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Soil lined facilities have been used extensively for the containment and disposal of waste liquids. Often slowly permeable natural clay-rich deposits were relied upon to retard the movement of liquids from landfills or surface impoundments. In some cases, remolded layers of soils with laboratory hydraulic conductivities of 10-7 cm-1 — or less have been constructed with the intention of retaining liquids. There is an increasing body of data which indicates that the hydraulic conductivity of both in situ clay deposits and recompacted clays may be greater than those measured on samples in the laboratory. In addition, these facilities have received a wide range of waste liquids with properties that differ greatly from those of water. In fact, most of the waste liquids which have been disposed in landfills are nonaqueous.
Water is well known for its ability to hydrate clay soils and cause them to swell, resulting in low conductivities. Many organic liquids are known to cause the inter layer spacing of smectitic clays to decrease from those which occur when the same clay is wetted with water. Thus, organic liquids could possibly cause hydrated clay-rich soils to shrink and crack, which could result in an increase in the conductivity of the soils intended to retain organic liquids.
A theoretical evaluation of the influence of dielectric properties of liquids on the thickness of the double layer between adjacent clay minerals suggests that the spacing should decrease when minerals are hydrated with liquids having dielectric constants lower than that of water. Most common organic liquids have dielectric constants considerably lower than that of water, suggesting that they should cause hydrated soil to shrink. X-ray observations of a smectitic clay mineral wetted with organic liquids confirmed that the inter layer spacings were less than those observed when the clay was wetted with water. Electrophoretic mobility studies indicated that organic liquids' with low dielectric constants cause suspended clay to flocculate. Flocculation studies using dispersed clays indicated that smectitic, micaceous, and kaolinitic clays all flocculated rapidly when they were added to organic liquids, which had low dielectric constants and which were only sparingly soluble in water. The clays also flocculated when placed in a solution containing greater than 50% water soluble organic liquid. Observations of bulk samples of the three abovementioned clays indicated that water wetted specimens swelled more than similar samples wetted with organic liquids.
Laboratory studies of conductivities using a range of organic liquids including both polar and nonpolar solvents, waste solvents, and commercial petroleum products indicated that the hydraulic conductivities of compacted soils to organic liquids were one to five orders of magnitude greater than those to water. Observation of the soils permeated with dye labeled organic liquids revealed the formation of platy structural units near the surface. The dye stains in the soil revealed that the organic liquids moved through cracks that penetrated the soil, which originally had a massive structure. Field test cell liners were constructed using three clays and two organic waste liquids. The conductivity measurements in the test cells confirmed the laboratory findings. The nonpolar solvent waste containing xylene which was used to permeate the 1.5 m square and 15 cm thick field test section of compacted clay, broke through many of the replications within two weeks. The acetone waste took as long as two years to break through the test sect ions; however, in the end, sections of each type of clay were also permeated by the acetone waste.
Field data and observations collected at active landfills and surface impoundments suggest that organic liquids have moved 10 to 1000 times faster than anticipated based on laboratory measurements made using water. Some of this increased mobility may be attributed to differences between laboratory and field conductivities, while the remainder is likely due to the impact of organic liquid on the properties of clay soils. There are now sufficient data available to provide a mechanistic explanation as to how organic liquids migrate rapidly through soils. These data suggest that organic liquids, which are only sparingly soluble in water or water soluble liquids in concentrations greater than about 50%, will dessicate clays causing them to shrink and crack. The liquids are then able to flow through the newly formed macropores in the soil much more rapidly than when the soils are wetted with water.