Clay membrane made of natural high plasticity clay: leachate migration due to advection and diffusion

Abstract Leachate containment in Denmark has throughout the years been regulated by the DIF Recommendation for Sanitary Landfill Liners ( DS/R 466 ). It states that natural clay deposits may be used as membrane material provided the membrane and drainage system contains at least 95% of all leachate created throughout the lifetime of the landfill. Presently, the recommendation is under revision partly because this clause was considered to be insufficient for securing the drinking water quality in underlying aquifers — a necessary containment of at least 99% is foreseen. The revision should consequently take into account advective ion transport as well as diffusion. Clay prospecting for clays rich in smectite has revealed large deposits of Tertiary clay of very high plasticity in the area around Rodbyhavn on the Danish island of Lolland. The natural clay contains 60–75% smectite, dominantly as a sodium-type. The clay material has been evaluated using the standardized methods related to mineralogy, classification, compaction and permeability, and initial studies of diffusion properties have been carried out. Furthermore, at a test site the construction methods for establishing a 0.15–0.3 m thick clay membrane have been tested successfully. At a natural water content of w =40–45% it is possible to establish a homogeneous membrane with hydraulic conductivity k −12  m s −1 giving an extremely low transport by advection. The hydraulic conductivity measured in oedometer tests used for establishing swell and deformation properties were shown to be very dependent on the stress level. It varies from k =10 −11 to 2×10 −13  m s −1 at vertical stresses from 5 to 4800 kPa and is clearly related to a reduced effective porosity diminishing with stress. Preliminary diffusion tests indicate a similar influence on the effective diffusion coefficient being much lower than anticipated using the total porosity. These properties are of major importance to the future use of clay membranes for the containment of hazardous waste. In order to explain these properties microstructural investigations were initiated to establish the boundary conditions for the evaluation of relevant transport processes. Consequently, this clay material may be used as raw material to be compacted for membrane purposes in a natural wet condition using high compaction effort comparable to the modified Proctor energy. It fulfils the requirements that are presently proposed in the new Danish Recommendation for Landfills and a number of other applications that are foreseen for clay embedment of hazardous waste.