Diffusion of landfill leachate through compacted natural clays containing small amounts of carbonates and sulfates

Abstract

Diffusion of a synthetic urban landfill leachate through compacted natural illitic clays and the role of reactive accessory minerals (carbonates and gypsum) in the geochemical behavior of major soluble ions are evaluated. The leachate is composed of NH þ 4 and Na + (0.25 M) balanced by Cl À (0.25 M), acetate (0.1 M) and HCO À 3 (0.15 M). The pH is 7.8 and it is typical of the mature stage of organic matter degradation within an urban landfill. Laboratory scale diffusion tests were performed over 4 months (long term experiment, LT) and 2 weeks (short term experiment, ST). The ST experiments were designed to allow the measurement of the Cl À gradient as a diffusion tracer in the compacted clay. In the LT experiments the chemical gradients were already at steady state, but geochemical reactions involving dissolution of gypsum and precipitation of calcite were observed. Evolution of pore-water chemistry, mineralogy, cation exchange properties, and the specific surface of clays were determined. Numerical simulations were carried out using the geochemical code RETRASO. Chloride transport, precipitation of carbonates, pH buffering, and Ca 2+ /NH þ 4 cation exchange reactions took place in the laboratory tests. Apparent Cl À diffusion coefficients were determined by direct modeling of the ST tests and validated with the LT experiments. These coefficients were considered in the simulation of transport coupled with experimentally calculated exchange constants and dissolution/precipitation reactions of gypsum and carbonates. Sulfate reduction coupled with acetate oxidation has been proven to be relevant in the sample with significant concentration of SO 2À 4 (Bailén). This process also includes significant precipitation of carbonates (mainly calcite) and causes a decrease of SO 2À 4 in the porewater.

Self-Diffusion of Water and Ions in Clay Barriers

“…Studies that focused on very low solid-water ratios (Nakashima et al, 1999), that used high concentrations of the diffusing species (Nakashima, 2002(Nakashima, , 2003bLoomer et al, 2013), or that studied the codiffusion of multiple species (De Soto et al, 2012) were excluded, as were data obtained with bentonites other than Na þ -, Ca 2þ -, or Na þ /Ca 2þ -exchanged bentonite (Manjanna et al, 2009;Melkior et al, 2009) and data obtained with clay-rocks that showed evidence of significant alteration (Cavé et al, 2009) or for which the clay mineralogy was not fully characterized (Cole et al, 2000;Hendry et al, 2009;Barbour et al, 2012). The studies included in the database are listed in Table 6.1 (for bentonite) and Table 6.2 (for clay-rocks).…”

“…In porous media with such low k values, at hydraulic pressure gradients representative of subsurface conditions, molecular diffusion is an important mass transfer mechanism on length scales up to hundreds of meters and millions of years, with implications in the isolation of landfills and contaminated sites (Johnson et al, 1989;Jo et al, 2006;Lange et al, 2009;De Soto et al, 2012), the persistence of dense nonaqueous-phase liquids (DNAPL) in contaminated aquifers (Parker et al, 2004;Chapman and Parker, 2005), the distribution of inert tracers in clay formations (Patriarche et al, 2004b;Lavastre et al, 2005;Mazurek et al, 2011), and the geologic storage of CO 2 (Ketzer et al, 2005;Gherardi et al, 2007) and high-level radioactive waste (HLRW) . Because of the important role of molecular diffusion in clay materials, extensive experimental efforts have aimed to characterize the associated diffusion coefficients.…”

Geochemical processes in compacted clay in contact with an acid landfill leachate: laboratory experiments and modelling results

“…The hydraulic conductivity (HC) of this clay is less than 1610 À9 m/s at the optimum density (Proctor dry density) of 1.85 g/cm 3 (the highest permeability allowed by the Spanish and EU regulations for liner shields; Real Decreto 1481/2001). This material is composed of illite (33%), smectite (12%), kaolinite (7%), calcite (11%), dolomite (5%), quartz (30%), albite (1%) and pyrite (De Soto et al, 2012).…”

“…Leachates are defined as the aqueous effluents generated from the rainwater percolation through the wastes, the biochemical processes in the waste cells and the inherent water content of the wastes themselves (Renou et al, 2008). The main biogeochemical and transport processes in landfill areas involving the interaction between the leachates and clay liners are solute diffusion (Foged & Baumann, 1999;Rosanne et al, 2003;Appelo et al, 2010;De Soto et al, 2012;Shackelford & Moore, 2013), anion exclusion (Bradbury & Baeyens, 2003;Molera et al, 2003;Montes et al, 2005;Van Loon et al, 2007;Kang & Shackelford, 2010;Ruiz et al, 2012), cation exchange processes (Christensen et al, 2000;Thornton et al, 2000;Malusis et al, 2003;Pivato & Raga, 2006;Birgersson & Karnland, 2009;Regadío et al, 2012), biologically mediated redox reactions (Christensen et al, 2000;Van Breukelen, 2003;Thabet et al, 2009;Li et al, 2011) and mineral precipitation/dissolution, which can modify the barrier porosity (VanGulck & Rowe, 2004a, b;Castelló et al, 2008). All these mechanisms affect the confinement properties of the clay barriers and should therefore be studied in detail.…”

Leachate seepage from landfill: a source of groundwater mercury contamination in South Africa

“…Sometimes, some of the leachate may percolate through the soil to contaminate the groundwater, particularly in an unlined landfill (Mor et al, 2006). Advection and diffusion are also transport processes responsible for contaminant mixing and movement of leachate into groundwater (De Soto et al, 2012).…”