One finding presented in the CEMRC 1998 Report was that there were significant differences in many analyte concentrations between the Near Field and Cactus Flats grids. Differences in soil texture were postulated as a possible cause for these differences. Aliquots of 73 of the 96 samples collected in 1998 were subsequently analyzed for soil texture in order to test this hypothesis.

Initial preparation of the samples consisted of passing the soil through a 2-mm sieve to remove rocks, roots and other materials. One half of the 96 samples were originally analyzed for inorganic analytes in 1998. These soil samples were homogenized in the laboratory using a riffler. The method of homogenization was shown previously to yield subsamples that differed from the overall mean count of a radioactive tracer (¹³⁷Cs) by no more than 7%.

Each sample was homogenized again before aliquots were split for inorganic/metal analyses. New aliquots obtained from samples analyzed and reported in the 1998 CEMRC Report had already been dried at 105° C. Therefore, no analysis for Hg was performed on this set of 48 samples. A 50-g aliquot was removed for radiochemical analyses and ~300 mL aliquots were used for gamma spectroscopy analysis. The samples for gamma analysis were sealed in a ~300 mL paint can and stored for at least 21 days to allow radon progeny to reach equilibrium with parent radionuclides.

Soil samples were analyzed by AAS for As and Se. ICP-MS was used to analyze samples for Ag, Al, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Eu, Fe, Hg, K, La, Li, Mg, Mn, Mo, Na, Ni, Pb, Sb, Sr, Th, Ti, Tl, U, V and Zn. The lower detection limits for both of these systems are in the low parts per billion range (Appendix K). Soil samples were analyzed by IC for chloride, nitrate, nitrite, phosphate and sulfate. A summary of CEMRC QA/QC for inorganic analyses is presented in Appendix K. The mean concentrations of these analytes reported herein for soils include only those values that are above detection levels. Thus, some estimates of the mean may be biased toward larger values.

Gamma spectroscopy analysis was conducted using high purity Ge (HPGe) detector systems for 2-3 d. The systems have HPGe, p-type, coaxial detectors of ~50% nominal efficiency. A set of soil matrix standards was prepared using NIST traceable solutions and used to establish matrix-specific calibration and counting efficiencies. For analyses of alpha-emitting radionuclides, 10-g aliquots of each sample were heated in a muffle furnace to combust organic material and spiked with a radioactive tracer to allow determination of the efficiency of extraction.

The samples for radiochemical analyses underwent total dissolution followed by NaOH fusion of the insoluble residues. Multiple precipitation, co-precipitation and ion-exchange and/or extraction chromatography procedures were then used to separate and purify the desired elements. The elements of interest were then precipitated with La, deposited onto filters, mounted and counted on an alpha spectroscopy system. A summary of CEMRC QA/QC for radioanalyses is presented in Appendix L.

Aliquots of 73 of the 96 samples collected in 1998 were analyzed by the NMSU SWAT laboratory for soil texture. Analyses were made using the hydrometer method (Soil Conservation Service, 1972, Soil Survey Laboratory Methods and Procedures for Collecting Soil Samples. SCS, USDA; Gee, G. W. and J. W. Bauder, 1986, Particle-size Analysis. In Kline, A. (ed.) Methods of Soil Analysis. Part I. Physical and Mineralogical Methods - Agronomy Monograph no. 9. American Society of Agronomy, Madison, WI) resulting in measurements of the percentages of sand, silt and clay.

Multivariate analysis of variance (MANOVA) was used to test for differences between the Cactus Flats and Near Field grids across analytes. Mean concentrations of all analytes were estimated by grid and by soil type and significant differences between means were identified using t-tests. Correlations of the concentrations of radionuclides to soil texture classes, and to concentrations of non-radioactive analytes, were computed using Pearson correlation coefficients. Student's t-tests were applied to pairwise differences between radionuclide concentrations reported by Duke Engineering (1998 CEMRC Report) and the concentrations determined by CEMRC.

Results and Discussion

Baseline Analyses by CEMRC

Results of the MANOVA showed that there were significant (p<0.05) differences between the two grids, and that the Cactus Flats grid generally had higher concentrations of metals than found on the Near Field grid. Of the inorganic analytes, 20 (Ag, Al, As, Ba, Be, Cd, Co, Cu, Eu, Fe, K, La, Mn, Ni, Pb, Th, Ti, U, V and Zn) showed significantly higher concentrations on the Cactus Flats grid as compared to the Near Field grid. Chloride showed a significantly lower concentration on the Cactus Flats grid.

Many metals, including radionuclides, are known to have an affinity for attaching to small particles in the soil (Muller, R. N. and D. G. Sprugel, 1977, Health Physics 33, 405; Muller, R. N. and G. T. Tisue, 1977, Soil Science 124, 191; Watters, R. L., et al., 1983, Radiochema Acta 32, 89; Little, C. A., 1980, Journal of Environmental Quality 9, 350; Tamura, T., 1975, J.Environ. Qual. 4, 350). Clay minerals are aluminosilicates and hydrated oxides, and usually account for the major adsorptive component of soils (Wild, A., 1994, Soils and the Environment. Cambridge University Press; Whicker, F. W. and V. Schultz, 1982, Radioecology: Nuclear Energy and the Environment. Vol. II. CRC Press). Therefore, the concentration of aluminum in soil can be used as a surrogate for estimating the clay content of soils. A correlation between the concentration of Al and the percent clay in soils from 73 of the samples was highly significant (r=0.51, p<0.001), although the correlation between Al and silt was stronger (r=0.69, p<0.001). Similar correlations can be seen for many of the analytes (Table 12). Thus, comparing the ratios of the metals to Al helps to normalize for the effect of soil texture on the concentrations. The ratios of the metals to Al were similar between the two grids (Table 13). Only the ratios of Mo and Tl to Al were significantly (p<0.01) lower on the Cactus Flats grid.

The analyte:Al ratios in soils were compared to ratios reported herein for aerosols (Particulate Concentrations and Inorganics in Near Surface Air). Because aerosols are generally smaller than soil particles, the aerosols tend to have higher mass concentrations of metals which adsorb to fine particles than do soils. The ratios of Ag, Ca, Cd, Co, Cr, Cu, La, Mn, Mo, Ni and V to Al were lower by a factor of 2 or more in the soils as compared to the aerosols at both Cactus Flats and Near Field. The ratios of Fe, Hg, K, Mg, Na and Th to Al were higher in the soils than in the aerosols by a factor of 2 or greater.

The average MDC for radionuclides determined by alpha spectroscopy was »0.04 mBq g^-1. The average MDC for radionuclides determined by gamma spectrometry was » 0.5 mBq g^-1. Radionuclide activities greater than MDC were detected in all soil samples except for ⁶⁰Co, ^234mPa (1 value >MDC), ²³⁸Pu (11 of 96 >MDC) and ²⁴¹Am (72 of 96 >MDC). Only values greater than the MDC were included in the statistical analyses (Table 14). MANOVA confirmed that the Cactus Flats and Near Field grids were significantly (p<0.05) different. Concentrations of the radionuclides were significantly (p<0.05) higher on the Cactus Flats grid than on the Near Field grid for all radionuclides except ²³⁸Pu (Fig. 21). The mean ^239,240Pu concentration (0.158 mBq g^-1) fell within the range reported by Kenny et al., (1995, Radionuclide Baseline in Soil Near Project Gnome and the Waste Isolation Pilot Plant, Environmental Evaluation Group, Carlsbad, New Mexico) at the WIPP (0 - 0.74 mBq g^-1) and was lower than background concentrations found at Hueston Woods and Urbana, Ohio (0.7 - 1.0 mBq g^-1) (Alberts J. J., et al., 1980, J.Environ.Qual. 9, 592) and at a series of 15 locations between Ft. Collins and Colorado Springs, Colorado (0.6 - 1.7 mBq g^-1) (Hodge, V. et al., 1996, Chemosphere 32, 2067).

The soil textures for all of the samples were very similar, with 81.2-95.8% sand, 1.5-13.4% silt and 1.4% to 5.7% clay. Eleven of the 73 samples were classified as loamy sand and the remainder as sand. All of the radionuclides except ²³⁸Pu showed significant (p<0.01) correlations with each of the soil texture classes (Table 12). In general, the percentage of sand, or equivalently, the sum of the percentages of silt and clay, was the best correlate for the concentration of radionuclides. However, for ¹³⁷Cs and ^239,240Pu concentrations the percentage of silt gave a slightly better correlation (Table 12).

In general, the mean ratios of concentrations of the radionuclides to Al were similar between the two grids. Only ^239,240Pu showed a significant difference in these ratios, with the ratio for Cactus Flats being significantly greater (p<0.05) than that for Near Field. Concentrations of the radionuclides were also positively correlated by location with the concentrations of many of the analytes. All of the radionuclides were significantly (p<0.01) correlated with Al. Pb showed stronger correlations than Al for all radionuclides except ²³⁵U.

Concentrations of two radioactive elements, uranium (²³⁴U, ²³⁵U and ²³⁸U) and thorium (²³²Th), were measured independently using ICP-MS and alpha counting methods. A comparison of the estimated concentrations for these analytes showed that the average ratio of the alpha estimate to the ICP-MS estimate for Th was 2.22. The average ratio of the alpha estimate of uranium to that of the ICP-MS method was 8.20. The larger estimates from the actinide methodology may reflect that the samples prepared for actinide analysis undergo more complete dissolution, including fusion of any residues, whereas the samples prepared for ICP-MS analysis undergo only acid leaching. In drinking water analyses reported herein (Radiological and Non-radiological Constituents in Selected Drinking Water Sources), where dissolved forms of the elements are expected to dominate, the average ratio (± SE) for U by the two methods was 1.02 (± 0.09), but ²³²Th was not detected in drinking water samples. A correlation analysis failed to show any relation between the alpha counting to ICP-MS ratios and soil texture. If the difference in the estimates is due to the difference in dissolution, it appears that the refractory components are not strongly associated with a particular particle size class.