Pergamon Phys. Chem. Earth, Vol. 23, No. 2, pp. 171-178, 1998

0 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

0079-1946/98 $19.00 + 0.00

PII: SOO79- 1946(98)00009-3

Colloid-Facilitated Transport of Contaminants in Groundwater: Mobilization of Transuranic Radionuclides from Disposal Trenches by Natural Organic Matter

J. F. McCarthy

Environmental Sciences Division, Oak Ridge National Laboratory

Received 25 April 1997; accepted 15 December 1997

Abstract The role of natural organic matter (NOM) in enhancing the field-scale transport of transuranic radionuclides (TRU) in groundwater is used as a case study to illustrate the value of integrated laboratory and field approaches to understanding colloid-facilitated transport. Field observations provide evidence that TRU are mobilized and co-transported by NOM when hydrologic processes bring the groundwater in contact with waste buried in shallow trenches. This hypothesis receives further support from laboratory speciation studies and geochemical modeling. However, laboratory sorption studies indicate that the groundwater NOM should sorb to, and thus be retarded by the mineral surfaces of the formation. This issue is resolved through field studies of NOM transport. Discrepancies between laboratory predictions and field results reveal that the key process in NOM transport in natural-as opposed to model laboratory--systems is competitive adsoprtion among NOM subcomponents. Unlike laboratory studies of adsorption of NOM to clean mineral surfaces, surfaces in natural systems are coated with groundwater NOM, and binding sites are “passivated” with respect to further binding of the same NOM. The hypothesis that highly mobile NOM enhances TRU migration was tested by using lanthanides as field tracers to determine the extent of retardation of the TRU- NOM complex. The lanthanides, which have sorption and transport properties similar to TRU, migrated as NOM complexes without significant retardation over flow paths of 75-m. It is evident that assumptions inherent in many risk assessments for existing waste facilities, and performance assessments for future repositories, must begin to account for the role that even typically low levels of groundwater NOM plays in contaminant mobility.

0 1998 Published by Elsevier Science Ltd. 1 Introduction The transport of contaminants in the subsurface is controlled

Correspondence to: J. F. McCarthy, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 378316036, USA. E-mail: jjj@oml.gov.

by both chemical factors related to the sorption of the contaminant to immobile surfaces of the aquifer and by hydrologic factors that determine the rate of flow of water within the aquifer. Typically, contaminant transport has been described as a two-phase system, with a mobile phase composed of ground water and a solid phase composed of the immobile aquifer surfaces. Chemicals with limited solubility in water, or with a high affinity for binding to either charged surfaces of minerals or to organic coatings on the aquifer media, are generally regarded as being relatively immobile in the subsurface. For many subsurface environments, where the concentrations of natural organic matter (NOM) in the ground water are very low and there are no stable inorganic colloids present in the mobile ground- water phase, this paradigm is reasonable. However, the presence of NOM or inorganic colloids in soil or ground water can compete with sorption sites on aquifer surfaces and effectively increase the apparent solubility of contaminants in the mobile ground-water phase, thus enhancing the mobility of the contaminant. This paper will focus on the effect of NOM on contaminant transport, using the NOM-facilitated transport of transuranic radionuclides (TRU) as a case study. NOM is defined here as the total dissolved organic matter or colloidal organic matter in ground water, and includes both humic and nonhumic material.

The Oak Ridge National Laboratory (ORNL), located in eastern Tennessee (USA), has been operated since 1943 as a multidisciplinary facility for production and research related to nuclear materials. Beginning with production of materials for nuclear weapons for the Manhattan Project in World War II, and continuing with production and separation of radioactive isotopes for a variety of weapon- and energy-related activities, the site has been used for disposal of waste contaminated with an array of radionuclides. While this legacy creates obvious problems and concerns with migration of radionuclides to the biologically-accessible environment, it also provides opportunities to examine mechanisms of radionuchde transport at the field-scale that may lead to improved strategies for remediation and for containment of future

JPCE z,:*-8’ 171

172 J. F. McCarthy

WAG5 North

OP.1 2

DP-9 Dw-

IDI -

TRU

Fig. 1. Site map of WAG-5N. Contour interval is 1.5-m.

25

0

I I I I I I , 244

1992 1993 1994 1995 1996

Fig. 2. Changes in water elevation (dotted line) and gross alpha activity (squares) in well 516, located 10-m downgradient from the TRU trenches. Note that levels of radioactivity increased very rapidly when the water level rose in the winter. The high gross alpha levels in 1994 are caused by innundation of the TRU trenches because water levels that were much higher than in normal years.

waste. The current study focuses on the migration of TRU, specifically the actinide elements, curium-244 and americium-241 (McCarthy et al. 1997) from unlined trenches through a shallow fractured groundwater flow system. The rapid migration is unexpected because these radionuclides have a high affinity for sorbing to the mineral oxides and layer silicates that are abundant in the saprolite (highly weathered shale) of the formation (Meyer et al. 1984. Equilibrium isotherms using saprolite from the Dismal Gap formation demonstrated that the migration of *“Am was expected to be >50,000 slower than the rate of groundwater flow (McCarthy et al. 1998).

This paper will describe field observation, laboratory studies and geochemical speciation modeling suggesting that the TRU are migrating with little retardation as NOM complexes. However, laboratory studies have demonstrated that NOM is strongly adsorbed to mineral oxides and layer silicates, suggesting that the NOM itself would have very limited mobility in the formation (Tipping 1981; Jardine et al. 1989; Baham and Sposito 1994; Gu et al., 1994, 1995). To explain the high mobility of NOM in groundwater, the field-scale transport of NOM will be described based on an injection experiment in a sandy, coastal plain aquifer. This experiment demonstrated that groundwater NOM can be transported almost conservatively, presumably due to “passivation” of the aquifer by previously adsorbed components of the groundwater NOM. Finally, the role of NOM in facilitating radionuclide migration will be confirmed in an innovative tracer experiment utilizing nonradioactive lanthanides as surrogates to determine the mechanisms and rates of TRU transport at ORNL.

2 Mobilization of Transuranic Radionuclides from Disposal Trenches by NOM The study area is the northern quadrant of Waste Area Group-5 (WAG-XV) on the U.S. Department of Energy Oak Ridge Reservation (ORR) in Roane and Anderson counties, Tennessee (USA), which is currently used for storage of TRU solid waste materials (Fig. 1). WAG-5N is underlain by the Dismal Gap Formation, a series of interbedded lenses of limestone, siltstone and shale. Wastes are buried in the saprolite formed from the Dismal Gap formation, and contaminant migration occurs through pathways in, through, or over the soil and saprolite (Solomon et al. 1991). The water table is located within the saprolite, but can vary seasonally by several meters. Elevated water tables during the winter and early spring can permit groundwater to contact the buried waste. The groundwater is near-neutral pH dominated by CaCO, (alkalinity m 350 mg/L as CaCO,; McCarthy et al. 1997). The TRU is mobilized and transported as a result of interacting hydrological and geochemical events, including:

(1) seasonal fluctuations in the water table that intercept the shallow disposal trenches and permit contact of

Colloid-Facilitated Transport of Contaminants in Groundwater 173

groundwater with the acdnide-contaminated waste; (2) storm-driven recharge that can lead to transient

saturation of trenches above the local water table; and (3) rapid transport and limited retention of the mobilized

actinides due to complexation with natural organic matter (NOM) in the groundwater.

The role of hydrologic factors is inferred from the temporal pattern of changes in gross alpha radioactivity seen in monitoring well GW-516 from 1993-1996 (Fig. 2). High levels of gross alpha activity are associated either with years when water levels are very high (in 1994 the water table measured in the TRU trenches was only l-m below the ground surface, inundating the buried waste) or years when precipitation came in the form of intense storm events. If precipitation is faster than the rate of water infiltration through the soil profile, perched water moves laterally downslope through the very shallow stormflow zone. The trenches are oriented perpendicular to the slope of the land surface, and the disturbed backfill material in the trenches could intercept large volumes of stormflow water collected in the recharge area upslope of the trenches. Since the undisturbed formation surrounding a trench is likely to have a lower hydraulic conductivity than the backfill in the trench, there may be transient periods associated with storm events when the trenches become partially filled with storm water. The moderate releases of actinides observed in well 516 in 1995 are likely due to such transient saturation of the

Gross

Alpha -

NOM

0 100 200 300

Volume (ml)

Fig. 1. Adsoption and co-elution of NOM and gross alpha activity to DEAE weak anion exchange resin. Well 516 groundwater was passed through a column of resin. The column was rinsed with distilled water and then eluted with base. A sharp peak of NOM (measured as total organic carbon) and gross alpha activity co-elutcd.

trenches above the local water table caused by several large storms, including a lOO-mm event in January 1995. In 1996, however, not only was there less rain, but precipitation tended to be slow, steady rains with most storms involving less than 25-mm. The low-intensity rains infiltrate through the soil profile rather than forming a perched stonnflow zone, leaving the trenches unsaturated, and the TRU isolated from the groundwater.

The rapid changes in gross alpha activity (Fig. 2) are not consistent with the expectation that the TRU would be highly retarded due to strong interactions with the surfaces of the formation. Additional evidence that TRU is moving much more rapidly than expected is seen by comparing the magnitude and timing of peaks of gross alpha activity in wells near the TRU trenches and in the seeps at White Oak Creek (Fig 3). Storm-driven releases of TRU seen in well DW-3 are also observed at approximately the same time 75- m downgradient in the seeps. Furthermore, the level of activity diminished by only approximately one-half over that flow path, suggesting that the TRU radionuclides are not sorbing to the formation.

The expected inorganic speciation of the actinides in the neutral-pH, carbonate-rich groundwater is as a actinide- carbonate cation that should be retained by the negatively- charge geological media (McCarthy et al. 1997). However, the gross alpha activity is not retained by a cation exchange resin, but is retained by a weak anion exchange resin (Fig. 4). The gross alpha activity co-elutes with the groundwater

l A DW-3

I

1 3

0%

4126 5103 5117 5131 Fig. 3. Gross alpha activity in well DW-3 and two seeps 75-m downgradient in White Oak Creek. Data from 1995.

1

174 J. F. McCarthy

NOM (McCarthy et al 1997). These data strongly suggest that the mobile TRU are migrating as anionic NOM complexes. Geochemical speciation modeling using a Charge Neutralization model (Kim and Czerwinski 1995) demonstrated that in the presence of even the low levels of NOM in the WAG-SN groundwater (-1 mg-C/L), the predicted form of the actinides would be as an anionic actinide-carbonate-NOM complex.

Although these data indicate that TKU are complexed with NOM, it is less clear why the NOM itself is not retarded. Unless the groundwater NOM migrates without retention, it is difficult to postulate that the TRU-NOM complex would be highly mobile. Yet laboratory studies have shown that NOM is strongly adsorbed by the mineral phases present in the WAG-SN formation. This apparent paradox is resolved by recognizing that surfaces of the formation have been in contact with the groundwater NOM over geological time, and binding sites on the surfaces would be saturated with NOM. Because the surfaces of the formation are “passivated” with respect to further adsorption of NOM-or the TRU-NOM complex-little adsorption of NOM would be expected. This concept was demonstrated in a series of NOM injection experiments (McCarthy et al 1993; 1996) that will now be described.

The field-scale transport of NOM was examined in a two-

3 Field-Scale Migration of NOM and NOM

well forced gradient injection experiment in a sandy, coastal plain aquifer in Georgetown, South Carolina. Specific

Subcomponents

objectives of this field experiment were to determine the extent to which the field-scale transport of NOM conforms to behavior predicted from laboratory understanding. Discrepancies between laboratory predictions and field observation may point out key processes controlling transport behavior in natural subsurface systems. One key aspect of this study was a focus on the effect of the

multicomponent nature of NOM on descriptions of its transport behavior. Although individual components of the complex mixture of molecules comprising NOM can not be isolated, NOM can be operationally fractionated based on molecular size (by filtration) or hydrophobicity (by XAD-8 chromatography). We will focus here on the effect of size on the transport behavior of NOM. Previous laboratory studies (Gu et al. 1994, 1995) using

iron oxide-coated sand as a surrogate for the natural sand in the Georgetown aquifer demonstrated that NOM adsorbed to the sand. There were some differences in the adsorption of two size fractions of the NOM (~3000 Dalton mol. wt. and >3000 Dalton mol. wt). However, the two size fractions differed only about 20% in their adsoprtion capacity Thus, the laboratory results predicted that both size fractions would be significantly retained by the aquifer, although the smaller size fraction (c3K mol wt.) would be expected to

NOM Injection --)

3 0.3

9 I

P 0.2

0.1 1 >

,

0 I I / I / I

0 200 400 600 800 1,000

Time(h) Fig. 5. Breakthrough of total NOM and of two size

fractions of NOM at the withdrawal well during the NOM injection at Georgetown. The breakthrough of the nonreactive tracer, chloride, is also shown.

migrate with slightly less retardation than the larger-size NOM.

NOM (33 mg-C/L) was injected continuously into a well- characterized and highly instrumented site (McCarthy et al. 1996) for 659-h. followed by injection of the groundwater from a upgradient well to maintain the hydrologic gradient until the end of the experiment at 43-d. Concentration histories of NOM and chloride were monitored at observation wells located at 1, 2, 3 and 4-m from the injection well and at the pumping (withdrawal) well located 5-m from the source well. The source of the NOM was a wetlands pond draining a mixed hardwood forest near the site. The chemical properties and adsorption behavior of the NOM is described in Gu et al. (1994, 1995). Spatial moments described the migration of the center-of-mass of NOM and conservative tracer (McCarthy et al. 1996). In addition to measuring the total NOM, groundwater was fractionated by size to determine differences in the migration of two size fractions of NOM. The NOM injection solution and groundwater contained significantly different proportions of the different NOM size fractions. Virtually all of the groundwater NOM (-1.5 mg-C/L) was in the <3K fraction, whereas most of the NOM injection solution was in the 3-1OOK fraction (McCarthy et al. 1996).

Several features of NOM transport were difficult to reconcile with laboratory studies of adsorption or with conceptualization of NOM as a single entity. The transport behavior of the bulk NOM was characterized by significant losses of mass relative to the nonreactive tracer, chloride

Colloid-Facilitated Transport of Contaminants in Groundwater 175

(Fig. 5). Yet the NOM plume co-migrated with the chloride for the first few days and overall retardation was relatively low. The multicomponent nature of NOM was postulated to be the principal underlying cause of this behavior. There were significant differences in the relative mobility

of the <3K and 3-1OOK fractions arriving at the withdrawal well (Fig. 5); however, the results observed in the field were significantly different than that predicted based on the laboratory isotherms. The <3K fraction was transported almost conservatively during the NOM injection, with a plateau similar to that for the conservative tracer (Figure 5). Chloride and the c3K NOM lost a similar proportion of mass at wells progressively more distance from the injection well, based on spatial moments analysis at the observation wells. The shape of the breakthrough curve for the bulk NOM at the withdrawal well appears to result primarily from its dominant fraction, the 3-1OOK size fraction. This was also reflected in the similar proportion of mass loss for the bulk NOM and the 3-1OOK fraction at wells progressively more distance from the injection well (datanot shown). The heterogeneity in the transport behavior of NOM subcomponents was also evident when NOM was analyzed with respect to other properties, including hydrophobicity and UVMS absorbance spectra (data not shown). The shape of the breakthrough curve for the bulk NOM reflects the composite breakthrough of the two fractions with quite different inherent mobilities. Totsche et al. (1997) have described a multi-species approach to modeling NOM transport based on differences in mobilities of NOM subcomponents.

The limited adsorption of the c3K fraction in the aquifer is postulated be due to “passivation” of the aquifer by previously adsorbed components of groundwater NOM. Almost all of the NOM in the Georgetown groundwater is <3K mol. wt., and aquifer surfaces may be saturated with respect to their capacity to adsorb NOM subcomponents in this fraction. Although the groundwater NOM and the <3K NOM in the injection solution are unlikely to have identical chemical composition, their adsorption behavior are postulated to be similar and result in little additional adsorption of this fraction during the injection. The 3-1OOK fraction has a higher adsorption affinity for aquifer surfaces, and its retention by the aquifer is postulated to occur through competitive displacement of previously adsorbed <3K NOM.

The behavior of NOM in the injection experiment is interpreted as the introduction of a multicomponent NOM mixture into a porous media previously equilibrated with groundwater NOM of different composition. Because the aquifer sediments were equilibrated with the groundwater NOM prior to the experiment, only small amounts of the <3K NOM in the injection solution was adsorbed, and migration was essentially conservatively during the NOM injection. The experiment introduced novel NOM

subcomponents into an aquifer in which binding sites on mineral surfaces were occupied by NOM subcomponents derived from the groundwater. Retention of the novel NOM required competitive displacement of more weakly-binding components of the groundwater NOM by higher-affinity NOM subcomponents in the injection solution. The interpretation of the field data in terms of competitive interactions is consistent with the laboratory results of Gu et al. (1996a, b), who demonstrated the competitive adsorption of NOM and model organic solutes, and competitive displacement of weakly-binding organics by those with stronger binding affinities.

For purposes of understanding the migration of groundwater NOM at the TRU-contaminated WAG-SN site, the implications of the Georgetown experiment are clear. In a groundwater situation such as at WAG-SN, disturbance to the equilibrium between solid- and solution phase NOM is minimal, and the results from the Georgetown study demonstrate that the transport of the NOM-contaminant complex may be approximated as that of a conservative solute. Contaminants bound to groundwater NOM can therefore be expected to exhibit greatly enhanced mobility. This hypothesis was tested at WAG-SN in a field-scale tracer study.

4 Lanthanide Field Tracers Demonstrate Enhanced TRU Transport by NOM The field observations of radionuclide dynamics at WAG-SN described earlier (McCarthy et al. 1997) made it possible to formulate reasonable hypotheses concerning the mechanisms of TRU mobilization and transport, but are inadequate to unequivocally identify mechanisms or rates of transport of contaminants that have been present in the formation for 25 years. Therefore, trivalent lanthanides were injected as field tracers to confirm that the mechanism of actinide mobilization involved complexation by NOM, and to determine the extent of retardation of the actinide-NOM complex. The nonradioactive lanthanidesNd(II1) andEu(II1) have a chemistry and sorption/complexation characteristics analogous to actinides (Moulin et al. 1992; McCarthy et al. 1998). and are often used by nuclear chemists as analogs for the trivalent radionuclides. The two lanthanides will be referred to collectively as rare earth elements (REE) for convenience. Because transient hydrological conditions induced by storm events were postulated to affect mobilization and transport processes, the tracer injection was designed so as to maintain the natural hydrologic responses to storms, and the injection was sustained over a long period of time to capture responses to several storms. Multiple nonreactive tracers were injected with the REE tracers to provide a basis for comparing the extent of reactivity of the REE migrating through this formation (Fig 6). We hypothesized that the mobile form of the REE tracers would be as NOM complexes and that the REE-NOM complex

176 J. F. McCarthy

would migrate at rates similar to the nonreactive tracers. which the Nd adsorbed or precipitated. The small size of the mobile REE is consistent with the small size expected for a REE-NOM or actinide-NOM complex (McCarthy et al. 1997).

A 73-d natural gradient injection experiment was conducted in two adjacent injection wells, DW-1 and DW-3 (Fig. 1; McCarthy et al. 1998) in April-June 1995. Both wells are approximately 5-m from the TRU trenches, but screened at different depths to establish the presence of vertically discrete flow paths. Tracers were injected at nearly constant concentrations without altering the natural hydrologic gradient. The REE tracer, Eu was injected in DW-1 and Nd into DW-3 (Fig. 1). Bromide was present in the stock solutions injected in both wells. Sulphur hexafluoride (SF-6) was also used as a nonreactive dissolved gas tracer in injection well DW-I.

Two vertically distinct flow regimes were observed from the tracer tests, consistent with hydrologic characterization of the site (McCarthy et al. 1998). The REE tracer injected into the deeper flow path from DW-3 (Nd) migrated rapidly along a 10-m long deep flow path and arrived at GW-516 at approximately the same time as the conservative tracer (Fig. 6). The irregularities in the bromide breakthrough reflect changing hydrologic conditions due to storm events. For example, the decline in bromide concentrations in the middle of the injection is due to influx of dilute recharge water from storms during this period. The dynamics of Nd concentrations appear to be well correlated with changes in NOM concentrations in the source well (DW-3; Fig. l), consistent with the hypothesis that REE migration is facilitated by complexation with groundwater NOM. The peaks of Nd correlated in time with the sharp increase in the NOM levels in DW-3. The increased NOM concentration is likely caused by mobilization of NOM from the shallow saprolite as a result of the storm events during the injection period. Close linkages between storm-driven recharge in this saprolite and changes in NOM concentrations have been demonstrated in this formation (McCarthy et al. 1997). The Nd in GW-516 groundwater was not retained by a 0.05pm

filter, demonstrating that the mobile form of the Nd was not a colloidal hydrolysis product or an inorganic colloid to

.

0.12

0.08

Fig. 6. Breakthrough of the reactive tracer. Nd, and the nonreactive tracer. Br, at well GW-516.

DEAE chromatography of groundwater from DW-3 yielded results for Nd similar to that for gross alpha activity shown in Fig. 4. Neodymium, NOM and gross alpha activity were retained on the DEAE-cellulose column and when the column was eluted with alkali, the Nd and gross alpha activity co-eluted with the NOM. Thus, the mobile form of the REE tracers appears to be an NOM complex.

The arrival of Eu tracer was observed along the shallow flow path from DW-1 to wells DP-9, DP-12 and DP-2, and at seeps in White Oak Creek (Fig. 1). Tracer arrival was intermittent, the timing probably reflecting transient recharge events that periodically saturated the shallow fracture system and connected discrete flow paths between the source well and our monitoring locations. The failure to observe consistent breakthrough of the tracers from the shallow source well, DW-1, is likely related to the discrete nature of flow paths in the fractured formations of the ORR, where the transport of contaminants and tracers is often confined to laterally and vertically narrow zones. Nevertheless, when large storms connected 50-m long flow shallow pathways, coincident arrival of nonreactive (Bi and SF-6) and reactive Eu tracer was detected in DP-2, DP-9 and DP-12 and in two seeps located 75-m from the source well.

Temporal moments analysis of the concentration histories of the tracers yielded apparent retardation factors < 2 (compared to retardation >50,000 predicted by batch isotherms using site-specific material but without NOM). We conclude that NOM facilitated the almost-unretarded transport of lanthanide tracers, and by analogy, demonstrated that complexation by NOM in groundwater can facilitate the long-distance transport of a contaminant.

5 Summary and Conclusions This paper has attempted to make several points regarding approaches to and results of studies on colloid-facilitated contaminant transport, including: l Evidence is presented documenting the significant NOM-

enhanced mobility of TRU radionuclides. This evidence includes field observations of correlations between hydrologic processes and gross alpha activity (Fig. 2). temporal correlations in peaks of gross alpha activity in a set of up- and downgradient monitoring locations (Fig. 3). DEAE anion exchange chromatography of groundwater (Fig. 4) and geochemical modeling, as well as field-scale tracer tests (Fig. 6) with nonradioactive trivalent REE tracers as surrogates for radioactive trivalent actinide contaminants. We are aware of no previous study that brings together a coherent hydrogeochemical demonstration of the mobilization and transport of radionuclides by NOM.

Colloid-Facilitated Transport of Contaminants in Groundwater 177

l The importance of field studies to the understanding of

processes expressed in natural subsurface systems is emphasized. Discrepancies between the laboratory and field results are informative. For example, the discrepancy between the adsorption of NOM subcomponents on a model sorbent that lacked any previously adsorbed NOM (Gu et al. 1995) and the Georgetown field results lead to our hypothesis that previous coatings of NOM on surfaces controlled NOM adsorption and transport behavior in the field. This field-derived hypothesis has now been confirmed in laboratory studies on the competitive interactions among NOM and model organic compounds (Gu et al. 1996a. b).

l Recognition that NOM is ubiquitous in natural subsurface systems, and coatings of autochthonous NOM on aquifer surfaces can have a significant effect on the retention of NOM subcomponents is crucial to understanding the behavior of NOM with respect to contaminant transport. For example, the enhanced transport effected by NOM at WAG-SN appears at first to be inconsistent with other contaminant sorption and transport studies have shown that NOM adsorbed to mineral surfaces can increase adsorption of both organic and inorganic contaminants. In some column studies, addition of NOM to input solutions containing contaminants can increase retention of the contaminants on the column (Murphy et al. 1990; Zsolnay 1992). These apparent discrepancies are readily resolved if NOM transport problems are recognized as multicomponent transport process involving competitive adsorption of subcomponents for binding sites. Introduction of NOM that differs from autochthonous groundwater NOM with respect to the nature or amount ofNOMsnbcomponents will induce disequilibria between the solution- and solid phase NOM, with the release of weakly binding components and retention of strongly binding NOM which may also have a high affinity for contaminant binding.

l An additional implication of the multicomponent, competitive nature of NOM transport processes is that if allochthonous sources of NOM are introduced to a subsurface system (from disposal or organic wastes, for example), the chemical properties, and-more importantly-the functional behavior of NOM with respect to contaminant, will vary with time and distance along a flow path. Additional disequilibria in NOM adsorption may also be induced by changes in pH or ionic composition along flow paths. Predictive modeling of these dynamic competitive interactions along a flow path is problematic (McCarthy et al. 1996).

l The importance of NOM transport processes to the flux of contaminants at a site will depend on the binding affinity of the contaminant for NOM and the concentration of groundwater NOM. It is evident that assumptions inherent in many risk assessments for existing waste facilities, and

performance assessments for future repositories, must begin to account for the role that even typically low levels of groundwater NOM plays in contaminant mobility. This demonstration that NOM can dominate the mobilization, transport and fate of groundwater contaminants, and that NOM-contaminant complexes are not retained even by highly reactive surfaces of the saprolite forces a re- evaluation of our approaches to describing contaminant transport.

5 Acknowledgments This research was supported by the Environmental Restoration Program and the Waste Management and Remedial Action Division of the Oak Ridge National Laboratory (ORNL) and the Subsurface Science Program of Office of Health and Environmental Research of the U.S. Department of Energy (DOE). ORNL is managed by Lockheed Martin Energy Research Corp. for DOE under contract number DE-AC05960R22464. Publication No. 4726 of the Environmental Sciences Division, ORNL.

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Zsolnay, A. 1992. Effect of an organic fertilizer on the transport of the herbicide atrazine in soil. Chemosphere 24(5): 663-669.