Environ. Sci. Technol. 1989, 23, 1098-1 102
Leaching from Solidified Waste Forms under Saturated and Unsaturated Conditions Steven B. Oblath" E. I. du Pont de Nemours and Company, Savannah River Laboratory, Aiken, South Carolina 29808
The leaching behavior of nitrate ion from a cementbased waste form containing low-level radioactive waste was shown to be identical under saturated and unsaturated soil conditions. Only in soils containing less than 2 w t 5% water did the leach rate decrease. The observation of identical leach rates under saturated and unsaturated conditions is explained by diffusion through the waste form being the limiting step. Diffusion through the soil decreases in very dry soil and the limiting step changes. These laboratory tests were verified by measurements on a similar, Portland cement based waste form in a field lysimeter. Introduction The methods of disposal of low-level radioactive waste in the United States are currently receiving increased attention ( I ) . This is due to both the need to dispose of RCRA hazardous wastes, which are also radioactive ( 2 , 3 ) , and the need to site low-level, radioactive waste disposal facilities in each of the regional compacts (4). Geology and hydrology of the chosen site, restrictions on the physical forms of waste that can be accepted for disposal, and planned closure methods all are being used to ensure minimal environmental impact from these new waste disposal facilities. Development of leach-resistant waste forms is a major area of research in this field. One of the commonly held beliefs in designing and siting a disposal facility for low-level radioactive waste is that the rate of leaching from the waste to the soil is dependent upon the degree of saturation of the soil (5, 6). Under saturated conditions, the waste is in continuous contact with groundwater, and leaching is assumed to be a t a maximum (5). Unsaturated conditions are described by multiplying the saturated leach rate by a correction factor based on the assumption that leach rates are lower under unsaturated conditions. This factor is typically the fraction of time that the waste is under saturated conditions, such as when percolating rainfall is passing through the burial trench (5). The U S . Nuclear Regulatory Commission (NRC) recommends this approach to determine the environmental impact from a shallow land burial facility (5). Saturated leach data are obtained from studies of the West Valley (NY) and Maxey Flats (KY) low-level waste disposal sites, which have both experienced continuous saturated conditions in their waste disposal trenches (7). While these data represent valid leach rates for saturated conditions, the NRC provides no data to support the assumption that waste-to-soilleach rates are a linear function of the degree of saturation. The current trend toward solidified, leach-resistant waste forms raises further questions about the validity of the assumption that lower leach rates result from a lower degree of saturation. A number of experimental and theoretical studies of the leaching of solidified waste forms have been published and reviewed (ref 8-10 and references therein). However, none of these studies provide direct measurement of leaching *Present address: E. I. du Pont de Nemours and Co., Jackson Laboratory, Wilmington, DE 19898. 1098
Environ. Sci. Technol., Vol. 23,
No. 9, 1989
behavior under unsaturated conditions. To the author's knowledge, no data have been published that provide a direct comparison of the leaching behavior of the same waste form under saturated and unsaturated conditions. The present paper describes work performed specifically to provide such a direct comparison, and an interpretation of the results. The behavior of solidified waste forms has been under investigation for some time at the Savannah River Laboratory (SRL) in support of the Defense Waste Processing Facility (11). Prior to the solidification of high-level radioactive waste into borosilicate glass a t the Savannah River Plant, radionuclides (mainly Cs and Sr) are chemically concentrated and separated from waste salt solutions. The resulting decontaminated salt solutions, containing primarily sodium nitrate and sodium hydroxide, are mixed with Portland cement to form a solid, low-level radioactive waste form. Referred to as saltstone, this waste form is more easily disposed of than the highly radioactive glass. The leaching behavior of saltstone has been under investigation at SRL to determine what effect disposal in an unlined landfill would have on the groundwater beneath the disposal site. As part of this broader study, the waste-to-soil leach rates from saltstone in both saturated and unsaturated soils have been directly measured a t the Savannah River Laboratory. The study included laboratory determinations of the leach rate under various degrees of saturation of the soil, using soils collected from the Savannah River Plant (SRP) disposal site. The results were then verified by using data obtained from an ongoing field lysimeter study that had employed a similar waste form. Experimental Section The basis of comparison for leach behavior in saturated or unsaturated soil was to use a standardized, immersion leach procedure. However, soil of varying degrees of saturation was used as the leaching medium, in place of water. Leaching in water was used as a control. Cylinders of saltstone were immersed in either water or soil in a closed container. The saltstone waste forms were periodically removed from the soil or water (leachate) and placed in fresh leachant. Leachate replacement occurred daily for the first week, twice for the second week, weekly for the next 4 weeks, twice a month for the next 2 months, and monthly thereafter for up to 1 year. The leachate from each interval was analyzed for nitrate, nitrite, sodium, and sulfate ions, as well as tritium and technicium-99. Both incremental and cumulative fractional releases were calculated, allowing quantitative description of the leaching behavior of the samples. The details of the leach testing followed the American Nuclear Society's ANS 16.1 leach procedure (12). However, several modifications were made to the procedure to adapt it to the needs of this study and allow soil to be used as the leachant. The test was extended to as long as 1year, to provide a better characterization of the waste. Replacement frequency of the leachate was changed to the schedule described above. Also, the initial rinse of the sample was omitted. When soil was the leachant, this rinse was impractical. For uniformity of data, the rinse step was
0013-936X/89/0923-1098$0 1.50/0
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Table I. Decontaminated Salt Solution Composition component
w t 5%
H 8 NaNOJ NaOH NaNO, NaAI(OH), Na,SO. NakO;
71 14 3.8 3.5 3.3 1.7 1.5
component
w t 90
athkr s&
0.28 0.12 0.11 0.05 0.05 0.04 0.55
omitted for all of the samples. Description of Waste Forms. Saltstone waste forms prepared with decontaminated salt wastes were used in all laboratory testing. Samples were cast as 0.12- or 0.25-L right cylinders and cured for a minimum of 28 days prior to leaching. The samples contained 48 wt % ASTM class C fly ash, 12 wt % API class H cement, and 40 wt % decontaminated salt solution, the chemical composition of which is given in Table I. Nitrate ion, which is one of the major components of the salt waste, was used to monitor the leach rates. The leach behavior for nitrate ion was demonstrated to he the same as that for tritium (as HTO), sodium, nitrite, and pertechnetate (TcO,J ions. A detailed characterization of the hydraulic properties of the saltstone samples used was not performed. However, all of the cylinders were prepared from a single batch of cement grout and were cured under identical conditions. Also, the choice of the leach procedure, being a static immersion test where flow is mimicked by periodic replacement of the leachant, eliminates flow of the leachate through the sample. Measurements of the permeability of similar batches of saltstone showed permeabilities of about 10" cm/s. Field studies and largescale leach tests employed 200-L right cylindrical waste forms. These were commercial power reactor waste forms with a Portland type In cement hinder. Descriptions of the waste forms and their detailed characterizations are given elsewhere (13,14). Nitrate is a minor component in these waste forms, and sodium ion and the major radionuclides 13Vsand W o were used to monitor leaching. Laboratory Tests. Initial leach tests were performed using distilled water as the leachant. Nitrate in the leachate was analyzed by ion chromatography, with a detection limit of l mg/L. Later tests employed either wet sand or SRP soil as the leaching medium. Under all conditions the waste form was fully surrounded by at least 3 cm of leaching medium. At intervals when the leachate was to be changed, the waste form was carefully removed from the soil bed and a stiff brush was used to remove any soil adhering to the waste form. Visual inspections showed that the brush did not cause erosion of its surface. The waste form was then placed in a new soil bed and fresh soil packed around it. The soil was packed to a uniform density of 1.6 kg/L, determined by weighing the fully packed container. Distilled water (0.35 L) was used to leach the nitrate and other salts from the spent soil. The water was filtered and analyzed for nitrate by ion chromatography. All soils used in these tests gave quantitative recovery of nitrate with a distilled water leach and contained no leachable nitrate prior to contact with the waste form. The SRP soil is a clayey sand typical of those found at the saltstone disposal site. It has not been fully characterized, but is estimated to contain between 10 and 20 wt 70silt and clay (kaolin), with the remainder being sand. Hydraulic conductivities of these soils are typically approximately 10" cm/s. The as-collected soil had a mois-
Flgure 1. Lysimeter cross section.
ture content of 8 wt %. This corresponds to 32% of saturation (volume basis), based on a measured soil density of 1.6 kg/L and an estimated porosity of 40 vol %. T w o portions of this soil were dried by spreading the soil in pans and allowing the water to evaporate. Analyses determined the final moisture contents to be 1 and 2 wt %, corresponding to 4 % and 8% of saturation. Immersion leach tests on commercial power reactor waste forms were carried out in the same manner as the smaller tests. Deionized water was the leachant and full-size (200-L) waste forms were used. Large fiber glass tanks served as the leaching vessels. The work was performed for the Savannah River Laboratory by Brookhaven National Laboratory (BNL) in a facility specially designed to handle these large, radioactive samples. Tests were carried out in strict accordance with the ANS 16.1 leach procedure (12)omitting the rinse step. Details of the leach facility and the results are reported elsewhere (13, 14). Field Experiments. Field tests employed lysimeters, which are described in detail elsewhere (15).The lysimeters consist of 1.8-m diameter, 3-m deep fiber glass tanks, each containing a waste form surrounded by native soil. Rainfall serves as the only source of leaching water. Measurement of total percolate flow and analysis of periodic samples of the percolate water provided data on the quantity of material that leached from the waste and migrated through the soil to the lysimeter sump. A diagram of a lysimeter is shown in Figure l. The waste forms were maintained under unsaturated conditions, with the water table located 1 m below the bottom of the waste form. The lysimeters are located around a central caisson and are provided with ports for collecting horizontal soil cores at two different depths below the waste form. Analysis of the radionuclide content of segments of cores collected from the lysimeter allowed determination of the amount of each radionuclide released by the waste form to the soil after a specific length of burial. These data are then directly comparable to the immersion leach results of a duplicate 200-L waste form. Results Leach Tests. Figure 2 shows typical results of the leaching experiments for saltstone samples. These were Environ. Sci. Technol.. Vol. 23. No. 9. 1989
1099
Table 11. Leach Results with Air-Dried Soils
soil
it,
LepPnd
0 MoinSand
% saturation relative leach rate DeR,cm2/s
sot1 - 32%sat
lwt%
water
water
water
100 1 (defined)
32 1
8 0.7
4 0.3
5 x IO+
5 x IO+
3 x IO+
5x
Table 111. Fractional Releases from Commercial Waste Forms
0 001
i
ow01
2wt%
water
Water
0 01
8wt%
0
20
60
40
80
IW
120
140
160
180
200
Day
Flgure 2. Nitrate leach rates for saltstone in wet sand or SRP soils (see text for details).
radionuclide
immersion leaching
mco *37cs
0.002 0.3
lysimeterb exponential linear 0.006 0.2
0.008 0.7
"Extrapolated to 2 years. bExponential and linear refer to fit of concentration profile based on measured concentrations in the two cores. See text for exdanation.
01,
5
0.01
1
1
A
Il
Lwnd
1
1
1
1
1
1
1
R
-
0 sot1 8%Ut
\
W
,
1
Soil - 32% !I
b,
OOWl
1
,*
'
..
-----E
*-.
' .
0 -90
30 60 0 -30 Radial Distance from Center of Lysimeter lcmi
-60
90
Flgwe 4. Profile of concentrations along horizontal core collected 0.3 m below waste form after 2 years of leaching.
diffusion adequately describes the leaching behavior. A t the highest moisture content (8 wt %), the leaching behavior and Deffare the same as in water (see Table 11). Both of the air-dried soils showed lower leach rates and effective diffusion coefficients. The driest soil produced a DeKan order of magnitude lower than that for saturated conditions, corresponding to a reduction in leach rate by a factor of 3. For the particular soil used, no reduction in leach rate from that in saturated soil was observed until the moisture content dropped to 8% of saturation. Field Tests. Coring of a lysimeter containing a 200-L commercial waste form occurred 2 years after waste-form emplacement. The horizontal cores (0.03-m diameter) were taken at depths approximately 0.05 and 0.3 m below the bottom of the waste form. Significant concentrations of 13'Cs and 6oCowere observed on the soil. Migration had been predominantly in a vertical direction. The radial concentration profile for 'Wo in the lower core is shown in Figure 4. The other profiles were similar. The total fraction of each radionuclide released from the waste form was estimated by integrating over the lysimeter volume, with the results shown in Table 111. Vertical concentration profiles were calculated, assuming both linear and exponential decreases with depth beneath the waste form. In each case the profiles were fitted to match the measured concentrations in both cores. Based on a vertical coring of a similar lysimeter (16),an exponen,tial profile is expected to be more realistic. The calculations include corrections for radioactive decay.
Also given in Table 111are the fractional releases for the same type waste form under immersion leaching conditions a t BNL (14).The values represent extrapolation to cover 2 years of leaching, assuming a diffusion mechanism and correcting for decay. Comparison indicates that the fraction of the radionuclides leached into unsaturated soil and water are in good agreement. This field study validates the laboratory results indicating that leaching from a cement-based, solidified waste form is the same under saturated and unsaturated soil conditions.
Discussion The expected behavior for a nitrate ion leaching from saltstone, with a diffusion coefficient D1,into a medium with diffusion coefficient D2 is given by Crank (17). Assuming infinite media, the solution to the diffusion equation is
c1 =
+ (D2/D1)1/2erf 1 + (D2/D1)1/2 co
(1
(2) where Co is the initial concentration in saltstone and C1 is the concentration at time t and distance X from the saltstone-leachant interface. Differentiating this with respect to X gives the flux a t the interface:
Using the assumption that (Dz/D1)i/2 >> 1this reduces to @ =
co($)IZ
(4)
which, upon rearrangement and accounting for the cylindrical geometry of the saltstone samples, is equivalent to eq 1. The diffusion coefficient for nitrate in water (D2)is 2 X cmz/s (18). Since the measured value (Dl) for saltstone in the immersion leach tests is 5 X cm2/s, the assumption that (D2/D1)1/2 >> 1is valid. This indicates that for the immersion conditions, diffusion of nitrate through the block is the limiting step in the leaching process. When soil is used as the leaching medium, the diffusion coefficient for the saltstone does not change. However, the diffusion coefficient for nitrate in the soil is lower than that of water and will depend on the degree of saturation. For saturated soil, the diffusion coefficient is probably not lower than that of water by more than a factor of 3 (19). Equation 4 would still be a good approximation, and the leaching would still be limited by the diffusion through the saltstone. As drier soils are used, the diffusion coefficient for nitrate ion in soil decreases. When the value becomes comparable to the value in saltstone (D2= Dl) the flux (leach rate) would be reduced, as predicted by eq 3. In sufficiently dry soil, diffusion through the soil could become the limiting step and waste-to-soil leaching would no longer be the determining factor for releases to the environment. This is consistent with the current measurements. The driest soil (at 4% of saturation) must have a diffusion coefficient for nitrate that is about the same order of magnitude as that of saltstone. Although this value was not determined for the particular dried soil that was used, it is consistent with what has been reported in the literature (19) for similar soils. The same explanation applies to the large, solidified commercial waste forms. The effective diffusion coefficients for 13'Cs and 6oCowere determined by immersion
leaching to be 1 X and 2 X 10-l2cm2/s. The value for cobalt is lower because a fraction of it is incorporated into the cement matrix (14).These values are much lower than the corresponding diffusion coefficients in water, and the rate-limiting step is again diffusion through the waste form. Sorption of these radionuclides onto the soil reduces their concentrations in the interstitial soil water, but does not affect the limiting step for leaching, which remains diffusion within the waste form. In the lysimeter, the degree of saturation of the soil will vary with season, rainfall patterns, etc. However, with the water table maintained 1 m below the waste form (12),the soil will never get dry enough to reduce the leach rate from the waste. As a result, within our ability to measure the waste-to-soil leach rate under unsaturated field conditions, it agrees with that predicted from the immersion leaching (saturated conditions).
Conclusions All of the results of this study indicate that the leach rate of saltstone waste forms is independent of the degree of saturation of the soil, over the range of soil conditions that can be expected a t SRP. While this work is sitespecific, both in terms of the wastes and of the soils tested, the results refute the validity of the commonly held belief that waste-to-soilleach rates are lower under unsaturated burial conditions. The waste-form-controlled releases observed in this study should be generic for many solidified waste forms, which have been specifically designed to limit the release of waste components. Whether waste-formcontrolled releases are observed will depend on both the characteristics of the waste forms and of the soil in which they are to be disposed (leached). We have shown that the degree of saturation of the soil has little effect on the rate of leaching of material from a waste form. However, this does not suggest that the amount of water present does not affect the rate of migration of a leached species away from the disposal site. The migration, even if retarded by sorption on the soil, will still be a function of the rate of water flow through the media surrounding and beneath the waste (20). Therefore, limiting the water flow past the waste may help to keep the leached radionuclides (or other waste components) near the waste zone and out of the general environment.
Literature Cited Post, R. G., Ed. Waste Management 86, Proceedings; University of Arizona: Tucson, AZ, 1986; Vol. 3. Bowerman, B. S.; Kempf, C. R.; MacKenzie, D. R.; Siskind, B.; Piciulo, P. L. In Proceedings of the Seventh Annual Participants' Information Meeting-DOE Low-Level Waste Management Program; National Technical Information Service: Springfield VA, 1986; p p 317-323; CONF-8509121. Kempf, C. R.; MacKenzie, D. R. In Proceedings of the Seventh Annual Participants' Information Meeting-DOE Low-Level Waste Management Program; National Technical Information Service: Springfield VA, 1986; pp 324333; CONF-8509121. Lash, T. R. In Waste Management 86, Proceedings; Post, R. G., Ed.; University of Arizona: Tucson, AZ, 1986; Vol. 1; pp 21-24. Oztunali, 0. I.; Re', G. C.; Moskowitz, P. M.; Picazo, E. D.; Pitt, C. J. Data Base for Radioactive Waste Management; U.S. Nuclear Regulatory Commission: Washington, DC, 1981; Vol. 3, NUREG/CR-1. Devgun, J. S.; Charlesworth, D. H. In Waste Management 87, Proceedings; Post, R. G., Ed.; University of Arizona, Tucson, AZ, 1987; Vol. 3, p p 205-212. Fischer, J. N.; Robertson, J. B. In Radioactive Waste Management, Proceedings of an International Conference Environ. Sci. Technol., Vol. 23, No. 9, 1989
1101
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on Radioactive Waste Management; International Atomic Energy Agency: Vienna, Austria, 1984; Vol. 3; p 537. (8) Godbee, H. W.; Joy, D. S. Assessment of the Loss of Radioactive Isotopes from Waste Solids to the Environment. Part I Background and Theory; Oak Ridge National Laboratory, Oak Ridge, TN, 1974; USDOE Report
ORNL-TM-4333. (9) Kienzler, B.; Koester, R. H. Nucl. Technol. 1985, 71(3),
590-596. (10) Atkinson, A. Radioact. Waste Manage. Nucl. Fuel Cycle
1983,4,371-378. (11) Baxter, R. G.; Maher, R.; Mellon, J. B.; Shafranek, L. F.; Stevens,W. R., 111In Waste Management 84, Proceedings;
Post, R. G., Ed.; University of Arizona: Tucson, AZ, 1984; Vol. 1,p 275. Available from American Nuclear Society, La Grange Park, IL. (12) American Nuclear Society American National Standard: Measurement of the Leachability of Solidified Low-Level Radioactive Wastes; ANSI/ANS 16.1; 1986. (13) Neilson, R. M. Jr.; Kalb, P. D.; Colombo, P. Lysimeter Study of Commercial Reactor Waste Forms: Waste Form Acquisition, Characterization, and Full-Scale Leaching;
Brookhaven National Laboratory: Upton, NY, 1982; USDOE Report BNL-51613. (14) Kalb, P. D.; Colombo, P. Full Scale Leaching of Commercial Reactor Waste Forms; Brookhaven National
Laboratory: Upton, NY,1984,USDOE Report BNL-35561. (15) Oblath, S. B.; Grant, M. W. Special Wasteform Lysimeters-Initial Three Year Monitoring Report; E. I. du Pont de Nemours & Co., Savannah River Laboratory: Aiken, SC, 1985; USDOE Report DP-1712. (16) Oblath, S. B.; Hawkins, R. H. Migration of Pu and Am Determined in Lysimeter Studies. Presented at the 189th National Meeting of the American Chemical Society,Miami Beach, FL; American Chemical Society: Washington, DC, 1985; Abstract NUCL 98. (17) Crank, J. The Mathematics of Diffusion, second ed.; Clarendon Press: Oxford, England, 1975; p 38. (18) Greenland, D. J.; Hayes, M. B. H. Chemistry of Soil Processes; John Wiley and Sons: New York, 1981;Chapter 2. (19) Porter, L. K.; Kemper, W. D.; Jackson, R. D.; Stewart, B. A. Soil Sci. SOC.Am. Proc. 1960,24, 460-463. (20) Jury, W. A. Chemical Movement Through Soil. In Vadose Zone Modelling of Organic Pollutants; Hern, S. C., Melancon, S. M., Eds.; Lewis Publishers: Chelsea, MI, 1986. Received for review: December 14, 1987. Revised manuscript received November 3, 1988. Accepted April 27, 1989. The information contained i n this article was developed during the course of work under Contract No. DE-ACO9- 76SR00001 with the US.Department of Energy.
Copper Molluscicides for Control of Schistosomiasis. 1. Effect of Inorganic Complexes on Toxicity Timothy N. O’Sulllvan, J. David Smith,” and J. Donald Thomas
School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ, U.K. Cyril F. Drake
Pilkington Controlled Release Systems, Unit 10, Raynham Road Industrial Estate, Bishops Stortford, Herts, CM23 5PB, U.K.
’of copper A flow system has been for the assessment toxicity toward the snail Biomphalaria glabrata using a standard medium in which cupric ion speciation is well-defined. This allows the effects of complexing agents generated by the snail to be controlled. Dose-response curves show that the toxicity expressed as LT,, closely follows the s u m of the concentrations of the species ~ ~ z + and ( ~ c ~~ o) H + ( ~ indicating ~), that other copper species are considerably less toxic. copper molluscicides are most effective at pH 7. At lower pH metal uptake is reduced by competition from H+a t active sites and at higher pH by complexation with OH- or HC03-. The toxicity at high copper concentrations is limited by precipitation of malachite.
Introduction The control of schistosomiasis (bilharzia), which afflicts more than 200 million people in at least 80 countries, remains one of the world’s most intractable public health problems. Several control measures, such as the provision of latrines, clean water supplies, chemotherapy, and mollusciciding have been evaluated ( 1 , 2 ) . There is now a consensus that, although each approach may reduce the prevalence of bilharzia, none is likely to achieve the breakpoint when applied in isolation (2). An integrated approach is therefore necessary. In this, snail control remains an essential component, for even if latrines and piped water supplies are provided, certain categories of people remain at risk. These include fishermen and farmers, by the nature of their work, and children, because 1102
Environ. Sci. Technol., Vol. 23, No. 9, 1989
of their compulsive need to use water for recreational purposes. It is difficult to prevent the water bodies with which they interact from being contaminated by parasite eggs released from the excreta of people and other mammalian hosts, such as rodents, primates, and domestic Many of the problems associated with the use of molluscicides, such as their high costs, lack of specificity, and environmental deactivation by naturally occurring inorganic and organic ligands Can be alleviated by the aPPlication of controlled-release formulations (CRFs) (3-8) capable of delivering toxicants a t the required rate. soluble copper phosphate glasses were among the “first generation” CRFs to be developed (7). However, the specificity of such CRFs to target organisms could be enhanced by the development of “second generation” CRFs, which release snail attractants. The use of “third generation” CRFs, which release both snail attractants and phagostimulants, would result in further improvement (3). Specific snail attractants and phagostimulants have been identified and it would be feasible to incorporate them into the porous glass systems that are now available (3-6). In the first and second generation CRFs the toxic copper species is released into the external environment of the snails, whereas in the third generation systems it would be delivered into the alimentary canal of the target snails following ingestion. This would greatly reduce the possibility of harm to fish and nontarget invertibrates. As a prerequisite for the eventual production of specifically targeted copper molluscicides it is necessary to develop sensitive chemical and biological assays to quantify the
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