facilitated transport of selected metals in aquifer material packed columns

Journal of Contaminant Hydrology, 14 (1993) 233 246 233 Elsevier Science Publishers B.V., Amsterdam

Facilitated transport of selected metals in aquifer material packed columns

Meredith E. N e w m a n a, Alan W. Elzerman a and Brian B. Looney b

aEnvironmental Systems Engineering, Clemson University, Clemson, SC 29634, USA bSavannah River Laboratory, Westinghouse Savannah River Corporation, Aiken,

SC 29808, USA

(Received December 31, 1991; revised and accepted May 27, 1993)

ABSTRACT

Newman, M.E., Elzerman, A.W. and Looney, B.B., 1993. Facilitated transport of selected metals in aquifer material packed columns. J. Contam. Hydrol., 14:233 246.

Several factors may enhance the mobility of metals in an aquifer. Metals in such systems may associate with inorganic or organic ligands that affect retardation or with mobile colloids which may provide a faster transport mechanism. Complexation and/or sorption of a fraction of the metal may result in mobility in excess of that predicted using a single equilibrium based distribution coefficient which was derived from a more simple system. Failure to predict enhanced contaminant mobility may result in false predictions of arrival times and concentrations, and underestimation of human exposure to a contaminant. In this paper the results of a series of laboratory column studies designed to investigate factors influencing enhanced mobility of metals (Cd, Cu, Cr and Pb) in an aquifer are presented.

The first of two peaks in effluent metal concentration resulting from a pulse input of metal waste to an aquifer material packed column was observed to coincide with a peak in effluent turbidity. SEM/EDX analysis showed metals to be sorbed to the surface of the turbidity causing colloids. These observations indicated association with mobile colloids may facilitate the transport of metals through soil packed columns. The mobilization and/or transport of colloids was apparently influenced by alterations in eluent chemistry, such as pH and ionic strength.

The results also indicated complexation can enhance Pb mobility. Greater sensitivity of mobility to alterations in groundwater chemistry was observed for Pb and Cr than for Cd and Cu.

INTRODUCTION

Observations of contaminant mobility in excess of that predicted by models incorporating single equilibrium based distribution coefficients for each metal have emphasized the inadequacy of such simplified models in

0169-7722/93/$03.50 (~;; 1993 Elsevier Science Publishers B.V.

234 M E . N E W M A N ET AL,

many field applications (Vinton et al., 1983; Looney et al., 1986; Fish, 1987; Buddemier and Hunt, 1988). Such observations of enhanced contaminant mobility might be explained by several mechanisms, for example: (1) macroscale non-homogeneities in the aquifer; (2) inappropriate application of Kd-based models (i.e. nonequilibrium reactions or nonlinear adsorption isotherms); (3) the presence of more than one mobile species of contaminant in solution; and (4) the presence of a mobile solid phase (e.g., colloidal phase) composed of the contaminant or to which the contaminant is adsorbed.

The first mechanism listed, the presence of macropores, is related to physical characteristics of an aquifer. The presence of macropores has been recognized, but their importance remains difficult to quantify because they are generally site specific (Oliphant and Tice, 1985).

The second mechanism of apparent enhanced transport, non-linear, nonequilibrium adsorption, is currently a topic of intense investigation. More sophisticated, and possibly more appropriate, models for subsurface contaminant transport have been developed which incorporate ion-exchange between two polar solutes (Valocchi et al., 1981; Valocchi, 1985; Parker and Valocchi, 1986) and/or mass transfer limited processes (Bahr and Rubin, 1987). However, these models have not yet been tested in field applications due to intractability of the coupled systems of differential equations, difficulty in obtaining required parameter values, and difficulty in assessing the applicability of a specific model to a specific field situation.

Although the third mechanism listed, chemical speciation, has received little attention, it may play a key role in subsurface contaminant transport. The complex nature of most hazardous wastes, generally the source of toxic contaminant inputs, provide many organic and inorganic ligands that can complex contaminants of interest. Other conditions within the aquifer, such as redox potential, pH, and the presence of additional ligands, may also alter the speciation of the contaminant (Champ et al., 1979; Egozy, 1980; Daum and Newland, 1982; Elrashidi and Oconner, 1982; Giordano et al., 1983; Christensen, 1985; Lewis et al., 1987). Since each species of the metal may behave differently, with its own characteristic Kd, variations in mobility may occur due to variations in speciation.

The last mechanism listed, the presence of mobile colloids of the contaminant or colloids to which the contaminant is adsorbed, has recently become a topic of investigation (Gschwend and Reynolds, 1987; McCarthy and Zachara, 1989). Colloid-enhanced contaminant mobility is controlled by chemical and physical characteristics of the contaminant and aquifer material. For example, adsorption of a contaminant to colloids is controlled by chemical characteristics of the contaminant and colloids, while mobility of colloids is influenced by physical characteristics of the soil matrix, such as effective

TRANSPORT OF SELECTED METALS IN AQUIFER MATERIAL PACKED COLUMNS 235

porosity, and electrochemical characteristics of the colloids and aquifer material.

The last two mechanisms of enhanced contaminant mobility discussed above involve association of the contaminant with an additional mobile component and hence are referred to as mechanisms of "facilitated transport". The significance of facilitated transport in an aquifer is the subject of this paper.

EXPERIMENTAL METHODS

In order to investigate the extent and causes of both colloid- and complexation-facilitated transport, the mobilities of Cd, Cu, Cr and Pb through columns packed with sandy loam aquifer material were investi- gated. Recognizing the application of laboratory-determined transport data to field situations is difficult, we attempted to simulate field conditions as closely as possible.

The sandy loam aquifer material used in the column studies typifies soil found in much of the coastal plain of the southeastern U.S.A. The aquifer material was collected at the Savannah River Site (SRS) near Aiken, South Carolina. Aquifer material was collected at 1.5-m intervals from 1.5 to 21 m below the surface and composited into a single sample. Specific characteristics of the aquifer material are listed in Table 1. The aquifer material was stored at 4°C prior to use. Because drying may produce hysteresis effects and otherwise

TABLE 1

Soil charactersitics

Parameter Value

pH (1:1 with distilled water) .1 Cation-exchange capacity, CEC .2 Fraction of organic carbon, foc ,3

Particle size distribution ( USDA standard classes): Sand

very coarse (1 2 mm) coarse (0.5-1 mm) medium (0.25 0.5 mm) fine (0.1-0.25 mm) very fine (0.05 0.1 mm)

Total sand Silt (0.002 0.5 mm) Clay (< 0.002 mm)

5.6 2.9 meq/100 g soil

< 0.01

5.9% 13.3% 27.3% 29.8%

5.5% 81.8%

5.3% 12.9%

• 1 Jackson (1974); ,2 Uehara and Gillman (1981); ,3 Allison (1960); ,4 Day (1965).

236 M.E. N E W M A N ET AL.

alter the mobility of colloids present in the aquifer material, the material was not dried.

Polyethylene columns (18 cm long and 1.5-cm internal diameter) were used with ~ 1 cm of Glass Wool ® inside the polyethylene caps at each end to contain the packing material. The inside surface of the columns was roughened with sandpaper before packing to decrease wall channeling and the percentage of unfilled voids. Columns were packed with ~ 1 cm of aquifer material at a time then tapped until no further consolidation was evident before more material was added. Rhodamine~R~M dye tracer studies indicated this packing technique produced no observable channeling effects.

Columns packed as described above had porosities of 30% (s.d. = ~:6%). Porosities measured in the aquifer from which the material was taken varied from 10% to 30% (Looney et al., 1986). The pore volume of the packed columns was ~10 mL (s.d. = +2 mL) and the total packed column volume was 32 mL, hence a porosity approximating field characteristics was obtained. Bulk soil densities resulting from the packing technique described above (1.65 g cm -3, s.d. = i 0 .34 g cm -3) also compared favorably with values measured in the aquifer from which the soil was taken (1.60 g cm -3) (Looney et al., 1986).

P6clet numbers calculated using dispersion coefficients estimated from chloride breakthrough curves (BTC's) (Parker and van Genuchten, 1984)

ranged from 100 to 200. The P6clet number is a dimensionless parameter relating advection to dispersion [(P~clet number) = (fluid velocity) × (column length)/(dispersion coefficient)]. Large P6clet numbers indicate dispersion is small relative to advection and consequently dispersion should not greatly influence transport.

Two hazardous waste solutions were synthesized to represent effluent from metal plating processes at SRS. Both wastes contained several inorganic ligands with which metals could complex. Each waste represented an extreme pH: 2.3 or 12. The exact composition of each simulated waste is listed in Table 2.

Two scenarios for hazardous waste inputs to aquifers are spills and leaching from a landfill or lagoon. These two cases may be represented as an instantaneous pulse input and a continuous step input, respectively. Conse- quently, the two metal waste mixtures described above were applied to freshly packed columns as both pulse and step inputs. The use of washed columns (columns eluted until the effluent characteristics have stabilized) simplifies data interpretation. However, much information from the initial unsteady conditions, including possible colloid mobilization, is lost when columns are washed. Therefore, freshly packed columns were used for all runs in this investigation.

Pulse inputs to packed columns were eluted with a synthetic rainwater


TABLE 2

Eluent compositions

Synthetic rain pH-2.3 waste pH-12 waste*

compound (rag L- i) compound (mg L l) compound (mg L- J ) added added added

KCI 0 .044 AIC13.6H20 8.95 A1CI 3.6H20 894.78 NH4CI 0.527 NaF 4.42 NaF 11.05 NH4NO 3 0 .360 Cd(NO3) 2 .4H20 1.37 Cd(NO3)2.4H20 1.37 HNO 3 0 .690 Cr(NO3)3-9H20 3.85 Cr(NO3) 3 -9H20 7.70 CaSO4 0 .306 Cu(NO3) 2.2.5H20 1.83 Cu(NO3)2.2.5H20 3.66 H2SO 4 1 .206 Fe(NO3)3.9H20 7.23 Fe(NO3)3.9H20 0.72 MgSO4 0 .150 NaNO3 2,939.70 NaNO 3 1,809.76 Na2SO4 0 .923 Pb(NO3)2 0.80 Ni(NO3)2.6H20 2.48

Zn(NO3)2.6H20 1.36 Pb(NO3) 2 1.60 Na2SO4 7.39 Zn(NO3)z.6H20 2.28

NaOH 1.84 Na2SO4 73.92

* Values are concentrations added, not concentrations remaining in solution.

solution representative of rains typical in the southeastern U.S.A. The composition of the synthetic rainwater solution is listed in Table 2. The pH, ionic strength and total dissolved solids (TDS) of the synthetic rain eluent were 4.6, 95.1 #eq L I and 4.1 mg L -1, respectively. For pulse inputs, 2 mL of waste were applied to the top of a freshly packed column followed immediately be elution with synthetic rain. For step inputs, the waste itself was the eluent. Eluent was pumped through the columns by a Manostat ~, peristaltic, cassette pump at a flow rate of 1.0 mL min 1 (corresponding to a velocity of 27 m day-l). All column runs were duplicated.

Total metal concentrations were determined by acidifying the appropriate fraction of column effluent then filtering it through a 0.2-#m Nuclepore ~ filter prior to metal analysis using a graphite furnace atomic absorption spectro- photometer (Perkin-Elmer ®, 1976; APHA, 1989). Anion concentrations in an unacidified, filtered fraction of each effluent sample were determined using a Dionex ® (QIC ®) ion chromatograph (APHA, 1989). Volume collected, pH and turbidity were also measured and recorded for each effluent sample. Turbidity was measured using a Hach ® Model 2100A Turbidimeter and Formazin ® standards.

Turbidity causing particles observed in the column effluent were filtered onto 0.2-#m Nuclepore ® filters, and examined using a JEOL ® scanning electron microscope (SEM). Samples were gold/palladium coated for ima- ging and carbon coated for energy-dispersive X-ray (EDX) analysis. Particle


surface composition was determined by EDX in combination with SEM. In addition, bulk particle compositions were determined by digestion with hydrofluoric acid followed by atomic absorption spectrophotometry (Perkin- Elmer ;~, 1976; APHA, 1989).

R E S U L T S A N D D I S C U S S I O N

Results of pulse inputs

Pulse inputs of pH-2.3 metal waste resulted in two distinct peaks in metal concentration in the column effluent as a function of effluent volume. The first of these peaks occurred at more than one pore volume and corresponded to a peak in turbidity of the column effluent (Fig. 1). The second peak probably resulted from retardation of the metals by adsorption onto soil surfaces, but could have contained more than one species of each metal.

A mass balance indicated that ~ 95% of the Pb and ~ 93% of the Cu applied to the column were recovered in the first 60 pore volumes (PV) eluted. However, only ~80% of Cd was recovered within the first 60 PV. Presumably the metal not recovered was retained by the aquifer material. Fifty-one percent of the Pb applied was eluted in the first peak and 44% in the second peak. Seventy-five percent of the Cu was eluted in the first peak while 18% was eluted in the second peak. For Cd, 51% was eluted in the first peak and 29% in the second. These data indicate that for Pb, Cu, and Cd more than 50% of the metal applied was eluted in association with mobile colloids.

The variability observed in the baseline metal concentration in the column effluent noticeably decreased after elution of the turbidity peak (Fig. 1). This observation indicated naturally occurring metals were mobilized from the aquifer material in association with colloids as well as ions in solution.

O. 15 1 ~ pb 1 300

...I ~ 'N--X - ~ - Cu

0,1- k ,~ ~ Cr 200~

L . o 0

0 ~ o 0 20 40 60

Pore Volume

Fig. 1. Results from a pulse input of pH-2.3 metal waste to an aquifer material packed column, showing metal concentration and turbidity in the column effluent.


Pulse inputs of pH-12 metal waste resulted in more erratic metal concentrations in the column effluent, probably due to experimental difficulties in application of the waste. As expected at pH 12, precipitation of the metals was observed immediately upon addition to the waste solution and continued for several days. The pH-12 waste was applied to the columns as a mixture of metals in solution and precipitates in suspension. Application of this mixture to the columns resulted in an input scenario intermediate between a pulse input and slower leaching from the metal precipitate. Mobile colloids of metal precipitate might also be included in this input scenario. At pH 12, loss of metals due to adsorption on container surfaces was also a problem, resulting in poor metal recoveries and reproducibilities for these runs.

Although the pH-12 waste presented experimental difficulties, the tests were performed as one approach to evaluating how wastes that actually have a pH of 12 will behave. However, due to poor metal recovery and reproducibility at pH 12, the results of single-metal pulse inputs at pH 12 are not presented. Some results for pH-12 wastes were obtained, as discussed later. Also, it was evident that early metal peaks corresponded to a peak in turbidity of the column effluent.

Turbidity peaks were also observed in the effluent of columns eluted with synthetic rain only (blank runs). The maximum turbidity observed in the column effluent (~ 250 NTU*) and the shape of the turbidity curve were similar for all runs involving pulse input of waste and blank column runs.

Columns eluted at flow rates of 0.05 and 2.0 mL min -1 (corresponding to velocities of 1.4 and 54 m day -1, respectively) also showed similar turbidity peaks, indicating the particles were not mobilized solely by increased shear associated with greater flow rates. Groundwater velocities measured in the aquifer from which this material was taken were ~ 0.25 m day -l (Looney et al., 1986).

Column orientation (vertical pumped top to bottom, vertical pumped bottom to top and horizontal) also did not affect effluent turbidity. Lack of effect of column orientation on effluent turbidity indicated gravitational sedi- mentation did not play a primary role in colloid removal from suspension in these column runs. Colloid formation during handling and column packing cannot be ruled out. However, a colloid phase existing in these aquifer material samples, and mobilized by alterations in groundwater chemistry, seems likely.

SEM showed the turbidity-causing particles to be composed mainly of three types: (1) 5-10-#m-sized clay and/or mineral flakes; (2) distinctly rod- shaped particles of ~5 #m in length; and (3) spherical-appearing particles of

* Nephelometric turbidity units.

240 M . E . N E W M A N E T A L .

0. l #m diameter. The spherical particles were present in greatest number, and appeared to form a flocculent coating over the larger particles and the entire filter. EDX analysis showed all three particle types to be composed primarily of A1 and Si in varying ratios, with lesser amounts of Fe. The appearance and composition of the turbidity causing particles remained relatively constant over the entire span of the turbidity peak eluting from the column. Micrographs of filtered supernatant, from the clear layer of turbid effluent samples that had been allowed to settle for over one month, showed a fraction of the smaller spherical particles and rod-shaped particles to have remained in suspension. Cu, Cr, Ni and Zn were detected by EDX on the surface of the smaller spherical particles and the rod-shaped particles. The observations that: (1) colloids may be formed and/or mobilized by alterations in eluent chemistry; and (2) that metals were observed to be transported in association with these colloids, indicated that if colloids are formed or mobilized in aquifers they may facilitate the transport of metals.

After elution with ~ 40 PV, the pH of the column effluent oscillated around the pH of the eluent (4.4) for both blank runs and pulse inputs of the pH-2.3 waste. The pH of the column effluent was altered to a greater extent by pulse inputs of pH-12 waste. After pulse inputs of pH-12 waste the pH of the column effluent increased rapidly to a pH approaching 12 and then decreased to a pH of ~ 8 within the first 25 PV. The pH remained relatively constant at a value of 8 for the remaining 42 PV.

Results of step inputs

Step inputs of pH-2.3 metal waste resulted in metal BTC's resembling those expected for a step input, with little spreading due to diffusion (Fig. 2). At the first appearance of eluent, a phenomenon referred to as the "snow plow" effect was observed, in which the metal concentration in the column

1.2

L)

60 .8 g 0

~ 0.6

~ 0.4

- / ~ 0 . 2 '

Z

\

Pb

-~ - Cd

- -~ eu

~ - Cr

0 10 20 30 40 50 80 70 Pore V o l u m e

Fig. 2. Results from a step input of pH-2.3 metal waste to an aquifer material packed column.

T R A N S P O R T OF S E L E C T E D M E T A L S IN A Q U I F E R M A T E R I A L P A C K E D C O L U M N S 241

effluent was greater than that of the eluent (Fig. 2). This snow plow effect probably resulted from ion-exchange mechanisms (Starr and Parlange, 1979; Chanmugathas and Bollag, 1987). The results of step inputs of metal wastes were more reproducible than those of pulse inputs.

Cr was immobile when applied in a step input of pH-2.3 waste. Cr was added to the synthetic waste as Cr(III), the form reported for the SRS waste (Table 2). Cr(III) is the most stable redox state of Cr in oxic environments. Oxidation of Cr(III) to Cr(VI) by oxygen only occurs at high temperature and pressure (Rollinson, 1973; Moore and Ramamoorthy, 1984; EPRI, 1989). The equilibrium-based speciation model, GEOCHEM (Morel and Morgan, 1972; Sposito and Mattigod, 1979), was used to estimate metal speciation in the waste solution. Because material was collected from an oxygenated aquifer, an oxygenated system was assumed for GEOCHEM and redox reactions were not considered for this case. 6EOCI4EM estimated 78% of the Cr in solution was present as free aquated Cr 3+ under these conditions.

Difficulties in application of the pH-12 waste solution to the columns, similar to those discussed under pulse inputs, were also encountered with step inputs of this waste. A constantly stirred waste mixture was applied to the column, resulting in a step input of metals in solution coupled with slow leaching of metals from precipitates. Metal concentrations in the column effluent exhibited patterns intermediate between those expected for pulse and step inputs (Fig. 3).

In contrast to the pH-2.3 waste, Cr was mobile when applied in the pH-12 waste (Fig. 3). OEOCHEM estimated 100% of the Cr present in solution was complexed with OH- in the pH-12 solution (an oxygenated system without consideration of redox reactions, as described previously). Forty-eight percent of the Cr was estimated to be in the form of Cr(OH) ° and 4% in the form of Cr(OH)4. These Cr complexes are expected to exhibit greater mobility than that of free aquated Cr 3+ ion (Dzombak et al., 1987; EPRI, 1989).

1.2

~ 0 .8

0

~. 0 .6

~ 0.2

Z

" Cu I Cd ~ Pb -I~- Cr

10 2 0 3 0 4 0 5 0 6 0

Pore Volume 70

Fig. 3. Results of a step input of pH-12 metal waste to an aquifer material packed column.

242 M.E. NEWMAN ET AL.

Lead was immobile when applied with the pH-12 waste (Fig. 3). GEOCHEM estimated 80% of Pb present in the pH-12 waste was complexed with OH--. This complexed Pb was largely Pb(OH) + (which was presumably adsorbed) with smaller portions of the sparingly soluble solid Pb(OH)2. Other studies have also observed that at pH> 6, Pb sorbs strongly to clay surfaces and iron oxides (Moore and Ramamoorthy, 1984; Dzombak et al., 1987; Theis, 1988, Evans, 1989; Benoit and Hemond, 1990).

The buffering capacity of the soil was rapidly exceeded by the extreme pH of both wastes when applied as step inputs. The pH of the column effluent reached that of the eluent after 2 PV. Unlike the rest of the column runs, no turbidity was detected in the effluent from step inputs of pH-2.3 waste. The eluent pH of 2.3 may have fallen below the pH of zero surface charge (pHzpc) of the matrix and/or turbidity causing colloids, resulting in a surface charge reversal. A charge reversal of either the matrix or the colloid surfaces might increase attachment of the colloids to the matrix thus reducing colloid transport. Also, the high ionic strength of the pH-2.3 waste may have reduced the diffuse layer thickness, resulting in coagulation and increasing filtration of the colloids. This observation indicated that changes in the chemical characteristics of groundwater, such as pH and ionic strength, may affect colloid mobility in aquifers. Further studies (Newman, 1990) confirmed these hypotheses.

Step inputs of single-metal solutions

Step-input runs of solutions adjusted to the pH of each waste but lacking the ions of the waste matrix and containing only one metal at a time were also performed to more thoroughly evaluate the effect of the waste mixture on metal transport. The competing and complexing ions present in the waste were absent in the single-metal solutions.

The BTC's for Cd and Cu when applied as single-metal solutions at pH 2.4 and 2.0, respectively, were similar to the BTC's of these metals when applied as part of the pH-2.3 waste solution (Fig. 4). The Kd-values, calculated from the inflection point of the BTC's (Relyea, 1982), for Cd and Cu when applied with the pH-2.3 waste were 1.6 and 0.8 cm 3 g-l, respectively, essentially the same as the Kd-values of 1.2 and 0.7 cm 3 g-1 calculated for Cd and Cu, respectively, when applied as single-metal solutions. These results indicate Cd and Cu mobilities were not significantly affected by the formation of complexes with anions in the test waste solution. However, previous studies have observed the adsorption of Cu and Cd to be highly dependent on the surface properties of the adsorbent and solution pH (Christenson, 1985; Anderson and Benjamin, 1990; Muller and Kester, 1990). Therefore the Kd-values reported here may be specific to the soil type used.

TRANSPORT OF SELECTED METALS IN AQUIFER MATERIAL PACKED COLUMNS

1.2

L) --. 1

cdO.B o

20 .6

~0.4

~0.2 Z

Pb

@- Cd Cu

10 20 30 40 50 Pore Volume

243

Fig. 4. Results from a step input of 1.0 ppm Pb, 0.5 ppm Cd and 0.5 ppm Cu solutions at pH 2.3 to aquifer material packed columns.

The Pb step-input BTC when applied as a pH-2.3 single-metal solution occurred considerably later than the BTC observed for Pb when applied as part of the pH-2.3 waste (Fig. 4). The Kd-value calculated for Pb when applied as part of the pH-2.3 waste was 0.8 cm 3 g-l , whereas the Kd-value calculated for Pb when applied as a single-metal solution was 4.8 c m 3 g-l . GEOCHEM estimated 3.2% of Pb present in the pH-2.3 waste solution was complexed with SO ]-, and smaller fractions of Pb present in the waste solution were complexed with CI- and NO3. The remaining Pb was present as the free aquated ion Pb 2+. Assuming Pb complexed with anions would be more mobile than free aquated lead (due to lesser charge), these observations indicate complexation may facilitate the transport of Pb under these conditions. Scott (1990) has demonstrated through groundwater contaminant transport simulations that small fractions (even 1% or 2%) of the metal present in a more mobile form may affect initial arrival times and concentration time profiles significantly. Speciation of Pb may account for field observations of apparent facilitated transport and expected species are consistent with the laboratory results reported here.

S U M M A R Y A N D C O N C L U S I O N S

The chemical characteristics of a groundwater, such as pH and the presence of competing and/or complexing ions, may greatly affect the mobility of metals in an aquifer. Results obtained from laboratory aquifer material packed column studies indicated complexation of metals, specifically Pb, may result in enhanced metal mobility in aquifers when the solution matrix supports the presence of more mobile forms, even when present at a few per cent of the total metal. These results suggest complexation may explain field observations of facilitated contaminant transport. Therefore,


speciation of metals must be considered in groundwater contaminant transport models.

Facilitated transport associated with a mobile colloid phase may also be important. The first of two peaks in metal concentration in column eluent resulting from a pulse input of metal containing waste to aquifer material packed columns was observed to coincide with a peak in effluent turbidity. S E M / E D X analysis showed metals to be sorbed to the surface of turbidity causing colloids. These observations indicated association of metals with mobile colloids may have facilitated the transport of metals through aquifer material packed columns. The mobilization and/or transport of colloids in laboratory columns was apparently influenced by alterations in eluent chemistry, such as pH and ionic strength. These observations indicate association of groundwater contaminants with mobile colloids may also explain field observations of facilitated transport. In order to incorporate colloid-facilitated transport o f pollutants in groundwater models, the effects of changes in groundwater chemistry on colloid formation, mobilization and transport must be quantified.

ACKNOWLEDGMENTS

We thank Todd Scott for his work using the model GEOCHEM. This research was funded in part by contract No. AX-0824637 from the

Savannah River Labora tory of E.I. Dupont de Nemours and Co. from April 1988 through September 1989, for which we are very grateful.

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facilitated transport of selected metals in aquifer material packed columns

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