Method for remediating near-surface contaminated soil

The present invention relates to a method for insitu remediation of soil.

Removal of contamination from surface and near-surface is often accomplished, by physically removing contaminated soils and subsequently treating the soils at elevated temperatures to vaporize and/or destruct the contamination. This is very expensive, and also exposes the contaminants to the atmosphere as the contaminated soil is being removed and handled. Insitu methods have therefore been proposed, both to reduce the costs of removing the contaminants and as a method to reduce the exposure to the contaminants. Heating soil by conduction from heater wells has been proposed, and a well heater useful in such a process is disclosed in U.S. Pat. No. 5,190,405. This well heater functions as both a means to inject heat, and as a means to remove vapors from the soil. A flexible sheet is placed over the surface in order to seal the surface to vapors that may be drawn into the soils from the surface (and impede the heating process) and to keep water from entering the soil from the surface. Insulation is placed on top of the flexible sheet to reduce heat loss from the system. The flexible sheet of this system must be capable of withstanding the temperatures required for removal of contamination at the surface. If contamination originated from below the surface, such as a buried tank or pipe, then surface temperatures below those required for removal of contaminants may be acceptable, but in many situations, surface temperatures are required that are well above temperatures that can be tolerated by silicone sheets or other pliable sheets. It is desirable to have a heater well design which is tolerant of such high surface temperatures.

U.S. Pat. No. 5,244,310 discloses a system for remediation of near-surface contamination using shallow heater and vapor extraction wells wherein the wells are placed through a frame. An insulated shield covers the frame. This system utilizes the proximity of the vapor extraction wells to the heat injection wells to ensure removal of the contaminants, and does not include any particular seal at the surface to ensure that excessive amounts of air do not penetrate the surface and keep the soil from reaching acceptable remediation temperatures. Air leaking into the contaminated soil also reduces the vacuum that can be applied. A lower pressure within the soil results in more efficient removal of contaminants. It would be preferable to have such a system wherein the surface is sealed in order to both eliminate air intrusion, which cools the soil and increases the amount of vapor to be treated by the vapor extraction system, and also to result in a lower pressure within the soil.

It is therefore an object of the present invention to provide a method for remediation of near-surface contaminants in which the surface is sealed to prevent significant intrusion of air into the contaminated soil. It is another object of the invention to provide such a method wherein heater-vapor extraction wells can be provided at a relatively low cost.

These and other objects are provided by a method to remove contaminants from near-surface soil, the method comprising the steps of: placing a plurality of metal sheets on the surface above the contaminated soil to form a contiguous cover over at least a portion of the contaminated soil; seal welding adjacent metal sheets together; cutting a plurality of holes in the metal sheet; providing a plurality of heater wells and a plurality of vapor extraction wells into the contaminated soil through the holes cut in the metal sheets; providing vapor extraction wellheads over the vapor extraction wells, the vapor extraction wellheads sealed to the metal sheets; and removing contaminants from the near-surface soil by heating from the heater wells and extracting vapor through the vapor extraction wells. The wells are preferably both heater wells and vapor extraction wells, and the wellheads are preferably bolted to a flange which is welded to the metal sheets to ensure a positive seal at the surface. Insulation is preferably placed over the metal sheets prior to heating of the soil, and a rain cover placed over the insulation to keep rain out of the insulation and away from the soil being heated. The edges of the contiguous cover is preferably sealed by a vertical metal sheet placed into the soil, with a 90 degree angled lip on top, and the lip attached to the edge of the continuous sheet to form the seal.

The method of the present invention forms a relatively inexpensive apparatus for decontamination of soils in a fast and energy efficient manner.

FIG. 1 is a cross section view of a heater-vapor extraction wellbore of the present invention.

FIG. 2 is a plan view of metal sheets laid out over edge metal sheets to isolate a volume of contaminated soil for remediation by the method of the present invention.

FIG. 3 is a sectional view of a joint between adjacent metal sheets.

FIG. 4 is a sectional view of the edge of the metal sheets used to isolate a contaminated volume according to the present invention.

FIG. 5 is a representation of a soil remediation system having a surface heater positioned below metal sheets.

FIG. 6 is a representation of a soil remediation system having a surface heater positioned above metal sheets.

Decontamination of soil by thermal conduction and vapor extraction is known, and is described in, for example, U.S. Pat. No. 5,318,116, incorporated herein by reference. The present invention utilizes such a method of decontamination wherein the contaminated soil is heated, insitu, to temperatures at which the contaminants are removed by vaporization and/or thermal destruction. Insitu water is vaporized, aiding in the removal of the contaminants by lowering the partial pressure of the contaminants required for removal of the contaminants by vaporization of the contaminants. Generally, the highest temperatures required are the vapor pressure of the contaminants at the pressures of the soil during decontamination. The present invention provides a method whereby the contaminants are removed by providing an economical and effective seal around the soil to be decontaminated prior to application of heat and vacuum. This seal is provided by covering the contaminated soil with a relatively thin metal sheet, or preferably strips of the thin metal sheet, and sealing the sheets together so that vapor intrusion into the contaminated soil is minimized. The edges of the sheets may be sealed to vertical sheets which extend through the soil downward a distance of at least several inches to possibly several feet, or preferably even below the lowest level of contaminants to be removed. Thus a relatively positive seal is provided around the volume to be decontaminated. This relatively positive seal minimizes migration of contaminants out of the volume being decontaminated, and minimizes flow of air and water into the volume being decontaminated.

Referring now to FIG. 1, a section of a heater-vapor extraction well useful in the present invention is shown. Heater assembly 101 is placed on the wellhead 107 . Thermocouple 102 is routed into the well for measurement of the temperature within the well. Electrical power for heater elements 124 is provided through insulators 116 from electrical connections 114 and 115 . Electrical element insulators 103 , preferably ceramic insulators, provide for expansion of the elements while providing separation of the electrical elements. Wellhead insulation 104 is provided to reduce heat loss from the top of the wellhead. Vacuum connection 105 provides communication from the inside of the wellhead to a vacuum source from which vaporized contaminants and water vapor is removed from the wellhead. Rain shield 106 is placed over thermal insulation 125 to prevent rain from getting to the thermal insulation and the soil being decontaminated. Preventing rain from getting to the soil being decontaminated is important in remediation by the method of the present invention because vaporization of a continuing influx of additional water greatly increases the energy input required, and could prevent temperatures of the soil to be decontaminated from reaching temperatures necessary for decontamination. Although insitu water is also often useful for reduction of the partial pressure of the contaminants and steam distillation of the contaminants from the contaminated soil, the amount of additional water added to the contaminated soil must generally be limited. If additional water is needed in the formation for additional steam distillation of the contaminants from the soil, the amount and distribution of this additional water must be controlled.

A pipe collar 108 supports a sand screen 121 inside of the wellbore. Sand 111 can be provided between the sandscreen and the soil to reduce the amount of soil drawn into the vacuum system.

Wellbore 113 penetrates contaminated soil 109 and additionally penetrates into native soil 110 to ensure that the contaminates are completely removed. End disk 112 is provided at the bottom of a removable heater can 122 . A lifting eye 117 is provided to allow for installation and removal of the heater can from the wellbore. Cap 123 can be added over the electrical connections to help keep the electrical connections dry and to prevent the electrical connections from accidentally shorting.

Metal sheet 120 is spread over the surface of the soil to be remediated, and holes are then cut into the metal sheet for placement of the wells. Studs 118 can be used to clamp the wellhead 107 to the metal sheet with flange 119 .

Referring now to FIG. 2, a top view of the metal sheets laid out across soil to be decontaminated is shown. Metal sheets 120 are preferably supplied in strips, so they can be transported in rolls. The metal sheets can be carbon steel if the corrosion environment to which the sheets are exposed during remediation will allow. For example, shim stock may be acceptable. If the corrosion environment to which the sheets are exposed would result in excessive corrosion, stainless steel may be needed. The sheets are preferably thin sheets, such as 0.01 to about 0.03 inches in thickness. The strips are preferably from 4 to about 8 feet in width to allow reasonably easy handling without requiring excessive seal welding to seal the seams between the strips of metal. The relatively thin sheets also allow a considerable amount of conforming to the contour of the surface of the contaminated soil. Even with the flexibility of the metal sheets of the present invention, the surface may be leveled to result in a more efficient remediation operation. For example, low spots may be filled with soil from mounds or hills to flatten the surface. Sides of the metal sheets may be formed into interlocking channels in order to result in a tighter fit and easier seal welding of adjacent metal sheets.

The volume enclosed by the metal sheets and edge sheets may be of regular (square or rectangular) shapes, or may be irregular, depending on variations in the geography and the concentrations of contaminants. Wellheads 107 , heater-vapor extraction wells 201 , heater wells 202 , and vapor extraction wells 203 are shown in a regular pattern through the metal sheets, and sealed to the metal sheets as shown in FIG. 2 . Edge metal sheet 402 is shown surrounding the volume of contaminated soil to be remediated. This volume may be the total volume to be remediated at the site, or may be a portion, with adjacent portions to be remediated separately. When a site must be remediated one section at a time (as may be required because of, for example, power availability constraints) preferably adjacent sections are sealed with metal sheets, and vapor collection wellheads are installed to collect vapors generated outside of the volume contained by the edge metal sheets of the section being remediated. Alternatively, the edge metal sheets could be provided in a trench that is wider than required by the edge metal sheets, and the trench filled with an insulating grout to minimize heat transfer from the volume being remediated to contaminated soil surrounding the volume being remediated.

The wellheads 107 are each connected to vapor collection manifold (not shown) and power supply (not shown). Thermocouple leads from each wellbore are also routed to a control system. The control system may be local, or central, or have a local component for control of electrical current to the heater, and a central element for control of the remediation process.

Optionally, thermocouples may be pushed into the soil to be remediated for monitoring of progress of heat fronts through the volume being remediated. Also, additional heaters 204 may be placed, either in shallow holes (as depicted in FIG. 5 ), or just placed on the surface of the metal sheets (as depicted in FIG. 6 ), or on the surface to be remediated, at midpoints between the heater wellheads. These additional heaters 204 add heat to the coldest point in the pattern, and significantly reduce the time required to complete removal of contaminants from these regions.

Referring now to FIG. 3, a cross section of interlocking channels between metal sheets is shown, with first metal sheet 301 interconnecting with second metal sheet 302 and seal welded with a bead of weld metal 303 . These interlocking channels may be readily field fabricated using a sheet metal break. The seal weld is preferable made after the two adjacent sheets are placed on the soil to be decontaminated.

Referring now to FIG. 4, an exemplary edge metal sheet to metal sheet joint is shown. A metal sheet 401 extends to a lip 406 on a edge metal sheet 402 . The edge metal sheet and the metal sheet are connected, for example, with sheet metal screw 403 , shown with a washer 404 . Gasket 405 is preferably provided in order to improve the seal between the two metal sheets. The gasket may be, for example, of a garlock material, or a high temperature silicone rubber. The edge metal sheet preferably extends down several inches or feet into the soil, or preferably, to below the lowest level of contaminants to be removed by the method of the present invention. The edge metal sheet may be placed in the soil by digging a trench, which is then back filled with dirt, or grouted into place, after the edge metal sheet is placed in the trench. Alternatively, the edge metal sheet may be pressed into the soil, either in long segments, or one short length at a time. When the edge metal sheets are pressed or pounded into the soil in short segments, they preferably have interlocking edges that can be sealed after adjacent pieces are in place.

The edge metal sheets may optionally be extended down to not only below the lowest level of contaminates to be removed, but may be extended further downward to control flow of subsurface water into the volume of soil to be decontaminated.