nodes that occur within the plume boundary. As an example, the 25 Lower Zone naphthalene grid file was filtered to include 7453 of the 4, grid nodes. Once the data were filtered to include only pertinent data, the location of the center of mass was calculated easily in Excel as follows. For each grid node, the center mass coordinate is calculated by dividing the summation of the product of each discrete and Z coordinates by the summation of all discrete Z coordinates. In a similar manner, the center mass Y coordinate is calculated by dividing the summation of the product of each discrete Y and Z coordinates by the summation of all discrete Z coordinates. The calculation of the center of mass for the 25 naphthalene plume in the Lower Zone is shown subsequently: 9 J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4: 85 94

7 Area (Hectares) Concentration (μg/l) ,5 2, 1,5 1, 5 Plume Area Average Plume Concentration Mass (kg) Plume Mass Parameter R 2 Regression Line Slope Trend Analysis (Linear Regression) 95 % Lower Confidence Limit 95 % Upper Confidence Limit Trend Analysis Conclusion Area Indeterminate Trend Concentration Decreasing Trend Mass Decreasing Trend Figure 4. Naphthalene plume stability analysis Upper aquifer zone. Center of mass coordinate; = 7453 P 1 P Z 1:7E þ 12 = = 498; 867 3:4E þ P Y 3 Z 1 Center of mass Y coordinate; Y = 7453 P Z = 1 1 Z 3:38E þ 11 =99; 429 3:4E þ 6 Once the and Y coordinates of the center of mass are calculated, then the location may be posted on a site map in Surfer or any geographic information system (GIS)-based graphics system. The center of mass locations for the naphthalene plume in the Upper and Lower zones are posted on the isopleth maps shown on Figures 6 and 7, respectively. Figure 8 shows the naphthalene plume center of mass locations through time for both the Upper and the Lower zones. As observed in Figure 8, the naphthalene plume center of mass has been stable in the Upper Zone and somewhat more variable in the Lower Zone. However, the naphthalene plume center of mass locations in both aquifer zones have migrated upgradient since 1987, and the Lower Zone naphthalene plume has migrated more than 88 m upgradient. This provides strong evidence that the plume is shrinking. Summary and Conclusions The analytical data collected at the site since 1987 provide strong evidence that the ground water contaminant plume at the site in both the Upper and the Lower aquifer zones is shrinking. If the plume stability analysis consisted solely of the common approach of evaluating changes in the J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4:

8 Area (Hectares) Concentration (μg/l) Plume Area Average Plume Concentration 2,5 2, 1,5 1, Mass (kg) Plume Mass Parameter R 2 Regression Line Slope Trend Analysis (Linear Regression) 95 % Lower Confidence Limit 95 % Upper Confidence Limit Trend Analysis Conclusion Area Indeterminate Trend Concentration Decreasing Trend Mass Decreasing Trend Figure 5. Naphthalene plume stability analysis Lower aquifer zone. plume boundary (i.e., area), then the result would have been stable trends. It would thus be difficult to implement a natural attenuation remedy based on this conclusion alone. However, the results of the plume stability analysis presented in this article exhibit clear and meaningful decreasing trends in naphthalene concentration and mass, which is a critical component when demonstrating natural attenuation at a site (U.S. EPA 1998). This is conceivably the best line of evidence to demonstrate when implementing natural attenuation as a remedial approach. Based on the observed decrease in contaminant mass over time, it can be concluded that a large portion of the observed decrease in contaminant mass is likely due to natural biodegradation. The plume stability analysis was originally developed to demonstrate compliance with the Government Performance and Results Act Environmental Indicator (EI) status for RCRIS Code CA 75 (Groundwater Releases Controlled). More specifically, this method provided a definitive answer to the question posed under CA 75: Has the migration of contaminated groundwater stabilized? This methodology was subsequently highlighted by U.S. EPA Region IV as a Resource Conservation and Recovery Act Showcase Pilot. In addition to being a very useful tool to assess the stability of a plume pursuant to EI CA 75, this method has been successfully implemented at several sites undergoing monitored natural attenuation. The plume stability analysis described in this article is an effective tool to clearly demonstrate the occurrence of natural attenuation of a contaminant plume. In order for the analysis to be meaningful, there should be a well-established monitoring well network that has been consistently sampled through the time of interest. This methodology assumes that there are a sufficient number of monitoring wells at a site. U.S. EPA (1998, 24) provides detailed discussion on the number of and optimum locations for monitoring wells. In order to conduct a statistical trend analysis, it is recommended that there be a minimum of four sampling events (U.S. EPA 1992). At many sites, samples are collected 92 J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4: 85 94

9 1987 Upper Zone Naphthalene Isopleth Map 25 Upper Zone Naphthalene Isopleth Map DA-3 DA-3 DB-44 DA-31 DB-44 DA-31 DA-28 DA-29 DA-19 DA-2 DA-21 DA-18 DA-32 DA-13 DA-7 DA-15 DA-11 DA-8 DA-1 Groundwater Flow Direction DA-28 DA-29 DA-32 DA-19 DA-2 DA-21 DA-13 DA-7 DA-15 DA-18 DA-11 DA-8 DA-1 DB-34 DA-27 DA-22 DA-17 DA-14 DA-3 DB-34 DA-27 DA-22 DA-17 DA-14 DA-3 DA-23 DA-23 DA-26 DA-24 DA-26 DA-24 DA-25 DB-4 DA-33 DA-5 DA-2 DA-25 DB-4 DA-33 DA-5 DA-2 DB-36 DA-38 DA Upper Plume Evaluation Plume Area: 13.3 Hectares Plume Average Concentration: 1,895 g/l Plume Mass: 346 kg DB-36 DA-38 DA Upper Plume Evaluation Plume Area: 1. Hectares Plume Average Concentration: 71 g/l Plume Mass: 96.1 kg Naphthalene Concentration ( g/l) , 5, 1, 25 5 Scale in Meters EPLANATION DA-15 Monitoring Well ID Plume Center of Mass Figure 6. Comparison of naphthalene isopleth maps in the Upper aquifer zone for 1987 and Lower Zone Naphthalene Isopleth Map 25 Lower Zone Naphthalene Isopleth Map DA-3 DA-3 DB-44 DA-31 DB-44 DA-31 DB-28 DB-29 DA-32 DA-19 DA-13 DA-7 DA-2 DB-15 DB-21 DB-18 DA-11 DA-8 DA-1 Groundwater Flow Direction DB-28 DB-29 DA-19 DA-2 DB-21 DB-18 DA-32 DA-13 DA-7 DB-15 DA-11 DA-8 DB-34 DB-27 DB-22 DA-17 DA-14 DA-3 DA-1 Groundwater Flow Direction DB-34 DB-27 DB-22 DA-17 DA-14 DA-3 DB-23 DB-23 DB-26 DB-24 DB-26 DB-24 DB-36 DB-38 DB-25 DB-39 DB-4 DB-33 DA-5 DA Lower Plume Evaluation Plume Area: 14.5 Hectares Plume Average Concentration: 1,888 g/l Plume Mass: 375 kg DB-36 DB-38 DB-25 DB-39 DB-4 DB-33 DA-5 DA-2 25 Lower Plume Evaluation Plume Area: 11.8 Hectares Plume Average Concentration: 1,72 g/l PlumeMass:173kg Naphthalene Concentration ( g/l) EPLANATION , 5, 1, 25 5 Scale in Meters DB-18 Monitoring Well ID Plume Center of Mass Figure 7. Comparison of naphthalene isopleth maps in the Lower aquifer zone for 1987 and 25. J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4:

10 25 Naphthalene Plume Extent Upper Zone Detail Lower Zone Detail DB meters DA DA DA Meters 1991 EPLANATION SCALE IN METERS Upper Zone Center Mass Location Lower Zone Center of Mass Location Figure 8. Naphthalene plume center of mass locations in the Upper and Lower aquifer zones. quarterly or semiannually; however, due to the long-term nature of remediation sites, it is recommended that a minimum of 4 years of sampling data be collected prior to conducting the trend analysis. This methodology is limited to contaminant plumes in porous media and may not be applicable for plumes in karst aquifers. Furthermore, this methodology is only applicable to dissolved contaminants in ground water and does not account for nonaqueous phase liquids. Although not required to conduct the analysis, the accuracy of this methodology could be improved through the use of multilevel or nested monitoring wells, which would be beneficial in the determination of plume thickness. For sites with multilevel monitoring wells, the methodology is easily modified by conducting the analysis for each level of the contaminant plume. Because this method is independent of chemical-specific information (other than laboratory data), this method is applicable to any contaminant dissolved in ground water. References Gilbert, R.O Statistical Methods for Environmental Pollution Monitoring. New York: Van Nostrand Reinhold. Mann, H.B., and D.R.Whitney On a test of whether one or more random variables is stochastically larger than in the other. Annals of Mathematical Statistics 18: U.S. EPA. 26. Data quality assessment: Statistical methods for practitioners. EPA/24/B-6/3. Washington, DC: Office of Environmental Information. U.S. EPA. 24. Performance monitoring of MNA remedies for VOCs in ground water. EPA/6/R-4/27. Cincinnati, Ohio: National Risk Management Research Laboratory. U.S. EPA Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA/6/R-98/ 128. Cincinnati, Ohio: National Risk Management Research Laboratory. U.S. EPA Methods for evaluating the attainment of cleanup standards. Volume 2: Ground water. EPA 23-R Washington, DC: Office of Policy, and Evaluation. Wiedemeier, T.H., M.A. Lucas, and P.E. Haas. 2. Designing monitoring programs to effectively evaluate the performance of natural attenuation. San Antonio, Texas: U.S. Air Force Center for Environmental Excellence. Wiedemeier, T.H., H.S. Rifai, J. Newell, and J.T. Wilson Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface. New York: John Wiley & Sons. Editor s Note: The use of brand names in peer-reviewed papers is for identification purposes only and does not constitute endorsement by the authors, their employers, or the National Ground Water Association. Biographical Sketches Joseph A. Ricker, M.S., P.E., corresponding author, is a senior engineer with Premier Environmental Services Inc., 153 North Main St. Suite 22, Collierville, TN 3817; (91) , x262; fax: (91) ; jricker@premiercorp-usa.com. 94 J.A. Ricker/ Ground Water Monitoring & Remediation 28, no. 4: 85 94