JOURNAL OF

Contaminant Hydrology

ELSEVIER Journal of Contaminant Hydrology 21 (1996) 269-279

Colloid and suspended particle migration experiments in a granite fracture

Peter Vilks, Don B. Bachinski Geochemistry Research Branch, Whiteshell Laboratories Pinawa, Man. ROE ILO, Canada

Received 15 December 1994; accepted 11 May 1995 after revision

Abstract

To determine the mobility of colloids (0.001-0.45 pm> and suspended particles (> 0.45 pm) in granite fractures, laboratory particle-migration and conservative tracer studies have been carried out in a natural fracture within a large granite block, with overall dimensions of 83 X 90 X 60 cm. Flow fields within this horizontal fracture were controlled through a set of 9 boreholes drilled orthogonally to the fracture. Laboratory experiments were performed using a range of average water velocities which contained values low enough to closely approximate the natural flow velocities of < 2 m yr -I in plutonic rocks of the Canadian Shield. The particles used had diameters between 0.02 and 22 pm, and included latex spheres, glass spheres and colloidal silica. Migration experiments were carried out with a filtered groundwater, ionic strength of 0.01 mol kg-‘, obtained from a granite fracture within the Whiteshell Research Area of Manitoba. Flushing experiments showed that suspended particles as large as 40 pm could be mobilized from the fracture surface. The mobility of suspended particles was significantly less than that of colloids. However, within the size range of colloids used in these studies (0.022-0.090 pm), colloid size did not affect colloid migration. Although, in general, colloids eluted ahead of the conservative tracer, colloid mobility was significantly reduced when the average groundwater velocity dropped below between 32 and 240 m yr-‘. Colloid transport was found to be very sensitive to flow path and flow direction.

1. Introduction

AECL Research is assessing the concept of nuclear fuel waste disposal in an engineered vault at a depth of 500-1000 m in plutonic rock of the Canadian Shield (Dormuth and Nuttall, 1987; Torgerson, 1990). Radionuclide transport in groundwater is the only likely path for radionuclide migration to the biosphere through the mass of rock surrounding a disposal vault. The transport of radionuclides in groundwater systems can be modelled as a chromatographic process in which radionuclides are partitioned

Elsevier Science B.V. SSDI 0166.3542(95)00053-4

270 P. Vilks, D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279

between a mobile fluid phase and an immobile solid phase (Freeze and Cherry, 1979). However, if radionuclides become attached to mobile solid particles to form radiocol- loids, the migration rate of radionuclides could be increased. There is evidence that particle transport underground does occur and may affect contaminant migration (Mc- Dowell-Boyer et al., 1986; Vilks and Drew, 1986; Mills et al., 1991). These particles include colloids, with diameters between 0.001 and 0.45 pm, and suspended particles larger than 0.45 pm (Vilks et al., 1991).

The literature contains theoretical models for describing particle transport through porous and fractured media (Yao et al., 1971; McDowell-Boyer et al., 1986; Hunt et al., 1987; Travis and Nuttall, 1987; Mills et al., 1991; Grindrod et al., 1993; Smith, 1993). However, due to the complex nature of natural particles and the geometry of subsurface flow paths, the ability of these models to predict particle transport in natural systems is limited. Particle migration experiments, guided by an understanding of colloid chemistry and filtration theory, are required to develop a better understanding of particle transport through fractures and porous media, and to calibrate existing transport models. Although particle migration experiments have been reported for porous media (Neretnieks, 1978; Eichholz and Craft, 1982; Gerba and Goyal, 1985; Tipping et al., 1991) few particle transport experiments have been carried out in fractures.

AECL Research initiated laboratory- and field-scale experiments to evaluate the mobility of colloids and suspended particles within granite fractures. Although field migration experiments are carried out on scales more relevant to a potential disposal site, laboratory-scale experiments have advantages for studying colloid transport. For exam- ple, there is more control over flow path and particle mass balance, and it is possible to use low flow rates which reasonably approximate natural conditions. In this paper we report laboratory-scale migration experiments carried out within a natural fracture in a large granite block (LB4) at the Large Block Radionuclide Migration Facility located at Whiteshell Laboratories (Vandergraaf and Drew, 1991). The objective was to investigate particle mobility as a function of particle size, water velocity, flow direction and flow path. In addition to studying the migration of particles introduced into the fracture, the mobilization of natural particles from fracture surfaces by water flow was also investi- gated.

2. Materials and methods

Granite block (LB4), having dimensions of 83 X 90 X 60 cm, was obtained from the Whiteshell quarry near Whitemouth in southeastern Manitoba, Canada. After selecting a section of granite outcrop containing an open fracture, the block was cut using a wire saw and a course grinding powder. The main fracture within the granite block (Fig. 1) dips - 9” from a high, near boreholes 7 and 8, towards side C-D. The granite block also contains several vertical fractures, whose orientation was approximated by their traces on the block’s surface and their intersections with boreholes.

Nine 3.8-cm-diameter boreholes were drilled orthogonally from the surface of LB4 until the main fracture was intersected. After saturating the fractures and interconnecting pore spaces with groundwater, the granite block was coated with a silicone-based rubber

P. Vilks, D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279

LARGEGRANITEBLOCK: LB4

271

B

- Experimental Flow Paths Main horizontal fracture

---___- Traces of inferred fracture planes D Inferred vertical fracture

Fig. 1. Schematic diagram of large granite blocks (LB4), showing the main fracture, the nine borehoies and the orientations of inferred fractures. The directions of migration experiments described in this paper are shown by arrows on the main fracture.

compound to seal the outer surfaces. To further eliminate evaporative loss, LB4 was immersed in a stainless-steel tank containing groundwater. The boreholes were isolated from the fracture with packers containing expanding rubber collars. The boreholes were then filled with groundwater above the packer and sealed at the surface of the block with rubber stoppers. The main fracture was accessed by stainless-steel tubes (0.208-cm inner diameter), passing through the packers and rubber stoppers. In several boreholes, which were intersected by vertical fractures above the packer, additional stainless-steel tubes were inserted to access the groundwater above the packer. Each stainless-steel tube was fitted with a valve, allowing a flow path to be established in the main fracture between any combination of boreholes. The combined water volume in the stainless-steel tubes used for injection and recovery was N 2 mL, which represented l-2% of the flow path volume.

The groundwater used in all experiments was taken from fracture zone 2 at the Underground Research Laboratory (URL), located within the Whiteshell Research Area (WRA) in southern Manitoba. This Na-Cl-HCO,-SO, water had an ionic strength of 0.013 mol kg-’ and a pH of 8.3. Before use, the water was filtered with a Millipore” tangential-flow ultrafiltration membrane (10,000 nominal molecular weight limit). The particles used as tracers (Table 1) consisted of latex spheres, silica colloids and glass spheres. Colloids and suspended particles were injected as separate pulses, with each pulse having a mixture of particle sizes. Uranine and Br- were used as conservative tracers.

272 P. Vi& D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279

All migration experiments were carried out with dipole flow fields established between selected pairs of boreholes. Water was injected into one borehole with a high-performance liquid chromatography (HPLC) pump and sampled from the other borehole with a fraction collector. The volume of eluted water was determined by weighing. Before selecting borehole pairs for migration experiments the hydraulic connection between each pair of boreholes was characterized by measuring the injection pressure while pumping groundwater into one borehole at a constant rate. The hydraulic transmissivity within fracture segments between borehole pairs was determined as described by Vandergraaf and Drew (1991).

The connecting flow paths between borehole pairs were flushed extensively with filtered groundwater using injection rates of 4-400 mL h-l to study the effects of groundwater flow on particle erosion from fracture surfaces, and to reduce particle concentrations within the fracture before starting migration experiments. Before the start of each migration experiment the flow path was flushed with groundwater for several hours to stabilize the flow field and to remove any previously injected tracers. Conservative and particle tracers were injected as lOO- or 200-mL aliquots, followed by tracer-free groundwater. After the completion of a migration experiment between a given borehole pair, groundwater was flushed back to the injection borehole from each neighboring borehole separately to evaluate the reversibility of particle filtration and the extent of particle dispersion within the main fracture. This flushing reduced particle concentrations to background levels. Initial experiments carried out with 72-nm latex colloids between boreholes 7 and 8 produced reproducible elution curves.

Total particle concentrations were determined by either turbidity measurements with a turbidimeter (Cole Parmer’ Model 8391-35) or by absorbance measurements with a spectrophotometer (Milton Roy @ Spectronic 601). A Microtrac’ Ultrafine Particle Size Analyzer (UPA) was used to determine the size distribution of particles in the 0.006- 2.5-pm size range. The size distribution and concentration of particles with sizes between 0.4 and 100 pm were measured with a Climet” CI-1000 particle counter, which uses forward light scattering and light obscuration to detect particles. Due to its ability to measure very small amounts of particles, the particle counter made it possible to use very low concentrations of suspended particles, that would not affect fracture permeability. The uranine tracer concentration was determined by absorbance measure- ments and the Br- concentrations were monitored with an Orion@ Br specific ion electrode, mounted in a flow cell.

3. Results and discussion

3.1. Particle mobilization from fracture by groundwater flow

The mobilization of natural particles in the main fracture was studied by injecting filtered (0.22 pm) groundwater into borehole 7 at injection rates of 4-100 mL h-‘, and measuring the particle concentration in water eluted from borehole 8. Particles with diameters as large as 40 pm were mobilized with an injection rate of 4 mL hh’. Scanning electron microscopy showed these particles to consist of clays, quartz, calcite

P. Vilks, D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279 273

Table 1 Particles used in migration experiments

Particle description

Colloidal silica: Ludox a Uniform glass microspheres Carboxylate modified hydrophilic latex spheres with carboxylic acid polymers grafted to particle surfaces Hydrophobic sulfate latex spheres stabilized with sulfate and hydroxyl surface functional groups

Particle diameter C, ( pm) (mg L-r)

0.022 24,000 2.1, 10.2, 22.1 0.42 0.087, 1.01 200-600

0.039, 0.072 100

source

DuPont Duke Scientific Interfacial Dynamics Corp.

Interfacial Dynamics Corp.

C, = initial particle concentration in the tracer solution.

and microorganisms. When the injection rate was increased to 100 mL h-’ there was a 200-fold increase in the concentration of particles larger than 2 pm that were mobilized, while the smaller particles were not affected. During any continuous injection the concentrations of eluted particles fluctuated up and down, without a major reduction in particle concentration. Stopping and restarting the flow did not have a significant effect on particle concentrations. Flushing for up to 5 days did not eliminate naturally-occur- ring particles. However, since the concentrations of these particle were significantly less than the concentrations of tracer particles used for migration experiments they did not interfere with the measurement of tracer particles.

3.2. Eftiect of groundwater flow rate

Migration experiments with tracer particles between boreholes 7 and 8 were carried out using injection rates of 1, 2, 15, 100 and 400 mL h-’ (Table 2). The average water velocities for these injection rates were determined from the borehole separation distance and the arrival time of the conservative tracer. Fig. 2 shows examples of elution profiles of particles and conservative tracer (Br-) for injection rates of 2 and 15 mL h-‘. Although at 2 mL h-’ the silica and latex colloids eluted ahead of the Br- and ahead of colloids injected at 15 mL h-‘, the recovery of colloids at 2 mL h- ’ (1.3%) was significantly lower than the recovery of the Br- (58%) and of the colloids injected at 15 mL hh’ (28%). Results in Table 2 show that colloid recovery dropped to 0.03% when the injection was 1 mL h-‘, and was as high as 83% at an injection rate of 400 mL hh’. These results indicate that the migration of colloidal particles in fractures is much more sensitive to water velocity than is the migration of dissolved conservative tracers. There appeared to be a very significant reduction in colloid transport when the injection rate dropped below a value between 2 and 15 mL hh’, corresponding to average water velocities of 32 and 240 m yr-‘, respectively.

3.3. Effect of flow path

The effect of flow path on particle transport was investigated by carrying out experiments in other borehole pairs (7-4, l-6 and 6-8) using an injection rate of 15 mL

274 P. Vilks, D.B. Bachinski / Journal of Contaminant Hydrology 21 (1996) 269-279

Table 2 Summary of migration results

Flow path Injection Average rate velocity (mL min-‘1 (myr-‘)

Particle type a

Size ( pm)

Percent tracer recovery ’

7 to 8 1 16 400 6,400

2 32

LS LS

S LC G

S LC G

S LC G

S LC G

S LC G

S LC G

S LC G

S LC G

0.039 0.039

7 to 8 0.022 0.87 (2, 10, 22)

37” 91”

5uB

Percent particle recovery ’

c 0.03 C 83

c 1.3 SP 0.2

7 to 8 15 240

7 to 8 100 1,600

7 to 4

4 to 7 15 291

1 to 6

6 to 1 15 183

6 to 8 15 276

15

15

437

183

0.022 0.087 (2, 10,221

0.022 0.087 (2, 10, 22)

0.022 1 22

0.022 1 22

0.022 0.0087 (2, 10, 22)

0.022 0.0887 (2, 10, 22)

0.022 0.0087 (2, 10, 22)

54B C 28.1

79B c 33.0

16’ C 63.3 SP 0.2

24’

43B

52B

35B

c 10.2 SP 0.2

c 9.7

c 74.5

c 13.0

SP 1.5

a Particle types are identified as follows: LS = sulfate latex: LC = carboxylate modified latex; S = silica colloid; G = glass microspheres. ’ “Uranine used as the conservative tracer; “bromide used as the conservative tracer. ’ Particle recoveries for colloids are marked with C, and recoveries for suspended particles are identified with SP.

hP I. While the flow path between boreholes 7 and 8 was relatively simple, the presence of intersecting vertical fractures may have affected the flow paths between the 7-4, l-6 and 6-8 borehole pairs. Furthermore, the flow paths defined by these holes were located in parts of the main fracture that have a dip of - 9”. Table 3 provides information on path length and hydraulic transmissivity for these borehole pairs, while Table 2 summarizes data on particle and conservative tracer recovery, as well as average flow velocities (m yr~ ‘> in these m igration experiments. For the same injection rate (1.5 mL

P. Vilks, D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279 275

ELUTION PROFILES FOR PARTICLES AND BROMIDE

BOREHOLE 7 TO 8 AT 2 mLlh BOREHOLE 7 TO 8 AT 15 mLlh

~~~!~~ iii,:_;;-1

0 100 200 300 400 500 60: 0 100 200 300 400 500 600 700

VOLUME (ml.) VOLUME (ml.)

Fig. 2. Elution profiles of silica and latex colloids and conservative tracer (Br- ) for migration experiments between boreholes 7 and 8, carried out at injection rates of 2 and 1.5 mL h-r. C is the eluted particle or Br-’ concentration and Co is the injected concentration.

ELUTION PROFILES FOR PARTICLES AND BROMIDE

BOREHOLE 7 TO 4 AT 15 ml/hr BOREHOLE 4 TO 7 AT 15 mllhr

0.6 0.15 - SILICA COLLOIO - SILICA COLLOID

0.5 - --.- BROMIDE -..- BROMIDE ;f\, ,.’ \ _ .._.. _..

0.4 - O.lO- 0

,;’ I

g 0 0.3 -

si i

! 0.2 - 0.05 -

0.1 -

0 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800

VOLUME (mL) VOLUME (mL)

Fig. 3. Elution profiles of silica colloids and conservative tracer (Br- 1 for flow in both directions between boreholes 4 and 7. Note that colloid transport is strongly favored for the flow direction from borehole 7 to 4. C is the eluted particle or Brr’ concentration and C, is the injected concentration.

Table 3 Hydraulic characterization of flow paths between boreholes used in migration experiments

Boreholes defining Path length Turbidity of Transmissivity flow path km) eluted water (cm’ s- r )

(NTU)

4 to 7 65.3 0.200 0.1694 7 to 4 65.3 0.135 0.0242 6 to 8 68.3 0.165 0.0539 8 to 6 68.3 0.165 0.0379 7 to 8 33.3 0.160 0.390 8 to 7 33.3 0.150 0.0321 1 to 6 45.8 28.5 0.0251 6 to 1 45.8 0.160 0.0308

The injection rate of water used for hydraulic characterization was 400 mL hh’. The turbidity of the eluted water provides an indication of particle mobilization during hydraulic testing.

276 P. Vi&, D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279

ELUTION PROFILES FOR PARTICLES AND BROMIDE

BOREHOLE I TO 6 AT 15 mL/h BOREHOLE 6 TO 1 AT 15 mL/h

VOLUME (ml.) VOLUME (ml.)

Fig. 4. Elution profiles of silica and latex colloids and conservative tracer (Br- ) for flow in both directions between boreholes 6 and 1. C is the eluted particle or Br-’ concentration and Co is the injected concentration.

hP ’ > average water velocities vary between different borehole combinations because of differences in the geometry of the flow paths established by the migration experiments.

Fig. 3 and 4 present the elution profiles of experiments carried out in both directions between the borehole pairs 4-7 and 1-6. Flow direction had a very significant impact on the migration of colloids within the flow paths defined by both sets of boreholes. When flow was from hole 7 to 4 (Fig. 3) the colloid eluted ahead of BY, and the colloid recovery of 63% significantly exceeded the recovery of the conservative tracer (16%). When flow was in the opposite direction the colloid had a significantly lower recovery (10%) and eluted behind Br- (24% recovery). The hydraulic connection between these boreholes was dependent on flow direction, with a lower transmissivity when flow was from 7 to 4. Flow from borehole 7 to 4 was also characterized by a broad Br- elution profile, which eluted sooner (average velocity of 437 m yr~ ’ > than when flow was from 4 to 7 (average velocity of 291 m yr-‘).

The hydraulic connection between boreholes 1 and 6 appeared to be less dependent on flow direction with equal hydraulic transmissivity and similar Br- elution profiles for flow in both directions (Fig. 4). The presence of two small peaks on the elution profile for flow from 1 to 6, may suggest the presence of some channelling for flow in this direction. Despite the similarity in the elution of conservative tracer, the colloid elution profiles from opposite directions were distinctly different. Colloid migration from 1 to 6 was characterized by a high recovery (75%) and a sharp elution peak, which was clearly ahead of the Br-. For flow in the opposite direction only 10% of the colloid was recovered, with a broad elution profile that was only slightly ahead of the Br-. The only common factor in the experimental results between these two sets of boreholes was that colloids migrated more efficiently toward the C-D side of the block.

As shown in Fig. 5, colloid migration from borehole 6 to 8 was characterized by a low colloid recovery (13%) and a broad elution profile appearing slightly ahead of the Br- elution (35% recovery). The two injection experiments made from borehole 6 (l-6 and 6-8) showed that colloids in this corner of the large block preferred to migrate towards the center of the block. Flushing experiments showed that a significant fraction of the colloids from the experiment between boreholes 6 and 8 had travelled toward borehole 9. Flushing experiments carried out from neighboring boreholes after the

P. Vilks, D.B. Bachinski/Journal of Contaminant Hydrology 21 (1996) 269-279

ELUTION PROFILES FOR PARTICLES AND BROMIDE

0.10

0.08

0.02

BOREHOLE 6 TO 8 AT 15 mL/h

S

0’ -’ . ’

0 200 400 600 600 1000 1200

277

VOLUME (ml.)

Fig. 5. Elution profiles of colloids, suspended particles and conservative tracer (Br- ) for the migration experiment from borehole 6 to 8. C is the eluted particle or Br-’ concentration and C, is the injected concentration.

completion of migration tests from boreholes 7 to 8 also demonstrated that colloids tended to migrate towards the C-D side of the granite block and into vertical fractures that intersected the upper parts of boreholes 5, 1 and 9.

3.4. Particle size

The UPA was used to perform particle size analysis on particles eluted from LB4 during migration experiments to determine whether particle size had an effect on migration. These results showed that, within the size range of 0.020-0.090 ,um, the particle size distribution of the eluted colloids was the same as that of the injected colloids. However, the migration of suspended particles was much less efficient than the migration of colloids (< 0.09 pm). Typically, the measured recovery of suspended particles (Table 2) was less than 15% of the recovery observed for colloids. In the experiment with the borehole pair 7-8 the suspended particles eluted at the same time as the colloids (Fig. 2), while between the borehole pair 6-8, suspended particles eluted ahead of both colloids and conservative tracer (Fig. 5). These results indicate that, compared to colloids, the migration of suspended particles is more strongly controlled by channelling, with a small fraction of the injected particles travelling in more open fractures which may have a higher local (microscopic) water velocity. Even within a single fracture or capillary, larger colloids may travel faster than smaller colloids because of hydrodynamic chromatography (de Marsily, 19861, in which larger particles are concentrated in fracture centers, which have higher water velocities compared to regions near the walls.

4. Summary and conclusions

These studies have shown that groundwater moving through fractures may contain natural particles eroded from fracture surfaces. Particle transport in rock fractures is

278 P. Vi&, D.B. Bachinski / Journal of Contaminant Hydrology 21 (1996) 269-279

affected by flow velocity, with colloid transport significantly decreasing at average flow velocities below 240 m yr ’ . Although suspended particles with diameters of l-40 pm can move in open fractures, their mobility is significantly less than that of colloids (< 0.09 pm). In many experiments the elution of colloids was slightly faster than the elution of the dissolved conservative tracer. However, only a small fraction of the colloids in groundwater is likely to be mobile for the low groundwater velocities ( < 2 m yr-’ > expected at WRA. Compared to dissolved tracers, the migration of colloids is more affected by flow path and flow direction, which can have a greater influence than flow velocity in some cases.

The results of this study indicate that field-scale experiments could overestimate the amount of colloid transport that would occur in deep groundwater within the Canadian Shield because field-scale experiments often use flow rates that are 4 orders of magnitude higher than natural average groundwater velocities. The factors causing colloid transport to be affected by flow path and flow direction need to be identified and whether these factors are still important at the high flow rates used in field experiments determined. Finally, the relationship between particle transport and particle size needs to be investigated within the size range of 0.1-1.0 pm, to identify the particle size above which there is a significant decrease in transport,

Acknowledgements

We thank Geoff Bellingham and Marcia Keeler, 1993 students of the Deep River Science Academy, Whiteshell Campus, for their contributions to several migration experiments. The Canadian Nuclear Fuel Waste Management Program is jointly funded by AECL Research and Ontario Hydro under the auspices of the CANDU Owners Group Group.

References

de Marsily, G., 1986. Quantitative Hydrogeology. Academic Press, New York, NY. Dormuth, K.W. and Nuttall, K., 1987. The Canadian nuclear fuel waste management program. Radioact.

Waste Manage. Nucl. Fuel Cycle, 8: 93-104. Eichholz, G.G. and Craft, T.F., 1982. Role of particulates in subsurface migration of wastes. In: Environmental

Migration of Long-Lived Radionuclides. Proc. Int. Symp., IAEA (Int. At. Energy Agency), Vienna, IAEA-SM257.48, pp. 541-555.

Freeze, R.A. and Cherry, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ. Gerba, C.P. and Goyal, SM., 1985. Pathogen removal from wastewater during groundwater recharge. In: T.

Asano (Editor), Artificial Recharge of Groundwater. Butterworth, Stoneham, MA. Grindrod, P., Edwards, MS., Brown, R.C. and Gealy, N.D., 1993. Colloid migration and dispersal within

saturated media: A modelling perspective. Intera Info. Technol., Henley-on-Thames, IM2421-4. Hunt, J.R., McDowell-Bayer, L.M. and Sitar, N., 1987. Colloid migration in porous media: An analysis of

mechanisms. In: C.-F. Tsang (Editor), Coupled Processes Associated with Nuclear Waste Repositories. Academic Press, New York, NY, pp. 453-469.

McDowell-Boyer, L.M., Hunt, J.R. and Sitar, N.. 1986. Particle transport through porous media. Water Resow. Res., 22: 1901-1921.

P. Vilks, D.B. Bachinski/Joumal of Contaminant Hydrology 21 (1996) 269-279 279

Mills, W.B., Liu, S. and Fong, F.K., 1991. Literature review and model (COMET) for colloid/metals transport in porous media. Ground Water, 29: 199-208.

Neretnieks, I., 1978. Some aspects on colloids as a means for transporting radionuclides. KBS (Kambrlnslesakerhet), Stockholm, Rep. KBS-TR-103.

Smith, P.A., 1993. A mode1 for colloid facilitated radionuclide transport through fractured media. PSI (Paul Scherrer Inst.), Villigen, ISSN 1019-0643.

Tipping, E., Thompson, D.W., Woof, C. and Longworth, G., 1991. Transport of haematite and silica colloids through sand columns eluted with artificial groundwaters. Safety Studies, Nirex Radioactive Waste Disposal. Nirex, Ltd., Harwell, NSS/R276.

Torgerson, D.F., 1990. The Canadian waste management research program. Proc. Symp. on Waste Manage- ment, Tucson, AZ, Feb. 25-Mar. 1, 1990, 1: 27-32.

Travis, B.J. and Nuttall, H.E., 1987. Analysis of colloid transport. In: C.-F. Tsang (Editor), Coupled Processes Associated with Nuclear Waste Repositories. Academic Press, New York, NY, pp. 517-528.

Vandergraaf, T.T. and Drew, D.J., 1991. Laboratory studies of radionuclide transport in fractured crystalline rock. Proc. 3rd Int. Symp. on Advanced Nuclear Energy Research, Mito, Mar. 13-15, 1991, pp. 385-393.

Vilks, P. and Drew, D.J., 1986. The effects of colloids on actinide migration. Proc. 2nd Int. Conf. on Radioactive Waste Management, Winnipeg, Man., Sept. 7-11, Can. Nucl. Sot., Ottawa, Ont., pp. 667-673.

Vilks, P., Miller, H.G. and Doern, D.C., 1991. Natural colloids and suspended particles in the Whiteshell Research Area, Manitoba, Canada, and their potential effect on radiocolloid formation. Appl. Geochem., 6: 565-574.

Yao, K., Habibian, M.T. and O’Melia, CR., 1971. Water and wastewater filtration: Concepts and applications. Environ. Sci. Technol., 5: 1105-1112.