Environ. Sci. Technol. 1996, 30, 3094-3101
New Method for Sampling Groundwater Colloids under Natural Gradient Flow Conditions NOAM WEISBROD,† D A N I E L R O N E N , * ,‡ A N D R O N I T N A T I V † Seagram Center for Soil and Water Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel, and Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
This paper describes a new method for the passive sampling of groundwater colloids using a multilayer sampler (MLS). It is based on the use of dialysis cells with large pore size (10 µm) membranes that are in dynamic equilibrium with the mobile colloid and liquid phases in the aquifer. Under laboratory conditions, the dialysis cells reached equilibrium with a suspension of latex microspheres (5 mg/L) after 44100 h and with a suspension of kaolinite (16-41 NTU; 20-50 mg/L) after 50-180 h. No fractionation was detected in the particle-size distribution between the kaolinite suspensions inside and outside the dialysis cells. Field profiles, obtained under natural gradient flow conditions in a sand and sandstone aquifer, showed large variability (up to 1 order of magnitude) in the colloid content within profiles (e.g., variation of 7 NTU (∼45 mg/L) between cells located at a vertical distance of 40 cm) and between them. The predominant colloidal particles found in the cells were aluminosilicates, CaCO3, silica, and organic matter. The membranes are suitable for sampling groundwater colloids over long periods of time, at least 36 days, in very turbid solutions (up to 50 NTU; ∼550 mg/L).
Introduction Colloids are an important mobile phase in aquifers and are carriers of contaminants (1-7). However, field data on colloid concentration, size distribution, and their chemical and mineralogical composition are still scarce due to unreliable sampling protocols (8, 9). Pumping has been shown to produce colloids and associated contaminants that are not naturally present in groundwater (10). Even at the recommended very slow pumping rate of about 100 mL/min (8, 9, 11), the resulting shear rate is 1 order of magnitude higher than that found under natural gradient flow conditions (12). Therefore, colloids may be artificially formed in the forced gradient fields caused by pumping, * Corresponding author e-mail address: cidaniel@weizmann. weizmann.ac.il; fax: 972-8-9344124. † The Hebrew University of Jerusalem. ‡ Weizmann Institute of Science.
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and the samples obtained may not represent natural colloids or their distribution in the aquifer. Microscale heterogeneity (scale order of centimeters) in the chemical composition and flow field of groundwater is also a well-known phenomenon (13, 14) that can influence colloid generation, stabilization, and transport. Sampling intervals achieved by pumping are generally >0.5 m, and a vertical profile of simultaneous samples is quite difficult to obtain. It is expected that a more adequate method for sampling colloids will enable (a) collection of colloids that are transported by the natural gradient flow field without the need for an external source of energy (pumping); (b) dynamic equilibrium with groundwater so that the concentration, composition, and size distribution of colloids in the sample vary if they vary in groundwater; (c) simultaneous sampling, at small vertical intervals (centimeters), of the microscale environments; and (d) sample integrity not to be biased either by the insertion of the sampling device or the retrieval of the sample. Ronen et al. (15) developed a passive method of sampling groundwater that employs dialysis cells to obtain, simultaneously, undisturbed discrete microenvironments within the aquifer. The present paper reports on the further development of the multilayer sampler (MLS) technique for the passive sampling of groundwater colloids. It is based on the use of dialysis cells covered with large pore size membranes that are in dynamic equilibrium with the mobile colloid and liquid phases in the aquifer. The methodology was tested for artificial and natural colloids in the laboratory in batch and flow experiments. It was also used to obtain field profiles of colloids and groundwater chemical composition in a sandy Coastal Plain aquifer.
Methodology Sampling Principle. The MLS is composed of a sequence of criss-crossed dialysis cells separated by seals that fit flush to the well screen (Figure 1). When the MLS is lowered into the groundwater, colloids percolate through large pore size membranes that cover the dialysis cells at both sides. Transport across a dialysis cell membrane is driven mostly by diffusion. However, for large pore size membranes, the influence of advective transport cannot be ruled out. Hereafter, the term “percolation” is used to denote such combined effects. In the field, the impact of MLS insertion and retrieval from a well on sample integrity is negligible. This is due to (a) the relatively long time the MLS is immersed in groundwater after deployment (weeks), which allows for re-equilibration with the natural conditions of the aquifer, and (b) the short time needed (minutes) for MLS retrieval as compared to the long equilibration time between the cell solution and groundwater (days; see Laboratory Equilibration Experiments section). As the MLS is retrieved from the well, the sample is preserved inside the cell by surface tension at the membrane-air interface. The MLS (15) enables acquisition of vertical profiles of the concentration and composition of colloids and groundwater chemistry at variable vertical intervals (e.g., 3.5 cm Figure 1a and 12.5 cm Figure 1b).
S0013-936X(96)00197-6 CCC: $12.00
1996 American Chemical Society
FIGURE 1. (a) MLS made of PVC with 30-mL dialysis cells (provided by Margan M.L.S. (1994) LTD) and (b) MLS made of stainless steel and Teflon with 160-mL dialysis cells. Both sides of the dialysis cells were covered with 10-µm Versapor membranes and were separated from one another by Viton seals. The two MLS depicted in the figure are suitable for 10 cm diameter (4-in.) wells. The PVC and stainless steel dialysis cells were used in the laboratory and field experiments, respectively.
Selection of Membranes. Commercially available large pore size membranes were tested for their ability to hold water inside the dialysis cells for at least 10 min, the estimated time required to retrieve the MLS and cap the dialysis cells. Versapor membranes (Gelman Sciences) were found to fit this criterion. This membrane is made of a white acrylic copolymer coating (coated equally on each side) over a nonwoven nylon fabric. The membrane is hydrophilic and has a mean pore diameter of 10 µm and a typical thickness of 190 µm. Laboratory Equilibration Experiments. The use of dialysis cells requires knowledge of both kinetics and rate of equilibration between groundwater and the solution inside the dialysis cell. These data are needed to determine the required time of contact between groundwater and the cell and also to estimate the possible impact of deployment and retrieval of the sampler on the sample integrity. Such information is known for different types of membranes and solutes (15) and must also be established, by equilibration experiments, for colloids and the 10-µm membranes. Equilibration experiments were conducted using fluorescent latex microspheres and kaolinite. Latex microspheres were selected because of their homogeneous size and shape and their reported stability. Kaolinite powder was selected as a model of a naturally occurring colloidal phase. The red amidine latex microspheres, supplied as a 2% (w/w) suspension (International Dynamic Corporation), had a diameter of 0.6 µm ( 6.3% and are hydrophobic and positively charged. The kaolinite powder (BDH, light) had a mean diameter of 0.4-0.7 µm. In planning the experiments, we faced two problems: (a) the difficulty of maintaining a homogeneous colloid solution for relatively long periods of time (days) and (b) the necessity to simulate the slow natural gradient flow
FIGURE 2. Schematic representation of the experimental setup for the flow-through test showing only one of the two Perspex tubes, each containing one dialysis cell. The cell was centered in the tube with three steel pins. The low flow velocity through the Perspex tube (100 m/yr) was maintained with a Mariotte bottle and needle valve. The 10-L solution, containing a 41 NTU (50 mg/L) kaolinite suspension, was continuously stirred.
field of many groundwater systems. Batch and flowthrough experiments were conducted. During the batch experiments, the solutions were mildly shaken at 50-60 revolutions per minute (rpm) to maintain the homogeneity of the solution. These experiments do not intend to reproduce aquifer conditions. They were performed to test the feasibility of the technique and to estimate equilibration rates for both natural and artificial colloids. In the aquifer, the colloids are not shaken; however, note that static conditions do not represent advective transport
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FIGURE 3. Percent equilibration of 30 mL PVC dialysis cells tested in the laboratory with fluorescent latex, red amidine microspheres, 0.6 µm ( 6.3%, which were hydrophobic and positively charged (International Dynamic Corporation). Total equilibration (dashed line) was estimated with respect to the expected theoretical mass balance; (a) 5 mg/L suspension of microspheres introduced into 30-mL dialysis cells. The outside 4 L of solution initially contained distilled water; (b) cells filled with distilled water. The outside solution contained a 5 mg/L suspension of latex microspheres. In both experiments, the outside solution was continuously stirred at 50 rpm (22 °C).
either. In the flow-through experiments, the dialysis cells were tested in a system that simulates natural groundwater flow conditions. Two experiments were conducted with latex microspheres where 14 PVC dialysis cells, 30 mL each, were introduced into a 4-L container that was shaken for 238 h during the experiment at 50 rpm. In the first experiment, the cells were filled with a 5 mg/L suspension of latex microspheres, and the container was filled with distilled water. In the second experiment, the cells were filled with distilled water and the outside suspension contained 5 mg/L latex microspheres. The concentration of microspheres inside the dialysis cells and in the outside solution, in the container, was analyzed at predetermined time periods to evaluate the equilibration rate. Two experiments were conducted with kaolinite suspensions, in which PVC dialysis cells were each introduced into a separate container that was shaken during the experiment at 60 rpm for 240 h. In the first experiment, the 30-mL cells were filled with a distilled water solution containing 41 NTU (50 mg/L) of kaolinite, and the containers were filled with 600 mL of distilled water. In the second experiment, the cells were filled with distilled water, and the outside solution contained a 16 NTU (20 mg/L) suspension of kaolinite. In both experiments, a dialysis cell was taken from its container for analysis every 24 h. An additional set of experiments was conducted to more closely simulate natural groundwater flow conditions. A PVC dialysis cell (external diameter 3.1 cm), filled with double-distilled water, was placed in the center of a Perspex
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FIGURE 4. (a) Decrease in the initial concentration of a 41 NTU (50 mg/L) kaolinite suspension contained in 30-mL PVC dialysis cells. Each cell was immersed in a container filled with 600 mL of distilled water and shaken at 60 rpm (22 °C). (b) Equilibration between 30-mL PVC dialysis cells filled with distilled water and a 16 NTU (20 mg/L) kaolinite suspension (22 °C). Each cell was immersed in a vessel containing 600 mL of suspension. Since the experiment was conducted at 60 rpm, some of the clay particles may have aggregated or dispersed, causing large turbidity variations in the outside solution (up to 50%). In both experiments (a and b), kaolinite content was measured as turbidity and also expressed as concentration (C, mg/ L) according to a calibration curve in which NTU ) -0.638 + 0.833C [mg/L]; R2 ) 0.995. TABLE 1
Kaolinite Content in a Flow-Through Experiment kaolinite content, NTU (mg/L) Perspex tube
dialysis cell
septum (around dialysis cell)
needle valve (outlet)
1 2
12.5 (15.8) 9.5 (12.2)
16.7 (20.8) 8.9 (11.5)
9.8 (12.5) 8.3 (10.7)
a The initial concentration of kaolinite particles in the Mariotte bottle (Figure 2) was 41 NTU (50 mg/L). The reduction in concentration to 8.9-16.7 NTU (11.5-20.8 mg/L) in the Perspex tube (sampled through the septum, Figure 2) was due to precipitation along the system. Water flow velocity was about 100 m/yr. Kaolinite content was measured as turbidity and expressed also as concentration (C, mg/L) according to a calibration curve where C [mg/L] ) (NTU + 0.638)/0.833; R2 ) 0.995. The total error in the data is (15%.
tube (internal diameter 6 cm) through which a 41 NTU (50 mg/L) suspension of kaolinite was injected at a velocity of about 100 m/yr, for 14 days (Figure 2). This low flow velocity was maintained with a 10-L Mariotte bottle and a needle valve. During the experiment, two Perspex tubes, containing one dialysis cell each, were connected to the Mariotte bottle. All laboratory experiments were conducted at 22 °C. In the laboratory, the colloid concentration was measured immediately after obtaining each sample. Field Profiles. Profiles of colloids and of the chemical composition of the groundwater were obtained in a research
FIGURE 5. Profiles of colloid content, mean particle diameter, dissolved organic carbon (DOC), nitrate, and chloride in well NGD. Also shown is the specific discharge profile as calculated by Shati et al. (17). Profile 1 was retrieved in October 1994, after the MLS had been immersed in groundwater for 20 days. Profile 2 was retrieved in January 1995, after the MLS had been immersed in groundwater for 35 days. The specific discharge profile was obtained from June to August 1994. The relationship between NTU units and concentration (mg/L) for the colloids in the well is shown in Figure 6.
well (NGD) located in the sand and sandstone Coastal Plain aquifer of Israel, south of Tel Aviv. NGD is a fully screened research well (PVC screens, internal diameter 10 cm) that was drilled in June 1993 by the air rotary method to a depth of 41.50 m. Groundwater was found at a depth of 8 m (16, 17). The MLS used to obtain profiles of colloids and groundwater consisted of individual units, connected in a modular way (Figure 1b). These are (a) a stainless steel frame into which a stainless steel dialysis cell (volume 160 mL) was inserted; (b) a flexible Viton seal that separated consecutive dialysis cells and loosely fitted the inner diameter of the well screen, thereby isolating the environment of each cell from that of its neighbors; (c) a Teflon ring to guide the sampler through the well; and (d) a holding segment to which a polypropylene rope was attached to lower and retrieve the sampler from the well. The distance between two consecutive dialysis cells was 12.5 cm. The cells were arranged in four groups of five cells and one
group of eight cells, covering a total depth interval of about 30 m. Polypropylene ropes (each about 7 m in length) connected one group of cells to the next. After the MLS was retrieved and to maintain the original groundwater conditions, the dialysis cells were sealed with a Teflon liner and a PVC cap. The cells were introduced into glass bottles and transported to the laboratory within 2 h. In the laboratory, the glass bottles and the dialysis cells were opened inside a glovebox in an argon environment within 24 h of retrieving the MLS from the well. A 30-mL aliquot was obtained from each cell for turbidity and particle size distribution analyses. An additional 30 mL was filtered through a 0.1-µm filter to analyze dissolved organic carbon, Cl-, and NO3-. For colloid characterization, an 8-12-mL sample (depending on colloid concentration) was obtained from each group of dialysis cells at different depths. The sample was filtered through a 0.05-µm membrane (polycarbonate, Nuclepore) and the membrane was washed with 10 mL of double-distilled water (Milli-Q)
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FIGURE 6. Colloid concentration, obtained in profile 1, vs turbidity. Calibration was performed with a combined sample of colloids obtained from different depths in the aquifer. The sample was airdried in the laboratory, diluted, and analyzed for turbidity. Note that the calibration represents an average estimation for the profile and might be slightly different from cell to cell, according to colloid size and shape.
to prevent salt deposition. To relate field concentrations of colloids to turbidity, a calibration was performed with a combined sample of colloids from different depths. This sample was air-dried in the laboratory and then diluted and analyzed for turbidity. Analytical Measurements. Turbidity of field and lab (kaolinite) samples was measured with a Hach 2100N turbidimeter with a precision of (10%. Fluorometry was performed with a Sequoia-Turner digital filter fluorometer, Model 112, with a precision of 10%. The analyses of particle size distribution (mean diameter) was performed with Brookhaven BI-90 particle size analyzer. Dissolved organic carbon (DOC) was analyzed with a Dohrmann DC-190 hightemperature TOC analyzer with a precision of (1 mg/L. Cl- and NO3- concentrations were measured with a Wescan 262 ion analyzer with a precision of (4%. For characterization of field colloid, the Nuclepore membranes with the colloids were gold or carbon-coated for analysis by scanning electron microscope/energy X-ray spectrometry (SEM/EDX; Philips 505 SEM).
Results and Discussion As explained above, the use of dialysis cells requires knowledge of the equilibration time of the cells with the colloid suspension of the outside groundwater solution. The reversibility of the percolation process, i.e., whether or not a cell accumulates/traps colloids, must also be tested. These properties were established using suspensions of synthetic latex microspheres and kaolinite. The microspheres, initially introduced into the cells, percolated into the outside solution, and equilibrium between the two solutions was reached after about 100 h (Figure 3a). When the microspheres were introduced into the outside solution, the dialysis cells approached equilibrium after 44 h, but equilibrium never exceeded 77% during the 240 h of the experiment (Figure 3b). Although latex microspheres are considered to be stable, in this long-term experiment it was difficult to achieve equilibrium (as calculated from initial concentrations in the system) since the concentration of microspheres in the outside solution decreases over time. Consequently, each consecutive cell in the experiment
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FIGURE 7. Photomicrograph of (a) a Nuclepore polycarbonate membrane (pore size 0.05 µm) after filtration of 12 mL of distilled water. (b) The same membrane after filtration of 12 mL of groundwater from a dialysis cell that had been immersed in the aquifer for 20 days. The scale bar is 1 µm. Both samples were gold-coated and observed by SEM at an acceleration voltage of 25 kV.
is in equilibrium with a solution having a decreasing concentration of latex microspheres. This phenomenon (which is beyond the scope of this study) may be the result of aggregation and settling and/or sorption to the materials of the experimental setup. The irregular equilibration pattern depicted in Figure 3 for different cells may be the result of solution inhomogeneity and/or heterogeneity in the membrane structure. Note that similar results were obtained in two cells sampled simultaneously after 160 h in the first experiment (Figure 3a), while in the second experiment, the difference between cells amounted to 27% for the same period of time (Figure 3b). It was found that the cells do not behave as traps with regard to kaolinite particles. The colloids percolate from the dialysis cell into the outside solution in about 50 h (Figure 4). Equilibrium between a 16 NTU (20 mg/L) kaolinite suspension and dialysis cells filled with distilled water is achieved in about 180 h (Figure 4b). Clay particles could either aggregate or disperse during the 10 days of the experiment due to kaolinite instability. This phenomenon is reflected by the large (up to 50%) turbidity variations of the outside solution (Figure 4b). The mean diameter of kaolinite in the dialysis cells and the outside solution was similar and ranged between 0.4 and 0.7 µm. The diameter range is a result of kaolinite’s natural size heterogeneity. The similar kaolinite mean diameter inside the cells and in
FIGURE 8. Photomicrograph and EDX analysis of (a) aluminosilicate (the scale bar is 1 µm); (b) SiO2 (scale bar is 10 µm); (c) organic matter (scale bar is 10 µm) and, (d) CaCO3 (scale bar is 10 µm). Samples were gold and carbon-coated and observed on SEM with an acceleration voltage of 20 kV. The aggregates were probably formed on the Nuclepore filter (pore size 0.05 µm), through which the content of the dialysis cells were filtered (Figure 7). Note the hydrophobic type contact angle between the aggregate of organic matter particles and the Nuclepore filter (c). EDX analysis could not be performed for organic matter with the present SEM system.
the outside solution demonstrates that there is no fractionation during percolation through the membrane. The laboratory flow-through experiment was conducted at a flow velocity similar to that observed in the Coastal Plain aquifer of Israel (18) (up to 100 m/yr). Under laboratory conditions, the solution is unstable at this flow velocity, and kaolinite particles were found to settle along the Perspex tube (Figure 2). Consequently, the concentration of kaolinite particles in the Perspex tube decreased during the 14-day experiment from 41 NTU (50 mg/L) to 8.9 NTU (11.5 mg/L). However, the colloid contents of the dialysis cells and the outside solution around it (Septum, in Table 1) were found to be similar (Table 1). Two field profiles of colloid content and mean diameter obtained in the NGD well are presented in Figure 5. Profile
1 was retrieved in October 1994, after the MLS had been immersed in groundwater for 20 days. Profile 2 was retrieved in January 1995, after the MLS had been immersed in groundwater for 35 days. Figure 5 also depicts the measured concentration of DOC, nitrate, and chloride and the profile of the horizontal component of the calculated specific discharge obtained in the well (17). Figure 6 depicts the relationship between turbidity and colloid concentration for the field samples. In both profiles (Figure 5), colloid content was found to increase with depth, concomitant with an increase in the specific discharge from 9.6 to 26.7 m/yr. The concentration of colloids in profile 1 varied from values close to 0 to 9.5 NTU (∼60 mg/L). The increase in colloid concentration with depth was concurrent with an increase in DOC (from 3 to 15 mg/L) and Cl- (from 30 to
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FIGURE 9. Photomicrograph of a Versapor membrane, 10-µm pore size (Gelman Sciences) (a) before use (SEM, acceleration voltage of 1 kV) and (b) after having been immersed in groundwater for 36 days (SEM, 20 kV). The concentration of colloids inside the dialysis cell covered with this membrane was 50 NTU (∼550 mg/L; see profile 2, 41 m; Figure 5). Note the heterogeneous structure of these membranes; the irregular equilibration pattern in Figure 3 may be the result of this heterogeneity.
150 mg/L). NO3- concentration decreased with depth (from 16 to ∼ 0 mg/L), probably as a result of denitrification. In profile 2, obtained in the same well 67 days after profile 1, a water parcel with a different chemical composition and colloidal content was detected. Colloid concentration in the lower part of profile 2 (>40 m) was up to 1 order of magnitude higher (50 NTU; ∼550 mg/L) than that observed in profile 1. DOC and Cl- concentrations decreased to 4 and 75 mg/L, respectively, and their concentration was rather constant with depth. NO3- concentration in the upper part of the profile increased up to 80 mg/L, but the variation pattern was similar to that found in profile 1, i.e., a decrease in NO3- with depth to 20 mg/L. The mean diameter of the colloids in both profiles were similar, varying between 300 and 1300 nm. The colloids that penetrated the 10-µm membrane of the dialysis cells were collected on 0.05 µm Nuclepore membranes (Figure 7). The dominant colloidal particles found in the cells were aluminosilicates (80-90%), CaCO3, silica, and organic matter (Figure 8). Such large variations in both chemical and colloidal compositions, between and within profiles, had previously been observed in the same aquifer (19). They are the result of groundwater microscale heterogeneity, which is observed under natural gradient flow conditions. Very high colloid concentrations, such as depicted in Figure 5, were also reported by Ronen et al. (19) for very small (3-cm) sampling
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intervals. In the present study, the dialysis cells were located between Viton seals and collected particles from 12.5-cm sections in the aquifer. Some cells were located in an aquifer section where the concentration of particles is high, while others are located in a section of the aquifer where the concentration is low. For example, in profile 1, two cells located at a vertical distance of only 40 cm (between 32.9 and 33.3 m) depict different colloidal contents (8 and 1.5 NTU; ∼50 and 5 mg/L, respectively). Similar variations of concentration were observed in profile 2, e.g., between 9.9 (1.5 NTU; ∼5 mg/L) and 10.1 m (10 NTU; ∼60 mg/L). Such details would have been lost by slow pumping, and the resultant colloid concentration would have reflected an average value. This value would probably be consistent with the average concentration estimated from the dialysis cells located in high specific discharge zones (18). Assuming an aquifer porosity of 35% (19) and using the calculated specific discharge (17; Figure 5), it is worthwhile noting that there were large changes in local water chemistry and colloidal concentration given that water moved only 5 m in the upper part of the profile and 14 m in the bottom part, during the time interval between the two sampling events. In the MLS, the dialysis cells are criss-crossed (Figure 1). Since the transport of colloids in the aquifer is by Brownian motion and advective flow is extremely slow, the orientation of the membranes inside the well should not influence the capture of colloids. Indeed, note that, within a sequence of criss-crossed dialysis cells, turbidity is in many cases very similar for consecutive pairs of cells having a different orientation and immersed in a flow field of similar specific discharge (Figure 5; e.g., profile 2, 25 m sampling segment). The structure of the membranes was not significantly changed after they had been in contact with groundwater for up to 36 days. The membranes did not clog up, even after being immersed in a solution with a colloid concentration of 50 NTU (∼550 mg/L) (Figure 9). The present method is considered to be a significant improvement to the passive sampling technique utilized by Ronen et al. (19) where colloids were sampled from MLS seals (separating one cell from another) and from nylon nets inside open dialysis cells (traps). The method used by Ronen et al. (19) to obtain vertical profiles of colloids under natural gradient flow conditions calls for extreme caution in the analysis of the data since (a) artificial colloidal suspensions may be created while inserting the MLS into the well and (b) samples may be perturbed while retrieving the MLS from the well. Moreover, since the method of Ronen et al. (19) is based on the accumulation of colloids either on the MLS seals or in the traps (and not equilibration with groundwater as in the method presented in this paper), it allows only determination of the average concentration of colloids for the total sampling period.
Acknowledgments The authors acknowledge the contributions of Robert Puls and Cindy Paul during the study-visit of N.W. to the Robert S. Kerr National Risk Management Research Laboratory of the U.S. EPA, in Ada, OK. They are also grateful to Yuri E. Freedman for fruitful discussions. They thank Margan M.L.S. (1994) LTD for providing the PVC dialysis cells. This work is part of the senior author’s forthcoming Ph.D. A patent application has been filed for the described MLS system by Yeda Research and Development Ltd. at the Weizmann Institute of Science.
Literature Cited (1) McDowell-Boyer, L. M.; Hunt, J. R.; Sitar, N. Water Resour. Res. 1986, 22, 1901-1921. (2) Buddemeier, R. W.; Hunt, J. R. Appl. Geochem. 1988, 3, 535-548. (3) McCarthy, J. F.; Zachara, J. M. Environ. Sci. Technol. 1989, 26, 496-502. (4) . McCarthy, J. F.; Degueldre, C. In Environmental Particles; Buffle, J., Van Leeuwen, H. P., Eds.; Lewis Publishers: Boca Raton, FL, 1992; pp 247-313. (5) Mills, W. B.; Liu, S.; Fong, F. K. Ground Water. 1989, 29, 199208. (6) Puls, R. W.; Powell, R. M. Environ. Sci. Technol. 1992, 26, 614621. (7) Ryan, J. N.; Gschwend, P. M. Water Resour. Res. 1990, 26, 307322. (8) Puls, R. W.; Eychaner, J. H.; Powell, R. M. Environmental Research Brief; U.S. Government Printing Office: Washington, DC, 1990; EPA/600/M-90/023; 12 pp. (9) Backhus, D. A.; Ryan, J. N.; Groher, D. M.; MacFarlane, J. K.; Gschwend, P. M. Ground Water 1993, 31, 466- 479. (10) Magaritz, M.; Amiel, A.; Ronen, D. Chem. Geol. 1992, 100, 147158. (11) Reynolds, M. D. M.Sc. Thesis, Massachusetts Institute of Technology, Cambridge, 1985, 99 pp.
(12) Ryan, J. N. M.Sc. Thesis, Massachusetts Institute of Technology, Cambridge, 1988, 250 pp. (13) Ronen, D.; Magaritz, M.; Gvirtzman, M.; Garner, W. J. Hydrol. 1987, 92, 173-178. (14) Ronen, D.; Magaritz, M.; Almon, E.; Amiel, H. Water Resour. Res. 1987, 23, 1554-1560. (15) Ronen, D.; Magaritz, M.; Levy, I. Ground Water Monit. Rev. 1987, 7, 69-74. (16) Muszkat, L.; Ronen, D.; Magaritz, M. Fate of organic contaminants during Transport from the Soil Surface to Groundwater. Final Scientific Report, Joint Israeli-German Research Project, 1994. (17) Shati, M. R.; Ronen, D.; Mandelbaum, R. Environ. Sci. Technol. 1996, 30, 2646-2653. (18) Ronen, D.; Berkowitz, B.; Magaritz, M. Ground Water,1989, 31, 33-40. (19) Ronen, D.; Magaritz, M.; Weber, W.; Amiel, A. J.; Klein, E. Water Resour. Res. 1992, 28, 1279-1291.
Received for review February 28, 1996. Revised manuscript received May 22, 1996. Accepted May 29, 1996.X ES960197O X
Abstract published in Advance ACS Abstracts, August 1, 1996.
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