Soils are inherently variable in their physical and chemical characteristics. Usually the variability is much greater vertically than horizontally, resulting from the variability in the processes that originally formed the soils. The soil variability, in turn, will result in variability in the distribution of water and contaminants and in the ease with which they can be transported within, and removed from, the soil at a particular site.
Many data elements are relatively easy to obtain, and in some cases more than one test method exists. Field procedures are performed for recording data or for collecting samples to determine the classification, moisture content, and permeability of soils across a site. Field reports describing soil variability may lessen the need for large numbers of samples and measurements to describe site characteristics. Common field information-gathering often includes descriptions of natural soil exposures, weathering that may have taken place, cross-sections, subsurface cores, and soil sampling. Such an effort can sometimes identify probable areas of past disposal through observation of soil type differences, subsidence, and backfill.
Soil particle-size distribution is an important factor in many soil treatment technologies. In general, coarse, unconsolidated materials, such as sands and fine gravels, are easiest to treat. Soil washing may not be effective where the soil is composed of large percentages of silt and clay because of the difficulty of separating the adsorbed contaminants from fine particles and from wash fluids. Fine particles also can result in high particulate loading in flue gases from rotary kilns as a result of turbulence. Heterogeneities in soil and waste composition may produce nonuniform feedstreams for many treatment processes that result in inconsistent removal rates. Fine particles may delay setting and curing times and can surround larger particles, causing weakened bonds in solidification/stabilization processes. Clays may cause poor performance of the thermal desorption technology as a result of caking. High silt and clay content can cause soil malleability and low permeability during steam extraction, thus lowering the efficiency of the process.
Soil homogeneity and isotropy may impede in situ technologies that are dependent on the subsurface flow of fluids, such as soil flushing, steam extraction, vacuum extraction, and in situ biodegradation. Undesirable channeling may be created in alternating layers of clay and sand, resulting in inconsistent treatment. Larger particles, such as coarse gravel or cobbles, are undesirable for vitrification and chemical extraction processes and also may not be suitable for the stabilization/solidification technology.
The bulk density of soil is the weight of the soil per unit volume, including water and voids. It is used in converting weight to volume in materials handling calculations, and can aid in determining if proper mixing and heat transfer will occur.
Particle density is the specific gravity of a soil particle. Differences in particle density are important in heavy mineral/metal separation processes (heavy media separation). Particle density is also important in soil washing and in determining the settling velocity of suspended soil particles in flocculation and sedimentation processes.
Soil permeability is one of the controlling factors in the effectiveness of in situ treatment technologies. The ability of soil-flushing fluids (e.g., water, steam, solvents, etc.) to contact and remove contaminants can be reduced by low soil permeability or by variations in the permeability of different soil layers. Low permeability also hinders the movement of air and vapors through the soil matrix. This can lessen the volatilization of VOCs in SVE processes. Similarly, nutrient solutions, used to accelerate in situ bioremediation, may not be able to penetrate low-permeability soils in a reasonable time. Low permeability may also limit the effectiveness of in situ vitrification by slowing vapor releases.
High soil moisture may hinder the movement of air through the soil in vacuum extraction systems and may cause excavation and material transport problems. High soil moisture also affects the application of vitrification and other thermal treatments by increasing energy requirements, thereby increasing costs. On the other hand, increased soil moisture favors in situ biological treatment.
The pH of the waste being treated may affect many treatment technologies. The solubility of inorganic contaminants is affected by pH; high pH in soil normally lowers the mobility of inorganics in soil. The effectiveness of ion exchange and flocculation processes may be negatively influenced by extreme pH ranges. Microbial diversity and activity in bioremediation processes also can be affected by extreme pH ranges.
Eh is the oxidation-reduction (redox) potential of the material being considered when oxidation-reduction types of chemical reactions are involved. Examples of these types of reactions include alkaline chlorination of cyanides, reduction of hexavalent chromium with sulfite under acidic conditions, aerobic oxidation of organic compounds into CO2 and H2O, or anaerobic decomposition of organic compounds into CO2 and CH4. Maintaining a low Eh in the liquid phase enhances anaerobic biologic decomposition of certain halogenated organic compounds.
Kow (the octanol/water partition coefficient) is defined as the ratio of a chemical's concentration in the octanol phase to its concentration in the aqueous phase of a two-phase octanol/water system. Kow is a key parameter in describing the fate of an organic chemicals in environmental systems. It has been found to be related to the water solubility, soil/sediment adsorption coefficient, and the bioconcentration factors for aquatic species. The physical meaning of Kow is the tendency of a chemical to partition itself between an organic phase [e.g., polycyclic aromatic hydrocarbons (PAHs) in a solvent] and an aqueous phase. Chemicals that have a low Kow value (<10) may be considered relatively hydrophilic; they tend to have a high water solubility, small oil/sediment adsorption coefficients, and small bioconcentration factors for aquatic life. Conversely, a chemical with a large Kow (>104) is considered hydrophobic and tends to accumulate at organic surfaces, such as on humic soil and aquatic species.
Humic content (organic fraction) is the decomposing part of the naturally occurring organic content of the soil. High humic content will act to bind the soil, decreasing the mobility of organics and decreasing the threat to ground water; however, high humic content can inhibit soil vapor extraction (SVE), steam extraction, soil washing, and soil flushing as a result of strong adsorption of the contaminant by the organic material. Reaction times for chemical dehalogenation processes can be increased by the presence of large amounts of humic materials. High organic content may also exert an excessive oxygen demand, adversely affecting bioremediation and chemical oxidation.
Total organic carbon (TOC) provides an indication of the total organic material present. It is often used as an indicator (but not a measure) of the amount of waste available for biodegradation. TOC includes the carbon both from naturally-occurring organic material and organic chemical contaminants; however, all of it competes in reduction/oxidation reactions leading to the need for larger amounts of chemical reagents than would be required by the contaminants alone.
Measurement of volatile hydrocarbons, oxygen (O2), and carbon dioxide (CO2) at sites containing biodegradable contaminants like petroleum hydrocarbons or sites with high TOC is useful in further delineating and confirming areas contaminated, as well as identifying the strong potential for bioremediation by bioventing. In addition, if the use of thermal combustion or certain oxidation systems is planned for off-gas treatment of extracted vapors, then adequate supply of air or oxygen will have to be provided to efficiently operate these systems.
Biochemical oxygen demand (BOD) provides an estimate of the aerobic biological decomposition of the soil organics by measuring the oxygen consumption of the organic material that can be readily or eventually biodegraded. Chemical oxygen demand (COD) is a measure of the oxygen equivalent of the organic content in a sample that can be oxidized by a strong chemical oxidant such as dichromate or permanganate. Sometimes COD and BOD can be correlated, and the COD/BOD ratio can give another indication of biological treatability or treatability by chemical oxidation. COD is also useful in assessing the applicability of wet air oxidation.
One of the major determining factors in the fate of biodegradable contaminants is the availability of sufficient electron acceptors (i.e., oxygen, nitrate, iron, manganese, sulfate, etc.) to support biodegradation. Internal tracers, such as trimethyl and tetramethylbenzenes, are normal constituents of fuels that are significantly less biodegradable than benzene, toluene, ethylbenzene, and xylenes (BTEX), yet have very similar transport characteristics. Thus, these 'internal tracers' can be detected downgradient of the remediation area, thereby demonstrating that monitoring wells are properly placed. The absence of BTEX is a result of biodegradation. The concentrations of these tracers can also provide a basis to correct for the contribution of dilution to contaminant attenuation.
Oil and grease, when present in a soil, will coat the soil particles. The coating tends to weaken the bond between soil and cement in cement-based solidification. Similarly, oil and grease can also interfere with reactant-to-waste contact in chemical reduction/oxidation reactions, thus reducing the efficiency of those reactions.