A common concern with landfills and waste containment facilities is the risk of groundwater contamination. Groundwater contamination due to inadequate or absent prevention measures has adversely affected hundreds of thousands of persons in the United States of America (EPA, 2000). As the tailings pond at the Magnola facility is intended to contain several hazardous organochlorine compounds, an evaluation of their planned groundwater contamination prevention measures is essential to an environmental assessment.
Magnola's current plan is to line the basin with an upper layer of bentonite in geotextiles, also called a geosynthetic clay liner (GCL) and a lower layer of high-density polyethylene (HDPE) (Magnola Pers., Comm.). The reliability of this design was assessed through an extensive review of the literature on geosynthetic clay liners and geotextiles.
Geosynthetics clay liners are a composite material
consisting of bentonite and geosynthetics. Bentonite in GCLs used in North
America is usually sodium bentonite (also called sodium montmorillonite),
and the geosynthetics are either two geotextiles or a geomembrane. GCLs are
usually 5-10 mm in thickness and come in rolls between 4-5 meters wide and
30-60 meters long (Daniel and Koerner, 1995). The bentonite paste contained
between the two geosynthetics is of very low permeability, and is thus a suitable
hydraulic barrier to water, leachate, or other liquids. Upon hydration the
bentonite swells, increasing the width of the hydraulic barrier, and the confining
geosynthetic layers prevent the bentonite phase from becoming discontinuous
(Daniel and Koerner, 1995).
The ability of GCLs to withstand high stresses, physical and chemical weathering, slope or swell shifting, punctures and other adversities common to waste containment facilities has been well documented (Koerner, 1997). The attribute of shear strength is conventionally tested in terms of the GCLs ability to withstand normal stress (stress in the vertical plane) and shear stress (stress in the horizontal plane). The aforementioned problems are important to the assessment of GCLs as a measure of preventing groundwater contamination because their occurrence compromises the lining's low permeability (Koerner, 1997).
Magnola's plan will be considered acceptable if it fulfills the fourth condition recommended by the Environnement et Faune Québec in their environmental analysis report. Translated, this condition is:
"That Métallurgie Magnola inc. arranges, subjacent with the silica-iron waste cell described in the impact assessment, a lining of bentonite of which the permeability is equal or lower than 10-6 cm/s"
The durability of the GCL-HDPE configuration was also assessed after preliminary research revealed numerous problems, which compromised previous waste containment designs. The permeability, as well as the durability of the GCL configuration was assessed through a literature review of acceptance testing and criteria.
The ideal method for testing the GCL would have been
to conduct standard quality tests as outlined by Koerner (1997) on samples
of the product to be used. Monetary constraints, however limited us to conducting
an extensive literature review for sources of information regarding the permeability
and durability of GCLs.
Several sources were obtained regarding geosynthetics and waste containment applications in general, and only results from studies using similar or identical lining processes were used to evaluate Magnola's plan.
The current plan is to use an upper layer of bentonite in geotextiles and a lower layer of high-density polyethylene (HDPE) to prevent groundwater infiltration. Research into and experience with waste containment linings over the past two decades indicated that the application of such geosynthetic clay liners (GCLs) is reliable and effective barrier against groundwater contamination (Daniel and Koerner, 1995). Typical issues of concern during the planning of a landfill lining are: 1) shear strength behavior, 2) weathering effects, 3) interaction between the liner and the contained waste, 4) slope stability and 5) atmospheric uptake, this last issue being discussed in the next section (Daniel and Koerner, 1995).
GCLs were found to be able to handle shear stresses of 325 kPa without deformation (Hewitt et al., 1996). While the density of the reprocessed tailings is not known, it should be noted that is highly unlikely that the load on the basin lining will come close to 325 kPa, which is equivalent to the pressure exerted by a water height of roughly 33 meters. In addition, studies by Merrill and O'Brien (1997) tested a bentonite GCL - HDPE configuration similar to the one planned by Magnola under three shear stresses of 138, 276 and 552 kPa, which were used to simulate 18, 36 and 76 meters, respectively, of solid refuse at inclinations under 18 degrees. These studies concluded that all tests indicated that the lining design would be adequate for the large landfill in question. It should also be mentioned that the safety criteria used in this study included substantial seismic considerations, further substantiating the reliability of GCL - HDPE linings.
Because the reprocessed tailings will be submerged in water, the potential effects of hydration on the lining must also be considered. While hydration has been known to lower the shear strength of bentonitic blankets, a long term study by Trauger et al (1997) has shown bentonite GCLs to be resistant to displacement after long periods of time. This study subjected a bentonite GCL to a soaking period followed by four1000 hour or longer consolidation/shearing phases. With each phase the normal and shear stresses were increased, with a maximum normal stress of 389 kPa and a maximum shear stress of 135 kPa during the last phase. The maximum shear displacement of 0.5 mm occurred during the first 500 hours of the last phase, with no movement observed during the last 500 hours. Despite being subjected to over 7200 hours of consolidating and shearing, the specimen did not deform and was in good condition at the end of the experiment. Further tests by Seibken et al (1997) applied a normal stress of 621 kPa and a shear stress of 311 kPa on a GCL for 500 hours. After an initial displacement of 9.2 mm, subsequent movement (creep) measured an additional 0.6 mm over 500 hours. These studies indicate that bentonitic blankets may shift slightly (several millimeters) within the first hundred hours, after which the rate of displacement will fall to a near-zero level. Depending on the conditions, non-uniform initial displacements may cause problems such as gaps or sections of increased permeability between the geosynthetic mats in some GCLs (Daniel and Koerner, 1995). However, Mr. Alain Bergeron, Magnola's senior environmental officer, indicated that the GCLs would be thermally fused together to form one continuous barrier, making such problems extremely improbable. No problems of weakness at the seams were noted in the tests on thermally locked GCLs by Seibken et al (1997).
It should also be mentioned that bentonite GCLs have demonstrated a strong resilience against effects of punctuation, desiccation, freezing and chemical weathering damage as described by Shan and Daniel (1991). They found their bentonite GCL was able to withstand 140 kPa of normal pressure with no change in swelling behavior. At normal pressures beyond 140 kPa, the bentonite granules were observed to compact, which reduced the swelling of the bentonite layer after hydration. The level of reduction was proportional to the compressive stress applied. Furthermore, the permeability of bentonic blankets was also determined to be 2 * 10-11 m/s at a low pressure of 14 kPa and 3 *10-12 m/s at a high pressure of 138 kPa (Shan and Daniel, 1991).
The study also observed that bentonite GCLs responded well to punctures 12 and 25mm in diameter. Punctures of 75 mm were not adequately sealed, increasing the permeability by five orders of magnitude. Such punctures, however, are highly improbable if the GCL is properly applied (Daniel and Koerner, 1995). The bentonite layer filled the holes fully upon swelling, re-establishing a permeability to a comparable level before the occurrence of the puncture (Table 1) (Shan and Daniel, 1991).
Table 1: Effect of Punctures (modified from Shan and Daniel, 1991)
Diameter of Punctures Permeability (m/s)
No punctures 2 x 10-11
12 mm 3 x 10-11
25 mm 5 x 10-11
75 mm >2 x 10-6
These tests demonstrated that bentonitic blankets
have some self-healing capability. Shan and Daniel (1991) concluded that small
holes or imperfections in bentonitic materials are probably of little consequence
so long as the bentonite is not impeded from swelling to fill the holes once
the material is hydrated.
Further tests by Shan and Daniel (1991) also looked at the effects of freeze-thaw cycles on the permeability of bentonitic blankets. The permeability of the test specimen before freezing was 2 * 10-11 m/s. After five freeze-thaw cycles, the permeability of the specimen was found to still be 2 * 10-11 m/s. Under these conditions, freeze-thaw cycles had no effect on permeability.
Research regarding the performance of GCLs has indicated that it is robust against many conventional problems facing other waste containment designs. Considering that the water depth of the pond will be approximately one meter, and that the study by Merrill and O'Brien (1997) demonstrated that a GCL - HDPE configuration could bear loads of 552 kPa (the equivalent of roughly 56.3 meters of water) without significant deformation, it can be concluded that the occurrence of shearing and deformation problems is highly improbable. The tests done by Seibken et al (1997) and Trauger et al (1997) both indicate that GCLs are slope-stable and shear-stable over a long time. Shan and Daniels (1991) demonstrated that bentonite provides "self-healing" characteristics, making it robust against the effects of common punctures, desiccation and freeze-thaw cycles.
In conclusion, the majority of research findings on GCLs indicate it to be an effective long-term barrier to groundwater contamination. It therefore seems that Magnola's plan to line the tailings pond with a GCL and a HDPE will achieve its objective permeability less than 1 x 10-8 m/s.
Problems related to pre-placement hydration :
It should be noted that two considerations must be taken into account concerning the deployment of the lining layer. Bentonite liners have been known to increase their conductivity to organochlorines by an average of three orders of magnitude when permeated first with organochlorine solutions rather than with water (Shan and Daniel, 1991; Park et al, 1996; Didier and Comeaga, 1997). More importantly, the premature hydration of GCLs has been known to severely reduce their effectiveness in terms of both permeability (Didier and Comeaga, 1997) and stability (Daniel and Koerner, 1995). As such it is important to stress that GCLs must be covered before a rainfall or snow event. The reason for covering the GCL is that hydration before covering can cause shifting of the bentonite as a result of uneven swelling or whenever compressive or shear loads are encountered (Daniel and Koerner, 1995).
Magnola intends to monitor for groundwater pollution through a series of perforated pipes beneath the basin that will conduct groundwater to a location where it can be tested for hazardous compounds (Alain Bergeron, Pers. Comm.). Regular water sampling will also be conducted to measure the concentrations of selected compounds in the tailings pond. These measures appear adequate to determine the occurrence and magnitude of groundwater contamination (and thus the effectiveness of the GCL).