Abstract:
Devices and methods for directing a non-aqueous phase liquid (NAPL) that migrate entrained in a fluid (e.g., a gas) from sediments in bodies of water by using trapping caps having an upwardly sloped surface toward an accumulation zone that contains a water table, in which migration of the gas carries the NAPL toward the accumulation zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/259,017, filed Oct. 27, 2008 now abandoned, and entitled “DEVICES AND METHODS FOR DIRECTING MIGRATION OF NON-AQUEOUS PHASE LIQUIDS FROM SEDIMENT,” which claims the benefit of U.S. Provisional Patent Application No. 60/982,626 filed Oct. 25, 2007, the disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     The invention relates generally to methods for directing migration of non-aqueous phase liquids (NAPLs) from NAPL-contaminated sediment, and relates more particularly to methods of directing NAPL migration from such sediment to the surface of a body of water using devices through which NAPLs and other contaminants cannot percolate. The invention also contemplates directing migration of other granular materials that may contain contaminants, such as sand and silt, in a fluid, such as in a gas (i.e., via advective flow) or in a liquid (i.e., via conductive flow), especially where ebullition may be a factor. 
     NAPLs are highly hydrophobic liquids that have a low solubility and a low surface tension. Consequently, they are not water-soluble and form a separate phase when mixed with water. For example, oil is a NAPL that does not mix with water, and oil and water in a glass will separate into two separate phases. NAPLs can be lighter than water (light NAPLs (LNAPLs)) or denser than water (dense NAPLs (DNAPLs)). Hydrocarbons, such as gasoline, oil creosote and tar, as well as chlorinated solvents, such as trichloroethylene, are examples of NAPLs. 
     NAPLs are often found at or near industrial sites or former industrial sites. For example, historic wastewater outfalls from manufactured gas plant (MGP) sites often contain NAPL-contaminated sediment, such as MGP tar (e.g., coal tar), as well as other organic matter. Likewise, current or historical wastewater discharges from other industrial sites, such as refineries, wood-treating facilities, asphalt plants, aluminum smelters, coking plants, steel mills, chemical manufacturing facilities and others, may also contain NAPLs (both DNAPLs and LNAPLs). 
     When organic matter in NAPL-contaminated sediment anaerobically biodegrades, carbon dioxide (CO 2 ) and methane (CH 4 ) gases are produced that can migrate toward the surface as buoyant bubbles, entraining and transporting the highly hydrophobic NAPLs upward through a water column to the water surface, even though NAPL by itself may be denser than water. When the gas is released to the atmosphere at the water surface, NAPLs typically form a surface sheen because the net density of the gas bubble/NAPL is less than water and/or because the surface tension of NAPLs is much less than that of water. The sheen and NAPL droplets then are able to migrate along the water surface, creating a potential human health hazard and environmental hazard (e.g., coal tar is a known human carcinogen). Likewise, NAPL droplets adhere to shoreline structures or floating objects, such as boats or buoys. Occasionally, NAPL droplets lose their buoyancy when the gas is lost or when the surface tension of the water is broken. The NAPL droplets then fall back through the water column only to be re-deposited on the sediment. NAPL migration can also occur when ebullition or some other force (e.g., turbulent prop wash) disturbs sediment and causes NAPLs to migrate from the sediment. LNAPL from the sediment may also move upward due to buoyancy. 
     Certain techniques for directing NAPL migration are known to one of ordinary skill in the art and are summarized by McAnulty &amp; McLinn. See McAnulty S &amp; McLinn E, “NAPL migration from contaminated sediment 2: implications for remedial design,” Paper A-031 in Remediation of Contaminated Sediments—2007 (Foote E &amp; Durell, eds. 2007); see also McLinn E &amp; McAnulty S, “NAPL migration from contaminated sediment 1: diagnosis and transport mechanisms,” Paper A-030 in Remediation of Contaminated Sediments—2007 (Foote E &amp; Durrell, eds. 2007), each of which is incorporated herein by reference as if set forth in its entirety. These techniques include, but are not limited to, the following: (1) removing NAPL-contaminated sediment; (2) placing filter caps over NAPL-contaminated sediment; (3) infilling over NAPL-contaminated sediment; (4) lowering the temperature of NAPL-contaminated sediment; (5) increasing the pressure over NAPL-contaminated sediment; (6) altering the properties of NAPL-contaminated sediment; and (7) preventing physical disturbance of NAPL-contaminated sediment. Each technique, however, presents its own advantages and disadvantages. The art, however, needs other devices and methods of controlling or preventing NAPL migration from NAPL-contaminated sediments. 
     BRIEF SUMMARY 
     In a first aspect, a method for directing a NAPL or other contaminant from a NAPL-contaminated sediment beneath a surface of a body of water toward an accumulation zone includes the step of interposing in a path of a plurality of NAPL-entraining gas bubbles (or LNAPL droplets) migrating between the sediment and the surface a device through which neither the gas nor the NAPL can percolate, the device defining, at or near one end, a vent or chimney in fluid communication with the accumulation zone and sloping upward toward the surface to an extent sufficient to direct the gas bubbles and any entrained NAPL from under the device to the vent or chimney and into the accumulation zone. The accumulation zone contains a water table or other barrier (e.g., a filter medium, such as a reactive core mat) that allows the gas to be liberated while accumulating residual NAPL. In some embodiments, the method also includes the step of treating the accumulated residual NAPL using any of the various techniques known to one of ordinary skill in the art. 
     It will be appreciated that the device can be constructed in situ, where NAPL accumulation in accord with the method is desired. The device for directing NAPL migration, including migration caused by ebullition, NAPL migration caused by surface tension, or NAPL migration caused by buoyancy, as well as other modes of NAPL migration, includes at least a control layer constructed of a material having a permeability sufficiently low to prevent percolation into the layer of the gas and the entrained NAPL. A suitable material for the gas control layer has an expected hydraulic conductivity no higher than about 10 −2  cm/s. In some embodiments, the device includes a grading layer which, in use, is provided between the sediment and the control layer. In other embodiments, the device includes an armoring layer which, in use, is provided between the control layer and the water surface. In still other embodiments, the device includes a transmission layer interposed between the grading layer and the control layer. In some embodiments, the gas transmission layer has a hydraulic conductivity approximately 100 times greater than the gas control layer. In yet other embodiments, the device includes both the grading and armoring layers, positioned as described. Still further embodiments include the grading, armoring and transmission layers, positioned as described. The grading, armoring and transmission layers can be constructed of conventional materials suited for such purposes as are constructed for use in sediment caps. 
     These and other features, aspects and advantages of the present invention will become better understood from the description that follows. In the description, reference is made to the accompanying drawings, which form a part hereof and in which there is shown by way of illustration, not limitation, embodiments of the invention. The description of preferred embodiments is not intended to limit the invention to cover all modifications, equivalents and alternatives. Reference should therefore be made to the claims recited herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and features, aspects and advantages other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such detailed description makes reference to the following drawings, wherein: 
         FIG. 1  shows a side sectional view of a NAPL-trapping cap device of the present invention; 
         FIG. 2  shows migration of NAPL-entraining gas bubbles along a surface of the NAPL-trapping cap device of  FIG. 1 ; and 
         FIG. 3  shows a side sectional view of a second embodiment of a NAPL-trapping cap device of the present invention. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to the inventor&#39;s observation that gas bubbles having entrained NAPL droplets that migrate from NAPL-contaminated sediments cause sheens to form on the surface of a body of water. This observation suggests that sheen formation can be controlled by directing gas bubbles having entrained NAPL droplets away from the water surface above the sediment to an accumulation zone where the gas can escape and where residual NAPL can be treated by methods known to one of ordinary skill in the art. 
     Referring to  FIG. 1 , the present invention provides a NAPL-trapping cap device  10  for directing NAPL migration from NAPL-contaminated sediment  12  in a body of water. The device  10  is constructed above the contaminated sediment  12 , a floor layer  14  of the body of water, such as a riverbed, and bedrock  16 . The device  10  includes a control layer  18  that includes at least an inferior surface  20  that is sloped upwards to direct gas bubbles and entrained NAPLs toward an accumulation zone  22 . The device  10  may also include a grading layer  24  which, in use, is located between the contaminated sediment  12  and the control layer  18  to support the control layer  18 . The device  10  may further include a transmission layer  25  interposed between the grading layer  24  and the control layer  18  that promotes efficient transmission of gas bubbles to the control layer  18 . Further still, the device  10  may include an armoring layer  26  which, in use, is located between the control layer  18  and the water surface  28  to prevent erosion of the control layer  18 . These and other aspects of the device  10  will be described in further detail below. 
     The slope of the control layer  18  in situ is upwards and begins at an outermost edge of a gas-facilitated contaminant transport zone  30  from the contaminated sediment  12 , and preferably begins outside the outermost edge of the contaminated sediment  12 , and ends at the accumulation zone  22 . Although the entire control layer  18  may be sloped, at least the inferior surface  20  of the control layer  18  is sloped upwards toward the accumulation zone  22 . The slope can, and will, vary depending upon the location of the contaminated sediment  12  (i.e., its depth in the body of water and its distance from the accumulation zone  22 ), but must be enough to direct gas bubbles that are being transported to the accumulation zone  22  for accumulation and/or treatment. If the slope is too low, gas bubbles may be trapped under the control layer  18 . Trapped gas bubbles may ultimately lead to failure of the device. See Mutch, R. et al., “Monitoring the Uplift of a Low-Permeability Sediment Cap Due to Gas Entrapment Beneath the Cap: Findings of the First 18 Months” (2005). An exemplary slope is 1/60 (an angle 0.95 degrees); as shown in laboratory testing, a slope of 1/60 is sufficient to direct gas bubbles provided other conditions are met as described in further detail below. However, construction of the control layer  18  with such a low slope may be difficult due to settlement of the layers during or after construction. As such, the slope is preferably 1/10 (an angle of 5.7 degrees) or greater. Laboratory testing has also shown that a slope of 1/10 is sufficient to direct gas bubbles. An exemplary upper limit for the slope may be 10/57 (an angle of 10 degrees) or even greater. However, the upper limit for the slope is limited by the underwater angle of repose, or the angle at which a specific material can be piled without sliding, of the material from which the control layer  18  is constructed. 
     A portion of the control layer  18  near the accumulation zone  22  defines a vent (“chimney”)  32 , which contains a water table  35  or other structure (i.e., a reactive core mat) that allows gas to escape from gas bubbles having entrained NAPLs, but retains NAPLs in the accumulation zone  22 . By “water table,” I mean a surface where water pressure equals atmospheric pressure. The vent  32  may also be defined in part by a shoreline surface or a variable fill layer upper surface  33 . The vent  32  may have a greater slope than other portions of the control layer  18  depending on the shape of the surface  33 . 
     The accumulation zone  22  is located adjacent the vent  32  of the control layer  18  may be defined in part by the surface  33 . The accumulation zone  22  is preferably located in an area where contaminants can be sequestered or treated so that the potential for human or environmental exposure is limited. The accumulation zone  22  is gas-permeable and is constructed from a granular material, such as sand or gravel. The accumulation zone  22  may additionally contain amendments (e.g., carbon, coke or organoclay). 
     Referring to  FIG. 2  and during operation of the device  10 , sediment at an appropriate depth (e.g. less than 17 feet) generates gas bubbles  36  that migrate upward. The gas bubbles  36  entrain NAPLs and form NAPL-entraining gas bubbles  38  that escape from the NAPL-contaminated sediment  12 . If the grading layer  24  is provided, the gas bubbles  38  first migrate through the grading layer  24  before contacting the control layer  18 . After contacting the control layer  18 , the gas bubbles  38  migrate along the upward slope of the inferior surface  20  of the control layer  18  to the accumulation zone  22 . The NAPLs are trapped between a high tide (or high water) line  40  and a low tide (or low water) line  42  when the gas bubbles  38  reach the water table, and because the accumulation zone  22  is outside the body of water, the gas is vented, leaving behind a NAPL residue  44  that forms an interfacial film. The NAPL residue  44  may be treated by any of the various techniques known to one of ordinary skill in the art. For example, NAPL residue can be removed or treated with an agent that alters the chemical properties of NAPLs. See McAnulty &amp; McLinn, supra. 
     Exemplary materials for the control layer  18  include, but are not limited to, the following: plastics, such as high-density polyethylene (HDPE); certain geotextiles (such as those available from Propex; Chattanooga, Tenn.); geocomposites (such as those available from CETCO; Arlington Heights, Ill.); a well-graded sand layer, or a clay layer (e.g., AquaBlok®; AquaBlok, Ltd.; Toledo, Ohio). In general, the material has resistance to chemical attack, has low gas permeability (estimated to be 1,000 millidarcys or less), has low NAPL permeability (estimated to be 1,000 millidarcys or less), and has sufficient strength to prevent rupturing under differential stresses caused by uplift forces from gas accumulation. Alternatively, the material for the control layer  18  may be selected in conjunction with the material for the transmission layer  25 , if included, as described in further detail below. 
     As described briefly above, the device  10  can include a grading layer  24  that provides a stable surface upon which to construct the control layer  18  with an appropriate slope for directing the gas bubbles  38 . The grading layer  24  helps provide a stable surface especially when the armoring layer  26  is present. Without the grading layer  24 , the weight of the armoring layer  26  may cause differential settlement of the material below the control layer  18 . The grading layer  24  should contact both the NAPL-contaminated sediment  12  and the control layer  18  to help support the control layer  18 . To further support the control layer  18 , the grading layer  24  should likewise have at its upper surface  34  an upward slope toward the accumulation zone  22 . Depending on the shape of the surface of the contaminated sediment  12 , the grading layer  24  may have generally uniform thicknesses as shown in  FIG. 1 . The grading layer  24  is gas- and contaminant-permeable, and can be formed of sand, gravel, crushed stone, or general fill. 
     The device  10  can include a transmission layer  25  that promotes more efficient transmission to the control layer  18  of the NAPL-entraining gas bubbles  38  than the grading layer  24 . The transmission layer  25  can be located above the grading layer  24 , if present, but below the control layer  18 . Alternatively, the transmission layer  25  and the grading layer  24  may be combined to form a single layer that performs the functions of both individual layers. The transmission layer  25  is gas- and contaminant-permeable, and is a granular material (e.g., sand, crushed stone or gravel), a geomembrane, or a composite of granular material and a geomembrane. Alternatively, the transmission layer  25  may include multiple sections (e.g., upper and lower sections) that comprise different materials, such as different types of crushed stone. The transmission layer  25  can contain amendments (e.g., carbon, coke or organoclay). 
     As described briefly above, the material for the transmission layer  25 , if included, may be selected in conjunction with the material for the control layer  18 . Specifically, the materials for the transmission and the control layers  25  and  18  may be selected considering the permeability contrast, or the ratio of the permeability of the transmission layer  25  to the permeability of the control layer  18 . The permeability contrast is preferably at least 100:1. Such a permeability contrast may be obtained by using poorly-graded gravel (with a permeability of 10° cm/s) as the transmission layer  25  and well-graded sand (with a permeability of 10 −2  cm/s) as the control layer  18 . A high permeability contrast, such as 100:1, permits gas bubbles to easily pass through the transmission layer  25  and prevents gas bubbles from passing through the control layer  18 . In addition, laboratory testing has shown that a permeability contrast of 100:1 may permit a relatively low slope, such as 1/60, to be used. Alternatively, a lower permeability contrast may be used although a higher permeability is recommended. A higher permeability contrast should be used as laboratory testing has shown that a permeability contrast of 10:1, obtained by using well-graded sand (with a permeability of 10 −2  cm/s) as the transmission layer  25  and silty sand (with a permeability of 10 −3  cm/s) as the control layer  18 , may result in failure of the device  10 . In this case, failure occurs because the gas bubbles attempt to permeate the control layer  18  instead of moving along the control layer  18  and through the transmission layer  25 . Failure may even occur if a relatively large slope, such as 1/10, is used with a permeability contrast of 10:1. 
     As described briefly above, the device  10  can further include an armoring layer  26  atop the control layer  18  to prevent erosion of the control layer  18  as a result of water flow or ice attack. Likewise, the armoring layer  26  provides weight to counter any uplifting forces caused by gas accumulation under the control layer  18 . The armoring layer  26  should be composed of a material with enough mass so that it will not be moved by the highest anticipated erosive forces. The armoring layer  26  can be a granular material (e.g., gravel, riprap or boulders), a geomembrane or a composite of granular material and a geomembrane. Alternatively, the armoring layer  26  can be articulated block mats (Nilex Geosynthetics; Denver, Colo.) or interlocking concrete blocks (e.g., Xbloc® from Delta Marine Consultants b.v.; Gouda, The Netherlands). 
     To construct the device  10 , appropriate grades must first be present to allow the gas bubbles  38  to be conducted along the sediment-facing surface  20  of the control layer  18 . In some, but not all, applications, an initial existing grade should be adjusted by constructing the grading layer  24  to ensure that the gas bubbles  38  migrate along the control layer  18  and transmit gas and NAPL toward the accumulation zone  22 . Initial bathymetry should be established to determine the amount of fill needed to provide appropriate grades. The grading layer  24  is constructed by a sand sprinkling device, by a clamshell, by washing sand from a barge, or by a large-diameter tube if constructed in the wet (i.e., if water is present in the area of construction). Likewise, the transmission layer  25  and/or armoring layer  26 , if present, are constructed similarly to the grading layer  24 . The entire device  10 , however, may be installed in the dry (i.e., if water has been temporarily removed from the area of construction). 
     The control layer  18  may be constructed using techniques that are akin to those for installing a landfill liner if the control layer  18  is a sheet of low-permeability material, such as a geotextile. Alternatively, the control layer  18  may be placed in a controlled manner from a barge if the control layer  18  is a clay layer, such as Aquablok®, or if the control layer  18  is a sand. The vent  32 , if present, may be constructed from the same gas- and contaminant-permeable material as the grading layer  24  or the transmission layer  25  (typically sand or gravel). Alternatively, the vent  32  may be a culvert that is plumbed so that gas and NAPL migration in the gas control layerl  8  is sloped towards the vent  32 . The vent  32  may further include amendments, such as organoclay or activated carbon, to help sequester migrated contaminants. 
     Referring now to  FIG. 3 , a second embodiment of a NAPL-trapping device  110  is disposed above a NAPL-contaminated sediment  112  in a body of water. The device  110  is also positioned above a floor layer  114  of the body of water, such as a riverbed, and bedrock  116 . As described above, the device  110  includes a control layer  118  that is sloped upwards to direct gas bubbles and entrained NAPLs toward an accumulation zone  122 . The accumulation zone  122  may include a passive gas vent  121  and a topsoil layer  123  to conceal the accumulation zone  122 . The device  110  also includes a transmission layer  125  interposed between the floor layer  114  and the control layer  118  that promotes efficient transmission of gas bubbles to the control layer  118 . The device  110  includes a bedding layer  127  disposed above the control layer  118 . The bedding layer  127  may be, for example, crushed stone. An armoring layer  126  is located between the bedding layer  127  and the water surface  128  to prevent corrosion of the other layers. 
     Unlike the first embodiment, the NAPL-trapping device  110  includes a toe drain  140  at a lower end of the device  110  opposite the accumulation zone  122 . The toe drain  140  may be formed by portions of the transmission layer  125  and the armoring layer  126  as shown in  FIG. 3 . As such, water can pass through the toe drain  140  relatively easily compared to the control layer  118 . That is, water can move into and outside the device  110  through the toe drain  140 , which may be advantageous during certain situations, such as tide changes, or when groundwater is discharging to surface water in the area to be capped. As a result, the toe drain  140  permits water to escape from the device  110  quickly and thereby prevents water from exerting excessive hydraulic forces on the control layer  118 . Such hydraulic forces could ultimately lead to failure of the device  110 . 
     The toe drain  140  is preferably spaced apart from the contaminated sediment  112  so that NAPL-entraining gas bubbles do not simply rise and pass through the toe drain  140 . Instead, the NAPL-entraining gas bubbles are directed along the control layer  118  toward the accumulation zone  122  as described above. The control layer  118  is preferably sloped so that the NAPL-entraining gas bubbles are directed toward the accumulation zone  122  even as a large volume of water exits through the toe drain  140  as the tide goes out. An exemplary slope in this case is at least 3.5:1, although shallower slopes have been found to be adequate for transmitting gas. 
     The toe drain  140  may include a device or structure that treats water passing there through. For example, the toe drain  140  may treat arsenic-contaminated water using known methods. In this case, the device  110  advantageously directs and treats NAPL-entraining gas bubbles and water in different manners. Specifically, the device directs NAPL-entraining gas bubbles in a first direction toward the accumulation zone  122  for treatment and directs contaminated water in a second direction toward the toe drain  140  for treatment. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although any methods and materials similar to or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. 
     The invention will be more fully understood upon consideration of the following non-limiting Examples. 
     EXAMPLES 
     Example 1 
     Utility of a Sand Layer for Directing NAPL Migration from NAPL-Contaminated Sediment During Ebullition 
     NAPL migration from sediments to the surface of water bodies has been reported frequently at sites with NAPL-contaminated sediments, such as sediments contaminated with coal tar and creosote. Transport of NAPLs from NAPL-contaminated sediment is facilitated by gas ebullition caused by anaerobic biodegradation of organic matter in the sediment. A remedy often specified for these sites is a sand cap or a sand cap amended with sorbent materials (e.g., coke breeze or organoclays), or a reactive core mat (RCM) consisting of a geomembrane sandwiched around a reactive layer containing sorbent material such as organoclay. However, through testing, I recognized that such an art-recognized sand cap or RCM for controlling NAPL migration was not effective. 
     I used a laboratory study to assess the effectiveness of a sand layer for directing NAPL migration from a NAPL-contaminated sediment. I used a test column consisting of a Plexiglas® tube containing a tar source buried beneath a 30 cm-thick layer of sand. Water was added to the column until 5 cm of standing water covered the sand layer. To simulate ebullition, I injected air into the base of the sand column at approximately 200 ml/min under a gage pressure of 0.1 atmosphere. The gas and NAPLs migrated primarily through channels and fractures in the sand, and was not filtered through a network of stable pores. Tar was transported through the sand layer in 12 hours and accumulated on the water surface for several hours before losing its buoyancy and falling back down to the sand surface. 
     After terminating ebullition, I froze the test column to preserve sedimentary structures in the sand. Upon dissection of the column, I found that tar had migrated through the simulated sand cap in small (2 mm) channels only a few sand grains thick, and that at least one channel was continuous through the thickness of the column. These results call into question the effectiveness of sand caps for directing NAPL migration from sediment in the presence of ebullition. 
     Example 2 
     (Prophetic): Utility of a Sloped Layer for Directing NAPL Migration from NAPL-Contaminated Sediment During Ebulliton 
     This example describes the utility of a sloped layer in treatment of a sediment having a tar deposit. By way of example, the inventor determined dimensions of a tar deposit based on results of a field sampling program at a large river having a daily average flow of 400 m 3 /s, and a peak flow in excess of 4,000 m 3 /s. The estimated velocity at the riverbed varied between 0 m/s at slack tide to nearly 3 m/s during extremely high flow. A bulkhead at the upstream edge of the cove created a large eddy where the river velocity was much less than in the main channel of the river. The sediment textures in the riverbed were characteristic of a high-energy environment—well- to poorly graded gravel with cobbles in the main channel of the river, and silty sand to well-graded gravel in the lee of the bulkhead. 
     Organic material in sediment in the lee of the bulkhead included sawdust, leaves, wood, coal ash, coal and MGP tar. Organic material in the main channel of the river was more sparse than in the sediments in the lee of the bulkhead, and was almost exclusively tar. The extent of the tar deposit was determined using several different methods including sediment probes, an underwater video survey, descriptions of roughly fifty sediment cores and chemical analysis of sediment samples. The tar deposit was more than 500 m long, and between 50 and 80 m wide; the tar deposit was elongated in the predominant downstream direction of river flow. Likewise, the tar deposit contained up to 4 m of tarry sediment in a sandbar at the sewer outfall; the central portion of the tar deposit contained more than 1 m of tarry sediment, 400 m downriver from the outfall. The tar deposit thinned and became discontinuous, moving laterally and longitudinally from the core of the tar deposit. 
     Sediment core samples were collected using a split-spoon sampler and a rotary drilling rig equipped with hollow-stemmed augers, as well as a barge-mounted Vibracore drilling rig. Sediment observations were made with an underwater video camera. In addition, shallow sediment samples were collected by a diver. 
     As gas bubbled up from the sediment at low tide from just below the surface of the river, gas samples were trapped and collected in a 10-liter, transparent, polyethylene tub. When a sufficient volume of gas was trapped in the tub, the gas composition was measured in the field with a portable infrared spectrophotometer (Landtec GA-90; Landtec; Colton, Calif.). The extent of the area of gas bubbling was determined by direct observations of the river collected over time. 
     Tar droplets and sheen samples were collected directly from the river&#39;s surface using Teflon® nets designed for collecting oil film samples (General Oceanics; Miami, Fla.). Positions on the river were determined using a handheld global positioning system (GPS Garmin eTtrex®; Garmin; Olathe, Kans.) for the ebullition observations. A differential GPS was used to determine the locations of borings. 
     Chemical analysis of sediment, tar droplet and sheen samples was performed for polycyclic aromatic hydrocarbons (PAHs) using a gas chromatograph/mass spectrometer in accordance with USEPA Method 8270. Tar density was determined using ASTM Method D455. 
     The gross composition of PAHs in the sediments of the tar deposit was similar to the composition of the tar droplets and sheens found on the river surface. Total PAH concentrations in all of these phases were similar (on the order of 100&#39;s to 10,000&#39;s of mg/kg). Most (75%) of the PAHs in the tar had a relatively low molecular weight (LMW PAHs), containing two or three benzene rings, whereas the remainder (25%) had a higher molecular weight (HMW PAHs), containing four to six benzene rings. The tar in the droplets and sheens were depleted with respect to LMW PAHs (e.g., BTEX and napthalenes) when compared to sediment. 
     The tar at the bottom of the river was denser than water (tar density was about 1.3 g/cm 3 ). For the tar to float from the riverbed to the river&#39;s surface, it needed to have a net density of less than 1 g/cm 3 . Interestingly, tar at the bottom of the river became light enough to float on the river&#39;s surface when enough gas was entrained in the tar so that the net density of the tar was slightly less than 1 g/cm 3 . After enough gas diffused out of the floating tar droplet, it became denser than water, and sank to the river&#39;s bottom. This sinking/floating/sinking behavior was also observed in tar samples in the laboratory (as described herein). 
     A second, related, tar migration mechanism occurred when tar droplets became entrained in gas bubbles migrating through sediment column. The tar preferentially accumulated in the gas bubbles because the surface tension of tar was less than the surface tension of the water. 
     The extent of bubbling (ebullition) and tar migration in the river over the tar deposit was observed on roughly thirty occasions over a three-year period. The tar deposit was at the head of the estuary, and spring tides in the area of the tar deposit were commonly more than 6 m. Based on more than 300 individual observations of tar migration, a roughly two hectare area of tar migration and ebullition at low tide was mapped. Tar migrated from the riverbed to the river surface in the nearshore area of the tar deposit. Tar migration typically was associated with an increase in the rate of gas bubbling from the river&#39;s bottom and that was more vigorous in the warm months, when river temperatures ranged up to 24° C. Winter river temperatures were around 1° C. or 2° C. The gas from the tar deposit was CH 4  (80 to 90% by volume) and CO 2  (6 to 12% by volume), which was consistent with anaerobic respiration of organic matter. See, Kehew A, Applied Chemical Hydrogeology (2001). 
     Changes in pressure exerted a strong influence on ebullition and tar migration. Ebullition was most vigorous at low tide in the nearshore portions of the tar deposit, where total PAH concentrations were high (100&#39;s to 1,000&#39;s of mg/kg), and the maximum water depth at low tide was less than 7 m. Gas bubbling was controlled by the water pressure over the riverbed. 
     Ebullition slowed, and eventually ceased when the tide came in because the depth of water, and hence the pressure over the tar deposit increased by 4 m to 7 m of water (0.36 to 0.63 atm). In a portion of the tar deposit located in deeper water, gas did not bubble to the river&#39;s surface at low tide, even in areas where several feet of tarry sediment were present and total PAH concentrations were very high (e.g., 10,000&#39;s of mg/kg). Bubble-facilitated tar migration, however, did not occur in deeper water because the tarry sediment did not accumulate enough gas to cause the tar to become lighter than water and float to the surface. Likewise, the rate of gas formation was not vigorous enough to cause bubble migration that would have entrained NAPLs and that would have facilitated transport of NAPLs. 
     In the channel of the river, the pressure was always greater than 7 m of water. In this area, either the gas bubbles never formed because the pressure stayed too high, or the gas dissolved at a fast enough rate such that bubbles did not accumulate to the extent that the tar became lighter than water. 
     In general, tar migration was a function of ebullition, surface tension of the tar and the river, water pressure, gas generation rates, and the presence of tar in the river sediment. Anaerobic biodegradation of the organic matter generated CH 4  and CO 2  gases in sediment. Gas built up in the sediments at high tide (when the confining hydraulic pressure was high) and was released episodically during low tide (when the hydraulic pressure was low). As gas bubbles percolated through the sediment, they became entrained in, and also entrained, particles of tar. The tar droplets became lighter than water once they contained enough gas, even though the tar alone (i.e., without the gas) was denser than water. The lighter-than-water tar droplets floated through a water column to the river&#39;s surface. In addition, tar droplets were transported to the water surface by being entrained in migrating gas bubbles. Once the tar droplets reached the water surface, they often were trapped there, because the surface tension of the water was greater than the surface tension of the tar. Hydrocarbons dispersed from the tar droplets and formed a sheen once they reached the river&#39;s surface. When the sheens formed, the surface area of the tar on the river increased, which thereby increased the potential for human exposure (e.g., dermal contact) with tar constituents. 
     Once the surface tension was broken or when sufficient gas leaked from floating tar droplets, the tar droplets became denser than water again and sank to the river&#39;s bottom, albeit some distance downstream from the point of release. The effect of this tar migration from the riverbed to the river surface increased the potential for human exposure to tar constituents. 
     In accord with the method, and as shown by way of example in  FIGS. 1-2 , a grading layer  24  of sand is provided over the entire area of the tar deposit 12. The grading layer  24  of sand is sloped from an edge of the tar deposit 12 located distal to an accumulation zone  22 . The sand is deposited over the tar deposit 12 from a barge. In this example, the grading layer  24  also serves as a gas transmission layer  25 . Next, a control layer  18  is constructed from clay, such as Aquablok®, or another material that may control gas flow. The control layer  18  is sloped at the same degree as the grading layer  24 , is larger than the area of known ebullition and NAPL migration (approximately two hectares) and has a chimney  32  located at an edge that is adjacent to an accumulation zone  22 . The control layer  18  is constructed on the grading layer  24  by dispersal on the water surface  28  using a barge. An armoring layer  26  is provided over the control layer  18 . The armoring layer  26  provides weight to counter any uplifting forces caused by gas accumulation under the control layer  18  and helps prevent erosion of the control layer  18  from forces above the control layer  18 . The armoring layer  26  is carefully constructed on the control layer  18  using a clamshell from a barge. The armoring layer  26  would be constructed of, e.g., eight- to twelve-inch stone. 
     As NAPL-entrained gas bubbles  38  escape from the NAPL-contaminated sediment  12  located below the device  10 , they travel upward toward the device  10  and ultimately contact the control layer  18 . The gas bubbles  38  then migrate along the upward slope of the inferior surface  20  of the control layer  18  to the accumulation zone  22 . Because the NAPLs cannot migrate above the water table, the gas is vented at the chimney  32 , leaving behind a NAPL residue  44 . The NAPL residue  44  is sequestered from human and environmental receptors within the accumulation zone  22 . The residue  44  may be treated in place by sorption to amendments in the fill material or is simply allowed to accumulate. 
     The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements within the spirit and scope of the invention as set forth in the appended claims.