HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Morton, L. S. Articles by Estes, G. O. Search for Related Content PubMed Articles by Morton, L. S. Articles by Estes, G. O. GeoRef GeoRef Citation Agricola Articles by Morton, L. S. Articles by Estes, G. O. Related Collections Pedology Soil Pollution Soil Mineralogy

DIVISION S-5—PEDOLOGY

Pedogenic Fractionation and Bioavailability of Uranium and Thorium in Naturally Radioactive Spodosols

^a Dep. of Plant Biology, Univ. of New Hampshire
^b Brookhaven National Lab

* Corresponding author (evansc{at}uwp.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Plant uptake of radionuclides into the human food chain is one of many vectors used for calculating exposure rates and performing risk assessment. This study provides a pedogenic assessment of soil–plant relationships in terms of their relationships to nuclide bioavailability. The objectives of this study were (i) to identify associations of U and Th with pedogenic fractions and with soil properties, and (ii) to evaluate U and Th bioavailability in terms of soil processes. We collected composite samples of leaves, stems, AE and Bs horizons from 10 plots containing a Spodosol with naturally high levels of ²³⁸U and ²³²Th. Additional replicate subsamples of fine (<63 µm) soil material were also extracted with citrate–bicarbonate–dithionite (CBD), ammonium oxalate (AO), sodium pyrophosphate (PP), and Mehlich reagent (M3). Concentrations of ²³⁸U and ²³²Th were determined for soils and plants by neutron activation analysis or by gamma spectroscopy. Results indicated that Th was translocated preferentially to plant leaves while U showed little preferential translocation. The CBD extractant removed the most U and Th from soils. The PP extraction ranked second for U, and AO ranked second for Th removal. The M3 extraction was equally inefficient in the removal of both U and Th. Neither Th, nor Th uptake, was closely associated with organic fractions. In contrast, U distribution and plant uptake of U were more closely correlated to organically bound oxide fractions.

Abbreviations: AO, ammonium oxalate • CBD, citrate–bicarbonate–dithionite • CD, sodium citrate–dithionite • M3, Mehlich reagent • PP, sodium pyrophosphate

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
THERE IS MUCH EVIDENCE to indicate that U and Th in soils can be associated with oxides, organic matter, and other colloidal materials or complexes (e.g., Langmuir and Herman, 1980; Burnett, 1988; Greeman et al., 1990; Longmire et al., 1991; Sheppard and Thibault, 1992; Payne et al., 1994; Rao et al., 1994; Willet and Bond, 1995). Certainly, soil constituents active in sorption reactions are both intuitively and demonstrably critical to radionuclide retention and distribution in soils, but relatively few of these studies have considered a natural soil setting under the influences of both pedogenic and plant cycling. Furthermore, bulk determinations of organic matter, sesquioxide, or clay content are often too broad to elucidate specific mechanisms of U and Th biogeochemical behavior. Similarly, measurements of total U or Th do not necessarily correlate well to plant concentrations (Sheppard and Sheppard, 1985; Sheppard and Evenden, 1988a; Sheppard et al., 1989; Sheppard and Evenden, 1992).

Fractionation of sorptive soil materials is often used to assign sorbed nuclides empirically to one of three broad fractions: water soluble, exchangeable, or non-exchangeable (Konoplev et al., 1993), as well as to clarify the scale and mechanisms of bioavailability. Selective extractions can also be used—again empirically—to categorize extracted fractions by association with constituents of known or calculated chemical properties. This can be done in sequence through the use of progressively harsh extractants on a single sample or on parallel batches of separate samples. One example of this practice is the use of a set of chemical extractants to characterize various oxide forms in spodic materials. Citrate–bicarbonate–dithionite (CBD), ammonium oxalate (AO), and sodium pyrophosphate (PP) are commonly used to extract total free oxides, amorphous oxides, and organically bound oxides, respectively (McKeague and Day, 1966; McKeague, 1967; Ross and Wang, 1993).

Although these fractions are operationally defined, rather than mechanistically defined, the extraction principles are useful in characterizing the chemical properties—and presumed behavior—of the extracted components. Among the materials extracted by these three procedures, a nested relationship is presumed, in that the organically bound oxides are a subgroup of the amorphous oxides, and that both are included in the total free oxides. Similarly, procedures have been devised to operationally define exchangeable and extractable cations—particularly plant nutrients. In this study we used several of these standard extractions, in parallel fashion. Our objectives were (i) to determine the primary associations of U and Th with pedogenically significant fractions and (ii) to evaluate contributions of these fractions to U and Th bioavailability.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Parent Material and Soils
Two locations with Redstone soil were chosen as sampling sites—Hurricane Mountain in Conway, NH, and Red Eagle Pond in Albany, NH. The Redstone Series (fragmental, mixed, frigid Typic Haplorthod) is a well-drained Spodosol formed in hydrothermally altered Conway Granite, which typically contains 11 to 14 mg kg^-1 U and 50 mg kg^-1 Th (Rogers and Adams, 1963; Richardson, 1964; Brimhall and Adams, 1969; Fehn et al., 1978). At each site, five 3- by 3-m plots were identified for study. Composite soil samples were collected from both the organically enriched but leached AE horizon (0–10 cm), and the upper part of the Bs horizon. Sampling depth did not exceed 25 cm, even though the Bs horizon often continued much deeper in the profile. Therefore, a total of 20 soil samples were collected from the field plots with 10 from each site: 5 each of the AE and Bs horizons.

Soil Extractions
Soil samples were air-dried for 3 d, then dry-sieved to pass a 62-µm mesh sieve. In soils of fragmental particle-size families, the whole-soil surface area is relatively small. Although large particles often appeared to be coated with spodic materials, we chose to analyze the finer fraction because the particles in the coarse fraction were very large (10-mm mean diam.). To maximize the amount of surface area, 500 to 1000 g of fine fraction was obtained from each sample and homogenized thoroughly before further subsampling. Individual subsamples were treated with one of four extraction methods. We chose three standard extractions for total, amorphous, and organically bound free oxides that have been extensively used in pedologic research as estimates of different iron oxide fractions in a soil. The fourth extraction is commonly used to assess exchangeable bases, macronutrients and micronutrients in soils.

Extractions were done according to modifications to original authors' work (Table 1), as described in Soil Sampling and Methods of Analysis (Carter, 1993). Abbreviations for the extractions used here are also presented in Table 1. Replicates and blanks were prepared for all extractions. Extractant solutions were analyzed for Fe, Al, Si, Mn, and P by direct current plasma spectroscopy. Following extraction, soils were dried, and soil residue was lightly ground with a mortar and pestle. Pre-weighed samples were packed tightly into a small flint glass tube. Soil depth in the tube was always <2.2 cm, to be within the 100% efficiency region of the well counter (see below). This corresponded to 1 to 2 g of soil per tube. The tubes were sealed with epoxy resin and stored for 90 d for equilibration. Subsamples of untreated soil were also oven-dried, ground, and packed in the same manner. The untreated samples provided baseline data for total U and total Th.

Plant Sampling and Analysis
Composite samples of native blueberry (Vaccinium angustifolium) leaves and stems were also collected from each of the 10 plots. Care was taken not to include any material that had external contact with the soil, such as extreme lower stems and soiled leaves. Plant samples were refrigerated immediately after sampling and processed within 24 h of collection. The plant samples were weighed, separated into woody stem and leaf samples, and washed thoroughly with distilled deionized water for not more than 2 min. Samples were then immediately put into an oven at 105°C for 24 h. Dried samples were ground in a Wiley Laboratory Mill and ashed in a muffle furnace at 550°C for 4.5 h.

When all replicates of the samples had been ashed and homogenized, the composited ashes were re-run at 550°C for an additional 4.5 h to ensure complete combustion. Total U and Th concentrations were determined by neutron activation analysis of the ashed samples.

Concentration ratios (CRs) for U and Th were calculated separately for stem and leaf tissue:

CR values were calculated twice for each plant tissue type: once using the AE horizon as the base and once using the Bs horizon as the base.

Data Analysis
Uranium and Th levels in extracted soil samples were expressed as weight concentrations and as extracted fractions. The weight concentration is simply the micrograms of U or Th per gram of soil. The extracted fraction is the percentage of U or Th removed by a particular treatment, calculated from the difference between the treated and untreated subsamples. Extracted fraction data were evaluated with a one-way ANOVA, testing the effect of the extraction type using "horizon" as a blocking factor. For significant ANOVAs, the Newman-Keuls test was used in post hoc multiple comparisons. One-way ANOVAs (StatSoft, 1996) were also used to test the effects of plant tissue type (stems versus leaves, blocking by site) on CR values.

To compare the extractability of U and Th, an additional one-way ANOVA was performed on the data from each individual extraction, testing the effect of nuclide and blocking by horizon. U and Th levels reported in these tables are expressed as weight concentrations or as extracted fractions. The weight concentration is simply the micrograms of U or Th per gram of soil. The extracted fraction is the percentage of U or Th removed by a particular treatment, calculated from the difference between the treated and untreated subsamples. A simple correlation matrix was created for each horizon type to assess bioavailability of the extracted nuclides. Concentration ratio values based on AE horizons were analyzed separately from those based on Bs horizons.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Uranium and Thorium Fractionation
Mean values of U and Th extracted by each procedure are shown in Table 2. Extracted fraction values are shown in Fig. 1. The blocking factor of horizon was not significant for either U or Th (P = 0.119 and 0.747, respectively), therefore AE and Bs horizons are grouped in Fig. 1. Sodium citrate–dithionite (CD) was the most efficient extraction for both U and Th (P << 0.001 for both nuclides. The PP extraction ranked second for U extraction efficiency (P < 0.005). Following the M3 and AO extractions, relative mean U content—on a g g^-1 basis—actually increased by 0.848 and 8.31%, respectively. This is probably due to loss of soluble material during the extraction procedure, which reduced the total sample mass. The AO extraction removed significantly (P < 0.05) more Th than the M3 and PP extractions. The M3 extraction was equally (P = 0.560) inefficient in the removal of U and Th.

View larger version (17K): [in this window] [in a new window]   Fig. 1. Fractionation of U and Th by selective extraction. Mean values indicated by points. Standard error indicated by boxes; whiskers indicate standard deviation.

View larger version (19K): [in this window] [in a new window]   Fig. 2. CR values for U and Th, by plant part and horizon. Mean values indicated by points. Standard error indicated by boxes; whiskers indicate standard deviation.

Thorium CRs of both leaves and stems were rather strongly, but inversely, correlated to total Th in both AE and Bs horizons (Table 4). Leaf concentrations and CRs were also inversely correlated to CD extractions, but only in Bs horizons. Pyrophosphate-extractable Th was highly correlated to Th concentrations—but not CRs—in leaves. In contrast, PP-extractable U was closely related to both concentrations and CRs of U in stems. Conversely, CRs—but not absolute concentrations—of Th in stems were most closely related to Th extracted by the M3 procedure. Correlations for extractable Th were also considerably weaker than correlations for extractable U.

To facilitate interpretations of pedogenic processes, correlations between extractable nuclides and Fe, Al, Mn, Si, and P were examined. Phosphorous was included because of its potential to associate strongly with oxide and organic fractions. Extractable elements were correlated with both extracted and residual U and Th. Again, AE and Bs horizons were treated separately because of their pedogenic significance. In Table 5, Th extracted by CD and AO procedures is strongly correlated to similarly extracted Fe in the eluvial AE horizons. Thorium is negatively correlated to AO-extracted Fe in the illuvial Bs horizons, however. No significant correlations were found between Al and Th, but extractable Mn was consistently correlated to residual Th. There were no significant correlations between the nominally organically bound PP-extractable elements and Th. However, following AO extraction, P in AE and Bs horizons was significantly correlated to residual Th.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Previous research on these soils (Evans et al., 1997; Morton, 1998) demonstrated that U- and Th-series nuclides are deficient in eluvial horizons, which implies a connection to pedogenic processes of translocation. These results suggest a further link to the translocation and formation of crystalline and amorphous oxides.

The results of the CD extraction indicate that nearly half of all Th in these soils is found in total free oxides. Although Th is associated with Fe oxides and extractable Fe, the exact method of Th translocation is not clear. A positive correlation between CD-extracted Fe and extractable Th in AE horizons is accompanied by a similar correlation to post-CD residual Th in Bs horizons. Thus, the relationship between CD-extractable Fe and Th seems to be related to eluvial-illuvial processes.

Correlations between AO-extractable Fe and Th are more readily interpretable. The positive correlation in the AE horizons, coupled with the negative correlation in the Bs horizon, suggests a possible relationship between Fe and Th. It is conceivable that the heavier Th is bound to amorphous—presumed early weathering stage—Fe oxides, and then acts to inhibit Fe translocation. Alternatively, Th weathered from primary minerals in the AE horizon becomes highly associated with amorphous (AO-extractable) materials, which are translocated. Following illuvial deposition, Fe mineral precipitation and aging transform the Th-associated oxides to a more crystalline—and less exchangeable—form, and Th thus becomes associated with these secondary products (Payne et al., 1994). Results presented in Table 5 suggest that Th associated with more crystalline (CD-extractable) Fe oxides is less susceptible to extraction than the Fe. This may be because the uniform valence of Th makes it impervious to the reduction phase of the CD extraction.

In contrast, the additions and transformations of organic matter seem to play a more significant role in U biogeochemistry. Uranium is consistently organically bound. For example, the PP extractant removed more U than AO and M3 extractants in both AE and Bs horizons. And in Bs horizons, U extracted by PP was highly correlated to stem CRs, and negatively correlated to PP-extractable Fe and Si. This trend indicates that processes in the organically enriched surface largely affect U. For example, U may be returned to the soil surface as a component of organic matter. It remains trapped in resistant fractions, such as lignin. Other organic compounds, however, which are more readily decomposed, release U and supply potential chelating agents, which facilitate U translocation. These relationships may clarify the homogeneity of U across pedogenic horizons.

In this study, both U and Th were efficiently removed from soils by the CD extraction, which in practice sums the AO and PP extractions plus the more crystalline secondary materials. These results agree well with those of an earlier study comparing soils with different pedogenic histories. Gueniot et al. (1988) studied a variety of soils, including a small sampling of quartz-rich sandy soils similar to those studied in New Hampshire, although the U content was significantly lower (<5 µg g^-1). In that study, 65 to 80% of U in Bs horizons was CD-extractable and 45 to 55% was PP-extractable, although U concentrations were not as uniform across horizons. The CD data here also agree reasonably well with a previous study on U, Th, and Ra fractionation in carbonate-rich soils from Pennsylvania, in which 8 to 23% of U and 18 to 51% of Th were CD-extractable (Greeman et al., 1990). Those results may be somewhat lower than ours because of a preliminary step in their extraction sequence to remove Mn oxides (hydroxylamine hydrochloride), which is not entirely selective for Mn and may also remove Fe (Tessier et al., 1979). Additionally, the Pennsylvania soils were not podzolized and were derived from argillaceous dolomite. Since carbonate desorbs oxide-bound U (Starik et al., 1958), this could also create some discrepancy.

Bioavailability
Neither U plant concentrations nor U CRs were linearly related to U soil concentrations, suggesting that soil U is partitioned quite explicitly into available and non-available fractions. The correlations between U stem concentrations and PP-extractable U indicate that the organically bound fraction may be consistently, but perhaps sparingly, available on a long-term basis, since there is no corresponding relationship to leaf tissues. These relationships are also weaker for AE horizons than for Bs horizons, suggesting that the translocation process somehow enhances bioavailability. This effect may be simply a response to enhanced concentration of U in the PP-extractable form within Bs horizons, especially during early stages of podzolization—that is, initial translocation. Short-term (e.g., annual) cycling of U in the AE horizon is also indicated by the correlation between leaf concentrations and CRs and CD loss of U.

Previous studies (Sheppard and Evenden, 1988b) found that U and Th CR values decrease as soil concentrations increase. This was true for CR Th in both leaves and stems, but not for CRU. To some extent, this indicates that Th uptake may plateau within a particular concentration range. The strength of these negative correlations does indicate that the high measured uptake of Th is probably real. If these high values were an artifact of surface accumulation that was not removed by washing, the correlations between plant CRs and soil Th would have been positive. The link between the two weakest Th extractants—PP and M3—to plant uptake can also be interpreted as support for the hypothesis that Th uptake is plant-limited, versus supply-limited. Furthermore, the PP and M3 extractions are the least harsh, suggesting that although there is little Th in those forms, PP- and M3-extractable Th are most accessible to the blueberry plants.

With respect to radionuclides in the soil solution, plants may be accumulators, indicators, or excluders (Baker, 1981). Accumulators tend to concentrate radionuclides in plant tissues, occasionally at levels in excess of soil levels. It is clear that the plants sampled for this study were not accumulators, since CR values were consistently very small. For purposes of comparison, the CRs calculated on a fresh weight basis for U are 10 times smaller than CRs for Th and Fe and 100 times smaller than CRs for other cations such as Ca (Morton, 1998).

Indicators are accumulators that demonstrate a predictable (i.e., linear) relationship between tissue levels and soil levels. Neither U nor Th concentrations in plant tissues were linearly related to soil concentrations. Blueberry plants can therefore not be used as indicator plants for Th or U in soil. This could also reflect both the low concentrations of U and Th relative to macronutrients and micronutrients and the relatively limited bioavailability of U and Th. Since plants do not require U or Th for either their structure or metabolism, the uptake of these nuclides is restricted to "passive cycling" due to imperfect selectivity by plants (Salisbury and Ross, 1992).

Excluders can discriminate against radionuclides or other pollutants until the root-bathing solution reaches extreme concentrations. Thus the Th plateau expressed as negative correlations between soil Th and Th CRs may be the response of an excluder plant.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
These results demonstrate the importance of soil-forming processes as long-term trends that can dramatically affect the pools of radionuclides in soil, plants, and in the ecosystem. Pedogenic processes impact the distribution of U and Th in these podzolized soils through the additions of organic matter, chelation, translocation, and aging of sesquioxides. During long-term pedogenesis, U is associated with the accumulation of organic matter and Th is associated with inorganic oxides. Although neither U nor Th has an appreciable "exchangeable" fraction, the isolation of specific U- and Th-rich soil fractions helped to identify connections between bioavailability and pedogenesis, which control ecosystem cycling of U and Th.

	ACKNOWLEDGMENTS

 
This research was funded by a grant from USDA NRICGP, #94-37101-0836. Work at Brookhaven National Laboratory was carried out under Contract DE-AC02-98 CH 10886 with the Department of Energy.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
L.S. Morton is currently at USDA-NRCS, Durham, NH. C.V. Evans is currently at Geology Dep., University of Wisconsin–Parkside, Kenosha, WI 53141-2000.

Received for publication November 5, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

• Baker, A.J.M. 1981. Accumulators and excluders: Strategies in the response of plants to heavy metals. J. Plant Nutr. 3:643–654.[ISI]
• Brimhall, W.H., and J.A.S. Adams. 1969. Concentration changes of thorium, uranium, and other metals in hydrothermally altered Conway Granite, NH. Geochim. Cosmochim. Acta 33:1308–1311.
• Burnett, W.C. 1988. Release of radium and other decay-series isotopes from Florida phosphate rock. Rep. 05-016-059. Florida Inst. of Phosphate Res., Bartow, FL.
• Carter, M.R. (ed.) 1993. Soil sampling and methods of analysis. Can. Soc. Soil Sci.. Lewis Publ., Boca Raton, FL.
• Evans, C.V., L.S. Morton, and G. Harbottle. 1997. Pedologic assessment of radionuclide distributions: Use of a radio-pedogenic index. Soil Sci. Soc. Am. J. 61:1440–1449.[Abstract/Free Full Text]
• Fehn, U., L.M. Cathles, and H.D. Holland. 1978. Hydrothermal convection and uranium deposits in abnormally radioactive plutons. Econ. Geol. 73:1556–1566.[Abstract]
• Greeman, D.J., A.W. Rose, and W.A. Jester. 1990. Form and behavior of radium, uranium, and thorium in Central Pennsylvania soils derived from dolomite. Geophys. Res. Lett. 17:833–836.
• Gueniot, B., C. Munier-Lamy, and J. Berthelin. 1988. Geochemical behavior of uranium in soils: Part I. Influence of pedogenetic processes on the distribution of uranium in aerated soils. J. Geochem. Expl. 31:21–37.
• Konoplev, A.V., N.V. Viktorova, E.P. Virchenko, V.E. Popov, A.A. Bulgakov, and G.M. Desmet. 1993. Influence of agricultural countermeasures on the ratio of different chemical forms of radionuclides in soil and soil solution. Sci. Total. Environ. 137:147–162.
• Langmuir, D., and J.S. Herman. 1980. The mobility of thorium in natural waters at low temperatures. Geochim. Cosmochim. Acta 44:1753–1766.
• Longmire, P.A., D.G. Brookins, B.M. Thomson, and P.G. Eller. 1991. Application of sphagnum peat, calcium carbonate, and hydrated lime for immobilizing uranium tailings. p. 623–631. In T. Abrajano, Jr., and L.H. Johnson (ed.) Scientific basis for nuclear waste management, XIV. Vol. 212. Materials Research Society, Pittsburgh, PA.
• McKeague, J.A. 1967. An evaluation of 0.1 M pyrophosphate and pyrophosphate-dithionite in comparison with oxalate as extractants of the accumulation products in Podzols and some other soils. Can. J. Soil Sci. 47:95–99.
• McKeague, J.A., and J.H. Day. 1966. Dithionite and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Can. J. Soil Sci. 46:13–22.
• Mehlich, A. 1984. Mehlich-3 soil test extractant: A modification of the Mehlich-2 extractant. Comm. Soil Sci. Plant Anal. 15:1409–1416.
• Morton, L.S. 1998. Radionuclide bioabsorption by blueberry plants growing on conway granite soils. M.S. thesis, Univ. of New Hampshire, Durham.
• Payne, T.E., J.A. Davis, and T.D. Waite. 1994. Uranium retention by weathered schists—The role of iron minerals. Radiochim. Acta 66/67:297–303.
• Rao, L., G.R. Choppin, and S.B. Clark. 1994. A study of metal-humate interactions using cation exchange. Radiochim. Acta 66/67:141–147.
• Richardson, K.A. 1964. Thorium, uranium, and potassium in the Conway Granite, New Hampshire, USA. In The natural radiation environment. Univ. of Chicago Press, Chicago, IL.
• Rogers, J.J.W., and J.A.S. Adams. 1963. Log normality of thorium concentrations in the Conway Granite. Geochim. Cosmochim. Acta 27:775–783.
• Ross, G.J., and C. Wang. 1993. Extractable Al, Fe, Mn, and Si. p. 239–246. In M.R. Carter (ed.) Soil sampling and methods of analysis. Can. Soc. Soil Sci., Lewis Publ., Boca Raton, FL.
• Salisbury, F.B., and C.W. Ross. 1992. Plant physiology. 4th ed. Wadsworth Publishing, Belmont, CA.
• Sheppard, S.C., and W.G. Evenden. 1988a. The assumption of linearity in soil and plant concentration ratios: An experimental evaluation. J. Environ. Radioac. 7:221–247.
• Sheppard, S.C., and W.G. Evenden. 1988b. Critical compilation and review of plant/soil concentration ratios for uranium, thorium, and lead. J. Environ. Radioac. 8:255–285.
• Sheppard, S.C., and W.G. Evenden. 1992. Bioavailability indices for uranium: Effect of concentration in eleven soils. Arch. Environ. Contam. Toxicol. 23:117–124.
• Sheppard, S.C., W.G. Evenden, and R.J. Pollock. 1989. Uptake of natural radionuclides by field and garden crops. Can. J. Soil Sci. 69:751–767.
• Sheppard, M.I., and S.C. Sheppard. 1985. The plant concentration ratio concept as applied to natural U. Health Phys. 48:494–500.[ISI][Medline]
• Sheppard, M.I., and D.H. Thibault. 1992. Desorption and extraction of selected heavy metals from soils. Soil Sci. Soc. Am. J. 56:415–423.[Abstract/Free Full Text]
• Soil Conservation Service/U.S. Department of Agriculture. 1972. Soil survey laboratory methods and procedures for collecting soil samples. Soil Survey Investigations Rep. 1 (revised). U.S. Gov. Print. Office, Washington, DC.
• Starik, I., F. Starik, and A.N. Apollonova. 1958. Adsorption of traces of uranium on iron hydroxide and its desorption by the carbonate method. Zh. Neorg. Khim. 3:1.
• StatSoft. 1996. STATISTICA for Windows [computer program manual]. StatSoft, Tulsa, OK.
• Tessier, A., P.G.C. Campbell, and M. Bisson. 1979. Sequential extraction procedure for speciation of particulate trace metals. Anal. Chem. 51:844–851.
• Tran, T.S., and R.R. Simard. 1993. Mehlich III-extractable elements. p. 43–49. In M.R. Carter (ed.) Soil sampling and methods of analysis. Can. Soc. Soil Sci., Lewis Publ., Boca Raton, FL.
• Willet, I.R., and W.J. Bond. 1995. Sorption of manganese, uranium, and radium by highly weathered soils. J. Environ. Qual. 24:834–845.[Abstract/Free Full Text]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Morton, L. S. Articles by Estes, G. O. Search for Related Content PubMed Articles by Morton, L. S. Articles by Estes, G. O. GeoRef GeoRef Citation Agricola Articles by Morton, L. S. Articles by Estes, G. O. Related Collections Pedology Soil Pollution Soil Mineralogy