Laboratory investigation of GCL performance for covering arsenic contaminated mine wastes

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Geotextiles and Geomembranes 39 (2013) 63e77

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Laboratory investigation of GCL performance for covering arsenic contaminated mine wastesq R. Kerry Rowe*, M.S. Hosney 1 GeoEngineering Centre at Queen’s-RMC, Queen’s University, Ellis Hall, Kingston, ON, Canada, K7L 3N6

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 July 2012 Received in revised form 5 June 2013 Accepted 26 June 2013 Available online 16 August 2013

The performance of three geosynthetic clay liners (GCLs) above arsenic-bearing gold mine tailings is examined based on a series of laboratory column experiments conducted over a two year period. The GCLs examined had either untreated or polymer-enhanced sodium bentonite and had different carrier geotextiles (a woven geotextile, a scrim-reinforced nonwoven geotextile, and a woven geotextile laminated with a polypropylene ﬁlm). After 24 months, the hydraulic conductivity (k) of GCL with untreated sodium bentonite increased by almost an order of magnitude from 3 1011 m/s to 2 1010 m/s with the swell index decreasing from 26 mL/2 g to 7e11 mL/2 g and the exchangeable sodium percentage (ESP) decreasing from 65% to 12e21%. Samples of this GCL exhumed after 9 months of moisture uptake and cation exchange and then subjected to 15 extreme wetedry cycles experienced an increase in k up to 6 1010 m/s. The GCL with polymer-enhanced bentonite maintained k less than 5 1011 m/s after 24 months (virgin k ¼ 1 1011 m/s) and experienced less cation exchange (ESP decreased from 78% to 39e42%) than the GCL with untreated bentonite. For this GCL, 15 extreme wet edry cycles gave k < 9 1011 m/s provided that the soil stress was 15 kPa or higher (1.0 m of cover soil above the GCL). The GCL with polymer-enhanced bentonite and carrier geotextile laminated with a polypropylene geoﬁlm demonstrated the best performance with k < 7 1012 m/s even with exposure to the extreme wetedry cycling. In all cases, the GCL prevented arsenic migration into the overlying cover soil over the two year period examined. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Geosynthetic clay liner Gold mine tailings Arsenic contamination Wetedry cycles Hydraulic conductivity Cation exchange

1. Introduction Tailings associated with gold mining often contain high arsenic concentrations (Parsons et al., 2012). Prior to the existence of regulations limiting the concentration of arsenic in drainage from mine waste, tailings were typically dumped into a convenient surrounding area, lake, or stream near the mine (Wong et al., 1999). These unlined gold mine tailings sites are potential environmental and health threats (Hopenhyan-Rich et al., 1996; Kaltreider et al., 2001; Walker et al., 2009; Meunier et al., 2010; DeSisto et al., 2011). Over the last few years, different techniques have been proposed to limit the environmental impacts of mine leachates (MEND Manual, 2001; Bussière, 2009). One approach is to restrict the ﬂow of water into gold mine tailings to minimize the formation q Dr. C.H. Benson acted as Editor with respect to the review of this paper. * Corresponding author. Tel.: þ1 (613) 533 3113/6933; fax: þ1 (613) 533 2128/ 6934. E-mail addresses: [email protected] (R.K. Rowe), Mohamed.hosney@ ce.queensu.ca (M.S. Hosney). 1 Tel.: þ1 (613) 583 8054; fax: þ1 (613) 533 2128. 0266-1144/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.geotexmem.2013.06.003

of potentially toxic leachates by using a water inﬁltration barrier such as a geosynthetic clay liner (GCL) above the tailings. GCLs have been widely used in a range of applications and have been the subject of considerable recent research addressing a number of aspects of their behavior (e.g., Azad et al., 2011, 2012; Eid, 2011; Kang and Shackelford, 2011; Zanzinger and Saathoff, 2012; Buckley et al., 2012; Chevrier et al., 2012; El-Zein et al., 2012; Ishimori and Katsumi, 2012; Rosin-Paumier and Touze-Foltz, 2012; Benson et al., 2012; Dominijanni and Manassero, 2012; Liu et al., 2013; Sirieix et al., 2013). One aspect that has attracted considerable recent attention is the effect of cation exchange on the hydraulic performance of GCLs. Previous research (e.g., Melchior, 1997; James et al., 1997; Meer and Benson, 2007; Benson and Meer, 2009; Buckley et al., 2012; Bradshaw et al., 2012) has shown that the exchange of monovalent sodium in a GCL for divalent ions in the surrounding soil decreases the swelling capacity of the bentonite and may increase the GCL hydraulic conductivity (k). Shackelford et al. (2010) assessed the hydraulic performance of two types of GCLs (a GCL with untreated sodium bentonite and a GCL that was treated to increase resistance to contaminants) being considered for use as a secondary liner

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Table 1 Conﬁgurations of pipes. Series

Tailings Location

GCL

Foundation

Cover

Wc (%)

Stress (kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 3

B B A B B B B A B C A B A B C A B C A B C

No No Yes Yes No Yes Yes No No No Yes Yes No No No Yes Yes Yes No No No

Yes Yes Yes Yes GMB GMB GMB GMB GMB GMB Yes Yes GMB GMB GMB Yes Yes Yes GMB GMB GMB

20 10 20 20 20 20 10 20 20 20 20 20 20 20 20 20 20 20 20 20 20

15 15 15 15 7.5 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

Note: The Series labeling scheme was selected such that for the same tailings, the cover conﬁgurations with a GCL used as a single barrier were listed ﬁrst followed by the conﬁgurations with GCL used with a GMB as a part of a composite liner.

component in a composite basal liner for tailings impoundment at a proposed zinc and copper mine. Using a ﬂexible wall permeameter under an effective stress of 28 kPa and a hydraulic gradient of 430 100, the GCLs were permeated with three liquids: (i) an onsite ground water (GW) with relatively low ionic strength (3.2 mM), (ii) a simulated process water (PW) with an ionic strength of 32e51 mM, and (iii) a simulated tailings leachate (SL) with an ionic strength of 350 mM. The pH was 7.2, 6.9e9.8, and 2.5 for GW, PW, and SL, respectively. The tests showed that when the GCL was permeated by PW or SL, there was an increase in the k (relative to samples permeated with GW) by 2e4 orders of magnitude for both GCLs. Benson et al. (2010) evaluated the engineering properties of two GCL products exhumed after being in service for 4.7e5.8 years from four test pits in a landﬁll cover comprised of (from top to bottom): 0.3 m top soil, 0.6e0.9 m silty sand, a geomembrane (GMB), and a GCL. The subgrade soil was silty clay having an ionic strength of 3e 5 mM and was dominated by divalent cations (the ratio of the molar concentration of monovalent cations to divalent cations, RMD, was 0.032e0.038 M1/2). The k values obtained using ﬂexible wall permeameters under an effective stress of 24 kPa and hydraulic gradient of 125 increased from 1.2 1011e2.6 1011 m/s for virgin GCLs to 1.4 1011e2.8 108 m/s. The swell index (SI) of all exhumed samples dropped from 25e36 mL/2 g for virgin GCLs to 8e11 mL/2 g. Benson et al. (2007) described a ﬁnal cover over a coal-ash landﬁll. The cover had a GCL overlain by 0.76 m of silty sand and underlain with two lysimeters to monitor percolation. The percolation rates (203e262 mm/year on average and up to 450 mm/year) recorded within 12e18 months after installation were higher than expected which was attributed to an increase in k of the GCL as a result of exchange of Ca and Mg in the surrounding soil for Na in the bentonite combined with dehydration of the GCL. This GCL was replaced by a GCL coated with a polyethylene geoﬁlm (0.1 mm thick). In this case, low percolation rates (2.6e4.1 mm/year) were maintained for more than 5 years. The combined effect of cation exchange and the exposure to wetedry cycles have been reported to increase the k of GCLs signiﬁcantly. For example, Lin and Benson (2000) conducted a

series of k tests on GCL specimens under an effective conﬁning stress of 17 kPa and hydraulic gradient of 80 using either deionized water (pH ¼ 6.5) or 12.5 mM CaCl2 solution (500 mg/L Ca; pH ¼ 6.2) as a permeating liquid. After 5 wetedry cycles, the k of specimens permeated using 12.5 mM CaCl2 increased to 2.8 109 m/s (i.e., 2 orders of magnitude higher than k for the specimens subjected to the same number of wetedry cycles with using deionized water as a permeating liquid). Hosney and Rowe (2013) investigated the ﬁeld performance of three GCL products covered with up to 1 m of local sand and gravel (with some silt and trace clay) in a test cover over tailings at the former Montague Gold Mine, Nova Scotia, Canada. Of the GCLs examined, one GCL had untreated sodium bentonite encapsulated between a woven and a nonwoven geotextile. The other two GCLs had polymer-enhanced sodium bentonite and a nonwoven cover geotextile but different carriers; one a scrim-reinforced nonwoven geotextile, the other a woven geotextile laminated with a thin polypropylene ﬁlm. After 2 years in the test cover, the GCL with untreated sodium bentonite experienced a signiﬁcant exchange of the sodium in the GCL with divalent cations in the tailings-cover soil porewaters. The exchangeable sodium percentage, ESP, dropped from 65% to 10e17% but the GCL still maintained k 5 1011 m/s at locations where there was 0.7 m of cover soil above the GCL. When the cover soil above the GCL was 0.5 m, the GCL with untreated bentonite experienced a similar change in the ESP but k increased to 1 1010 m/s. The ESP of GCL with polymerenhanced bentonite decreased from 78% to 19e28% and k was 5 1011 m/s when there was 0.5 m of cover soil above the GCL. The k of the GCL with polymer-enhanced bentonite and a carrier geotextile coated with a geoﬁlm was controlled by the thin geoﬁlm and the k value remained around 5 1012 m/s regardless the thickness of the cover soil. These results are encouraging; however the location of the ﬁeld test cover was selected primarily based on accessibility. Thus, while the tailings at the test cover location are representative of part of the site, they do not represent the most extreme conditions at this site. The porewater of tailings at other locations at the site were characterized by a higher concentration of divalent cations and a lower pH than the tailings at the test cover location. The laboratory research reported herein is a parallel study to the ﬁeld research reported by Hosney and Rowe (2013). The objective of this paper is to examine the effect of interaction between arsenic-rich gold mine tailings extracted from the abandoned Montague Gold Mine site at three different locations (representing the range of typical tailings at the site) and the three different GCL products used in the ﬁeld trial on the hydraulic performance of the GCLs under isothermal laboratory conditions after up to two years of exposure. Additional objectives include reporting the effects on GCL performance of: (i) the chemical composition of tailings porewater, (ii) the presence of local till as a foundation between the GCL and the tailings, (iii) the use of the local sand and gravel as a cover soil above the GCL, (iv) the initial water content of the tailings and sand and gravel cover soil, and (v) the applied stress. The combined effect of exposure to wetedry cycles and cation exchange between gold mine tailings/local till and bentonite in the GCL on the hydraulic performance of the three GCL products is also examined. In addition, arsenic migration through the GCLs is investigated.

2. Experimental program In August 2009, 1.5 m3 of tailings were extracted from the former Montague Gold Mine site located near the city of Dartmouth in Nova Scotia, Canada. Tailings with different characteristics

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(described later) were extracted from three different locations (denoted as Location 1e3). The extracted tailings were compacted into PVC pipes with an internal diameter of 0.1 m. The total thickness of tailings in each pipe was 0.3 m. The tailings in each pipe were then covered by one of the three examined GCLs (denoted as GCL AeC). The pipes were prepared in 21 different conﬁgurations (Table 1) involving the use of the tailings extracted from the three locations at Montague Gold Mine and the three different GCLs. Some pipe conﬁgurations had the three different GCLs resting directly (with no foundation layer) on the same tailings (e.g., Series 8e10; Table 1) while other conﬁgurations examined the effect of the tailings from the three different sampling locations on the same GCL (e.g., Series 4, 12, and 17). To evaluate the effect of the chemistry of pore ﬂuid from which the GCL was initially hydrated, some pipe conﬁgurations had a 0.15 m layer of soil (described later) with lower concentration of divalent cations than the tailings as a foundation layer between the GCL and tailings (e.g., Series 3). In some cases, a local soil (the same soil used in the foundation layer) was placed as a cover directly above the GCL to simulate the use of a GCL as the sole hydraulic barrier (e.g., Series 1). Other conﬁgurations had an intact GMB over the GCL (i.e., to simulate a case of a composite liner) to allow an examination of the effect of the cation exchange between GCL and underling soil/tailings alone (e.g., Series 5). The tailings and the soil in the foundation and cover layers were compacted to a dry density of 1750 and 1500 kg/m3, respectively, at a water content (Wc) of 20% (ﬁeld capacity water content and typical of that often found on site). The lowest water content measured in the ﬁeld in May 2009 was 10%, thus two tests were prepared at this water content (Series 2 and 7) to examine the effect of tailings and soil water content on GCL performance. A stress of 15 kPa (equivalent to overburden pressure of about 1 m soil) was generally applied above GCL. In one case (Series 5), a stress above the GCL of 7.5 kPa was applied to examine the effect of the stress on the GCL performance. After preparation, the pipes were sealed and stored under controlled isothermal conditions at 20 C. Two sampling events took place; one at 9 months and one at 24 months after test setup. The values of k, swell index, and exchangeable cations were obtained for each exhumed sample. The combined effect of cation exchange and exposure to wetedry cycles was evaluated using the GCLs exhumed after 9 months. After each test termination, tailings, soil, and GCL specimens were recovered to measure their arsenic concentrations in both solid and aqueous phases. 3. Test methods Directly after exhumation of the GCL samples from the pipes after 9 and 24 months, GCL specimens with nominal diameters of 70 mm were permeated using ﬂexible wall permeameters under falling head/rising tail conditions to obtain k values following ASTM D5084-03 (ASTM, 2004) using tap water (pH ¼ 7.4, Ca2þ ¼ 39 mg/L, Mg2þ ¼ 10 mg/L, Naþ ¼ 19 mg/L, Kþ ¼ 2 mg/L, S2 ¼ 12 mg/L, ionic strength ¼ 3.2 mM) as the permeant liquid. The tap water was selected because there is no practical difference between the chemistry of the tap water and the porewater in the cover soil (presented later). Also, using tap water, the standard permeating liquid connected directly to the ﬂexible wall permeameters, there was no need to use a pressure interface chamber between the ﬂexible wall panel and the tested specimen (which is not the case if a non-standard liquid such as a synthetic cover water is being used). To evaluate the effect of the permeant liquid on the hydraulic conductivity (and for other reasons mentioned later), the

65

k values of samples exhumed after 9 months were measured again using a synthetic cover water as a permeant liquid and the k values obtained from this case were compared to the values obtained with tap water. The average conﬁning pressure was 7.5 or 15 kPa. The hydraulic gradient was 70 except that a lower hydraulic gradient of 30 was used for the GCL product laminated with a polypropylene geoﬁlm to minimize the risk of the side wall leakage (Petrov et al., 1997). To investigate the effect of the wetedry cycles, it was assumed that the GCL would only experience drying during the warmest months of the year (JulyeSeptember). Based on the climate recorded at the closest weather station (Halifax Citadel station, 44 390 0000 N, 63 350 0000 W; located about 8.5 km from the test cover) during the reference period 1971e2000 (Environment Canada, 2012), the average temperature for the period Julye September was 17.6 2.1 C. The average relative humidity was 82% (5%).The average annual precipitation was 1508 mm with an average of 311 mm during the period JulyeSeptember. During the last 4 years(2009 and 2012), the average temperature from July to the end of September was 18.7 C (it varied between 16 and 28 C), the average relative humidity was 82%, and the average daily minimum humidity between July and September was 35%. The annual precipitation was 998e1237 mm with 240e335 mm of this falling during JulyeSeptember (Environment Canada, 2012). Based on the climate recorded between 2009 and 2012, typically three main rainfall events took place in JulyeSeptember (one per month on average). Each of these rainfall events involved intermittent rainfall over 12 6 days. The time span between the end of one rainfall event and the beginning of following event was 11 4 days. This would provide the possibility of up to three wetedry cycles per year. The technique developed to apply the wetedry cycles is described below. After the k values with respect to tap water reached equilibrium for the GCLs exhumed from the pipes after 9 months, the permeant liquid was replaced by synthetic water with a chemical composition similar to the porewater chemistry of the cover soil (i.e., characterized by concentrations of divalent cations Ca2þ ¼ 41 mg/L, Mg2þ ¼ 9 mg/L, and Mn2þ ¼ 7 mg/L; S2 ¼ 24 mg/L, pH ¼ 5.5, ionic strength ¼ 7.1 mM, and ratio of the monovalent soluble cations (in cmol/kg) to the divalent soluble cations (in cmol/kg) (MDR) of 1.6 0.3 (the number of tested samples, n ¼ 14)). The purposes of this stage were to: (i) establish a baseline for the k with respect to synthetic cover soil porewater for samples subjected to cation exchange only, and (ii) to replace the tap water in the pores of bentonite with synthetic cover water. When the GCLs were permeated with this synthetic cover water, k tests were running until the termination criteria speciﬁed by the ASTM D5084-03 (ASTM, 2004) were satisﬁed and at least 1 pore volume (PV) of synthetic cover water had passed through each sample. Jo et al. (2005) permeated a GCL with using deionized water (concentrations of Ca and Cl were <0.02 mg/L and <0.04 mg/L, respectively), 100 mM NaCl, 100 mM KCl, or CaCl2 with different concentrations (5, 10, 20, 50, 100, 500 mM) as permeant liquids. Tests showed that permeation of GCL with deionized water, 100 mM NaCl, or 100 mM KCl resulted in no signiﬁcant change in k regardless of the duration of permeation (up to 3.7 years) and number of pore volumes (up to 60 PVs). In contrast, permeation with CaCl2 resulted in an increase in k at a rate that depended on the concentration, with slower change occurring for weaker solutions. For 5 mM CaCl2 solution, the k increased from an average of 2.8 1011 m/s to 1.7 1010 m/ s after 2.5e2.9 years of permeation with 200e562 PVs. Because the synthetic cover water used in the research reported herein only has a molar concentration of Ca of 1 mM, it is not expected that the k will signiﬁcantly change within a reasonable period of time (<3 years). Thus, no attempt was made to run the k tests when the

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synthetic cover water was used as a permeant liquid until chemical equilibrium was established. After permeation with the synthetic cover water, the GCLs were then placed in a test apparatus (Fig. 1) comprised of two stainless steel tubes each with an internal diameter of 65 mm and height of 25 mm to apply the wetedry cycles. The 70 mm GCL specimen was sandwiched between two smooth perforated steel plates and placed between the two tubes. The steel plates rested on a grove in the tubes (Fig. 1). A stress of 7.5 or 15 kPa was applied to the GCL using springs. To simulate a rainfall event, before connecting the test apparatus to the elevated tank, the test apparatus was ﬂipped around and the lower chamber was ﬁlled with the synthetic cover water using a syringe (to allow purging of air) then the test apparatus was connected to an elevated tank ﬁlled with synthetic cover water at a water level of 1.0 m above the GCL (Fig. 1) and the sample was allowed to hydrate for 4 days at 20 C and 50e70% relative humidity. Assuming steady-state (veriﬁed later), the hydration that occurred in 4 days with 1000 mm head approximates that which would occur over 1 month at 133 mm head (i.e., if an entire month’s rain was sitting on the GCL). To the extent that hydration is not completed in 4 days, the samples would be expected to experience less hydration than would occur over a month in the ﬁeld and hence the results are likely conservative. For the GCL product with the carrier geotextile laminated with polypropylene ﬁlm, the GCL was installed in the test apparatus with the geoﬁlm oriented to allow interaction between the bentonite and the synthetic cover water during hydration. At the end of the hydrating cycle, the elevated tank was disconnected and the test apparatus placed in a temperature controlled room at 25e29 C and 30e40% relative humidity for 10 days to allow the sample to dry. The period of the wet cycle after the third wetedry cycle (three wetedry cycles approximate the exposure over a summer season in one year) was extended to 30 days to simulate exposure to rain over the rest of the year. This process was repeated 5 times (total of 15 wetedry cycles) to simulate the effect of 5 years of exposure at ﬁeld. The k value of each GCL was measured after the wetedry cycles using synthetic cover water as permeant liquid following the same procedure described earlier. The objective of this test is to simulate the exposure conditions in the ﬁeld as much as possible, however the process adopted here represents an extreme worst case since it assumes that the cover soil that exists above the GCL is not present to provide buffering of the drying cycle by the moisture stored in the cover soil as is the case in the real cover. Thus the change in

properties assessed from these sequences is considered to represent an upper bound to what could happen over 5 years in the ﬁeld. Swell index tests were conducted according to ASTM D5890-06 (ASTM, 2006) except that air-dried bentonite was used instead of oven-dried bentonite. This change was made because oven heating of the GCL could cause: (i) changes in bentonite mineralogy (Gu et al., 2001; ASTM D7503-10 (ASTM, 2010)) and (ii) changes in the polymers in GCLs with polymer-enhanced bentonite (Redding and Burns, 2000; Schenning, 2004). Specimens from each exhumed GCL sample were air-dried in a fume hood until their mass reached equilibrium and then the bentonite was extracted from these air-dried GCL specimens and a portion of bentonite was used to measure its water content. Finally, a portion of the air-dried bentonite containing 2 g of solids was used for the SI test. The technique described in ASTM D7503-10 (ASTM, 2010) was followed to measure the soluble cations, bound cations, and cation exchange capacity of bentonite recovered from the exhumed GCLs. Inductively coupled plasma-mass spectrometry (ICP-MS) was used for the chemical analysis of the extracts from the soluble cations and bound cations stages. The concentration of the ammonia was measured in the extract from the cation exchange capacity test to quantify the nitrogen in the extract and calculate the cation exchange capacity. The total metal concentration in the bentonite, tailings, and cover soil were obtained by digesting 0.5 g of oven-dried specimen for 2 h at 95 C with a 3:1 HCl:HNO3 mixture. After cooling, the specimen was diluted to 25 mL with deionized water and analyzed for metals by ICP-MS. Metals that were over limit (above the analysis range) were re-digested as described above and diluted to the analysis range of the ICP-MS as described by Chen and Ma (2001). The total soluble ions in the porewater of bentonite were measured by following the method described by Lange et al. (2007), by shaking GCL specimens with 35 mL of double distilled water (1:10 soil to solution ratio) for 12 h and centrifuging; the supernatant was analyzed by ICP-MS. The shake ﬂask extraction technique described by Price (2009) was followed to measure the readily extractable elements from the tailings and cover soil samples. In this test, a 250 g of tailings/ soil sample was mixed with 750 mL deionized water and gently agitated on a rotary extractor for 1 day. The sample was left stand for 3 h to allow the suspended materials to settle and then the leachate was ﬁltered through 0.45 mm membrane ﬁlter paper and analyzed by ICP-MS. The water content of exhumed GCLs, cover soil, and tailings samples, was measured following ASTM D4643-08 (ASTM, 2008). 4. Materials

1000 mm (not to scale)

4.1. GCLs

Hole 70 mm PVC plate Stainless steel rod Springs 50 mm

Perforated steel plate GCL Wetting liquid

Perforated steel plate Stainless steel tube PVC plate

Valve

Fig. 1. Schematic of the wetedry apparatus.

Three needle-punched (reinforced) GCL products were examined: (1) Bentoﬁx Thermal Lock NSL (GCL A; denoted herein as “untreated”) comprised of a layer of 4491 g/m2 (standard deviation, SD ¼ 401 g/m2, n ¼ 14) granular sodium bentonite encapsulated between a 123 g/m2 (SD ¼ 13 g/m2, n ¼ 4) slit-ﬁlm woven carrier geotextile and a 231 g/m2 (SD ¼ 17 g/m2, n ¼ 4) staple ﬁber needlepunched nonwoven cover geotextile; (2) Bentoﬁx Thermal Lock NWE (GCL B; “polymer-enhanced”) comprised of a layer of 5009 g/ m2 (SD ¼ 336 g/m2, n ¼ 14) of a polymer-enhanced granular sodium bentonite between a 253 g/m2 (SD ¼ 23 g/m2, n ¼ 4) scrimreinforced nonwoven carrier geotextile and a 235 g/m2 (SD ¼ 15 g/m2, n ¼ 4) staple ﬁber needle-punched nonwoven cover geotextile, and (3) Bentoﬁx Thermal Lock CNSE (GCL C; “coated”) consisted of a layer of polymer-enhanced granular sodium bentonite with a dry mass per unit area of 5238 g/m2 (SD ¼ 608 g/

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

m2, n ¼ 14) sandwiched between a slit-ﬁlm woven carrier geotextile and a staple ﬁber needle-punched nonwoven cover geotextile. The woven carrier geotextile in GCL C was coated with a low permeability polypropylene geoﬁlm to lower the hydraulic conductivity and reduce cation exchange between the bentonite and the underlying soil. The polymer used to enhance the bentonite in GCLs B and C is proprietary and details are not available. For these GCLs, the bentonite and polymer are mixed for a sufﬁcient duration to achieve uniformity in the blending at ambient temperature. The polymer/bentonite is not cured in any way. In all three products, the needle-punched ﬁbers were thermally fused to the carrier geotextile to enhance the reinforcing bond.

67

Tailings samples were extracted from the site at depths of 0e 0.1 m (surface), 0.1e0.2 m, and 0.2e0.3 m from three locations and analyses of grain size, total metals (by acid digestion), and porewater composition (as per Price, 2009) were conducted for each depth range. Based on the grain size distribution, tailings at Locations 1 and 3 were classiﬁed as silty sand while tailings at Location 2 was classiﬁed as sand according to the Canadian Foundation Engineering Manual (CFEM, 2006). The tailings from Location 1 (latitude 44 420 5400 N and longitude 63 3101400 W) had the lowest total arsenic concentration of the three locations. The total arsenic concentration for tailings from 0 m to 0.3 m was 4500e6500 mg/kg (0.45e0.65 wt.%) which is about 450 times higher than the Canadian soil quality guidelines (12 mg/kg, Environment Canada, 1996). The total iron, calcium, and sulphur concentrations were 42,000e53,000 mg/kg (4.2e 5.3 wt.%), 2650e3390 mg/kg, and 270e900 mg/kg, respectively. The inferred concentration of the key constituents in the tailings porewater from Location 1 at different depths is given in Table 2. The arsenic porewater concentration was 16.0e21.5 mg/L which is more than 3 orders of magnitude higher than the maximum allowable value by the Canadian drinking water quality guidelines (0.01 mg/L, Health Canada, 2010) and 1e2 orders of magnitude higher than the Canadian Metal Mine Efﬂuent Regulations (0.5 mg/ L, MMER, 2012). The concentrations of soluble divalent cations Ca,

Mg, and Mn were 37e61, 18e38, and 0.6e2.8 mg/L, respectively. The concentration of iron was 0.5e4.2 mg/L. The pH (6.3e6.6) was near neutral and almost constant with depth. The ionic strength of porewater varied between 11 and 19 mM and MDR was 0.9 (SD ¼ 0.3, n ¼ 14). The tailings at Location 2 (44 420 5500 N, 63 3101900 W) had the highest total arsenic concentration and the lowest pH of porewater measured at the site. The total arsenic concentration for tailings from 0 m to 0.3 m was 24,100e123,000 mg/kg (2.4e12.3 wt.%) which is 2000e10,000 times the Canadian soil quality guidelines. The total iron, calcium, and sulphur concentrations were 57,000e 138,000 mg/kg (5.7e13.8 wt.%), 471e1080 mg/kg, and 2760e 21,400 mg/kg, respectively. The high arsenic and iron concentration are the result of the historic disposal of an arsenopyrite-rich concentrate on the tailings, which has partially oxidized to a mixture of arsenic minerals dominated by scorodite (FeAsO4$2H2O) (DeSisto et al., 2011). The arsenic porewater concentration was 12e 30 mg/L (Table 3). The concentrations of soluble divalent cations Ca, Mg, and Mn were 17e27, 19e24, and 0.9e4.9 mg/L, respectively. The iron concentration was 2.8e116 mg/L. The pH of 2.7e3.5 was measured for all samples exhumed from 0 m to 0.3 m. The ionic strength of the porewater at Location 2 varied between 24 and 54 mM and MDR was 1.1 (SD ¼ 0.2, n ¼ 14). The tailings at Location 3 (44 420 5700 N, 63 31’2300 W) had the median total arsenic concentration of the three locations examined but the highest concentration of soluble divalent cations. The total arsenic concentration in tailings for depths from 0 m to 0.3 m was 9700e39,100 mg/kg (1e4 wt.%). The total iron, calcium, and sulphur concentrations were 41,200e76,600 mg/kg (4.1e7.7 wt.%), 4420e6250 mg/kg, and 6650e13,600 mg/kg, respectively. The concentration of arsenic in the porewater (Table 4) was 3.5e 9.7 mg/L. The concentrations of soluble divalent cations Ca, Mg, and Mn were 390e572, 122e571, and 12e117 mg/L, respectively. The iron concentration was 3.1e4.5 mg/L. The pH values were 3.9, 3.5, and 5.5 at depths 0e0.1 m, 0.1e0.2 m, and 0.2e0.3 m, respectively. The ionic strength of porewater was 68e190 mM and MDR was 0.2 (SD ¼ 0.1, n ¼ 14). The soil (till) used for the cover and foundation layers was classiﬁed as sand and gravel, with some silt and trace clay (CFEM,

Table 2 Potential porewater concentration of tailings exhumed from Location 1.

Table 3 Potential porewater concentration of tailings exhumed from Location 2.

4.2. Tailings and cover soil

Parameter

Units

Tailings from Location 1 0e0.1 m

Ionic strength pH Conductivity MDR Aluminum (Al) Arsenic (As) Barium (Ba) Calcium (Ca) Iron (Fe) Magnesium (Mg) Manganese (Mn) Potassium (K) Sodium (Na) Zinc (Zn) Sulphur (S) Fluoride (F) Chloride (Cl) Nitrite (NO 2) Nitrate (NO 3) Sulphate (SO2 4 )

mM pH mS/cm e mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

Parameter

0.1e0.2 m

0.2e0.3 m

Units

Tailings from Location 2 0e0.1 m

Mean

SD

Mean

SD

Mean

SD

11 6.6 74 0.5 4.2 16.0 1.9 61.0 0.5 17.5 1.9 27.5 23.4 1.4 15.8 <0.05 48.1 0.4 6.7 42.2

2 0.3 3 0.1 2.5 1.5 0.7 5.7 0.1 1.8 0.6 0.7 4.0 0.5 2.0 e 5.2 0.2 2.3 4.3

18 6.6 69 1.2 17 21.5 3.5 37.0 4.2 31.5 2.8 37.4 66.0 8.7 11.6 1.5 46.5 <0.05 1.3 16.7

3 0.4 1 0.3 2.5 3.3 1.2 4.8 0.8 9.7 1.4 9.0 9.7 1.4 1.2 0.1 2.5 e 0.5 1.4

19 6.3 93 0.9 13.7 19.6 3.2 60.0 1.5 37.8 0.6 62.9 58.3 5.2 40.7 <0.05 34.4 <0.05 0.5 132

1 0.4 5 0.1 5.5 0.7 0.2 6.3 0.2 7.1 0.3 11.0 3.8 3.8 4.7 e 2.0 e 0.1 5.4

Note: Each mean and standard deviation (SD) is based on ﬁve measurements.

Ionic strength pH Conductivity MDR Aluminum (Al) Arsenic (As) Barium (Ba) Calcium (Ca) Iron (Fe) Magnesium (Mg) Manganese (Mn) Potassium (K) Sodium (Na) Zinc (Zn) Sulphur (S) Fluoride (F) Chloride (Cl) Nitrite (NO 2) Nitrate (NO 3) Sulphate (SO2 4 )

mM pH mS/cm d mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

0.1e0.2 m

0.2e0.3 m

Mean

SD

Mean

SD

Mean

SD

54.5 2.7 1452 1.2 22.6 29.8 0.2 16.7 115.9 22.4 4.9 43.3 10.3 2.9 432.9 1.2 27.8 <0.05 0.7 1539

4.7 0.4 10 0.8 7.4 2.6 0.1 4.6 3.3 3.1 1.8 9.7 1.5 1.0 24.3 0.1 13.2 e 0.1 23

24.0 3.4 457 0.9 43.6 13.3 1.7 20.3 2.8 24.4 0.9 27.6 16.0 2.5 208.1 0.7 25.8 <0.05 0.7 467

1.2 0.1 7 0.6 3.6 1.4 0.7 4.3 0.0 7.1 0.3 6.0 2.4 0.3 28.2 0.0 0.7 e 0.0 9

34.6 3.5 379 1.1 87.2 12.1 0.5 26.6 8.0 19.3 1.2 24.8 21.7 3.6 225.7 1.0 40.5 <0.05 1.5 591

1.0 0.1 7 0.7 28.2 1.8 0.3 0.2 1.6 4.7 0.0 2.0 5.6 0.8 4.8 0.0 1.2 e 0.0 11

Note: Each mean and standard deviation (SD) is based on ﬁve measurements.

68

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

5. Results and discussion

Table 4 Potential porewater concentration of tailings exhumed from Location 3. Parameter

Units

0e0.1 m

Ionic strength pH Conductivity MDR Aluminum (Al) Arsenic (As) Barium (Ba) Calcium (Ca) Iron (Fe) Magnesium (Mg) Manganese (Mn) Potassium (K) Sodium (Na) Zinc (Zn) Sulphur (S) Fluoride (F) Chloride (Cl) Nitrite (NO 2) Nitrate (NO 3) Sulphate (SO2 4 )

mM pH mS/cm e mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

5.1. Swell index and exchangeable cations

Tailings from Location 3 0.1e0.2 m

0.2e0.3 m

Mean

SD

Mean

SD

Mean

SD

145.4 3.9 1572 0.2 2.0 7.4 0.6 536 3.5 349 81.7 87.4 50 21.7 909 1.0 41.5 <0.05 <0.05 3767

7.8 0.3 15 0.1 0.8 1.7 0.1 48 0 56 11.9 8.7 13 7.9 14 0.9 3.6 e e 253

189.9 3.5 1760 0.2 1.6 3.5 1.4 572 4.5 571 116.5 156.2 54 30.8 1431 <0.05 53.0 <0.05 1.5 4784

5.9 0.5 10 0.1 0.2 0.6 0.5 5 0.4 74 45.5 12.8 5 13.8 165 e 0.1 e 0.5 7

67.7 5.5 686 0.2 <1.0 9.7 0.7 390 3.1 122 11.6 59.0 26 0.1 440 <0.05 30.6 <0.05 0.8 1521

5.5 0.3 5 0.1 e 3.1 0.3 53 0 55 7.7 9.9 12 0.1 48 e 0.4 e 0.1 28

During moisture uptake by GCLs, divalent cations transfer from the tailings and/or adjacent soil to the GCLs by advectiveediffusive transport. This would be expected to result in cation exchange with sodium in the bentonite. However, the degree of the cation exchange could depend on the cover conﬁguration as discussed below. 5.1.1. Effect of cover soil above the GCL The effect of the cover soil was investigated by comparing the changes in SI and ESP for the polymer-enhanced GCL B in Series 4 and 6 (Tables 1 and 5) relative to values for the virgin GCL B (SI ¼ 24 mL/2 g, ESP ¼ 78%; Table 5). In both cases, GCL B was separated from Location 1 tailings by 0.15 m of foundation soil. In Series 4, there was also cover soil in contact with the GCL from above allowing hydration from water containing divalent cations on both sides. After 9 months, for Series 4 the SI decreased from 24 mL/2 g to 17 mL/2 g and the ESP decreased from 78% to 46% (Table 5). For Series 6, which simulated a composite liner with an intact GMB above GCL B and where hydration was only from below the GCL, the SI decreased from 24 mL/2 g to 20 mL/2 g and the ESP from 78% to 67%. The Wc of the GCLs was essentially identical for these two cases (124% for Series 4 and 127% for Series 6) and hence the restriction to hydration from one side in Series 6 had no effect on the level of hydration but did appear to reduce the degree of cation exchange that had occurred during 9 months. This is because with exposure of the GCL to porewater with divalent cations from both sides (Series 4), migration of the divalent cations by advection and diffusion could cause the cation exchange to occur starting

Note: Each mean and standard deviation (SD) is based on ﬁve measurements.

2006). The total arsenic concentration was 10 mg/kg in the solid phase and below the detection limit (<0.03 mg/L) in the porewater. The porewater was characterized by concentrations of divalent cations Ca, Mg and Mn of 41 mg/L, 9 mg/L and 7 mg/L, respectively, pH of 5.5, ionic strength of 7.1 mM, and MDR of 1.6 0.3 (n ¼ 14).

Table 5 Swell index, exchangeable cations, and hydraulic conductivity with tap water for GCL samples exhumed after 9 and 24 months. Series

9 months SIa (mL/2 g)

24 months

Nac Virgin A Virgin B Virgin C 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 a b c d

26 24 25 15 24 14 17 18 20 24 15 19 23 14 18 12 17 23 9 15 19 10 13 25

kd (m/s)

Exchangeable complex (mole fraction)b

0.65 0.78 0.76 0.44 0.78 0.25 0.46 0.64 0.67 0.81 0.26 0.68 0.75 0.28 0.49 0.24 0.49 0.74 0.23 0.42 0.51 0.24 0.29 0.71

K 0.03 0.03 0.03 0.05 0.03 0.02 0.03 0.08 0.05 0.03 0.02 0.02 0.02 0.02 0.03 0.01 0.02 0.03 0.02 0.04 0.03 0.01 0.02 0.02

Ca 0.24 0.16 0.18 0.40 0.16 0.55 0.43 0.26 0.25 0.14 0.55 0.25 0.16 0.55 0.41 0.56 0.41 0.20 0.58 0.42 0.37 0.57 0.54 0.20

SIa (mL/2 g)

Mg 0.08 0.03 0.02 0.10 0.04 0.18 0.08 0.02 0.03 0.02 0.17 0.04 0.07 0.16 0.07 0.19 0.08 0.03 0.18 0.12 0.09 0.19 0.16 0.07

11

2.8 10 1.4 1011 5.0 1012 1.5 1011 1.6 1011 2.7 1011 2.0 1011 2.9 1011 1.4 1011 1.3 1011 2.8 1011 1.8 1011 5.7 1012 2.8 1011 1.6 1011 3.0 1011 2.0 1011 5.3 1012 3.0 1011 1.9 1011 4.2 1012 3.0 1011 2.5 1011 6.7 1012

Average of three tests using air-dried bentonite. Average of two tests. Exchangeable sodium percentage (ESP). For virgin GCLs, k is at effective stress ¼ 15 kPa, tap water was used as a permeant liquid.

26 24 25 13 20 7 14 18 20 24 e e e 11 17 e e e 7 14 18 e e e

kd (m/s)

Exchangeable complex (mole fraction)b Nac

K

Ca

Mg

0.65 0.78 0.76 0.37 0.50 0.13 0.40 0.66 0.65 0.79 e e e 0.21 0.42 e e e 0.12 0.39 0.45 e e e

0.03 0.03 0.03 0.09 0.08 0.02 0.07 0.06 0.04 0.05 e e e 0.05 0.08 e e e 0.05 0.10 0.06 e e e

0.24 0.16 0.18 0.41 0.32 0.63 0.44 0.24 0.28 0.13 e e e 0.59 0.43 e e e 0.52 0.34 0.33 e e e

0.08 0.03 0.02 0.13 0.09 0.21 0.09 0.04 0.04 0.03 e e e 0.16 0.07 e e e 0.30 0.18 0.17 e e e

2.8 1011 1.4 1011 5.0 1012 5.0 1011 1.7 1011 1.0 1010 1.7 1011 2.7 1011 1.7 1011 1.4 1011 e e e 3.5 1011 3.0 1011 e e e 2.1 1010 3.8 1011 6.0 1012 e e e

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

from both sides of the GCL. However, when the GMB was above the GCL (Series 6), the diffusion only occurred from one side. In addition, the lower side of the GCL was hydrated from the foundation soil porewater and matric suction in the bentonite caused the water to migrate through the entire thickness of the GCL to hydrate the upper side. As water migrated from the lower to the upper side of the GCL, it can be hypothesized that there was removal of divalent cations (calcium and magnesium) from the porewater due to cation exchange between the porewater and the lower bentonite. Thus, the bentonite in the upper part of the GCL may have been initially hydrated with water having a lower calcium concentration than the bentonite on the lower side. This is likely the reasons for the higher values of SI (20 mL/2 g for Series 6 versus 17 mL/2 g for Series 4) and ESP (67% for Series 6 versus 46% for Series 4) when there was a GMB above the GCL. A similar situation was evident from the SI (20 mL/2 g for Series 6 versus 14 mL/2 g for Series 4) and ESP (65% for Series 6 versus 40% for Series 4) after 24 months (Table 5). Interestingly, after 24 months the water content for Series 6 with a composite liner (Wc ¼ 136%) was greater than for Series 4 with a single GCL (Wc ¼ 120%) (Table 6). There may be several possible explanations for this but one is that less cation exchange can result in higher water content since Rowe and Abdelatty (2012) observed that the water content of a GCL decreases (under isothermal conditions) with increasing cation exchange (other things being equal). 5.1.2. Effect of foundation soil For GCL B after 9 months, the presence of a foundation layer below the GCL (Series 4) resulted in slightly higher SI and ESP (17 mL/2 g and 46%, respectively; Table 5) than when the GCL rested directly on tailings from Location 1 (15 mL/2 g and 44%, respectively, for Series 1). A similar observation can be made after 24 months with the SI and ESP (14 mL/2 g and 40%, respectively) for Series 4 being slightly greater than for the GCL resting directly on tailings from Location 1 in Series 1 (13 mL/2 g and 37%, respectively). The relatively small beneﬁt of the foundation layer in this case likely arises from the fact that the concentrations of divalent cations in the foundation soil were only a little less than in the upper 0.1 m of tailings from Location 1. This hypothesis is indirectly supported by the observation that the foundation layer between GCL and tailings exhumed from Location 2 reduced the cation

Table 6 Initial and exhumed water content and degree of saturation of GCLs. Series

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Initial conditions

9 months

24 months

Wci (%)

Sri (%)

Wc (%)

Sr (%)

Wc (%)

Sr (%)

8 4 9 11 13 14 7 7 10 7 7 13 9 10 7 10 10 8 10 8 7

10 5 12 10 12 14 7 9 11 7 8 13 11 10 7 13 12 11 14 9 7

139 72 97 124 127 127 73 85 132 23 103 130 80 127 26 90 127 118 78 118 21

98 62 92 93 90 89 59 90 98 19 97 90 88 96 24 91 89 98 91 92 19

120 78 80 120 129 136 94 e e e 89 132 e e e 72 113 123 e e e

93 68 94 97 91 94 79 e e e 94 99 e e e 87 91 98 e e e

69

exchange relative to that when the GCL rested directly on the tailings to a greater extent than at Location 1. Where GCL B rested directly on tailings from Location 2 and is covered with GMB (Series 14; Table 5), after 9 months the SI and ESP decreased to 17 mL/2 g and 49%, respectively. Where there is no GMB above the GCL (i.e., a single liner system, Series 12), it is expected that the SI and ESP would decrease more than for the case of a composite liner system as discussed earlier, however the existence of a 0.15 m foundation layer below the GCL (Series 12) neutralized this effect of exposure to cover soil. The foundation layer was even more effective between GCL and the tailings exhumed from Location 3. Comparing results for a composite liner system (GCL B þ GMB) resting directly on tailings exhumed from Location 3 (Series 20) with that for GCL B in a single liner system where the GCL is exposed to cations from the cover soil and rests on a foundation layer above the tailings from Location 3 (Series 17), the single liner system behaved much better than the case of composite liner system because of the presence of the foundation layer. The beneﬁt of the foundation layer between GCL B and tailings from Location 2 and 3 arise because the porewater of the foundation layer was not as acidic as that from the tailings exhumed from Location 2 and its divalent cation concentrations were much lower than for the tailings from Location 3. 5.1.3. Effect of GCL type The initial SI and ESP for GCL A (untreated) were 26 mL/2 g and 65%, respectively, compared to 24 mL/2 g and 78% for GCL B (polymer-enhanced) discussed above. Considering the situation where there is cover soil over the GCL and a foundation layer between the GCL and the tailings from Location 1, after 9 months (Table 5) GCL A (Series 3) had experienced a greater decrease in SI and ESP (to 14 mL/2 g and 25%, respectively) than GCL B (to 17 mL/ 2 g and 46%, respectively; Series 4). Likewise after 24 months (Table 5) GCL A (Series 3) had experienced a greater decrease in SI and ESP (to 7 mL/2 g and 13%, respectively) than GCL B (to 14 mL/ 2 g and 40%, respectively; Series 4). This difference is predominantly attributed to the effect of the polymer in the bentonite of GCL B; a secondary factor may be the greater mass per unit area of bentonite in GCL B than GCL A. The effect of the type of GCL on the level of cation exchange can also be investigated from the cases where there was cover soil over the GCL and a foundation layer between the GCL and the tailings from Location 3 (Series 16e18; Table 5). After 9 months, the SI for GCLs A (untreated), B (polymer enhance) and C (coated) were 9, 15 and 19 mL/2 g, respectively, and the ESP was 23%, 42% and 51%, respectively. After 24 months, the SI for GCLs AeC were largely unchanged at 7, 14 and 18 mL/2 g, respectively, and the ESP was 12%, 39% and 45%, respectively, showing smaller change for GCLs B and C than GCL A. Thus, in terms of cation exchange, GCLs B and C with polymer-enhanced bentonite performed much better than GCL A. GCL C with the polymer-enhanced bentonite and polymer ﬁlm preventing hydration (and diffusion of cations) from the tailings performed better than GCL B. 5.1.4. Effect of tailings characteristics The effect of the porewater chemistry of tailings on the performance of GCLs was dependent on whether or not there is a foundation layer between the GCL and the tailings. For GCL B when there was a 0.15 m foundation layer separating the GCL from tailings, the porewater of tailings had almost no effect on the GCL performance which was dominated by the cations from the soil above and below the GCL. For example, the SI after 9 months was 17, 18 and 15 mL/2 g and ESP was 46, 49 and 42% where a foundation layer separated the GCL and the tailings from Locations 1 (Series 4), 2 (Series 12), and 3 (Series 17), respectively. After 24 months, the SI was only marginally lower than after 9 months at 14,

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

5.1.5. Effect of the water content of tailings, cover, and foundation soil For the case where tailings and soils were compacted at a water content of 10% and GCL B was separated from tailings by a foundation layer and covered by an intact GMB (Series 7), there was no change in either the SI or ESP after 9 or 24 months compared to virgin GCL (Table 5) although the water content of the GCL had increased to 73 and 94%, respectively (Table 6). When GCL B in a single liner system was in direct contact with tailings from Location 1 (Series 2), higher cation exchange occurred (SI ¼ 20 mL/2 g; ESP ¼ 50%; Wc ¼ 78%) than the case of GCL in a composite liner system with a foundation layer (SI ¼ 24 mL/2 g; ESP ¼ 79%; Wc ¼ 94%) after 24 months. However despite this cation exchange in Series 2, the effect of cation exchange for GCL B over tailings at 10% water content was much less than over the same tailings at a moisture content of 20% after 24 months (SI of 13 and 20 mL/2 g and ESP of 37% and 50%, GCL Wc of 120% and 78% for Series 1 and 2, respectively, Tables 5 and 6). It is possible that more cation exchange will occur with time due to diffusion; especially for the soil/ tailings at an initial moisture content of 10%. 5.1.6. Summary of changes in Swell index and exchangeable cations In summary, for all cases examined, after 24 months GCL A had experienced a signiﬁcant reduction in SI (from 26 mL/2 g to 7e 11 mL/2 g) and in ESP (from 65% to 12e21%). Better performance was observed for GCL B in similar conﬁgurations to GCL A where SI and ESP decreased to 14e17 mL/2 g and 39e42%, respectively. GCL C was better than GCL B with SI and ESP of 18 mL/2 g and 45%,

respectively when exposed to the cover soil. The change in the SI of GCLs was strongly correlated with the change in the ESP as shown in Fig. 2a. The same trend was observed for GCLs exhumed from the test cover constructed at Montague Gold Mine after 1 and 2 years in service. 5.2. Hydraulic conductivity under isothermal conditions The initial hydraulic conductivity of virgin GCLs AeC, at an average effective stress of 15 kPa with tap water as the permeant liquid was 2.8 1011, 1.4 1011, and 5.0 1012 m/s, respectively, while under an effective stress of 7.5 kPa the initial hydraulic conductivity of virgin GCL B was 1.8 1011 m/s. Table 5 shows the k values of exhumed GCLs after 9 and 24 months. Cation exchange between sodium in bentonite and divalent cations (mainly calcium and magnesium) in the porewater of tailings/soil may be expected to cause a reduction in the thickness of the diffuse double layer and an increase in the free pore space available for ﬂuid ﬂow and hence increase the k value (Egloffstein, 2001). However, bentonite also may consolidate under the overburden stress and this mechanism will reduce the bulk void ratio and the size of the pores thereby reducing the k value (Petrov and Rowe 1997). The change in the k value will be the result of the net effect of the two mechanisms. Based on k values measured after 9 months, it appears that despite the signiﬁcant cation exchange that took place in some cases, the net effect on k values was negligible

(a) 30

Swell Index, SI (mL/2 g)

17 and 14 mL/2 g over tailings from Locations 1 (Series 4), 2 (Series 12), and 3 (Series 17), respectively. Likewise there was very little difference in the ESP values for these cases at 9 and 24 months. In contrast when the tailings were the only source of hydrating water and cations (i.e., a composite liner directly on the tailings), the effect of the tailings was substantial. For GCL B in direct contact with tailings the SI after 9 months was 19, 17, and 13 mL/2 g over tailings from Locations 1 to 3, respectively (Series 9, 14 and 20; Table 5) and even more signiﬁcantly the ESP was 68, 49, and 29% over tailings from Locations 1 to 3, respectively. Thus, the tailings from Location 1 had a minor effect, Location 2 a greater effect, and Location 3 had the most signiﬁcant effect on the GCL when placed directly over the tailings. The conclusions drawn earlier for the effect of the foundation layer and type of tailings on the performance of GCL B do not seem applicable for GCLs A and C as there is no clear effect of these parameters on the level of the cation exchange that took place between GCLs A and C and surrounding tailings/soil. In addition there was no clear effect for the cover layer above GCL A on the GCL performance. After 9 months, the ESP of all GCL A samples was reduced from 65% to 23e28% and the exchangeable calcium percentage increased from 24% (virgin) to 55e58% whether the GCL A was in a composite liner system and rested directly on tailings (i.e., Series 8, 13, and 19; Table 5) or in a single liner system separated from tailings by a 0.15 m foundation layer (i.e., Series 3, 11, and 16; Table 5). This was also the case after 24 months when GCL A samples exhibited a reduction in ESP to 12e21% and an increase in the exchangeable calcium percentage to 52e63% for all cases examined (Table 5). On the other hand, GCL C in a composite liner is largely isolated from tailings by the polymer geoﬁlm (Series 10, 15, and 21; Table 5). In these cases, this geoﬁlm below and the GMB above the bentonite minimized the uptake of moisture (the Wc only increased from 7% to 21e26% in 9 months; Table 6) and hence GCL C only experienced at most a slight decrease in the SI from 25 mL/2 g to 23 mL/2 g (this may be just natural variability of the bentonite in GCL C) and a negligible change in ESP (Table 5).

25 20 15 GCL A - 9 months GCL A - 24 months GCL B - 9 months GCL B - 24 months GCL C - 9 months GCL C - 24 months Hosney and Rowe (2013)

10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

Exchangeable sodium percentage, ESP (-)

(b) 10-9

Hydraulic conductivity, k (m/s)

70

SI <15 mL/2 g

SI >15 mL/2 g

10-10

GCL A - 9 months GCL A - 24 months GCL B - 9 months GCL B - 24 months GCL C - 9 months GCL C - 24 months Hosney and Rowe (2013)

10-11

GCL C exhumed from the test cover

10-12 0.0

0.2

0.4

0.6

0.8

1.0

Exchangeable sodium percentage, ESP (-) Fig. 2. Variation in (a) swell index, and (b) hydraulic conductivity with exchangeable sodium percentage for GCLs exhumed from pipes after 9 and 24 months. Results from Hosney and Rowe (2013) for GCLs exhumed from the test cover at Montague Gold Mine after 1 and 2 years in service are also presented.

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

from a practical perspective (i.e., 2.7 1011e3.0 1011 m/s for GCL A; 1.3 1011e2.9 1011 m/s for GCL B, and 4.2 1012e 6.7 1012 m/s for GCL C; Table 5). The net effect of cation exchange and consolidation could be a time-dependent phenomenon. k values of exhumed GCLs after 24 months showed that the effect of cation exchange varied from one GCL to another based on the reduction in the ESP that had occurred. The greatest effect of the cation exchange was for GCL A where k increased to 1.0 1010e2.1 1010 m/s (SI ¼ 7 mL/2 g, ESP ¼ 12e13%) when the GCL was above tailings from Locations 1 and 3, even with presence of a foundation layer in between GCL A and the tailings. The k value for GCL A over tailings from Location 2 was much lower at 3.5 1011 m/s (SI ¼ 11 mL/2 g, ESP ¼ 21%). This result is a little surprising since the cover and foundation soil above and below the GCL would be expected to dominate the behavior of all GCLs above the tailings at all three locations; it may be because the bentonite mass per unit area of this speciﬁc sample (4730 g/m2) was higher than those for GCL A above tailings from Locations 1 and 3 (4350 g/m2 and 4430 g/m2, respectively) which seems to lead to (i) a decrease in the level of cation exchange and (ii) increase the self-induced stress created by the needle punching and reduce the bulk void ratio (Rowe et al., 2004). For all cases examined, the k value of the GCL B samples exhumed after 24 months was less than or equal to the typically speciﬁed value of 5.0 1011 m/s, hence was still low. The k values of GCL C were not affected by the reduction in the ESP and was less than 7.0 1012 m/s for all exhumed GCL C samples; a value controlled by imperfection in the geoﬁlm rather than the bentonite. Hosney and Rowe (2013) measured the hydraulic conductivity of GCLs B and C exhumed from the ﬁeld test cover constructed at Montague Gold Mine after 2 years in service. The hydraulic conductivity of GCL B samples permeated with tap water was 2.5 1011e5.2 1011 m/s while for GCL C the hydraulic conductivity was 3.6 1012e6.5 1012 m/s. To investigate the effect of the thin plastic sheet coating GCL C on its hydraulic conductivity, the thin plastic sheet was removed from the previously tested GCL C samples and the hydraulic conductivity of these samples was measured again. In this case, it was found that GCL C samples had similar hydraulic conductivity to GCL B. Fig. 2b shows the correlation between the hydraulic conductivity of exhumed GCLs after 9 and 24 months with the ESP. Data from ﬁeld results for exhumed GCLs (the same three products used in the laboratory tests herein) after 1 and 2 years in a test cover above the tailings from Location 1 (Hosney and Rowe, 2013) are also plotted. GCLs with ESP greater than 25% exhibited no signiﬁcant

71

increase in k value (i.e., all were <5 1011 m/s). Below an ESP of 25%, the k value started to increase; becoming most signiﬁcant for ESP < 15%. Samples which had ESP > 25% typically had SI > 15 mL/ 2 g and experienced only a relatively small change in k. The k value for GCL C samples was not affected by the change in ESP, because it was mostly controlled by the very thin geoﬁlm rather than the bentonite.

5.3. Effect of extreme wetedry cycling on hydraulic conductivity The ﬁndings discussed above were for samples under isothermal laboratory conditions. The effect of the wetedry cycles on samples hydrated and allowed to experience cation exchange for 9 months before the wetedry cycling is presented in Table 7. When the k value for GCLs exhumed after 9 months of test setup reached equilibrium with respect to tap water, the permeant liquid was switched to synthetic cover water. The k value obtained using the synthetic cover water (ko) after more than 1 pore volume was exactly the same as for tap water (Tables 5 and 7), although this is not surprising since there was no major difference in the chemistry of the two waters. During the wetedry cycles, the water content varied between 92e109% after the wet cycle to 36e38% after the dry cycle for GCL A, 128e163%e41e59% for GCL B, and 138e50% for GCL C. As the water content of the GCLs after each wet cycle (each wet cycle included hydration of GCL for 4 days with synthetic cover water at 1 m head) is typically slightly higher than or equal to the water content of the GCLs after being tested in the ﬂexible wall permeameters for longer period of time (16e29 days) under 0.7 m head, it might be inferred that steady-state hydration was achieved during the 4 days of the wet cycle. After the dry cycles, there was a measurable reduction in the thickness of the GCLs (Table 7) however there was no measurable reduction in the diameter. After the GCLs had been exposed to 15 wetedry cycles, the ratio between the hydraulic conductivity at this time (kafter; Table 7) to the initial value (ko; Table 7), is shown in Fig. 3 as a function of the ESP at exhumation. The GCL A samples exhibited an increase in k by a factor of 13e23 (3.9 1010 k 6.5 1010 m/s). The k values of the GCL B samples at 15 kPa increased by a factor of 1.3e5.0 (k < 9 1011 m/s), while that at 7.5 kPa increased by a factor of 16e4.5 1010 m/s. GCL C experienced no change in k since this was controlled by the geoﬁlm and not the bentonite. The level of wetedry cycling simulated in these experiments represents an extreme worst case since the effect of the cover soil in limiting moisture loss from the GCL was not considered. However, even with this extreme drying, the k

Table 7 Hydraulic conductivity of GCLs exhumed after 9 months before and after wetedry cycles. Series

Before wetedry cycles Time (days)

1 2 3 4 5 6 7 11 12 16 17 18 a b c

46 46 46 46 71 53 53 46 46 46 71 165

PV 1.83 1.24 3.56 1.76 1.64 1.04 1.10 1.63 1.10 1.93 3.03 1.04

After wet cycle a

ko (m/s) 11

1.5 10 1.6 1011 2.7 1011 2.0 1011 2.9 1011 1.4 1011 1.3 1011 2.8 1011 1.6 1011 3.0 1011 1.9 1011 4.2 1012

After dry cycles c

c

After wetedry cycles

Wc (%)

H (mm)

Wc (%)

H (mm)

Time (days)

PV

kafterb (m/s)

147 162 109 128 162 163 170 92 132 104 140 138

9.3 9.0 6.7 7.5 9.2 11.1 9.9 6.3 7.5 7.7 8.2 8.4

48 49 38 41 53 56 59 36 41 36 49 50

8.4 7.7 5.4 6.8 8.3 10.2 8.6 5.1 6.8 5.7 7.3 7.5

27 26 24 26 29 28 29 18 18 16 16 130

4.93 3.10 17.12 1.58 11.27 2.04 0.73 7.91 1.49 3.60 2.88 0.65

7.5 1011 6.5 1011 6.4 1010 2.6 1011 4.5 1010 5.8 1011 2.0 1011 6.5 1010 7.7 1011 3.9 1010 8.9 1011 3.3 1012

Hydraulic conductivity with using synthetic cover soil porewater as a permeant liquid before wetedry cycles. Hydraulic conductivity with using synthetic cover soil porewater as a permeant liquid after wetedry cycles. H, sample thickness. The thickness was measured using a line laser (Dickinson et al., 2010). Each data point is an average of 255 readings.

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

25 GCL A GCL B GCL C

kafter/ko

20

15 GCL B under 7.5 kPa

10

5

0 0.0

0.2

0.4

0.6

0.8

1.0

Exchangeable sodium percentage, ESP (-) Fig. 3. Ratio of the hydraulic conductivity (with respect to synthetic cover soil porewater) exhumed after 9 months after (kafter) and before (ko) the wetedry cycles versus ESP for GCL samples at exhumation (stress ¼ 15 kPa unless otherwise noted).

values were <7 1010 m/s for GCL A and <9 1011 m/s for GCL B for the level of cation exchange that had occurred at 15 kPa stress. 5.4. Moisture uptake 5.4.1. Factors affecting the moisture uptake The initial (off the roll) water content (Wci) of all GCLs was between 7% and 14% (Table 6). Because the bentonite in the GCL initially has a high suction, the GCL uptakes water from the surrounding tailings/soil. The water uptake process from tailings/soil will continue until the suctions in GCL and surrounding tailings/soil reach equilibrium. The experiments indicated that some of the parameters examined had a direct effect on the GCL water uptake, and hence water content, while the effect of some other factors was indirect. The primary factors directly affecting the water uptake over 24 months were: (i) the water content of the tailings/cover soils, (ii) the type of bentonite, and (iii) the polypropylene geoﬁlm coating. For example, when the initial water content of the foundation layer below GCL B in a composite liner was 10% (Series 7; Table 6), GCL B had a Wc ¼ 73% (degree of saturation, Sr, of 59%; Sr values were calculated knowing the bulk void ratio (Petrov et al., 1997) and the wet and dry mass of GCLs, thus they represent an estimate of the degree of saturation) after 9 months and Wc ¼ 94% (Sr ¼ 79%) after 24 months compared to 127% (Sr ¼ 89%) and 136% (Sr ¼ 94%) after 9 and 24 months, respectively, when the foundation soil had initial water content of 20% (Series 6; Table 6). The effect of the polymer in the bentonite was to increase Wc after 9 months to between 118 and 139% for GCL B (polymerenhanced) compared to 78 and 103% for GCL A (untreated) when the GCL was in contact with tailings/soil compacted at Wc ¼ 20%. Although the water content of the products with untreated bentonite and polymer-enhanced bentonite was signiﬁcantly different, the degree of saturation was almost the same (viz: Sr of 89e98% for GCL B and 88e97% for GCL A). Rayhani et al. (2011) and Anderson et al. (2012) reported that a GCL with scrim-reinforced nonwoven carrier geotextile will have lower water content for a given degree of saturation than a GCL with woven carrier geotextile when used over a silty sand or clayey sand subsoil because the better anchorage of the needle-punched ﬁbers by the scrimreinforced nonwoven geotextile is more effective in constraining the bentonite swell and gives lower void ratio and moisture uptake for a given degree of saturation. However, this is not the trend observed for GCLs A and B herein because the polymer in GCL B appears to result in higher moisture uptake. When GCL C was in a

composite liner resting above tailings (i.e., Series 10, 15, and 21) the GCL Wc only increased slightly to 21e26% (Sr ¼ 19e24%) after 9 months because the thin geoﬁlm between the bentonite and the underlain tailings minimized the water uptake. On the other hand, when there was a cover soil above GCL C (Series 18) the increase in Wc and Sr was similar to that for GCL B. Rowe and Abdelatty (2012) demonstrated that cation exchange can lead to a reduction in water content under isothermal conditions and this appears to have also been the case here although the effect is more complicated in this case due to the larger number of variables examined. Here, the presence of (i) a foundation layer below the GCL, (ii) cover soil above the GCL, and (iii) the type of tailings all indirectly affected the GCL water content since they all affect the cation exchange between GCL and tailings as noted earlier. Fig. 4 shows that GCLs having an ESP > 40% and surrounded by tailings/soil at Wc ¼ 20% typically have 120% Wc 140%. For ESP < 40%, the water content of GCLs was highly affected by the reduction in the ESP (e.g., Wc ¼ 72% and 103% at ESP ¼ 12% and 28%, respectively, Fig. 4). Plotting the ratio of the water content after 24 months to that after 9 months (Wc 24 months/Wc 9 months) versus ratio of ESP at 24 to 9 months (ESP24 months/ESP9 months), it was found that the greater the exchange between monovalent cations in the bentonite (mainly sodium) and divalent cations (mainly calcium and magnesium), the lower its ability to retain water (Fig. 5). The stress level (7.5 or 15 kPa) had no signiﬁcant effect on the water content for the low stresses examined (e.g., Wc ¼ 127% for Series 5 and 132% for Series 9, Table 6) and only a small effect on the degree of saturation at 9 months (Sr ¼ 90% for Series 5 and Sr ¼ 98% for Series 9). This is likely because at these low stresses, the constraint of bentonite swelling was dominated by the locked-in stress induced by needle-punched ﬁbers (Lake and Rowe, 2000). 5.4.2. Rate of moisture uptake and source of water hydrating the GCL Scalia and Benson (2011) showed that the rate of water uptake by the GCL and the porewater chemistry of the soil from which the GCL uptakes water are two important factors affecting the physicochemical and hydraulic behavior of GCLs. When in contact with tailings/soil at Wc ¼ 20%, GCL B experienced a relatively rapid increase in water content to 100% (Sr was typically >80%) within 1e3 weeks and then the rate of water uptake slowed (Fig. 6a). For this conﬁguration, and others when the GCL was covered by GMB, the stress on the GCL was applied by placing concrete block above the GMB. Thus, it was possible to measure the water content of the GCL at different points of time other than at the termination of a test at 9

150

Water content, Wc (%)

72

125

100

75 GCL A GCL B 50 0.0

0.2

0.4

0.6

0.8

1.0

Exchangeable sodium percentage, ESP (-) Fig. 4. Variation in water content of GCLs A and B exhumed after 9 and 24 months versus exhumed ESP.

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

1.4

Wc24months/Wc9months

1.2 1.0 0.8 0.6

Y= 0.2446+0.9047x

0.4

R2 = 0.8301

0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

ESP24months/ESP9months Fig. 5. Ratio between water content of exhumed GCLs after 24 months to water content after 9 months versus ratio of ESP after 24 months to ESP after 9 months.

and 24 months. To do so, the concrete block and the GMB above the GCL were removed; the mass of the GCL was measured; and then the GCL, the GMB, and the concrete block were placed back into the pipe. The period of time taken to measure the mass of the GCL (few minutes) is not considered to have been sufﬁcient to cause a signiﬁcant moisture loss or gain to the system. The water content

73

proﬁle for tailings/soils extracted from Series 6 (GCL B above tailings from Location 1; separated from the tailings by a 0.15 m foundation layer and covered by an intact GMB) after 9 and 24 months is shown in Fig. 6b. In this case, and for other cases where GCLs were in contact with tailings/soil compacted at 20% water content and covered by intact GMB, the GCL took up water from the underlying soil to a depth of 0.3 m below the GCL but there was no effect on the water content below 0.3 m (e.g., Fig. 6b). When GCL B was placed over tailings compacted at Wc ¼ 10% (Series 7, GCL B above tailings from Location 1; separated from the tailings by a 0.15 m foundation layer with Wc ¼ 10% and covered by an intact GMB), the water uptake may be still ongoing with the Wc being 94% (corresponding to Sr ¼ 79%) after 24 months (Fig. 7a). Thus, for a given GCL and adjacent soil, the rate of the water uptake is a function of the water content of the adjacent soils, as well as grain size distribution/water retention curve of the adjacent soil and water retention curve of the GCL as shown by previous research (e.g., Rayhani et al. 2011; Anderson et al. 2012; Siemens et al. 2012). Fig. 7b shows the variation of water content with depth for tailings/soil exhumed from Series 7 after 9 and 24 months. The high suction of adjacent tailings/soil due to its low water content (10%) makes it difﬁcult for the GCL to uptake water. After 24 months, the GCL had only taken up water from the 0.15 m immediately below the GCL. The source of the water hydrating GCLs depends on the cover conﬁguration. When the GCL rested directly on tailings without cover soil above the GCL (i.e., in case of a composite liner with an

(a)

(a)

160

120

Water content, Wc (%)

Water content, Wc (%)

140 120 100 80 60 Series 6- Pipe 1 - 9 months Series 6- Pipe 2 - 9 months Series 6- Pipe 1 - 24 months Series 6- Pipe 2 - 24 months

40 20 0 0

100

200

300

500

600

700

80 60 40

0

800

0

0

50

50

GCL B

100

Wci

Foundation

Depth (mm)

200 250

Tailings

300

450 10

200

Series 6- Pipe 1 - initial Series 6- Pipe 1 - at 9 months Series 6- Pipe 2 - initial Series 6- Pipe 2 - at 24 months

300

400

500

600

700

800

12

14

Time (days)

Wci GCL B

100

150

400

100

(b)

0

350

Series 7- Pipe 1 - 9 months Series 7- Pipe 2 - 9 months Series 7- Pipe 1 - 24 months Series 7- Pipe 2 - 24 months

20

Time (days)

(b)

Depth (mm)

400

100

Foundation

150 200 Tailings

250 300

Series 7- Pipe 1 - initial Series 7- Pipe 1 - at 9 months Series 7- Pipe 2 - initial Series 7- Pipe 2 - at 24 months

350 400 450

12

14

16

18

20

22

Water content, Wc (%) Fig. 6. Variation of (a) GCL water content with time, and (b) tailings/soil water content with depth initially and after 9 and 24 months when GCL B was in a composite liner separated from Location 1 tailings by a 0.15 m foundation layer and with 15 kPa effective stress above the liner. The initial Wc of tailings/soil ¼ 20% (Series 6).

0

2

4

6

8

10

Water content, Wc (%) Fig. 7. Variation of (a) GCL water content with time, and (b) tailings/soil water content with depth initially and after 9 and 24 months when GCL B was in a composite liner separated from Location 1 tailings by a 0.15 m foundation layer and with 15 kPa effective stress above the liner. The initial Wc of tailings/soil ¼ 10% (Series 7).

74

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

intact GMB overlying GCL), all the water hydrating the GCL must come from the tailings. The worst case scenario for this case was when the GCL rested directly on tailings from Location 3 (i.e., the tailings with highest divalent cation concentration). Fig. 8a shows the water content proﬁle for tailings/soil at 9 and 24 months for Series 1, where GCL B rested directly on Location 1 tailings and was covered with soil compacted at an initial Wc ¼ 20%. In this case, about 72% of the water hydrating the GCL came from the cover soil compared to 28% from the tailings. The difference in the water uptake from the cover and tailings on either side of the GCL may be due to the difference in the grain size distribution and hence the water retention curves of the cover soil relative to the tailings. When the GCL rested on a foundation layer and was covered with soil, most of the water hydrating GCL came from the foundation and cover which had porewater with a lower ionic strength and a higher MDR than the tailings (Fig. 8b). 5.5. Arsenic uptake When GCL B rested directly on the tailings exhumed from Location 1 (Series 1, Fig. 9a), after 24 months the arsenic (As) concentration in the porewater of the upper 0.1 m of the tailings below the GCL decreased from an average of 14.5e17.5 mg/L to 4.5e 8.0 mg/L and the uptake of water by the GCL and diffusion from the

tailings caused a reduction in As concentration to the full depth of the 0.3 m considered. As a result of this movement of arsenic, after 24 months the concentration of As in the porewater of GCL increased from below the detection limit (<0.03 mg/L) to 5.5 mg/L while the total As concentration measured by acid digestion of bentonite increased from 5 mg/kg to 142 mg/kg. When GCL B was separated from tailings from Location 3 by 0.15 m of foundation soil (Series 17, Fig. 9b), the As concentration in the porewater of the top 0.1 m of the tailings decreased from 5.7e9.1 mg/L to 3.9e4.0 mg/L after 24 months. The As concentration in the porewater of bentonite increased from below the detection limit to 3.8 mg/L and the total As concentration increased from 5 mg/kg to 30 mg/kg. In all cases examined, there was higher As uptake by the GCL when the GCL rested directly on tailings than when there was a foundation layer between the GCL and tailings. This is because the foundation layer operated as a diffusion barrier to the migration of arsenic over time. Based on the cation exchange capacity tests on exhumed GCLs, there was no measurable As in the exchange sites. The As retained within the bentonite was probably adsorbed to the iron oxyhydroxide surfaces (Lange et al., 2007, Castlehouse et al., 2010, Hosney and Rowe, 2013). The lack of mobility of the As was evident by the non-measurable increase in the arsenic concentration in the porewater of the cover soil above the GCL despite the increase in the total and porewater As concentrations in the GCL.

(a)

(a)

0

0

100

100

at 9 months at 24 months

Depth (mm)

Depth (mm)

Cover

Cover

200

Wci

GCL B

300

Tailings

400 Series 1- Pipe 1 - initial Series 1- Pipe 1 - at 9 months Series 1- Pipe 2 - initial Series 1- Pipe 2 - at 24 months

500

5

10

15

GCL B

300 Asi

400

Asi

20

600

25

0

Water content, Wc (%) 0

100

100

Depth (mm)

Depth (mm)

Wci GCL B Foundation

500

Tailings

Series 4- Pipe 1 - initial Series 4- Pipe 1 - at 9 months Series 4- Pipe 2 - initial Series 4- Pipe 2 - at 24 months

700 0

15

20

25

5

at 9 months at 24 months Cover

200

Cover

200

600

10

As concentration (mg/L)

0

400

5

(b)

(b)

300

Asi

Tailings

500

600 0

200

10

15

GCL B

300

Foundation

400 Asi

500

Asi

700

20

25

Water content, Wc (%) Fig. 8. Variation of tailings/soil water content with depth initially and after 9 and 24 months when GCL B was (a) resting directly on tailings from Location 1 (Series 1), and (b) separated from Location 1 tailings by a foundation layer (Series 4). In both cases, the stress above the GCL ¼ 15 kPa and Wc of tailings/soil ¼ 20%.

Tailings

Asi

600

0

5

10

15

As concentration (mg/L) Fig. 9. Variation in arsenic concentration with depth in cover soil and tailings porewater when GCL B was (a) resting directly on Location 1 tailings (Series 1), and (b) separated from Location 3 tailings by a 0.15 m foundation layer (Series 17). In both cases, the GCL is in direct contact with the overlying cover soil with 15 kPa effective stress on the GCL; Wc of tailings/soil ¼ 20%.

R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

The initial As concentration in the porewater of the foundation and cover layers was less than the detected limit (<0.03 mg/L). For cases when the GCL was separated from tailings by a foundation layer, after 24 months of exposure, the As concentration in the 85 mm zone of the foundation layer directly above the tailings typically increased (0.06e2.6 mg/L) to a value higher than the maximum allowable by the Canadian drinking water quality guidelines (0.01 mg/L) and the Canadian Metal Mine Efﬂuent Regulations (0.5 mg/L). However, there was no measurable migration of arsenic to the cover soil above the GCL, regardless the presence or absence of a foundation layer, in any case examined. 6. Summary and conclusions The performance of three different reinforced (needle-punched) GCLs (denoted as GCL AeC) being considered for a cover over gold mine tailings has been examined through a series of laboratory simulations over a period of 24 months. GCL A had granular sodium bentonite encapsulated between a slit-ﬁlm woven carrier and needle-punched nonwoven cover geotextile. GCL B had polymerenhanced granular sodium bentonite between a scrim-reinforced nonwoven carrier and needle-punched nonwoven cover geotextile. GCL C had a layer of polymer-enhanced granular sodium bentonite sandwiched between a needle-punched nonwoven cover geotextile and a slit-ﬁlm woven carrier coated with a low permeability polypropylene geoﬁlm which was placed down (toward the tailings). For these simulations, tailings were extracted from three locations at the former Montague Gold Mine in Nova Scotia, Canada. The tailings have the particle size of sand to silty sand and had the following characteristics: Location 1: solid phase arsenic (As) concentrations of 4500e 6500 mg/kg (compared to the Canadian soil quality guidelines of 12 mg/kg) and porewater As concentrations of 16.0e 21.5 mg/L (compared to 0.01 mg/L allowable in the Canadian drinking water), 6.3 pH 6.6, concentrations of soluble divalent cations of 37e61 mg/L Ca and 17e38 mg/L Mg, an ionic strength of 11e19 mM, and a ratio of total charge of soluble monovalent cations to total charge divalent cations (MDR) of 0.9. Location 2: solid phase As concentrations of 24,100e 123,000 mg/kg and porewater As concentrations of 12e30 mg/ L, 2.7 pH 3.5, concentrations of soluble divalent cations of 17e27 mg/L Ca and 19e24 mg/L Mg, an ionic strength of 24e 54 mM, and MDR of 1.1. Location 3: solid phase As concentrations of 9700e39,100 mg/ kg and porewater As concentrations of 3.5e9.7 mg/L, 3.5 pH 5.5, concentrations of soluble divalent cations of 390e570 mg/L Ca and 120e570 mg/L Mg, an ionic strength of 68e190 mM, and MDR of 0.17. The cover soil (and, where used, foundation soil between the tailings and the GCL) was a locally derived till composed primarily of sand and gravel, with some silt and trace clay. It had a solid phase As concentration of 10 mg/kg and a porewater As concentration below the detection limit (<0.03 mg/L) and pH of 5.5. The porewater concentrations of the key cations were 41 mg/L Ca and 9 mg/ L Mg, ionic strength of 7.1 mM, and MDR of 1.6. The study evaluated the effect on GCL performance of: (i) the type of tailings, (iii) the presence of a foundation layer between the GCL and the tailings, (iii) the effect of cover soil versus an intact geomembrane directly above the GCLs, (iv) the initial water content of the tailings and soil, and (v) the applied stress. In addition, the combined effect of cation exchange and exposure to wetedry cycles

75

on GCL performance and arsenic retention in GCLs has been reported. Unless otherwise noted, the stress on the GCL was 15 kPa. The following conclusions were reached for the speciﬁc materials and conditions examined over a period of 24 months: 1. The initial hydraulic conductivities, k, of the GCLs at 15 kPa were 2.8 1011 m/s, 1.4 1011 m/s, and 5.0 1012 m/s, for GCLs AeC, respectively. 2. When GCL B (with polymer-enhanced bentonite) was separated from Location 1 tailings by a 0.15 m foundation layer and overlain by cover soil, the SI and ESP of the GCL decreased from 24 mL/2 g and 78% to 14 mL/2 g and 40%, respectively, but k only increase marginally to 1.7 1011e2.0 1011 m/s. In the absence of the foundation layer there was slightly more decrease in SI and ESP and k increased to 5 1011 m/s. This was the highest value of k measured for GCL B under isothermal conditions. 3. When GCL B was used in a composite liner separated from Location 1 tailings by a 0.15 m foundation layer, the SI and ESP of the GCL only decreased to 20 mL/2 g and 65%, respectively, and k hardly changed (1.4 1011e1.7 1011 m/s). Thus, this cover soil directly over the GCL reduced the swelling capacity of GCL but even so the effect on k was negligible from a practical perspective. 4. The foundation layer had a signiﬁcant beneﬁt for GCL B especially over Locations 2 and 3 tailings where it reduced the cation exchange between GCLs and tailings and resulted in less increase in k. 5. Under similar conditions, GCL A (with untreated bentonite) experienced more cation exchange than GCL B, and GCL A experienced an increase in k to 1 1010e2 1010 m/s when the SI reduced to 7 mL/2 g and ESP to 12%, even with presence of a foundation layer in between GCL A and the tailings. 6. The thin polypropylene geoﬁlm in the carrier geotextile of GCL C prevented signiﬁcant cation exchange between the GCL and the tailings and its performance was independent of the type of tailings. The cation exchange that did occur was a result of interaction with the cover soil above the GCL, but even in that case the value of k was dominated by the geoﬁlm and there was no apparent change in k. 7. When the GCL B was in contact with tailings/soil compacted at 10% water content in a composite liner system, there was no practical change in the SI, ESP, or k over 24 months. When GCL B was not covered by a geomembrane and was in direct contact with the cover soil at 10% moisture content, there was a little more cation exchange (SI ¼ 20 mL/2 g ESP ¼ 50%) but only a negligible increase in k to 1.7 1011 m/s over the 24 months of the experiment. 8. Simulation of 15 extreme wetedry cycles after 9 months of GCL hydration and cation exchange increased the k value of GCL A to 6.5 1010 m/s and for GCL B to 9 1011 m/s (provided that the effective stress 15 kPa). There was negligible change in k for GCL C. 9. The main direct factors affecting the water uptake by GCLs were: (i) the type of bentonite in the GCL (untreated or polymer enhanced), (ii) the presence or absence of a polypropylene geoﬁlm on the carrier geotextile, and (iii) the water content of tailings/cover soil adjacent to the GCL. 10. When the GCL was used as part of a composite liner with no foundation soil between the GCL and the tailings, the tailings were, necessarily, the sole source of water for hydrating the GCL. When there was a cover soil in contact with the GCL and no foundation layer, more than 70% of water hydrating the GCL was from the cover soil and less than 30% was from the tailings. The presence of a cover and foundation layers resulted in

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R.K. Rowe, M.S. Hosney / Geotextiles and Geomembranes 39 (2013) 63e77

essentially all water hydrating the GCL coming from these layer with little, if any, water from the tailings. 11. Arsenic migration upwards from tailings (by advection and diffusion) was most signiﬁcant when there was no foundation layer. However, in all cases the arsenic was well retained in the GCL and there was no migration of arsenic to the cover soil above the GCL. Based on the results obtained from this research and from Hosney and Rowe (2013) for GCLs tested in a laboratory and ﬁeld scales for covering gold mine tailings with characteristics described earlier for 2 years, the optimum cover conﬁguration above the tailings might include a layer of GCL with polymer-enhanced bentonite (GCL B) underlain by a 0.15 m clean foundation layer and overlain by >0.7 m of clean soil. A better performance could be achieved by placing a GMB layer above the GCL and/or using a GCL with a polymer-enhanced bentonite and a coated carrier geotextile (GCL C).

Acknowledgments The research reported in this paper is supported by an NSERC Strategic Grant. The authors very gratefully acknowledge the assistance with the ﬁeld construction and sampling provided by Dr. Michael Parsons (Natural Resources Canada) and the NS Department of Environment. The value of discussions with Drs. H. Jamieson and M. Parsons and Ms. S. DeSisto is gratefully acknowledged as is the assistance of our industrial partners Terraﬁx Geosynthetics Inc., Terraﬁx Environmental Technologies Inc., TAG Environmental Inc, AMEC and SRK. The authors are also very grateful for the careful review of the manuscript by Drs. H. Jamieson and M. Parsons.

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Laboratory investigation of GCL performance for covering arsenic contaminated mine wastes

Laboratory investigation of GCL performance for covering arsenic contaminated mine wastes

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