Methods for Testing the Effect of Hydraulic Gradient on the Polymer Elution of Polymer GCLs

Abstract

Geosynthetic Clay Liners (GCLs) were developed in the 1980’s as an alternative landfill liner or cover material. GCLs are primarily comprised of a layer of processed sodium bentonite (Na-B), generally 7-10 mm thick, sandwiched between two geosynthetic fabrics or membranes. The layer of bentonite is held in place with some combination of adhesive, needle punching, or stitch-bonding. Because of the susceptibility of Na-Bentonite GCLs to chemical incompatibility (e.g., increases in hydraulic conductivity) with aggressive (e.g., high ionic strength) permeant solutions, there has been a demand for chemically resistant GCLs, such as those composed of a dry blend of polymer and bentonite. The mechanism for decreased hydraulic conductivity (K) in these GCLs is hypothesized to be physical clogging of the intergranular pore space with polymer hydrogel. Subsequent increases in K are hypothesized to partially result from elution of polymer from the pore space, which may be exacerbated from seepage forces associated with permeation at high hydraulic gradient. ASTM D5084 recommends using a hydraulic gradient less than 30 for materials with K values less than 1*10-7 m/s, while laboratory hydraulic conductivity testing procedures generally have much higher hydraulic gradients. Higher gradients are used because of the extremely long test durations associated with low hydraulic conductivity values of GCLs. The effect of hydraulic gradient on conventional Na-B GCLs has been studied extensively and shown to have a negligible effect on K values for gradients as high as 2500. As dry blended polymer GCLs (DB GCLs) continue to gain popularity, it is important to understand the effect that hydraulic gradient can have on the hydraulic

conductivity and polymer elution of these GCLs, in order to ensure that laboratory compatibility testing is applicable to field compatibility. The primary objective of this study is to determine if high hydraulic gradient and flow rate have an effect on polymer elution and hydraulic conductivity of DB polymer GCLs. Three GCL types were used in this study, one Na-B GCL, Bentomat, and two DB GCLs, Resistex and Resistex Plus. GCLs were permeated with one of five permeant liquids: DI water, 50 mM CaCl2, and three synthetic Coal Combustion Product (CCP) leachates. The three CCP leachates were selected from an Electric Power Research Institute (EPRI) database representing CCP disposal facilities in the United States and include flue gas desulfurization residual (Typical FGD), high ionic strength leachate (High Strength), and trona ash leachate (Trona). A constant flow pump method was developed in accordance with ASTM D5084 and D6766 to afford precise control of flow rate and corresponding hydraulic gradient. A total of four constant flow pump tests were performed with the following materials: Test 1; Bentomat GCL permeated with DI water, Test 2; Bentomat GCL permeated with 50mM CaCl2, Test 3; Resistex Plus GCL permeated with 50 mM CaCl2, and Test 4; a replicate Resistex Plus GCL permeated with 50 mM CaCl2. A total of four falling head tests following ASTM D6766 and D5084 were performed with the following materials: Test 5; Resistex Plus GCL permeated with High Strength leachate, Test 6; Resistex Plus GCL permeated with Trona leachate, Test 7; Resistex GCL permeated with Trona leachate, and Test 8; Resistex Plus GCL permeated with Typical FGD leachate. Test 1 and Test 2 gave hydraulic conductivity values of 3.8*10-11 m/s and 1.3*10-7 m/s, respectively. Test durations and K values for Tests 1 and 2 are comparable to those

found in literature using the falling head test method. Test 3 showed unusually low K values from 0 – 8.5 PVF, and achieved chemical equilibrium at ~35 PVF at K = 2.1*10-7 m/s. Incremental flow rate changes from 20 to 40, 40 to 80, and 80 to 120 ml/hr were tested at ~40, ~50, and ~60 PVF, respectively. A negligible amount of change was observed in K and Total Organic Carbon (TOC) concentrations after each increase in flow rate. Test 4 is still in progress at ~1.5 PVF, and is performing similarly to test 3. Tests 5 – 8 were all running for ~1000 days at an average hydraulic gradient of 190, showed low hydraulic conductivity values (< 1.0*10-11 m/s), and showed relatively consistent hydraulic conductivity values for at least 600-1000 days. For test 5 and test 6, the average hydraulic gradient was doubled and cell pressure was increased in order to keep average effective stress nearly constant. A slight increase in effective stress (i.e. from 20 kPa to 26.5 kPa) was implemented to prevent low minimum effective stresses, because ASTM D5084 recommends keeping effective stresses > 7 kPa or else risk separation of the membrane from the rest of the specimen. After the hydraulic gradient increase, both tests showed a hydraulic conductivity increase of more than 3 orders of magnitude within 1-2 days, from 8.0*10-12 to 8.6*10-9 m/s for test 5 and from 3.8*10-12 to 4.8*10-9 for test 6. The change was accompanied by a spike in TOC followed by a decrease in test 6, but only a decrease in TOC for test 5. For test 7 and test 8, the gradient was increased incrementally to give average values of 190, 233, 276, 319, 380, and 470. Slight increases in effective stress (i.e. from 20 to 21.4, 22.9, 24.4, 26.5, and 29.5 kPa respectively) were implemented. Each test was allowed to permeate for approximately 1 week at each interval to allow for any alterations to occur. Test 7 and test 8 showed constant hydraulic conductivity values at each hydraulic gradient interval.

Tests 1 and 2 showed that a constant flow pump can be used to find K values of Na-B GCLs equivalent to those found using the falling head method in a comparable amount of time. Tests 3 and 4 showed that there is an extended period of time where the constant flow method causes the K values of DB GCLs to be significantly lower than those measurements found using the falling head method. The precise mechanism for this observation isn’t fully understood at this time. Nevertheless, this is a significant disadvantage as it could take months to years longer for a constant flow pump test to reach chemical equilibrium. Test 3 also showed that when K is high, an increase in hydraulic gradient causes no change in polymer elution or hydraulic conductivity. Tests 5 and 6 showed that an immediate increase in hydraulic gradient from 190 to 380 can cause the hydraulic conductivity of DB GCLs to increase by greater than 3 orders of magnitude in as little as 1 to 2 days. The increase in hydraulic conductivity will likely be accompanied by a short surge of polymer elution followed by very low elution. Tests 7 and 8 showed that by incrementally increasing the hydraulic gradient from 190, 233, 276, 319, 380, and 470, and allowing one week for any change to occur at each gradient, the hydraulic conductivity can remain constant. There appears to be some mechanism during an incremental increase rather than an immediate increase that allows the hydraulic conductivity to remain low at gradients as high as 500. The precise mechanism for this observation is unknown at this time.