Geosynthetic clay liners containing bentonite-polymer composites (BPC-GCLs) are used for lining waste containment systems containing aggressive (e.g., high ionic strength, extreme pH) leachates. Elution of polymer from pore spaces controls the long-term hydraulic conductivity of BPC-GCLs. Leachates that promote polymer elution lead to opened pore spaces and higher hydraulic conductivity in BPC-GCLs; however, the mechanisms that control elution are not well understood, and no index tests are available to quickly screen BPC-GCLs for conditions that promote elution. A variety of proprietary polymers are used in commercially available BPC-GCLs, each of which may interact differently with aggressive leachates. Thus, tests are needed to evaluate the potential for polymer elution from BPC-GCLs.

The relationship between hydraulic conductivity and polymer elution was explored for commercially available BPC-GCLs permeated with solutions representative of aggressive leachates from industrial waste streams. Characteristics of the effluent, including polymer content, viscosity, pH, and EC, were monitored throughout testing. Intrinsic permeability was calculated using viscosity of the effluent, and pore-scale phenomena were assessed using scanning electron microscopy (SEM). BPC-GCLs with lower hydraulic conductivity (<10^-9 cm/s) produced low volumes of effluent with high concentrations of dissolved polymer, whereas BPC-GCLs with higher hydraulic conductivity (>10^-9 cm/s) produced high volumes of effluent with low concentrations of dissolved polymer. Intrinsic permeability increased proportionally with hydraulic conductivity, indicating that additional pore spaces were opened during permeation. SEM images of the BPC-GCLs before and after permeation indicate that the microstructure of the polymer gel changed from broad sheet-like structures that filled pores to thin stringy structures that were ineffective in blocking flow paths. These observations indicate that hydraulic conductivity of BPC-GCLs increases as flow is directed through pores depleted of polymer at a higher velocity, reducing the area and time of contact between the eluent and the polymer.

Flow stress of the viscoelastic polymer hydrogel was measured to characterize how moisture content and solution chemistry affects whether the polymer component of the BPC behaves more like a liquid (more prone to elution) or a solid (less prone to elution). As the moisture content of three commercial polymer gels increased, the flow stress decreased, indicating that polymer would elute more readily. This is in agreement with the conceptual model developed from the permeation experiments: as polymer is depleted in a pore space, the moisture content of the remaining polymer increases, leading to a higher flow rate and lower dissolved polymer content of the effluent. Flow stress appears to be a practical index test for screening BPC-GCLs for compatibility with aggressive leachates.