Landfill leachate effects on sorption of organic micropollutants onto aquifer materials

Journal of Contaminant Hydrology, 9 (1992) 307-324 307 Elsevier Science Publishers B.V., Amsterdam

Landfill leachate effects on sorption of organic micropollutants onto aquifer materials

T h o m a s Larsen a, Thomas H. Christensen a'l * Fred M. Pfeffer b and Carl G. Enfield b

aDepartment of Environmental Engineering, Groundwater Research Centre, Building 115, Technical University of Denmark, DK-2800 Lyngby, Denmark

b R.S. Kerr Environmental Research Laboratory, U.S. Environmental Protection Agency, Ada, OK 74820, USA

(Received May 21, 1990; revised and accepted September 5, 1991)

ABSTRACT

Larsen, T., Christensen, T.H., Pfeffer, F.M, and Enfield, C.G., 1992. Landfill leachate effects on sorption of organic micropollutants onto aquifer materials. J. Contam. Hydrol., 9: 307-324.

The effect of dissolved organic carbon as present in landfill leachate, on the sorption of organic micropollutants in aquifer materials was studied by laboratory batch and column experiments involving 15 non-polar organic chemicals, 5 landfill leachates and 4 aquifer materials of low organic carbon content. The experiments showed that hydrophobic organic micropollutants do partition into dissolved organic carbon found in landfill leachate potentially increasing their mobility. However, landfill leachate interacted with aquifer materials apparently increases the sorbent affinity for the hydrophobic micropollutants. The combination of these two mechanisms affected the observed distribution coefficients within a factor of two, in some cases increasing and in other cases decreasing the sorption of the chemicals. No means for prediction of the effect is currently available, but from a practical point of view, the effect of landfill leachate on retardation of organic micropollutants in aquifer material seems limited.

INTRODUCTION

Understanding organic chemical transport through soil and aquifers is requisite for making intelligent decisions on waste disposal and on the remedial actions necessary when wastes are disposed of improperly. Ground- water supplies more than 99% of the drinking water in Denmark (Linde- Jensen et al., 1976); while 25% of all freshwater use in the U.S.A. is groundwater (McCarty et al., 1981). These figures, coupled with the enormous cost of groundwater remediation, underscore the importance of understanding the

* Corresponding author.

0169-7722/92/$05.00 O 1992 Elsevier Science Publishers B.V. All rights reserved.

3 0 8 x LARSEN ET AL.

phenomena that can affect organic chemical transport through soil and groundwater.

The transport of organic chemicals through aquifer material is governed by many physical, chemical and biological processes. Sorption of neutral organic compounds (NOC's) has usually been described utilizing theories and concep- tualizations developed for liquid chromatography. In these models, it is assumed that the concentration of a chemical in solution is low relative to its solubility, that chemicals in solution act independently of each other, and that sorption is driven by the favourable thermodynamics of solute removal from solution through sorption on a stationary and unchanging hydrophobic phase (Horvath et al., 1976). See Boesten and Leistra (1983), Rao and Jessup (1983), and Brusseau and Rao (1989) for reviews of mathematical models used to describe the effects of advection, dispersion, sorption and transformation on solute transport through soil and aquifer material. The degree of organic chemical sorption, and therefore the retardation relative to the movement of water is a function of solute hydrophobicity and the amount of hydrophobic sorbing phase. Properties that reflect chemical hydrophobicity, such as aqueous solubility, octanol/water coefficient (Kow), and reverse-phase chro- matographic retention time, have been successfully used to predict sorption (Carlson et al., 1975; Kenaga and Goring, 1980; Brown and Flagg, 1981; Karickhoff, 1981; Chin et al., 1986; Weber et al., 1986). In addition, the importance of solid organic carbon as a hydrophobic sorbing surface has been repeatedly demonstrated (Karickhoff et al., 1979; Chiou et al., 1983).

Enfield et al. (1989) questioned the assumption that chemicals in solution act independently of each other and demonstrated that dissolved organic carbon (DOC) in the fluid phase could increase the mobility of hydrophobic compounds, while Bouchard et al. (1988) and Lee et al. (1989) questioned the assumption of a stationary and unchanging hydrophobic phase and demonstrated that a cationic surfactant could significantly increase the partitioning of neutral organics in an aquifer material. Landfill leachate is a complicated mixture of organic chemicals which has been extremely difficult to characterize (Chian and DeWalle, 1977; Harmsen, 1983; Weis et al., 1989). It is not possible to predict if a landfill leachate will facilitate the transport of organic chemicals of environmental concern because of: (1) partitioning into DOC in the fluid phase as proposed by Enfield et al. (1989), (2) since most uncon- solidated soils with a high hydraulic conductivity are also low in natural organic carbon (Larsen et al., 1991), the landfill leachate will react with the soil increasing the retardation of the chemical in a manner similar to that proposed by Bouchard et al. (1988) and Lee et al. (1989), or (3) the normal assumptions of an unchanging stationary phase and independent action of contaminants in the fluid phase are appropriate.

The objective of this study was to investigate the influence of landfill

LANDFILL LEACHATE EFFECTS ON SORPTION OF ORGANIC MICROPOLLUTANTS 309

leachate on the sorption of selected neutral organic pollutants on aquifer material. The study was performed utilizing pristine aquifer material from four sites near sanitary landfills in Denmark and five different landfill leachates. The study was divided into three experiments: (1) the ability of DOC in landfill leachates to partition hydrophobic contaminants was measured with two compounds in two leachates; (2) dynamic column studies were performed with twelve compounds, one leachate and three aquifer materials to determine the overall significance of the landfill leachate on sorption of several common contaminants with relatively low Kow-values (column studies are preferred by the authors for simultaneously studying many compounds exhibiting little sorption); and (3) batch studies were performed with one compound, three aquifer materials and three leachates to study the influence on the partitioning of leachate concentration and eventual differences in combinations of leachate and aquifer materials. The components used in the various phases are shown in Tables 1-3.

MATERIALS AND METHODS

Aquifer materials

The aquifer materials used in this study originate from four drillings in Denmark and were taken by sand bucket. All samples were air-dried at room temperature and sifted to < 2 mm. Characteristics of the aquifer materials are shown in Table 1. Size distribution was measured by the hydrometer method. In addition the size distribution was measured for the < 75-#m fraction in a grain-size analyzer giving the continuous weight distribution in equivalent spherical diameters, Micrometrics ® Sedigraph 5000 ET. The specific surface

TABLE 1

Texture, organic matter content as TOC and specific surface area of the studied aquifer materials

Finderup Vasby Vejen Rabis

Coarse sand, 0.2-2 mm (%) 34 63 90 84 Fine sand, 0.02-0.2 mm (%) 63 34 8 15 Silt, 0.002-0.02 mm (%) 2 2 1 0 Clay, <0.002mm (%) 0 1 1 I TOC (%) 0.213 0.012 0.029 0.016 Specific area (m 2 g- l ) 2.5 0.8 1.4 0.4 Used in experiment No. 3 2, 3 2, 3 2

TOC is calculated from TRC values according to Mebuis (1960), see text for further explanation.

3 1 0 i. LARSEN ET A L

area of the aquifer material was measured by N 2 sorption according to the Brunauer-Emmett-Teller isotherm.

Since direct determination of total organic carbon (TOC) in low organic carbon soils, especially in soils with significant amounts of carbonates, is difficult (Powell et al., 1989; Ball et al., 1990), a standard wet dichromate oxidation method measuring the total reduction capacity (TRC) was chosen for the aquifer materials. Pedersen (1989) showed a highly significant correlation between TRC as measured by the wet oxidation method and TOC in a Danish aquifer. The method used to measure the TRC is a modification of a method by Mebuis (1960) used to determine TOC in top soils. From Mebuis the conversion factor between TRC and TOC is found to be: TOC (%) = 3.75.10 -5 TRC (mg O2/kg).

Leachates

Five leachates were used in the study: four from Denmark and one from Oklahoma, U.S.A. The Oklahoma sample was collected in Norman, Oklahoma, from a well located in a closed municipal landfill and screened 2 m below the soil surface. The sample was filtered using an all glass apparatus and 0.45-/~m Millipore ~ filter (HA ® mixed cellulose acetate and cellulose nitrate) with Millipore ® prefilter (AP ® borosilicate glass). The filtrate was stored until used at 4°C in glass 4-L bottles with minimum headspace. The Forlev leachate was sampled twice, once in 1987 and once 1990. The Forlev, the S~rup and the Vejen leachates all originate from the drainage systems of old methanogenic landfills showing low values of DOC. The untreated groundwater used for the column studies was obtained from the municipal drinking water plant, Dybendal, Lyngby, Denmark. All Danish leachates were sampled and filtered using a fibreglass filter. The leachates were kept at 4°C until use. Table 2 lists the organic content of the leachates. TOC of the leachates was either

TABLE 2

Organic carbon content of the leachates

Leachate TOC Used in experiment (mgl ~ C) No.

Norman 200 1 Forlev 1987 355 1, 2 Groundwater 5 For|ev 1990 330 3 Sorup 313 3 Vejen 609 3

LANDFILL LEACHATE EFFECTS ON SORPTION OF ORGANIC MICROPOLLUTANTS 31 i

determined by a Beckman ~ 915 or a Dohrmann ~ DC-80 as described by Powell et al. (1989).

Organic micropollutants

Several organic micropollutants were used in this study. Table 3 lists the octanol/water partition coefficient and the water solubility of the compounds. Hexachlorobenzene (HCB) and benzo-a-pyrene (BaP) were chosen as model compounds for the study of partitioning into DOC because of their high hydrophobicity. HCB is found in many pesticide formulations as a by- product and BaP is found in the wastes at coal gasification plants and wood treating plants. For the column studies, the more soluble compounds were chosen (cf. Table 3). The least soluble compound was biphenyl. The list of compounds used for the column experiments include commonly found contaminants such as chlorinated aliphatics, oil and gasoline components, and tar components. Phenanthrene was chosen for the batch experiments as a compound having moderate solubility. In batch experiments, compounds were ~4C labelled. Because of their low solubility and low specific activity, ~4C labelled HCB and BaP were used at ,--50% of their water solubility.

TABLE 3

Characteristics of the organic compounds ranked by their octanol/water partition coefficient

Compound log Kow Molar Solubility Used in weight (mg 1-1 ) experiment (g mol- 1 ) No.

Benzene 2.15 78 1,780 .1 2 Trichloroethylene 2.29 132 1,100 .1 2 1,1,1-Trichloroethane 2.49 131 1,360"~ 2 Tetrachloroethylene 2.60 162 400 * 2 2 Tetrach|oromethane 2.62 150 785"3 2 Toluene 2.80 92 515 *2 2 Indene 2.92 116 - 2 o-Xylene 3.12 107 175"1 2 1,4-Dichlorobenzene 3.38 147 79 *4 2 1,2-Dichiorobenzene 3.38 147 145,4 2 Naphthalene 3.59 128 32,1 2 Biphenyl 4.09 *2 154 8 *5 2 Phenanthrene 4.46 *6 178 1.3 *6 3 Hexachlorobenzene 5.5 *6 285 0.006 *6 1 Benz(a)pyrene 6.0 *6 252 0.004 *6 1

All values of IogKow are based upon the compilation by Hansch and Leo (1979) except otherwise noticed: *j Mackay et al. (1980); ,2 Hasset et al. (1983); ,3 Schwille (1981); ,4 Friesel et al. (1984); *SChristensen et al. (1987); *6Isnard and Lambert (1989).

3 1 2 I LARSEN ET AL.

Fig. 1. Outline of the experimental setup used for the determination of dissolved organic carbon/water partition coefficient.

Experiment 1. Partitioning into leachate DOC

Boiling flasks (125 mL) were connected with a 60 ° adaptor as shown in Fig. 1. Landfill leachate (50mL) was placed in one flask and deionized water (50 mL) was placed in the other flask. Both flasks were spiked at 50% of its water solubility with either 14C labelled HCB or BaP. The 14C labelled compound supplied by Sigma ~ was initially dissolved in a very small amount of hexane and diluted with methanol to make the spiking solution. The labelled compounds were allowed to distribute between the water and the leachate compartment by vapour phase exchange. Both flasks were monitored as a function of time by extracting 1-mL samples from both flasks, adding 6 mL of Beckman ~ HP cocktail, and counting on a Beckman j~ 7800 scintillation counter for 60 min, or until the 95% confidence limit of _+ 1.40% was obtained, whichever occurred first. All experiments were run in duplicate, and the results were averaged for presentation.

Experiment 2. Dynamic column studies with and without landfill leachate

Columns 30cm in length and 10cm in diameter were packed with aquifer material and equilibrated with groundwater or Forlev 1987 leachate. Constant influent experiments were run at a constant flux comparable to a pore velocity of 10myr i [see Larsen et al. (1989) for description of the experimental arrangement]. The columns were equilibrated with leachate before the compounds and a hydraulic tracer (tritium) were added to the influent initiating the experiment. Samples were taken when each 100 mL had passed the column. The effective pore volume was ,-~800mL in all three aquifer materials (Rabis, Vejen, Vasby). The stock solution was kept at I°C and spiked with 0.2% sodium azide to inhibit biodegradation. All equipment in contact with the samples was made of stainless steel. Compensation gas to


the feeding vessel was equilibrated with the investigated compounds before use to prevent losses due to volatilization (Larsen et al., 1989). Samples were extracted using pentane in alkaline solution. The compounds were quantified with gas chromatography using an internal standard. A DANI ® 8500 equipped with flame ionization and electron capture detectors and a 30m x 0.53mm JandW Scientific ® DB-5 capillary column was used for determination. Along with samples from the column outlet, inlet samples were analyzed to determine concentration stability. Standard deviation of the inlet concentration was 10-15%.

Experiment 3: Batch studies with various combinations of leachates and aquifer materials

An 8.5-mL reagent tube with Teflon®-lined screw cap was filled with 10 g of aquifer material. Leachate spiked with the model compound phenanthrene below solubility was added and the tube was shaken to eliminate pore trapped air. More stock solution was added to minimize headspace. The tubes were placed in a rotational shaker and slowly rotated end over end to obtain complete mixing. Along with the samples, blanks without any aquifer material were included. The samples were rotated for 96 hr and then centrifuged for 15 min at 2000 g to separate solute and solids. A 1-mL aliquot of supernatant was used for scintillation counting by a Packard ® TriCarb 2000 with automatic quench correction. Sorption isotherms were determined by these batch methods for various dilutions of leachate and various combinations of leachate and aquifer material.

Data analysis

The partitioning data in experiment 1 are evaluated in terms of Kp-values expressing the concentration of the compound associated with the DOC relative to the concentration of the compound in water by:

Kp = (C,-Cw)(DOC.Cw)-'

where C~ and Cw are the concentration of the compound in the leachate (1) and water (w) compartment, respectively; and DOC is the dissolved organic carbon in the leachate determined as carbon. The unit of Kp is (mg compound per kg C)-(mg compound per L water) - l , or L per kg C.

The column breakthrough curves (BTC's) of experiment 2 were analysed using a non-linear least-squares routine fitting the observed data points to the one-dimensional convective-dispersive transport model assuming local equilibrium, no transformation and flux boundary conditions (LEA model). Analytical solution of the LEA model can be found elsewhere (van Genuchten

3 1 4 I L A R S E N E T AL.

120000-

100000-

80000-

60000"

40000"

20000"

0 0

K p

z&

r) O 0 0

oFor lev HCB I Forlev BaP

c, Norman BaP

I I I I I 100 200 300 400 500 TIME (hr)

Fig. 2. Observed dissolved organic carbon/water partition coefficient for hexa-chlorobenzene (HCB) and benzo(a)pyrene (BaP) in Norman and Forlev 1987 leachate as a function of equilibration time. See text for the definition of Kp.

and Alves, 1982). BTC's for 3H20 were used to determine the hydraulic properties of the columns. Brusseau et al. (1991) showed that the kinetic impact on the transport in an experiment using the same setup was negligible; therefore, the equilibrium assumption is valid. From the BTC's, distribution coefficients, K d were estimated.

The equilibrium sorption isotherms of experiment 3 were analyzed using a linear model of the form:

S = K d C

where S is the calculated sorbed concentration; C is the measured liquid concentration; and K a is the distribution coefficient.

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

Experiment 1

The partitioning experiment showed that the organic micropollutants were concentrated in the leachate compartment, indicating that DOC in landfill leachate may sorb specific organic compounds as demonstrated for HCB and BaP having relatively high Kow-values. The results are shown in Fig. 2 as Kp-values vs. equilibration time. Kp expresses the ability of DOC on a carbon base to sorb the components relative to water.

The estimated values for log Kp are 4.3 and 4.8 for HCB and BaP, respectively. The time required to approach equilibrium for HCB was much quicker than the time required for the BaP which can be expected based on their vapour pressures (HCB = 1.09-10-SmmHg @ 20°C; BaP = 5 .6 .10 -gmmHg @ 25°C; Mabey et al., 1982). The partition coefficients for the BaP with the Forlev 1987 leachate and the Norman leachate are similar. The obtained results are discussed in brief in view of literature information on partitioning into humic materials.


According to Hutchins et al. (1985), Carter developed a humic material/ water partition coefficient vs. an octanol/water partition coefficient relationship as:

log Kp = 0.7091og Kow + 0.748 (1)

West (1984) measured partitioning of hydrophobic materials to groundwater humic materials and developed the following correlation between water solubility and humic material/water partition coefficient:

log Kp = - 0.9231og S(mg L-1 ) + 3.294 (2)

A comparison between these two regression models which yield log Kp's for HCB of 4.6 and 5.3, respectively, and 5.0 and 5.5 for BaP, respectively, show Carter's relationship to give slightly lower estimates for Kp. The values obtained in this experiment are very similar to those estimated by Carter and slightly less than those projected by West (1984).

The partitioning of trace organic pollutants to natural and synthetic humic material has recently been considered. Landrum et al. (1984), using Sep Pak ~ ClS-cartridges to separate humic bound and freely dissolved pollutants, measured by log Kp for Aldrich ® humic acid equal to 3.9 for anthracene and 5.1 for p,p'-DDT, which has the same Kow as HCB, and 5.95 for BaP. Alberts et al. (1989) found the log Kp for BaP in estuarine organics to range from 3.4 to 4.3. Gauthier et al. (1987) found that Kp with pyrene for a number of humic materials isolated from marine and terrestrial environments also varied by a factor of 10 and was strongly dependent on the aromaticity of the humic material, whereas Chiou et al. (1979) emphasized molecular size and polarity of the humic material in controlling the partitioning. Malcolm and MacCarthy (1986) compared the characteristics of commercial humic acids and concluded that they were significantly different from each other and from humic materials extracted from soils.

The variation in Kp depending on the "quality" of dissolved humic and fulvic acid is substantiated by the geographical and temporal variation of Kp for natural DOC from lakes and seawater (Landrum et al., 1984; Whitehouse, 1985). Although there is significant evidence that DOC from different sources behaves differently, the variations observed for partitioning to the DOC in landfill leachate appear not to vary by more than a factor of 2 for the two

-samples evaluated in this study. The DOC found in landfill leachate also appears to behave in a manner similar to other humic substances which have been previously studied. The partitioning suggests that transport of neutral organics theoretically may be affected due to the presence of DOC in the fluid. However, the actual significance will depend on the concentration of DOC in the leachate and the Kow-value of the compound as shown below.

316 I LARSEN ET AL.

12

1

0 8

0.6

0 4

0.2

0

C/C0

i;' ~ : -

I -' 7'4,

2 J 4 ~ / ; ,a

o 2

/~ PDCB Le.

' PDCB Gr.

Tritium Gr.

- - Tritium Le.

i i

4 6 Pore volumes

Fig. 3. Example of breakthrough curves for 1,4-dichlorobenzene (PDCB) in Vejen aquifer material for columns with leachate (Forlev 1987) and with groundwater. Tritium breakthrough curves are included as reference.

Experiment 2

The dynamic column experiments determining the effect of landfill leachate on sorption of 12 common contaminants with low Kow-values (see Table 3) under realistic flow conditions and solid/solute ratio yielded breakthrough curves as illustrated in Fig. 3 for p,p'-dichlorobenzene (PDCB) in groundwater and leachate, respectively. PDCB is retarded relative to the hydraulic tracer and apparently the retardation in the presence of leachate is larger than in the presence of groundwater.

The information from all the breakthrough curves in terms of estimated Kd-values is compared in Fig. 4 for the three aquifer materials. The comparison is made in terms of plots of Kd-values determined for columns with leachate vs. Kd-values determined for columns with groundwater.

For a given aquifer material the impact of the leachate on the distribution coefficient was consistent. The data points for the Vejen aquifer material (Fig. 4a) and the Vasby aquifer material (Fig. 4b) apparently reveal a compound sorption in the column with leachate which is equal or greater than the sorption in the groundwater system. This suggests that the leachate is increasing the distribution coefficient of the soil in a manner similar to the mechanism proposed by Bouchard et al. (1988) and Lee et al. (1989) and that the increase in distribution coefficient is more significant than the increase in mobility due to the DOC as proposed by Enfield et al. (1989). In contrast, the Rabis aquifer material (Fig. 4c) shows a slight decrease in the calculated distribution coefficient as a result of the leachate. If partitioning to the DOC were the only factor which should influence the distribution coefficient, all of the data should have been showing a decrease in distribution coefficient as a result of the leachate and all of the data points should have been below the 1:1 line displayed in the figures. Regression analysis (r 2 = 0.97-0.99) on the observations presented in Fig. 4 yielded slope values that were statistically

LANDFILL LEACHATE EFFECTS ON SORPTION OF ORGANIC MICROPOLLUTANTS 3 | 7

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 0

0.6

0.5

0.4

0.3

0.2

0.1

0

1.6

1.4

1.2

1

0.8.

0.6

0.4

0.2,

0 0

Kd in leachate [ml/g]

C,

1:1

K d in groundwater [ml/g] I I I I I I I

0.1 0.2 0.3 0.4 0.5 0.6 0.7

K d in leachate [ml/g]

~ ~ ~ ' ) ~ 1:1

K d in groundwater [ml/, , , , , g]

0.1 0.2 0.3 0.4 0.5 0.6

Kd in leachate [rnl/g]

1:1

o o

ml Kd in groundwater [ /g] " I I I I I I I |

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Fig. 4. Distribution coefficients obtained in Forlev 1987 leachate vs. distribution coefficients obtained in groundwater for twelve compounds (see Table 3) using three aquifer materials: (a) Vejen; (b) Vasby; and (c) Rabis. Solid line has a slope of one.

significant from 1.0 at a 95% confidence level: 0.83, 1.49 and 1.16 for Rabis, Vejen and Vasby, respectively.

Experiment 3

The column studies in experiment 2 revealed differences between the three studied aquifer materials in combination with the single type of leachate used in the experiments. In experiment 3 the batch studies involved 3 leachates and 3 aquifer materials in order to reveal any differences in the effect on the

318 I L A R S E N E1 A L

2

15

1

0 .5

0

2-

1.8-

1.6 ~ 1.4.

1.2-

1

0.8

0.6 0.4

0.2

0

1 0 0

9 0

80~

7 0

60.

50.

4O

3O

2O

10

0

3.5- K~j [ml/g]

3

2 .5

i

100

K o [ml/g]

i

100 200

Ko [ml/g]

(a)

' ' ' ~ ' ' D O C 200 300 400 500 600 700 [mg/I]

i

300 400

(b)

' ' ' ' D O C 500 600 700 800 [mg/I]

(c)

' ' ' ' ' ' ' D O C 100 200 300 400 500 600 700 [mg/I]

Fig. 5. Distribution coefficients of phenanthrene in three aquifer materials as a function of leachate DOC in three aquifer materials: (a) Vasby; (b Vejen; and (c) Finderup, using three leachates. Forlev indicates Forlev 1990 leachate.

sorption behaviour by various combinations of leachate and aquifer material. For the model compound phenanthrene, Fig. 5 shows the measured Kd-values for various dilutions of the three employed leachates expressed in terms of DOC concentration. The results show either no impact due to the leachate or a reduction in the distribution coefficient by as much as a factor of 2. Fig. 5 also reveals that both leachate and aquifer material are significant factors affecting the sorption behaviour, e.g. the Vejen leachate has for all three aquifer materials a significant effect at high DOC concentrations, while on the


other hand sorption onto the Vasby aquifer materials seemed to be less affected by the presence of DOC than were the other two aquifer materials.

The results of experiment 3 cannot be compared directly to experiment 2, because the leachates involved, due to the time elapsed between the two experiments, were not identical.

Effects on retardation

From the experimental data it seems that the general effect of the dissolved organic matter in the leachate independent of the compound's hydrophobicity is to decrease or increase by as much as a factor of 2 the distribution coefficient. The leachate can either increase or decrease the net distribution to the aquifer material, but the complex nature of the leachate and aquifer material interactions has merely just been identified and currently no means exist to predict the effect of a given leachate on the sorption of specific micropollutants onto a given aquifer material.

The effects of the landfill leachate DOC on sorption of specific micropollutants as identified in these experiments may be evaluated in terms of changes in relative solute transport velocity by the equation:

vJv . = (1 +p/EKd)-'

where v, and v. are the pore flow velocity for the specific solute (s) and the water (w), respectively; p is the bulk density of the aquifer material; e the porosity; and K d the distribution coefficient for the specific micropollutant. In Fig. 6 the above relation is simplified to:

v,/v. = ( l + S K d ) - '

% Water velocity

100

10

1

0.1

0.01 , , , , 0 1 2 3 4

I ~ " EXPER. [ NO EFFECT FAC. MOD.

I I

S 6 log Kow

Fig. 6. Compound relative velocity as a function of octanol/water partition coefficient assuming an fo~ of 0.05% and a DOC of 300mgL -~ . EXPER. represents the data range found in this study. NO EFFECT is based on the equation of Schwarzenbach and Westall (1981), representing no effect of DOC on sorption of specific organics. FAC.MOD. is the facilitated transport model as presented by Enfield et al. (1989) assuming partitioning into DOC.

320 T LARSEN ET AL.

and Ka is estimated for varying conditions as a function Kow. The curve marked NO EFFECT shows the expected relative solute velocity assuming no effect of DOC present in the solution. Ko is in this case estimated from Schwarzenbach and Westall (1981) assuming a typical aquifer material content of 0.05% Corg. The band covering the NO EFFECT curve is also based on Schwarzenbach and Westall (1981) but here the K s-values have been multiplied or divided by a factor of up to 2, as experienced in the above presented studies. It is seen that a factor of 2 may be substantial with respect to sorption as such but seen in perspective of the relative solute transport velocity, the effect of DOC in landfill leachate has much less significance. For specific micropollutants of low Kow-values, which are the most common organic groundwater pollutants, for example having a no effect Ka-value of 0.1, the relative solute transport velocity is of the order 65% and accounting for the presence of DOC it would be in the interval 50-80% according to the results of this experiment. For hydrophobic compounds, for example having a no effect Kd-value of 10, the relative solute transport velocity is of the order of 2%, and accounting for the presence of DOC, it would be in the interval I-4%.

Fig. 6 also presents the relative solute velocity estimated by the facilitated transport model (Enfield et al., 1989), using Carter's relationship (eq. 1) to estimate the partitioning into the leachate DOC, here set at 300 mg L-~ C. Obviously the effects of landfill leachate DOC on transport of specific organic micropollutants do not obey the facilitated transport model. Above a log Kow of 4 large discrepancies are encountered.

FUTURE RESEARCH NEEDS

The conducted experiments constitute a first generation approach to evaluate the effects of landfill leachate DOC on the sorption of specific organic micropollutants on to low-Corg aquifer materials with a view to its practical implications.

Basically three mechanisms seem to be involved in the problem: sorption of specific micropollutants onto aquifer material, partitioning of specific micropollutants into DOC in the leachate, and interactions between leachate and aquifer material.

The first mechanism has been fairly well studied previously (cf. the introduction) and the conducted column experiments involving groundwater also observed increasing retardation for increasing compound Kow-values. However, with respect to the aquifer materials, which have a very low content of Corg (< 0.1%), the governing sorptive sites are not well identified. Larsen et al. (1991) showed for 20 aquifer materials low in Corg that the carbon content only accounted for half the variance in K d-values. One major problem


associated with this aspect is the lack of precise methods to determine Corg when carbonate is also present in the sediment.

The second mechanism was proved in this experiment since HCB and BaP did partition into the DOC of the leachate, and the calculated Kp-values seemed to be of the right order of magnitude. Less hydrophobic compounds have been reported to partition into humic material, and although landfill DOC is not completely identical with humic material, it is likely that less hydrophobic compounds will also partition into landfill DOC, but of course to a less extent.

The third mechanism accounting for the interactions between leachate and aquifer material is not well understood. Kjeldsen (1986) reported for three different soils and aquifer materials studied in column experiments that the retardation factor (R = 1 + p/EKd, cf. previous sections) for dissolved organic matter (measured as chemical oxygen demand, COD) was for methanogenic landfill leachate of the order of 1.05. This is identical to a Kd-value of ,,~0.01mLg -1. The retardation of DOC in Kjeldsen's experiment was apparently very minute. Applying the results of Kjeldsen (1986) to the batches in experiment 3, assuming an initial leachate DOC of 300 mg L-i C and an aquifer OC of 0.05%, equilibrium conditions would correspond to a DOC of 285 mg L-~ C and an Corg value of 0.0503%. Both differences would be very hard if not impossible to identify analytically. The calculation shows that the increase in aquifer material OC is insignificant and could not account for any differences in micropollutant Kd-values as observed. Either the data by Kjeldsen is not applicable to the aquifer materials shown here, or the interaction between leachate and aquifer material is more sophisticated than simple sorption.

Identification of the mechanisms behind the observed complex patterns would need sophisticated methods to characterize the aquifer material, e.g. some of the approaches recently investigated by Ball et al. (1990), and to characterize the DOC of the leachate, e.g. some of the approaches presented by Weis et al. (1989). However, in our opinion, such elaborate studies would primarily be of academic interest, since the effects of landfill leachate DOC identified in this study is only of minor importance (cf. Fig. 6) from a practical point of view. The uncertainty that the presence of DOC in landfill leachate adds to prediction of organic micropollutant transport is definitely not exceeding the uncertainty caused by the many other factors governing their transport.

CONCLUSIONS

The DOC in landfill leachate does have an impact on the sorption of organic micropollutants onto aquifer material. The assumption that organic

322 ~ LARSEN ET AL

compounds in solution do not interact with each other, or the assumption that the stationary phase is unchanged, is not always valid. However, the impact of these assumptions on the transport of contaminants is significantly less than might have been expected. With the five landfill leachates and four aquifer materials studied, it appears that landfill leachate may react with the aquifer material increasing the distribution cofficients and, simultaneously, DOC enhances the mobility of the micropollutants.

It is not possible to predict at the present time the impact of a given leachate on a given aquifer material without experimental measurements. In most environmental situations the error introduced by assuming that organic contaminants in landfill leachate have the same mobility as in water, will be small in comparison to other errors and uncertainties related to describing the solute transport in the aquifer.

ACKNOWLEDGEMENTS

Although the research described in this article was funded in part by the U.S. Environmental Protection Agency through in-house efforts at the Rober t S. Kerr Environmental Research Laboratory, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.

The Danish contribution to this paper has been funded in part by the National Agency for Environmental Protection and the Commission of the European Communities.

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Landfill leachate effects on sorption of organic micropollutants onto aquifer materials