Water table soil contamination

Pressly Associates, Inc., an environmental consulting firm in Brookhaven, New York, researched treatment options (including PF) for a dry cleaner site contaminated with tetra-chloroethene (PCE). According to the firm, a 2-acre plume exists at the site to a depth of 35 ft below ground surface (bgs) (or 30 ft below the water table). Soils at the site consist of fine to medium sand, silt, and sand and gravel. The firm claims that capital costs for various treatment options are as follows ... 

Of course the presence of a Hquid phase of hydrocarbon in a soil gives rise to vapor contamination in the vadose zone above the water table. This can be treated by vacuum extraction, and the passage of the exhaust gases through a biofilter (see above) can be a cheap and effective way of destroying the contaminant permanently. 

In Situ Air Stripping. An innovation to conventional pump and treat air stripping is in situ air stripping. Two horizontal wells are installed, one below the water table and one in the vadose zone. Air is injected in the lower well while contaminated soil vapor is extracted by vacuum through the upper well. 

Soil vapor extraction (SVE) is a relatively new yet widely applied technology for the remediation of soils contaminated with volatile organic compounds (VOC) in the unsaturated zone above the water table (vadose zone). The process consists of generating an airstream through the contaminated soil subsurface in order to enhance the volatilization of organic contaminants and thus remove them from the soil matrix.913... 

For in situ soil flushing, large volumes of water, at times supplemented with surfactants, cosolvents, or treatment compounds, are applied to the soil or injected into the groundwater to raise the water table into the contaminated soil zone. Injected water and treatment agents are isolated within the underlying aquifer and recovered together with flushed contaminants.50-52,85... 

However, while the contaminant properties are important considerations in the selection and design of an MPE system for a given site, the applicability of MPE is more dependent on media properties, primarily hydraulic conductivity, transmissivity, depth of the water table, and soil moisture.46... 

In some configurations, the vacuum used in MPE increases the effective drawdown of ground-water (i.e., the increase or lowering of the depth of the groundwater table) locally near the pumped well. This has the effect of increasing exposed soil in the saturated zone and the removal of volatile contaminants located above and below the original water table. 

Table 1 indicates primary pollutant sources and waste modes, and Table 2 indicates the primary and secondary sources and associated pollutants. The primary sources of soil contamination include land disposal of solid waste sludge and waste-water industrial activities and leakages and spills, primarily of petroleum products. The solid waste disposal sites include dumps, landfills, sanitary landfills, and secured landfills. 

If contamination is the reason for sampling, samples at depth along the transect line will also be required. Depending on the soil depth, depth to the water table, the severity of contamination, and the type of soil, deep samples may need to be taken at each location. As with the transect, depth samples must be taken until the contaminant or analyte of interest is found to be at background levels. Thus, several depths may need to be taken at each location along the transect. 

A study of the vertical profile of chlorinated solvents in the soil, enables the source of contamination to be distinguished for atmospheric inputs a peak occurred a short distance below ground, whereas for inputs from groundwater the concentration increased progressively as the water table was approached. 

Groundwater can be found in the traditional sense at the water table below which the soil pore spaces are essentially saturated and the water is free to move, and in the unsaturated zone (or vadose zone) above the water table. It is possible for water to migrate through both of these zones, transporting dissolved components (or contaminants). The interaction of the various forces involved will determine the direction and rate of migration. 

When contaminant air rises above the water table into the vadose zone, the VOCs are captured by soil-venting extraction, escape to the atmosphere, or are treated as they encounter indigenous bacteria present in that zone. 

Bioslurping is most effective in fine- to medium-textured soils, where there is a significant quantity of LNAPL and associated soil contamination at the water table, with minimum drawdown and groundwater extraction. The practical maximum depth of liquid recovery is suction lift (27 ft of water column). 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

Key parameters include temperature, pounds of steam injected (or similar factors for air), and duration and depth of treatment. Steam at a pressure 3.5 to 4.2 kg/cm2 (50 to 60 psi) can heat contaminated soil to 155°C (310°F). The recovery process involves the use of wells to depress the water table and ensure capture of released free-phase NAPLs and vapor-phase hydrocarbons at or near the surface. A conceptual schematic is shown in Figure 10.8. 

LNAPL hydrocarbon pools occurring as perched zones and generally overlying the water table which serves as a continued source of both soil and groundwater contamination. 

The zone between land surface and the water table, which forms the upper boundary of the groundwater region, is known as the vadose zone. This zone is mostly unsaturated— or more precisely, partially saturated— but it may contain a saturated fraction in the vicinity of the water table due to flucmations in water levels or capillary rise above the water table. The near-surface layer of this zone—the soil—is generally partially saturated, although it can exhibit periods of full saturation. Soil acts as a buffer that controls the flow of water among atmosphere, land, and sea and functions as a sink for anthropogenic contaminants. 

These phenomena do not occur in a static domain chemical compounds migrate and are redistributed along the soil profile, down to the water table region and within the fully saturated aquifer zone, by flowing water. The extent of this redistribution and the kinetics of the geochemical interactions are controlled by the very nature of fluid flow in porous media, the water chemistry, and of course the properties of the soil and contaminant(s). 

Chapter 5 of the document reviews the UFs used by UK Government departments, agencies, and their advisory committees in human health risk assessment. Default values for UFs are provided in Table 3 in the UK document with the factors separated into four classes (1) animal-to-human factor, (2) human variability factor, (3) quality or quantity of data factor, and (4) severity of effect factor. The following chemical sectors are addressed food additives and contaminants, pesticides and biocides, air pollutants, drinking water contaminants, soil contaminants, consumer products and cosmetics, veterinary products, human medicines, medical devices, and industrial chemicals. 

Remediates contaminants in the smear zone between the water table and the overlying dry soil, resulting in faster site remediation. 

Many site-specific factors can infiuence the cost of VESTRIP treatment. Soil properties that can influence the cost of any SVE system include permeability, porosity, depth and stratigraphy of the contamination, site heterogeneity, and seasonal water table fiuctuations. In general, the more permeable and homogenous the soil, the more efficiently any SVE will operate, and the lower treatment costs will be (D22449H, p. 4-4). 

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