Appendix_H_Geochem.pdf

Appendix H – Geochemical Modeling

Appendix H

Geochemical Modeling

Evaluation of Chromium Analyses in Overburden The relationship between total chromium (analyzed by XRF) and hexavalent chromium

(determined by subcontract laboratory) was evaluated for use in the Leapfrog® model to determine the extent of hexavalent chromium contamination in the overburden greater than 64 mg/kg. Table H-1 presents the overburden samples which were analyzed for both total chromium and hexavalent chromium.

Table H-1: Comparison of Hexavalent and Total Chromium in Overburden

Sample

Location

Total Cr

Composite

DTL

Cr (6+)

Analytical

Lab Sample

Location

Total Cr

Composite

DTL

Cr (6+)

Analytical

Lab

(mg/kg) Q (mg/kg) Q (mg/kg) Q (mg/kg) Q

E60 N20 157 16.8 E110 N30 291 50.6

E60 N32 113 2.7 J E120 N10 1204 151

E65 N53 155 1.3 J E120 N10 1137 256

E72 N18 1486 86 J E120 N10 83 8.9

E72 N18 183 9.7 J E125 N18 1096 82.5

E75 N25 584 33.3 J E125 N18 131 3.4

E72 N32 599 49.5 J E125 N32 468 98.5 J

E83 N38 709 124 J E135 N5 150 19.3 J

E83 N38 224 6.1 J E135 N50 124 1.7 J

E83 N53 119 1.1 J E138 N32 124 1.8 J

E85 N25 4371 285 E138 N37 147 3 J

E85 N25 1563 284 E150 N30 1967 205

E85 N25 351 125 E150 N30 379 142

E85 N25 121 23.1 E150 N30 409 28.6

E88 N9 187 26.3 E150 N38 732 78.9 J

E90 N20 166 3.1 J E150 N38 165 17.8 J

E90 N35 5846 150 E151 N9 8754 516

E90 N35 1016 50 E151 N9 3095 214

E90 N35 199 16.8 E160 N5 172 21.3 J

E90 N38 150 1.4 J E163 N7 1704 139

E98 N30 226 32.3 J E163 N7 93 2.7

E103 N18 1200 45.6 E170 N50 206 6.7 J

E103 N18 66 0.71 E173 N3 185 10.8 J

E110 N10 231 9.7 J E178 N20 142 2.6 J

Appendix H Geochemical Modeling

These results were plotted to examine the linear correlation between the two analyses (Figure H-

1); the equation of the regression is y = 0.0533 + 24.048, and the coefficient of correlation (r2) =

0.8363. The value of total chromium that approximately equals 64 mg/kg hexavalent chromium was determined to be 1,100 mg/kg. The Leapfrog® model interpolated the extent of contamination for the following: hexavalent chromium over 64 mg/kg (Figure 4-7), and total chromium greater than 300 mg/kg, 500 mg/kg, 700 mg/kg, and 1,000 mg/kg (Figure 4-8 and

Figure 4-9). Using the model interpolations of the extent of contamination, it was determined that greater than 500 mg/kg total chromium most closely represents greater than 64 mg/kg hexavalent chromium.

Figure H-1: Comparison of Hexavalent and Total Chromium in Overburden

Evaluation of Chromium Analyses in Concrete Total chromium determination by XRF was completed on concrete using two different methods.

In the field, total chromium was measured by direct measurement on the surface of the materials before collection (Direct). At the CDM Smith DTL, the concrete was ground to less than 2 millimeters, homogenized, subsampled (SOP 7-11), and analyzed by XRF (Composite). Six concrete samples were analyzed at the subcontract laboratory for total chromium (digestion by

SW-846 Method 3050 and analyses by SW-846 Method 6020); these results are presented in

Table H-2.

The concrete cores that were collected in the field were sliced vertically, and the depth profile of each core was analyzed for total chromium by XRF (Table 4-4). From these results, the depth of chromium penetration throughout the slab (with the exception of the trenches) was determined to be a thin surficial layer. The total chromium results from the subcontract laboratory (Table H-

2) are similar to the composite total chromium by XRF. This confirms both the accuracy of the

XRF but also the presence of underlying less contaminated material.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

H e x a v a le n t C h ro m iu m m g k g

Total Chromium (mg/kg)

Correlation of Total and Hexavalent Chromium in Overburden

Table H-2: Comparison of Total Chromium Methods of Analysis in Concrete

Sample

Location

Total Cr

Direct

Field

Total Cr

Composite

DTL

Total Cr

Analytical

Lab

(mg/kg) Q (mg/kg) Q (mg/kg) Q

E25 N15 695 90 91 J

E50 N50 77 81 40 J

E60 N20 7456 162 200 J

E85 N25 664 2348 2600 J

E88 N9 2701 641 690 J

E110 N27 1614 694 560 J

In order to evaluate the total chromium direct screening measurements that were taken in the field, the two methods for measuring total chromium by XRF (composite and direct) on the concrete cores were compared (Table H-3). These results were plotted to examine the linear correlation between the two analyses (Figure H-2); the equation of the best fit trendline is y =

6.0935, and the coefficient of correlation r2 = 0.6876.

Table H-3: Comparison of Total Chromium Analysis by XRF in Concrete

Sample

Location

Total Cr

Direct

Field

Total Cr

Composite

DTL

(mg/kg) Q (mg/kg) Q

E150 N30 25200 976

E150 N35 9817 224

E150 N50 13900 352

E151 N9 843 107

E170 N80 14500 368

E25 N15 695 90

E50 N50 77 81

E50 N60 65 96

E60 N20 7456 162

E85 N25 664 2348

E88 N9 2701 641

E90 N35 919 64

E100 N30 6785 292

E103 N18 4349 213

E110 N27 1614 694

E110 N30 7074 139

E117 N8 2701 344

E120 N10 13900 1159

E120 N25 623 114

E120 N60 1206 138

E125 N18 1269 200

E140 N20 33900 5760

Figure H-2: Comparison of Direct and Composite Total Chromium in Concrete

Utilizing the correlation that was determined in the previous section (64 mg/kg hexavalent chromium = 500 mg/kg composite total chromium), the “high” value for total direct chromium that corresponds to the composite chromium value of 500 mg/kg was calculated to be 3,000 mg/kg. For graphical depictions, a more conservative value of 2,000 mg/kg was chosen.

The analyses for hexavalent chromium that were completed at the CDM Smith DTL (extraction by

SW-846 Method 3060 [modified by SOP 7-12] and HACH portable spectrophotometer [Powder

Pillow method 8023]), and by a subcontract laboratory (SW-846 Method 3060 [alkaline extraction] and SW-846 Method 7199 [ion chromatography]) were compared. These results are presented in Table H-4.

Table H-4: Comparison of Total Chromium Methods of Analysis in Concrete

Sample

Location

Cr (6+)

DTL

HACH

Cr (6+)

Analytical

Lab

(mg/kg) Q (mg/kg) Q

E85 N25 631 U 2630 J

E25 N15 0.3 J 44 J

E50 N50 0.6 J 7.5 J

E60 N20 5.5 J 78 J

E88 N9 1.0 J 110 J

E110 N27 28 J 343 J

These results do not correlate; however, due to the small sample size further calculations were not attempted. The extent of contamination in concrete was, therefore, primarily determined using total chromium results. Because the concrete contamination was found to be of limited

10000

15000

20000

25000

30000

35000

40000

0 1000 2000 3000 4000 5000 6000 7000

D ir e ct

T o ta l

C h ro m iu m m g k g

Composite Total Chromium (mg/kg)

Correlation of Direct and Composity Total Chromium by XRF in

Concrete concern (as discussed below and within the report), further subcontract laboratory analysis of hexavalent chromium in concrete was not completed.

Leaching Calculations As described in Section 3.2.1.3, surface leaching tests were completed on eight of the collected concrete cores (Table 4-5). Samples were selected to represent the range of chromium contamination. The leach results were then used to complete simple modeling to determine the potential for any of the concrete to serve as a continuing source of contamination (from exposure to precipitation events).

Modeling was completed (Table H-5) considering various states of the building: maximum area exposed due to the removal of the roof, minimal area exposed assuming that the current conditions would not change, and representative release considering a slightly larger area of exposure. Currently, rain water enters the building through both doors on the east wall, as well as through collapsed south wall. Rainfall events for the area were determined from Rainfall

Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years. For the model, 24-hour rainfall events were considered, with the return periods of 100 years, 50 years, and 1 year (rainfall amounts 8-9 inches, 7-8 inches, and 3-3.5 inches, respectively).

Table H-5: Leach Modeling Parameters and Results

Condition

Susceptibility to Rain

Events

Area of

Concrete

Rainfall in 24 hours

Leach

Results

Concentration in Rainwater

After Leaching

Concentration in Soil

(ft2) (inches) (mg/L) (mg/L) (mg/kg)

Maximum

Release

Entire slab

(no roof) 16200 3 4.25 15.66 2.94

Minimum

Release

Areas currently exposed

3000 9 0.048 0.06 0.01

Representative

Release

Slightly larger area exposed 4400 7 4.25 7.23 1.36

The results of the leach tests were used to determine whether chromium contamination leaching from the building material (surface of concrete) would result in hexavalent chromium concentrations greater than 64 mg/kg in overburden. Assuming no reductive capacity remains in the soil (a conservative estimate), the maximum leaching of chromium from the building material would result in approximately 3 mg/kg hexavalent chromium in overburden.