Methods

    Results reported herein are for soil samples collected during 1998 from a grid of 16 locations surrounding the WIPP site (the Near Field grid) and a grid of 16 locations approximately 12 miles southeast of the WIPP (the Cactus Flats grid, Fig. 2). The 16 sampling locations composing each grid are distributed over approximately 16,600 hectares. At each of the 32 locations, soil was collected at three randomly selected sites within a 25-m radius of the selected reference point. Individual sampling sites were selected on the basis of the following criteria: relatively flat topography, minimum surface erosion, and minimum surface disturbance by human or livestock activity. At each sampling site, approximately 12 L of soil were collected from within a 50 cm x 50 cm area, to a depth of approximately 2 cm. Soil samples were excavated using a trowel, sieved to remove all particles 1 mm, and placed in plastic bags for transport and storage. Sampling equipment was cleaned between collections.

    The soil samples were homogenized in the laboratory using a riffler. An aliquot of soil was removed for inorganic analyses, and the remainder of the sample was dried at 105°C. The soil aliquot used for inorganic chemical analyses was not oven dried because heating it to 105°C would vaporize any mercury in the sample. 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%. A 250-g aliquot was removed from each homogenized sample and pulverized in a shatter-box prior to analysis for radionuclides.

    Soil samples were analyzed at CEMRC for As, Cd, Sb, and Se, using a Perkin Elmer 5100 Graphite Furnace Atomic Absorption Spectrometer (GFAA) system. A FIAS-100 attachment and gold amalgamation system was used with the AA spectrometer for the measurement of Hg. A Perkin-Elmer Optima 3300 DV Inductively Coupled Plasma-Emission Spectrometry (ICP-ES) was used to analyze samples for Al, Ba, Be, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Sr, V, and Zn. Soil samples were analyzed for chloride, floride, nitrate, phosphate, and sulfate using a Dionex DX 500 Ion Chromatograph system equipped with an AS-14 separator column. The reported mean concentrations of these analytes in soil include only those values that were above detection levels. Thus, some estimates of the mean may be biased toward larger values.

    Of the 96 samples collected, 37 were submitted for radiochemical analyses. These included one sample from each of the 16 locations of the Near Field grid, and one sample from each of 15 locations in the Cactus Flats grid. The analysis of one sample from Cactus Flats grid was delayed and could not be included in this report. Additionally, a NIST traceable reference soil, replicate samples for five locations, and two split samples were included as quality control checks. Both the replicates and the sample splits were included in the statistical analyses of the results. Forty-eight samples were also submitted for elemental and ionic analyses, including two samples from each of the 16 locations on the Near Field grid, and one sample from each of the 16 Cactus Flats locations.

    An aliquot of approximately 10 g of each sample was used for analyses of actinides. Duke Engineering and Services (DES) (Bolton, Massachusetts) analyzed the samples for ²³⁴U, ²³⁵U, ²³⁸U, ²³⁰Th, ²³²Th, ²²⁸Th, and ^239,240Pu. Samples were recounted if activity measurements were less than the sample specific detection limit and had a sample specific concentration detection limit that exceeded 0.074 mBq g^-1 for ^239,240Pu, 0.15 mBq g^-1 for ²³⁵U, and 3.7 mBq g^-1 for ²³⁴U, ²³⁸U, ²²⁸Th, ²³⁰Th, and ²³²Th. All radionuclide recoveries were 30-110%. Mean minimal detection concentrations (MDCs) observed for these actinides were 0.02 mBq g^-1 for ^239,240Pu; 0.34 mBq g^-1 for ²³² Th, 1.37 mBq g^-1 for ²³⁰Th, 1.20 mBq g^-1 for ²²⁸Th; 3.0 mBq g^-1 for ²³⁸U; and 3.1 mBq g^-1 for ²³⁴ U and ²³⁵U.

    Soil types were identified for each of the 32 sampling locations using soil survey maps (Chugg, J.C., et. al. 1971, Soil Survey Eddy Area, New Mexico, U.S. Department of Agriculture, Soil Conservation Service; Turner, M.T., et. al. 1974, Soil Survey of Lea County, New Mexico, US Department of Agriculture, Soil Conservation Service) (Table 8). In addition to the above analyses, a 1-L aliquot of soil from each of four soil types found on the Near Field grid was sent to A&L Plains Laboratory (Lubbock, Texas), yielding soil texture information for each of the four soil types identified in the Near Field grid.

    Mean concentrations of all analytes were calculated by grid and by soil type, and 95% confidence intervals were computed for the estimated means using Student's t distribution. Significant differences between means were identified using a two-sided t-test. Multivariate analysis of variance (MANOVA) was used to test the hypothesis of no significant grid effect on analyte concentrations.

Results and Discussion

    The soil textures for all of the soils were very similar, having 88-90% sand, 2-4% silt, and 6-10% clay. The Berino Complex soil is classified as a loamy sand, while the Berino Dune Complex, Kermit-Berino Fine Sand and Maljamar Fine Sandy Loam are identified as primarily sands.

    The Cactus Flats grid had significantly higher soil concentrations of Al, As, Ba, Be, Ca, Co, Cr, Cu, Fe, K, Mn, Ni, Pb, V and Zn than were found on the Near Field grid (p < 0.05) (Fig. 21). Results of the MANOVA confirmed that there were significant differences between the two grids (p < 0.05), characterized by generally higher metal concentrations in samples from the Cactus Flats grid than in those from the Near Field grid. However, the ratios of these metals to aluminum (which normalized for the proportion of fine soil particles in the sample) are similar between the two grids (Table 9). This suggests that the observed difference in mean concentration is the result of a larger fraction of fine particles in the soil at the Cactus Flat grid. Soil concentrations of chloride for the Cactus Flats grid were significantly lower than their respective concentrations on the Near Field grid (p < 0.05). Furthermore, the ratios of Be, Ca, Co, Cu, Fe, K, Na, Ni, Pb, Se, Sr and Zn to Al were considerably higher in the aerosols than in the soils at both Cactus Flats and Near Field (Table 9). Although soil type did not appear to have an influence on analyte concentrations, a statistical analysis was prohibited because of low and variable numbers of samples collected within each soil type (Fig. 22).

    Radionuclide activities greater than MDC were detected in all but one sample. Activity concentrations in individual soil samples were 5.4 - 12 mBq g^-1 for ²³⁴U, 0.20 - 0.65 mBq g^-1 for ²³⁵U, 5.6 - 12 mBq g^-1 for ²³⁸U, 4.2 - 17 mBq g^-1 for ²²⁸Th, 4.5 - 16 mBq g^-1 for ²³⁰Th, 4.7 - 15 mBq g^-1 for ²³²Th, and -.0015 - 0.40 mBq g^-1 for ^239,240Pu. Concentrations of the radionuclides were significantly higher on the Cactus Flats grid than on the Near Field grid (Fig. 23) (p < 0.05). Results of the MANOVA confirmed that the two grids were significantly different (p < .05). However, the mean concentrations of the radionuclides relative to Al were similar between the two grids. Concentrations of the radionuclides at both sites were positively correlated with Al (Fig. 24) suggesting a fine particle effect as previously noted for metals. The overall mean (+ 95% CI) of 1998 measurements of ^239,240Pu concentration in samples from the Near Field grid was 0.090 (+ 0.022) mBq g^-1, which is significantly lower than the overall mean for measurements made in 1997 on samples from the Near Field grid (0.14 (+ 0.041) mBq g^-1). Concentrations of ^239,240Pu measured in samples from the 16 locations of the Near Field grid in 1998 showed no correlation with the concentrations of ^239,240Pu measured in samples from the same locations in 1997 (r² = 0.028, n = 16). These differences are likely the result of differences in analytical methods employed by the laboratories that performed the analyses (Accu-Labs Research in 1997 and DES in 1998) and natural variability.

    The ^239,240Pu concentrations reported for 1998 are somewhat higher than those reported as background levels in Ohio (about 0.2 mBq g^-1) (Muller, R. N. and D. G. Sprugel, 1977, Health Physics 33, 405). The indication that background concentrations of both fallout radionuclides and non-radioactive metals are lower on the Near Field grid than the Cactus Flats grid suggests that there are fundamental differences in the processes which influence the transport and fate of contaminants across the region. Differences in the kinetics of vertical transport of the contaminants in the soils, perhaps due to differences in soil composition at the 32 locations, could also explain the observed pattern. The soil types across the locations were relatively similar, being classified as sandy in all locations. However, the similarity between the two grids in the concentrations of the radionuclides and other metals relative to aluminum suggests that the proportion of clay in the soils could be a correlate of, and perhaps the controlling factor for, mass concentrations of the radionuclides and other metals. Differences in the relative amounts of soil organic materials also could explain the patterns in radionuclide activities and trace metal concentrations; this possibility could be tested by combustion of the soil organics.

    Clay minerals are aluminosilicates and hydrated oxides that usually account for the major adsorptive component of soils (Wild, A., 1994, Soils and the Environment, Cambridge University Press, Cambridge; Whicker, F. W. and V. Schultz, 1982, Radioecology: Nuclear Energy and the Environment. Vol. II, CRC Press, Ann Arbor, Michigan). The failure to see any relationship with soil type may reflect that the criteria used to classify soils may not be adequate for explaining the differences in the kinetics of transport, or that the soil maps have insufficient resolution to show the true soil types associated with the individual samples.

    These results demonstrate that significant variability in background levels of contaminants in soil exist in areas having relatively small differences in soil texture, and for contaminants such as Pu, where deposition is thought to be relatively uniform. The differences between the two grids may arise from differences in initial deposition or from a difference in the kinetics of particulates after initial deposition. There are no obvious factors that would suggest that the Cactus Flats grid would tend to have enhanced deposition as compared to the Near Field grid. Although topography and vegetative cover vary somewhat across both grids, they are generally similar on both grids. The presence of dunes at some locations on both grids indicates that some areas are subject to greater levels of soil erosion, deposition and vertical mixing, thus potentially depleting surface concentrations of some constituents.