AN APPROACH FOR DETERMINATION OF AVERAGE RESIDUAL CONCENTRATION OF PCBs IN SOIL

Transcription

1 AN APPROACH FOR DETERMINATION OF AVERAGE RESIDUAL CONCENTRATION OF PCBs IN SOIL Stacie A. Popp, Senior Project Leader ROY F. WESTON, INC. One Weston Way West Chester, Pennsylvania ABSTRACT A pertinent factor in the determination of the clean-up criteria for a site contaminated with polychlorinated biphenyls (PCBs) is estimation of how much material will remain in the ground following remediation. In the past the theory of average residual concentration of PCBs and other contaminants in soil has been used by agencies and private sector personnel in the environmental field for evaluation of clean-up scenarios, input into risk assessment-type evaluations, and evaluations of post-remediation conditions (Le., how "clean" a site is). This paper presents a calculation procedure that was utilized to estimate the average residual concentration (ARC) of PCBs in soils. The procedure was used for numerous industrial sites for various clean-up scenarios using characterization data and assumed postremediation conditions. This paper includes a discussion of the assumptions utilized for the ARC calculations. The ARC calculation procedure utilizes the following: o Surface soil characterization data stored in an electronic database. A commercially available computer modeling program to produce soil isoconcentration lines and calculate surface areas of extent of contaminants. (b Classical deterministic statistical methods. 8) A commercially available computer software statistical package. BI Several clean-up scenarios. This paper includes discussions of the details of the step-by-step procedure employed to produce the end result ARC values. 566

2

3 ~ INTRODUCTION L The concept of average residual concentration (ARC) has been found to be a useful tool for agencies and private sector personnel in the environmental field for dealing with sites that have been contaminated with PCBs and other types of contaminants. The ARC tool can be utilized for evaluation of clean-up scenarios, input into risk assessment-type evaluations, and evaluations of post-remediation conditions (i.e., how "clean" a site is). This paper presents one approach to calculation of the average residual concentration (ARC) of PCBs in soils. The procedure was used for numerous industrial sites for various clean-up scenarios using characterization data and assumed post-remediation conditions. - - BACKGROUND Assessment programs were conducted at numerous operating industrial facilities which consisted of characterization of extent of PCBs in surface soils, subsurface soils, sediments, and groundwater. Characterization activities included collection of surface soil samples (0 to 6 inch homogenized) which were analyzed for PCBs. In general, soil samples taken on the site property were collected at nodes of 25 feet by 25 feet (or larger) sampling grids, setup around potential source areas, until sample concentrations at or below a characterization level of 10 ppm total PCBs were achieved. Samples taken beyond the site property were collected at nodes of 50 feet by 50 feet (or larger) sampling grids setup parallel and adjacent to fencelines that were downslope of potential source areas, until sample concentrations at or below 5 ppm total PCBs were achieved. In addition, surface soil samples were collected to characterize to 1 ppm or to confirm 1 ppm computer-projected soil contour lines, as described in more detail in the following paragraph. Results of PCB analyses of surface soil samples were entered into the a database package available from Roy F. Weston, Technical Information Management System (TIMS). SOIL ISOCONCENTRATION CONTOUR LINES Surface soil sampling data from the site characterizations (total PCB concentrations) were utilized in a computer mapping program, Contour Plotting Sy~tem-3~~ (CPS-3), which uses a convergent gridding algorithm to generate isoconcentration contour lines representing selected PCB concentrations. Classical deterministic statistical methods and field confirmation sampling were used to confirm the 1 ppm contour lines. 567

4 SURFACE AREA ESTIMATES The areal extent of PCBs in surface soils was estimated for PCB concentration ranges of greater than or equal to 25 ppm, between 10 ppm and 25 ppm, between 5 and 10 ppm, and between 1 ppm and 5 ppm utilizing computer programs to calculate the surface areas within the projected contour lines. The surface areas were calculated for use in determining fractional area factors used in the average residual concentration calculations. The characterized area is defined as the total on-property area of a site where the areal extent of PCBs is greater than or equal to a concentration of 1 ppm, based on the CPS-3 isoconcentration contour lines. In certain areas, such as near upslope fencelines, where the extent of PCBs was not as accurately modeled by computer for reasons such as limited data, polygon-type hand calculations were used to estimate the areal extent. The surface area estimates were calculated for each concentration range for each site. CALCULATION OF AVERAGE RESIDUAL CONCENTRATION (ARC) ARC is defined as an area-weighted statistical estimation of the average residual concentration of PCBs left in the surface soil in the characterized area of the site following excavation and prior to backfill. Information used for the ARC estimates was site characterization surface soil data and the computer-generated/hand calculated surface areas. In addition, classical deterministic statistical methods and the Statistical Analysis SystemTM (SAS) computer software package were utilized to calculate the ARCS values. The step-by-step procedure to calculate the ARC for a particular site is as follows: Surface soil data with concentrations greater than or equal to 1 ppm total PCBs were obtained from the TlMS database. Property surface areas for areal extent of PCBs were calculated for four areas (A, B, C, D) of each site: - Area within 25 ppm contour lines (Area A). - Area between 10 and 25 ppm contour lines (Area B). - Area between 5 and 10 ppm contour lines (Area C). - Area between 1 and 5 ppm contour lines (Area D). 568

5 When calculating a weighted ARC, a weight factor was first calculated using the on property surface areas. For example, all surface soil samples with concentrations greater than or equal to 25 ppm were considered to be in the region within the 25 ppm contour line which carries a weight equivalent to that of the area of the region. In addition, it was assumed that each sample carried equal weight within its particular region; therefore, the weight factor, W, for each sample was calculated for a particular sample (i) where n equals the total number of samples within the area. A SAS computer program was used to sort the site database by A, B, C, D areas. Following this, a "cleanup scenario" of 10 ppm total PCB concentrations was applied to the database, as depicted in Figure 1. Two different cases to simulate a cleanup scenario of 10 ppm total PCBs were implemented prior to calculation of the ARC value. This was accomplished by reassigning total PCB concentrations of surface soil samples that were contained in an area that was assumed to be remediated (excavated and prior to backfill) to the post-remediation concentration. For cases 1 and 2, site areas with concentrations greater than the designated cleanup level (Le., 10 ppm total PCB concentration) were considered to be remediated. For case 1 it was assumed that, following remediation, concentrations of samples in site remediated areas would be 10 ppm. For case 2, it was assumed that concentrations of samples in the site areas would be 5 ppm following remediation. This is due to the assumption that, when trying to achieve an cleanup level of 10 ppm, in actuality remediation efforts would overshoot the target and on the average would accomplish a cleanup lower than the target of 10 ppm total PCBs. After sorting the data, assigning weight factors for each data point and reassigning PCB concentrations for the "remediated" areas, the PROC UNIVARIATE procedure of SAS was used to calculate the weighted averages for each site. The procedure for calculating the weighted average, X (ARC value) is as follows: Where Si means the sum over i samples and xi is the total PCB concentration in ppm for sample i. 569

6 c.. 570

7 After the weighted average (ARC) x and the standard deviation of the data set were calculated using PROC UNIVARIATE, the upperbound value of the ARC value was calculated for a 90 percent confidence level using the K, the variance (square of the standard deviation), and a statistical equation based on the 1-tail probability of the "student" t-distribution. This is shown graphically in Figure 2. The result is the upperbound of the residual concentration which shows that, for a 90 percent confidence limit, that if sets of sampling data were obtained several times ('hew'' characterization data to 1 ppm PCBs was obtained from the site with existing conditions) then 90 out of 100 times that you gathered data, the average residual concentration of each set of data (or each sampling event) would calculate to be less than or equal to the upperbound value. RESULTS AND CONCLUSION The above described technique was utilized to calculate ARC values for numerous industrial sites. The results of these calculations showed that, for a cleanup scenario of 10 ppm total PCB concentrations, ARC values of less than 10 ppm would be achieved based the cleanup scenario of 10 ppm total PCB concentrations for the sites. The ARC values ranged from approximately 4 to 7 ppm for a number of sites for which this method was applied with post remediation concentrations of 10 ppm in the remediated areas. Some example results are shown in Table I. The ARC procedure is based on classical mathematical and statistical techniques and was found to be useful for evaluation of various cleanup scenarios and useful for discussions concerning site risk assessments and post remediation conditions. 571

8 FIGURE 2 ONE-TAIL PROBABILITY OF t-distribution 9 0 % C 0 N FI DEN C E I NT E RVAL FOR AVERAGE RESIDUAL CONCENTRATION 0.10 TAIL AREA PROBABILITY ACCEPTANCE REGION - - X ui 572

9 TABLE 1 ESTIMATED AVERAGE RESIDUAL CONCENTRATION BASED ON SITE CHARACTERIZED AREA, ppm 10 PPM CLEANUP TO 10 PPM ARC" Upperbound ARC Site (PPm) (90% C.L.)b A B C D E F 'ARC = Estimated weighted mean value of average residual concentration based on site characterized areas bupperbound ARC = Upperbound (with 90% confidence) value of weighted mean ARC based on weighted standard deviation. Cleanup scenario of 10 ppm with post-remediation concentration of 10 ppm. 573

10 CONTAMINANT INVESTIGATION IN A KARST REGI9k BY / J. E. BENTKOWSKI, P.G. ATLANTA, GEORGIA Abstract / to have been contaminated. under the authori historical aerial photos aided in this information. Soil sample locations were surface and subsurface soil samples were the water and sediment samples g. The location of the springs roughly coincided with ideration for contaminant monitoring in karst regions. Introduction has occurred or is likely to occu nt at the sqe and to determine if a release of these substances, the approach to this inve USEPA, Region IV, as concentrate on source Permanent groundwat not part of normal SSIs. temporary groundwater 574