A multitude of technologies have been developed to remove subsurface contamination or to treat zones of soil or groundwater contamination in place. Although the mechanics of these technologies vary widely, from the injection of microbes that digest contaminants to the vacuum extraction of vapor in the soil, they all share one common requirement --they must come in contact with the contaminated media. Access to contaminants can be accomplished through trenching or other excavation, vertical drilling (either by auger or rotary methods), or horizontal directional drilling. HDD has many benefits as an access technology, with two significant advantages being:
This chapter summarizes the common technologies in use for site remediation that are compatible with horizontal directional drilling, as well as some of the emerging methods. Extraction Methods Extraction cleanup methods remove contaminants or contaminated media from their native location and convey them to the surface where they are treated. Currently, this is the most common technology applied at hazardous material cleanup sites, although some other methods, notably bioremediation, are gaining favor. Above the water table, horizontal wells are often used for removal of soil vapor, vapors produced by the evaporation of gasoline, volatile solvents or other volatile chemicals that have been released into the soil. Below the water table, horizontal wells can either extract contaminated groundwater directly or can be used to mobilize contaminants for collection elsewhere. Horizontal infiltration or injection wells may be used to reinject treated groundwater back into the subsurface. Soil Vapor Extraction
When these compounds enter the soil as a liquid, much of the liquid is adsorbed as free product onto the surface of the soil particles. Figure 1 shows a typical distribution of contaminants within the vadose zone, that portion of the soil column that lies above the water table. A portion of the compound evaporates, or volatilizes, to uniformly fill the spaces between soil particles, creating soil vapor. The amount of each kind of compound that volatilizes and enters these spaces is governed by several physical laws. If some of this soil vapor is removed, more of the free product evaporates to maintain the equilibrium between liquid and gaseous phases of the compound.
Horizontal wells are often used for soil vapor extraction, particularly since candidate sites are often beneath structures such as gas stations, chemical plants and refineries. The design and installation of SVE wells must address two key site variables: water table fluctuation and leakage into the atmosphere. If an SVE well penetrates into the water table, the vacuum system will ingest water when operated, reducing system efficiency and potentially damaging the equipment. Because water tables often rise and fall with the seasons, it is important to be aware of not only the current depth to water but the highest elevation that the water table is likely to reach. If a system must be installed at a site with a fluctuating water table, a dual-phase extraction system, described below, should be considered. Leakage, or short-circuiting, is another potential pitfall for the installer. Soil vapor extraction works because an evenly distributed region of lower vapor pressure (i.e., a partial vacuum) causes liquid contaminants to volatilize and move toward the extraction well. If the soil is homogeneous, with no major fractures or low-porosity zones, the area in which this occurs is usually most effective closer to the well and tapers off in efficiency farther away. A network of wells is usually designed to give even coverage throughout the contaminated zone.
Hot Air or Steam Injection Some contaminants, such as diesel or fuel oil, do not volatilize easily at normal ambient temperatures, making soil vapor extraction ineffective. If additional wells are installed to inject hot air or steam (steam injection wells), the higher temperatures will mobilize these heavier hydrocarbons and permit their removal. Free-Product Removal Some compounds or contaminants, such as gasoline, are lighter than water and tend to float on the surface of the water table. When accumulations of the contaminant have sufficient depth (a few inches), they are referred to as free product. Figure 4 shows how a free-product layer may exist with other phases at a contaminated site. Several types of extraction systems have been devised to remove free-product layers. Horizontal wells can be used for free-product removal but are less flexible in this application than vertical wells or trenches.
Groundwater Remediation Groundwater may be contaminated by a variety of chemical releases, from the VOCs discussed above to metals, pesticides or radioactive materials. Some of these contaminants require sophisticated treatment systems to remove them; others are relatively simple to strip from the water. As in the soil vapor, contaminants that are soluble in water will diffuse throughout a volume of groundwater to equalize their concentration. Unlike most soil vapor, groundwater can flow downgradient, essentially downhill along bedding planes or other geological structures. As the contaminants are swept downgradient with the groundwater from the site of the release, or source area, they form a contaminant plume. Figure 6 shows a cross-section of a site with a groundwater plume. One of the main concerns of environmental regulators is that a plume can travel off-site, often for thousands of feet, affecting other users along the way or potentially entering surface waters. Remediation of a groundwater plume can include removal or in-place destruction of the contaminants, construction of barriers to prevent migration of the contaminated groundwater, or combinations of these technologies. Of the methods typically used for contaminant removal, Groundwater extraction (also celled pump-and-beat) and air sparging are the most common. Groundwater Extraction Groundwater extraction wells operate in a fashion similar to wells used for household or municipal water supplies. A well or group of wells is situated where the highest concentration of contamination is located and water is pumped from that location, treated aboveground, and either reinjected or disposed of in a nearby sewer or surface stream Vertical wells have historically been used for this purpose; however horizontal wells are gaining favor because they can place a longer screen in contact with the contaminant plume and may be steered to follow a plume. Horizontal wells also require fewer pumps and manifolds than vertical well networks. Figure 7 illustrates a groundwater extraction system using a horizontal well, superimposed over the complex vertical well system that might be required to treat the same plume. Shallow horizontal wells may also be used to infiltrate treated water back into the ground. This can be of benefit at sites where connection to a municipal sewer is impractical since discharge to the ground surface or to a surface stream is generally highly regulated. Injection Systems Air Sparging Air sparging makes use of the physical principles outlined in the soil vapor section to remove contaminants that are dissolved in the groundwater. If clean air is bubbled through contaminated groundwater, some of the dissolved contaminant moves into the air (called partitioning) and is transported to the top of the water table and released into the soil vapor. Air sparging systems usually incorporate a pair of wells to promote this mass transfer. Figure 8 shows a typical layout. The actual sparge, or air injection well ("A"), is placed some distance below the water table and used to inject compressed air into the groundwater column. A soil vapor extraction well ("B") is situated vertically above the air sparge well to draw off the volatilized contaminants. Many designs have several sparge and extraction wells spaced at regular intervals across a site to cover a large area of contamination. Horizontal wells have several advantages over vertical wells in this application. The ability to gain access to obstructed areas and to follow plumes is described earlier. Horizontal sparge wells have also been demonstrated to be far more effective than vertical wells in dispersing air across a broad area; however, proper design is critical to ensure even air distribution from end to end within the well. Rather than using the slotted casing typical of soil vapor or groundwater extraction, air sparge wells most frequently use solid pipe with small (ca. 0.125 inch) holes drilled at intervals of a few inches along the desired interval. This limits the percentage of open area within the pipe, permitting suitable air injection rates with smaller compressors. The design of the pipe diameter, hole size, spacing and compressor capacity is critical to ensure a usable flow rate and uniform air delivery.
Chemical Injection Another form of treatment, particularly for groundwater, is the injection of chemical compounds to either reduce the toxicity of contaminants or to prevent their migration. Chemicals might be introduced to oxidize contaminants to change them into less harmful compounds. Other chemicals might be used to flocculate contaminants or solidify a region of contaminants, e.g., through pressure grouting. Because many of these treatment methods are "one shot" injections, these wells typically require less extensive well development than longer-term treatment alternatives. Bioremediation Bioremediation methods include the injection of bacteria or fungal colonies to metabolize the contaminants, or theintroduction of air, water or nutrients to enhance the growth of native microorganism populations, which then break down contaminants such as petroleum hydrocarbons. Bioventing is the injection of air at low volume and low pressure to stimulate bacteria growth. Well installation is much the same as for a soil vapor extraction well within the vadose zone, with the same concerns for avoiding short circuits. Sampling To date, HDD systems have not been extensively used for sampling. Two sampling applications are possible. First, soil sampling can be accomplished on a limited scale by attaching a sampling tube to the end of the drill string at intervals during the drilling process, pressing it into the bore-hole face, then tripping out to remove the captured sample. While common in vertical drilling, this sampling has not proved practical in HDD due to difficulties in keeping the borehole open and in recovery of the sample. Sampling of groundwater might be accomplished within a specific interval within a completed well by using packers to isolate the desired region, then withdrawing a sample with a sampling pump. HDD wells are also being installed beneath landfills or other infrastructure to contain leak sensors that detect the presence of certain index chemicals. Summary Many of the treatment technologies available for contaminated site remediation are compatible with HDD as an installation tool. Various extraction and injection systems have been successfully constructed, including:
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