Abstract:
A method is described for containing dispersed particulate or soluble matter during contaminated sediment remediation operations in the marine environment. The preferred method comprises the use of a specially designed marine vessel that establishes a negative differential pressure gradient between a defined remediation control zone and the external ambient water environment, thereby preventing the release of contaminants dispersed during the remediation process. The preferred method also includes the use of marine vessels to remove contaminants contained within said control zone and to provide for the management of solids collected in the process.

Description:
RELATED APPLICATIONS 
     This application is based in part upon provisional patent application No. 60/274,898, filed Mar. 9, 2001, entitled “Secure Dredging System for Contaminated Sediment Clean-up,” provisional patent application No. 60/309,731, filed Aug. 2, 2001, entitled “Hydropneumatic Sheet Pile Seal,” the examinable application entitled “Contaminated Sediment Removal Vessel,” Ser. No. 10/094,064, filed Mar. 8, 2002 and this application is a continuation of PCT/US01/09025, filed Mar. 20, 2001. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This application relates to a process for isolating sections of the lakes, reservoirs, rivers, streams, and other water bodies so that contaminated sediments may either be removed or treated in-situ, while at the same time containing and preventing the release of particulate and soluble matter to the ambient water environment. 
     BACKGROUND OF THE INVENTION 
     Removal of sediments that accumulate on the bottom of natural and artificial water bodies is commonly practiced to permit navigation of ships and/or to maintain designated water depths. This type of sediment removal is commonly referred to as maintenance or navigational dredging. Sediments are sometimes removed to clean up the bottom of these water bodies when such sediments are found to be contaminated and pose a threat to public health and/or the ecosystem. This type of sediment removal is commonly referred to as environmental dredging. In some cases the objectives of the sediment removal activity are both maintenance and environmental. The in-situ treatment of contaminated sediments to render contaminants inert without the need to remove the sediments from the waterway is another remediation approach that is being researched as an alternative to dredging. At the present time, however, sediment removal or dredging is the primary remediation method commercially practiced. 
     Current methods of maintenance or environmental dredging can be divided into two general categories. They include mechanical dredging and hydraulic or vacuum dredging. The fundamental difference between these categories is the equipment used and ultimately the form in which the sediments are removed. Mechanical dredges typically remove the sediments directly with clamshell-type buckets at a relatively low liquid to solid ratio (i.e., relatively little water is entrained in the sediments compared to hydraulic dredging operations). Hydraulic or vacuum type dredges agitate the bottom channel to dislodge the sediment, and pump (vacuum) the sediment from the waterway. In hydraulic dredging operations the sediment is transported in a slurry with water acting as the transportation medium. This results in a water sediment mix with a high liquid to solid ratio. The sediment in the slurry must later be segregated from the water carrier. This is typically accomplished using large impoundment areas where the sediment is extracted by settling and the water (effluent) is returned to the originating waterway. 
     The removal of bottom sediments, whether by a mechanical or by a hydraulic dredging operation, involves some form of raking, grabbing, penetrating, cutting, or hydraulically scouring of the waterway or channel bottom. During such operations, sediments are readily suspended into the water column, dispersed and lost. In addition to sediment loss due to sediment disturbance and resuspension, in the case of mechanical dredging operations, sediment loss will occur when the bucket leaks sediments due to improper bucket closure resulting from debris stuck in the bucket, inadequate bucket sealing mechanisms, and the displacement of water contained within the bucket that occurs when solids enter the bucket during the excavation. While hydraulic dredging operations may have the advantage of a vacuum system that can assist in capturing some resuspended solids during bottom scouring operations, the large volumes of water that must be withdrawn and processed during such operations limit the feasibility of hydraulic dredging operations to areas where large impoundments are available. In addition, the presence of tides and currents can be expected to significantly reduce the efficiency of capture of resuspended solids by vacuum dredges when compared to operations that occur in quiescent waters. 
     Sediment resuspension and loss during dredging is a particular concern in environmental dredging operations where sediments are contaminated and the resuspension and dispersion of such sediments can result in ecological and human health impacts. This concern is underscored by the fact that most contaminants are generally associated with or bound to the fine particles, which are also those particles that are most easily resuspended and dispersed during the dredging operation. In addition to particulate resuspension, the potential release of soluble contaminants that may be present in the pore space of contaminated muds or may be subject to dissolution from the mud particle upon resuspension during dredging operations is also a concern. 
     Other problems associated with environmental dredging operations include the lack of suitable methods to monitor the actual loss of sediment that occurs during the excavation process, the lack of appropriate methods to monitor and ensure that the cleanup is being properly effected, and the absence of suitable methods to make certain that the handling of such sediments, during marine-to-land transfer and land-based transfer of such materials, do not result in liquid leakage or loss of sediments. 
     Current approaches for monitoring sediment loss during the excavation typically involve the use of discrete upgradient and downgradient subsurface sampling stations. Water samples collected at these stations are used to assess the increase in solids or turbidity loading to the waterway during the excavation. Given the unpredictability of subsurface currents, the discontinuous dredging operation, and discrete (spike) loadings that can be expected during dredging operations, the collection of representative samples is difficult. In addition, a determination of sediment loss can only be made after the release has already occurred. 
     To achieve target cleanup goals at a contaminated sediment site, due to the resuspension and the redeposition of sediment that occurs during conventional mechanical and hydraulic dredging operations, second and third passes to clean the contaminated area are routinely common. Even with multiple passes of a contaminated area, targeted specifications are still difficult to achieve. Current methods to assess the effectiveness of the cleanup of the subsurface sediments after the excavation typically involve the collection of core samples at discrete locations in the dredge area. Due to the expected variability in the spatial distribution of contamination in bottom sediments, particularly after the sediment is disturbed in a dredging operation, the collection of representative bottom sediment samples is also a difficult proposition. If the remediation effort has not met the required specifications, dredging equipment must be remobilized and returned to the location for additional cleanup at significant expense. 
     The removal and management of contaminated sediments during environmental dredging operations is best accomplished by collecting the sediments in a secure manner, dewatering the sediments, and transferring the sediments in such a way that there will be little risk for spills or loss of material. In mechanical dredging operations, sediments are typically placed in solids barges that require off-loading facilities that make use of additional cranes and buckets to remove the sediments from the solids barge for land-based management. Such operations are extremely messy and difficult to manage, and present a relatively high risk for further environmental contamination. Hydraulic dredging operations, as previously noted, require the construction of facilities for collecting the slurry and segregating the solids from the slurry, and thickening and/or dewatering the collected solids prior to transport to the disposal site. These facilities must be constructed in close proximity to the dredging operation and also increase the risk of local environmental contamination. 
     Most of the advances in environmental dredging technology in recent years have focused on the development of improvements in the design of buckets or vacuum dredges that tend to reduce or control the disturbance of the bottom of the waterway during the sediment excavation process (Ouwerkerk, R. and H. Greve (1994). “Developments in Dredges During the Last Decade.” Pages 690-699 in Dredging &#39;94, Proceedings of the Second International Conference on Dredging and Dredged Material Placement, Edited by: E. C. McNair, Jr., American Society of Civil Engineers. 1994; Zappi, P. A. and D. F. Hayes. “Innovative Technologies for Dredging Contaminated Sediments.” Improvement of Operations and Maintenance Techniques Research Program, U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss. Miscellaneous Paper EL-91-20. September 1991; and Herbich, J. B. Handbook of Dredging Engineering, McGraw Hill, Inc., New York. 1992). 
     The primary objective of most mechanical bucket modifications has been to create as tight a seal as possible on the bucket through the installation of rubber backing compressible seals and sensors (proximity switches) to ensure the bucket is closed prior to lifting in the hope of minimizing spillage. A dredging apparatus referred to as a “cable arm clamshell bucket,” advertised by Cable Arm Incorporated of Trenton, Mich., claims to reduce turbidity levels through its use of electronic sensors to detect bucket closure and compressible seals. While these types of systems offer improved designs, they do not eliminate the impact due to raking, upswell, and water displacement that will occur on excavation. In addition, such systems are incapable of effectively addressing leakage that will occur when debris is caught in the bucket and prevents full bucket closure. 
     Numerous modifications and subcategories of hydraulic dredges have been developed to attempt to mitigate problems associated with hydraulic dredging particle resuspension (Cleland, J. Advances in Dredging Contaminated Sediment, Scenic Hudson, Inc., 1997). Many have specific features that are designed to reduce sediment dispersion resulting from cutterblade, jetting, or raking mechanisms. None of these methods, however, have proven to be highly effective in areas where currents are present, nor do they significantly reduce or have suitable provisions for managing large volumes of contaminated water that are generated in the process. Large impoundment areas and suitable treatment methods are still needed to contain fine contaminated particulates that are drawn up with these sediments. 
     In addition to the use of improved mechanical or vacuum dredge systems, physical barriers such as silt curtains (flexible, impermeable canvas or rubber-like sheets that are deployed by attaching ballast to the bottom of the fabric and floats to the top to hold the curtain in a vertical configuration) or sheet piles have been employed during environmental dredging operations in attempts to isolate the area of dredging and contain contaminated particulates that are dispersed into the water column during the excavation process. These physical barriers do not prevent the migration of soluble contaminants. Silt curtains can provide some temporary particulate containment, but lack secure containment due to the porous nature of joints and curtain underflow, and their use in waterways with currents or tides is, in most cases, impractical. The application of fixed sheeting along the periphery of a dredge area is costly and, due to the porous joints of sheet piles, cannot guarantee complete particulate containment. Finally, while the referenced physical barriers provide some containment, once the barriers are removed the resuspended particles that were temporarily contained are free to migrate from the dredge area. 
     Although some advancement has been made in bucket or vacuum dredging, little progress has been made in developing physical barriers that effectively contain resuspended particulates or soluble contaminants during a dredging operation. 
     Smith, in his U.S. patent, Dredge Environ Protection Assembly, U.S. Pat. No. 4,386,872, Jun. 7, 1983, does describe an approach for temporarily isolating an area within a waterway during a dredging operation to prevent disturbed sediment from spreading. Smith proposes the use of a barge-mounted assembly where the barge (an open hopper barge) system is floated over the area to be dredged and a series of separate large side panel members (preferably ⅜ inch thick steel, 24 feet long, and 18 feet high) are lowered from the barge to the river channel to isolate the dredge area. Smith&#39;s invention requires that the area within the panels be pumped dry prior to the excavation. Smith further describes the use of panel support columns that would be lowered to a depth below the bottom necessary to resist the water pressure that would be exerted on separate side panel members. Smith&#39;s invention provides for the use of a series of support column caissons (preferably 4 feet in diameter) that would be lowered to the bottom to support the entire vessel when the inner panel area is dewatered. Smith describes these caissons as “stilts” to support the barge above the channel. Smith makes no mention as to how these caissons would support the barge on a soft muddy foundation, but does state that the columns would be lowered to a depth below the bottom surface of the waterway that would be required to support the barge. Smith describes in his patent how the panels would be lowered and set up end to end until they reach the bottom with fluid-tight seals located between the side panels and the panel support columns. Smith describes these fluid tight seals as “rubber sealing strips” to prevent infiltration of water through the panels into the dredge area. 
     Although Smith in his invention proposes equipment to physically separate the dredge area from the ambient water environment, Smith&#39;s invention lacks practical utility. His plan to pump the area dry would exert significant hydrostatic pressure on his proposed sliding panel wall and its support structure. Smith fails to recognize that continuous pumping will almost certainly be required to maintain his dredge zone in the dry condition that his invention requires, and he does not address the disposition of the water that would be pumped from his dredge zone, which can be expected to contain contaminants. 
     Smith&#39;s invention requires caissons to support the entire weight of the barge. This would necessitate that his support caissons be driven to a firm foundation such as bedrock. Such a foundation could be well below the bottom of the channel, further complicating the practicality of the invention. Smith does not describe any means to drive the panels below the initial placement depth in the event it becomes necessary to excavate deeper to remove additional contaminated sediment during a dredging operation. His proposed use of rubber sealing strips sandwiched between his panel support columns and panels, to prevent water infiltration, is theoretically plausible, but Smith fails to address how these strips would be attached nor how they would be sufficiently secure to withstand the abrasive forces they would encounter during deployment and redeployment of the side panels (if his system is to be mobile as implied). Smith does not address how the joints between multiple panels that lay one upon another, if needed, would be sealed. 
     Smith implies in his patent that an inclined non-horizontal bottom channel, which would result in a gap between the bottom panel and the inclined bottom surface, could be mitigated by attaching one or more extension panels to the bottom panel to block the flow of water. Smith states that such lower extension panels could be attached to the upper panels with bolts. Smith fails to address how these gaps would be identified, how the extension panels would be sized, and how bolts would be attached. One can only assume that Smith is considering the use of divers to bolt extension panels to all locations where gaps in the bottom seal are identified. Even so, Smith fails to address how these lower extension panels would be made watertight. Smith&#39;s invention fails to account for debris such as logs, rocks, etc. that will invariably be encountered at the bottom of the channel at most dredge locations, preventing a side panel from fully penetrating the subsurface, thereby resulting in additional gaps under the panel. Large gaps would result in an inability to pump the inner dredge area dry, which is a key feature of Smith&#39;s invention. In summary, the authors believe that Smith&#39;s invention lacks the means to make it a practical mobile vessel for removing contaminated sediments in a dredging application. 
     Other inventors have focused their efforts on the development of physical barriers in the marine environment to contain spills or to provide dry areas for construction. 
     Santamaria, in his U.S. patent, Barrier for Water Treatment, U.S. Pat. No. 6,089,789, Jul. 18, 2000, describes a curtain barrier system that can be deployed to isolate two bodies of water for the purpose of treating the water contained on one side of the barrier without mixing of the two water bodies. Santamaria&#39;s invention includes two flexible water impermeable curtains, similar to conventional silt curtains, that are deployed vertically, joined to a bottom web that contains ballast material to hold the barrier to the lagoon bottom and end anchors (on shore) to prevent the movement of the barrier. 
     While Santamaria describes in his invention a system to isolate two water bodies, Santamaria does not relate his system to a contaminated sediment dredging operation. His invention is intended to provide for the treatment of water on one side of an area of a lagoon that is separated from the remaining lagoon locations. Santamaria&#39;s invention is similar to commercially available silt curtains that are held in place by a ballasted bottom and a buoyed float at the top. Santamaria describes features of his impermeable flexible curtain that differentiate it from silt curtains used in commercial applications. While Santamaria&#39;s invention does provide some measure of separation, it cannot be used in the center of a channel where it cannot be anchored, nor would it be effective in sealing in a location where there are currents, and as such its application is primarily intended for a quiescent lagoon. 
     Strange, in his U.S. patent, Marine Pollution Containment Device, U.S. Pat. No. 4,889,447, Dec. 26, 1989, describes a containment barge consisting of two self-propelled semi-circular hulls that can be oriented in a manner that contains marine spills. The device described by Strange is designed as a rapid deployment device and provides for the mobile containment of oil spills at the surface. It does not extend vertically to the bottom of a channel, which would be necessary to address contaminated sediment dredging containment issues. 
     McClellan, in his U.S. patent, Mobile Cofferdam, U.S. Pat. No. 5,277,517, Jan. 11, 1994, describes a mobile cofferdam vessel that consists of a watertight rectangular shell that can be filled with water to submerge the cofferdam vessel in place, or have the water evacuated to float the vessel. The submerged vessel is designed to replace conventional stationary cofferdams that establish a zone that could be dewatered for use in constructing on the bottom of a waterway. McClellan describes the use of a pump to dewater the interior of the cofferdam to define a dry working environment. 
     McClellan&#39;s invention is intended for construction of elongated structures (such as tunnels or piles) below a waterway where a physical barrier is submerged by sinking a physical structure at a desired location and dewatering the area inside the physical structure so that construction can take place. The application is not designed nor intended for contaminated sediment excavation and removal. 
     It is relevant to this invention that interlocking sheet piles are commonly used in the construction industry to act as barrier walls to prevent soil movement or to prevent or minimize liquid permeation. Typical sheet piles are comprised of individual steel plates that are vertically aligned with one another as they are sequentially driven into the ground. Interlocking joint connections are used to connect the sequentially aligned sheets. 
     While interlocking joint connections of conventional sheet piles effectively hold and align the sheets, they are not specifically designed to prevent seepage of water through the interlocking joint connection. 
     Reducing sheet pile seepage is important in applications where at least one of the objectives of the installation is to divert, intercept or control water flow. In most applications this flow is usually subsurface or groundwater flow. In the marine environment where contaminated sediments must be excavated and sheet piles are used as physical barriers to segregate the excavation area from the main water body, reducing sheet pile seepage rates is important. 
     There are several known methods for sealing sheet piles. Most of these methods make use of sealants, which are cement-based, bituminous-based, or consist of low permeability materials, such as bentonite or combinations of the above, to seal the voids in the sheet pile joint connection. Some related patents include Hunsucker (U.S. Pat. No. 3,302,412), Zanelli et al. (U.S. Pat. No. 5,163,875), Cherry et al. (U.S. Pat. No. 5,437,520). While many of the above inventions have the potential for sealing sheet pile joints, they also require special sheet pile construction and in addition cannot be reused once the sheet piles are removed (i.e., piles must be cleaned and the sealant applied each time the piles are used). 
     Wheeler and Harvey (U.S. Pat. No. 5,938,375) describe a method in which sealant material is inserted into a form-like housing that is designed to vertically align with the seam between adjacent sheet piles. Sealant material is inserted into this housing to reduce seepage. The housing is designed so that it may be installed on conventional sheet piles without the use of specialized sheets. While such is an advantage claimed by the authors of the patent, the use of applied sealants limits the potential for reusing the sheet pile if redeployment is required without having to clean and completely reseal the system. 
     Others have proposed the use of specially designed sheet piles with a variety of interlocking surfaces to increase the surface area contact between adjacent sheets and hence reduce seepage through the sheets. Some related patents include U.S. Pat. No. 5,320,454 and Wickberg and DeGrout (U.S. Pat. No. 5,921,796). While increasing surface contact will undoubtedly reduce seepage, once installed the removal of such sheets are extremely difficult so they tend to weld together and bind. Such an approach is not practical in applications that require installation and reuse of the sheet piles in a continuous operation. 
     As will be shown, the invention being disclosed in this application incorporates a novel approach that makes use of a new sealing system that can reduce flow through the seam of sheet piles installed in the water environment, and has particular application in the cleanup of contaminated sites where sheet piles are used to segregate the dredge zone from the ambient water body. 
     At the present time, essentially all contaminated sediment remediation strategies involve the removal of contaminated sediments via one or more of the aforementioned dredging methods. There are presently ongoing research efforts being conducted to seek methods for treating sediments in-situ. The primary objective of an in-situ treatment strategy is to chemically or biologically render the offending contaminants within the sediments inert, thereby eliminating the need for dredging. One of the major impediments to the implementation of such an in-situ treatment strategy is a system that can deliver and contain chemical or biological additives that might be used in such a treatment strategy and to prevent the migration of sediments that may be disturbed if the aforementioned additives are mixed with the sediments. 
     In addition to ongoing efforts to develop in-situ treatment technology, dredge muds present at the bottom of waterways are at times in a fluff-like form, making the mechanical excavation of such sediments extremely difficult. This is because a mechanical bucket will be scooping mostly water when attempts are made to excavate such a material. Just as the introduction of chemical or biological reagents can assist in rendering contaminants inert, it is possible to introduce conditioning agents that will promote the densification of fluff-like bottom muds to permit the more effective excavation of such sediments. Such a strategy, however, is also limited by systems that can isolate the area to be conditioned to avoid the loss of conditioning agents and disturbed sediments during the chemical delivery and mixing process. 
     In contrast to the prior art, the invention being disclosed herein relates to the development of a system that isolates a contaminated sediment area and produces a negatively pressurized control zone into which mechanical or hydraulic dredging apparatus can be placed and used to remove contaminated sediment or additives can be introduced to treat or condition the sediments. The invention does not require the dewatering of the subject dredge area. The invention does not require the use of large columns to support a vessel. The invention is not susceptible to large gaps in the barrier that would result in major water infiltration, and the invention provides a practical means to deploy and redeploy the vessel during continuous operations along a waterway. As will become apparent, the maintenance of this negatively pressurized control zone ensures a secure zone that prevents the release of both particulate and soluble matter, and can be readily monitored and evaluated to establish or certify that the operation (dredging or treatment) has met its specified objectives. These features are not included in the prior art. In addition to the establishment of the control zone, the inventors have included a new method for managing dredge solids removed by securing and stabilizing said solids at the dredging site to minimize the opportunity for spills associated with conventional solids management approaches. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the invention to provide for the removal or the in-situ treatment or conditioning of contaminated sediments from the bottom of lakes, rivers, reservoirs, and other water bodies in a manner that prevents the release and transport of particulate matter, soluble matter or additives from a contaminated site during dredging, treatment or conditioning operations. 
     It is also an object of the invention to establish boundaries of a control zone where the excavation, treatment or conditioning of sediments can occur in a contained manner. 
     It is a further object of this invention to make such a control zone mobile to permit redeploying the invention along a contaminated waterway. 
     It is a further object to establish a negative differential pressure between said control zone and the ambient water environment. 
     It is yet another object of the invention to establish such a negatively pressurized control zone by deploying a sealed, vertical barrier wall beneath a marine vessel and by pumping water from the inner volume of the control zone to establish a differential pressure gradient between the control zone and the ambient water environment. 
     It is also a further object of the invention that such a sealed, vertical barrier consist of sheets and sheet pile joint seals to respectively define the surface area and volume of the control zone, and to reduce the permeability through the joints of the sheets and hence the rate of flow required to establish said negatively pressurized control zone. 
     It is still a further object of the invention to treat water collected from the control zone, if necessary, during the pumping process with a polymeric membrane filtration treatment system to enable the discharge of this purified water back into the ambient environment. 
     It is yet a further object of the invention that such a polymeric membrane filtration treatment system be incorporated into a mobile water treatment vessel, to avoid the need for land-based facilities. 
     It is also a further object of this invention to incorporate mechanical and/or hydraulic dredging equipment for use within this control zone, if sediment removal is desired, to improve the effectiveness of the subsurface cleanup. 
     It is a further object of this invention to provide the means to contain all contaminated matter within said control zone and remove such matter until such time as the contaminated area can be certified as complying with the desired specification of the cleanup. 
     It is a further object of the invention to incorporate a dredge solids stabilization and transportation vessel that can manage the contaminated solids that are removed in a dredging process, and transport such solids to an off-loading facility in an environmentally sound manner. 
     SUMMARY OF THE INVENTION 
     In keeping with these and other objects of the invention, a system is described that provides for the containment of particulate and soluble matter within an isolated zone by the use of a specially designed marine vessel that establishes a negatively pressurized control zone that can be used with either mechanical or hydraulic dredging equipment to remove contaminated sediment, or with systems that can introduce and mix reagents into the contaminated sediments for in-situ treatment or conditioning purposes. The physical boundary of the control zone is established by deploying a vertical barrier wall in the form of sheet piles and impermeable sheet joint seals located within and below an open barge-like vessel. 
     While the use of sheet piles and impermeable sheet joint seals can assist in establishing the physical boundary of the control zone in which suspended contaminants are contained, the driving force used to ensure that neither soluble nor particulate matter is released from this zone during dredging, in-situ treatment or in-situ conditioning operations is provided by the introduction of a pumping system located inside the control zone. This pumping system withdraws water from the control zone to establish a negative pressure gradient directed toward the inside of the control zone from the ambient water environment. Development of this negative pressure gradient between the ambient water surface and the control zone water surface elevation ensures that all flow through the vertical barrier is into the control zone, thereby preventing the release of any liquid or particulate matter contained within the confines of the control zone into the ambient water environment. 
     Liquid pumped from the control zone, where agitation due to dredging, treatment or conditioning occurs, can be expected to contain contaminated particulate matter and, in some cases, soluble contaminants that must be treated prior to release into the ambient water environment. To achieve the appropriate degree of treatment for the collected water, the invention makes use of a polymeric membrane treatment system capable of removing particulate matter in the range of 0.01 to 10 microns, and soluble contaminant removal, if warranted. 
     Most polymeric membrane filtration systems in commercial operation at the present time utilize positive pressure as a driving force to pass liquids through the membrane. In recent years, immersed vacuum-driven hollow fiber membranes have been introduced commercially. An immersed hollow fiber membrane filter operating under negative pressure is a polymeric filter that achieves filtration by drawing water through a thin fiber (membrane) surface into the hollow annular inner core of the fiber. Either pressurized or immersed hollow fiber membrane filtration systems are suitable for use in the subject application. 
     Once the subject control zone is established, dredging can be undertaken using either mechanical or hydraulic dredging techniques, or reagents can be introduced and mixed with bottom sediments with assurance that any suspended material or soluble contaminants will be confined to the control zone or pumped out to the membrane treatment system for removal. As will subsequently be described, the preferred dredging operational mode makes use of both mechanical and hydraulic dredging equipment to remove contaminated sediments. The preferred in-situ delivery and treatment mode makes use of pumping and sediment agitation equipment to deliver the treatment reagent to the subsurface sediment, simultaneously mixing and disbursing the reagent within the sediment layer to be treated. 
     To minimize the pumping rate required to maintain a differential elevation and hence a negative pressure gradient between the control zone and the ambient water environment, the inventors have developed a specially designed sheet pile joint seal that is designed to reduce the flow of water through the joint between the individual sheet piles. The sheet pile joint seal included in the invention consists of a flexible shroud and a sealing tube that is deployed from the vessel and slides down the outer portion of the vertical barrier wall to secure each sheet pile joint. Pressurization of the sealing tube, by use of water pressure or air pressure or both, is used to achieve a seal that substantially reduces seepage through sheet pile joints deployed in a water column. The effectiveness of the seal is enhanced by the established pressure gradient, which acts to force the flexible shroud and sealing tube into the sheet pile joint, thereby reducing the flow of water through the joint. 
     To provide the means to effect the removal, treatment or conditioning of contaminated sediments, and to treat the collected water and manage all sediments without the need for land-based facilities in the vicinity of the cleanup, the inventors have further developed a system in which all components of the subject invention are adaptable to mobile marine vessels. This includes the specially designed control zone vessel, referred to as a contaminated sediment remediation vessel, the membrane water treatment system, and the solids stabilization and transportation system. 
     The system and vessels included in the invention are designed to provide mobile platforms that can readily be deployed and redeployed in order to provide for ongoing remediation over an extended length of contaminated waterway. 
     While other inventors have proposed physical barriers for separating a dredging area from an ambient waterway to prevent the escape of contaminated particulates, no prior inventions consider the use of a low pressure control zone that is not dewatered to achieve said objective. No prior invention considers the possibility of treating water pumped from said control zone to remove soluble contaminants that may be released during the excavation. No prior invention makes use of sheet piles deployed from a mobile vessel as the vertical barrier wall with the means to penetrate the channel bottom and bottom debris if necessary to achieve a bottom seal. No prior invention provides for individual sheet pile joint seals that can readily be deployed to the desired depth, and can assist in sealing the low pressure control zone to prevent the outflow of contaminants and reduce the inflow of water into said zone. No prior invention provides the means to readily deploy and redeploy a vertical, low permeability barrier in a practical manner during a cleanup operation. No prior invention provides the means to introduce and mix additives to treat or condition sediments, in-situ, in a contained manner. No prior invention provides the means to manage solids without land-based solids handling facilities near the dredge site. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention can be best understood in conjunction with the accompanying drawings in which: 
     FIG. 1 is a conceptual perspective of a control zone within and below an open-bottom vessel; 
     FIG. 2 is a conceptual perspective of a continuous wall of discrete sheet piles deployed within and below said open-bottom vessel to define the limits of the control zone; 
     FIG. 3 is a conceptual schematic showing the deployment of a sheet pile joint seal system design to reduce the flow of water through the sheet pile joints; 
     FIG. 4 is a schematic illustrating a floating skimmer-pumping system used to establish an elevation difference and hence pressure gradient between the inner control zone and external water environment; 
     FIG. 5 is a plan drawing of a contaminated sediment remediation vessel. 
     FIG. 6 is a profile drawing of a contaminated sediment remediation vessel. 
     FIG. 7 shows Section A—A and FIG. 8 shows Section B—B from FIG. 5, both of which depict views of one possible gantry arrangement for deploying sheet piles from the contaminated sediment remediation vessel. 
     FIG. 9 is Detail 1, referred to in FIG. 5, which shows a plan view of the vertical barrier comprised of individual sheet piles connected at the joints. 
     FIG. 10 is Detail 2, referred to in FIG. 9, which shows the configuration of the inner hull falsework on the contaminated sediment remediation vessel. 
     FIG. 11 presents a plan view of the joint seal shroud deployed at an interlocking sheet pile-joint. 
     FIG. 12 shows a plan view of the sealing tube deployed inside the joint seal shroud. 
     FIG. 12A shows a profile view of a deployed shroud and sealing tube. 
     FIGS. 13,  14 ,  15 , and  16  present a series of plan, profile and sectional views of a membrane water treatment system deployed on an independent vessel. 
     FIG. 17 is an isometric view of the individual vessels used in an integrated contaminated sediment dredging operation. 
     FIGS. 18,  19  and  20  present a series of plan, profile and sectional views of an in-situ contaminated sediment treatment operation. 
     FIG. 21 is an isometric view of a plurality of vessels used in a contaminated sediment treatment operation deployed on a water body, showing a water treatment system deployed on a contaminated sediment remediation vessel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention consists of a contaminated sediment remediation vessel that is comprised of an open bottom barge vessel  101 , shown in FIG. 1 in a perspective view, beneath which a low pressure control zone  102  is established. As shown in the perspective view in FIG. 2, sheet piles  103  connected along the inner hull of said vessel are deployed and vibrated into place on the bottom of the waterway from the interior four sides of the rectangular opening to define the physical boundary of said control zone. 
     An additional feature of the invention, shown in the perspective view in FIG. 3, is the use of individual sheet pile joint seals  104  designed to reduce the flow of water through the sheet pile joints from the ambient water environment into the control zone. Such a sealing system can be readily deployed from the vessel. The sheet pile-seal system is used in conjunction with a water pumping operation to induce a negative pressure gradient inside of the control zone, relative to the external hydrostatic pressure outside the control zone. Inducing a negative pressure gradient prevents any liquid or particle migration from inside the control zone to the external water body during the remediation operation. 
     To achieve this negative gradient, as shown in the perspective view in FIG. 4, a pumping system including a floating suction well or pump  106  is located within the control zone  101  and is connected by piping or hoses  107  to a water treatment system located on a separate vessel, of if convenient, on the contaminated sediment remediation vessel. FIG. 4 illustrates the development of a differential head or differential water elevation  108  between the ambient water surface  109  and the control zone water surface elevation  110 . Such a differential head supplies the energy to induce the negative pressure gradient. The floating suction well provides the means to select the depth below the surface from which water is pumped by either skimming water from the top of the control zone or by extending piping below the surface to the desired depth. 
     FIGS. 5 and 6 show plan and profile views, respectively, of the contaminated sediment remediation vessel. FIG. 5 shows a plan view of the limits of the control zone  102  within the vessel  101 . FIG. 5 also shows the location of a gantry  141  and gantry rails  142  used to direct the movement of a vibratory driver that assists in deploying and removing individual sheet piles during vertical barrier deployment and removal operations. While in many instances individual sheet piles can be suitably deployed by simply dropping the sheet piles into the subsurface, the use of a vibratory driver provides the means to set the sheet piles to the desired depth and to penetrate subsurface debris, such as logs and rocks, should they be located in the subsurface at a location that might interfere with the penetration of the sheets. 
     Mooring winches  143  and spud wells  144 , designed to establish the location of the vessel over the dredge zone, are also shown in FIG.  5 . FIG. 6 shows the plurality of individual sheet piles  103 , connected to the inner hull of the vessel floating atop of the water surface  109  above the river bottom  145  to be remediated. 
     FIGS. 7 and 8 show views of Section A—A and Section B—B (from FIG.  5 ). These figures provide sectional views of the gantry  141 , directing movement of a vibratory driver  146  traveling along the gantry rails  142 . The vibratory driver  146  attaches to the tops of sheet piles  103  after having been moved into place by the trolley and hoist  147  having a hoist wheel  148 . Also shown in FIG. 8 is a diesel hydraulic power pack mounted on the gantry  149  and motorized wheels  150 . 
     FIG. 9 shows a plan view of a continuous wall of individual sheet piles  103  joined together by sheet pile joint interlocks and attached to the inner hull  121  by means of a falsework. FIG. 10 shows a detail of the inner hull  121  and said falsework  122 , which consists of clips  123  and an H-beam  124  that is connected to an individual sheet pile  103 . The individual sheet piles are shown connected side by side by an interlocking system. Said interlocking system typically consists of interlocking U-shaped sockets  125 , as shown in FIG.  10 . 
     To assist in sealing the seams of the interlocking sheet pile joints, which can be a source of water infiltration into the control zone, the inventors have developed a joint seal system capable of being deployed from the top of the vessel during mobilization of the vertical barrier. Reducing water infiltration into the control zone is necessary to reduce the pumping rate required to maintain the negative pressure gradient between the control zone and ambient water environment. The subject joint seal system, shown in FIGS. 11,  12  and  12 A, consists of a flexible shroud and a sealing tube system, together referred to by the inventors as a hydropneumatic seal. 
     To permit deployment of said sealing tube system from the deck of the contaminated sediment remediation vessel, two circular rods  131 , shown in FIG. 11, are welded onto the edge of a flat plate  132 . Each rod and plate is welded to the edge of each sheet pile  103  where the U-shaped sockets  125  from adjacent sheet piles are attached. The circular rod and plate run the entire vertical length of the sheet pile. The two circular rods  131  at each joint, shown in FIG. 11, act as shroud guides and provide a sliding and locking mechanism over which slotted shroud tubes  133 , shown in FIGS. 11 and 12, can slide over the circular rods  131  to deploy the seal shroud  134 . The seal shroud  134  is a flexible rubber membrane (such as neoprene) that is attached to the ends to the two hollow rods  133 . Metal brackets framing the shroud  134  provide rigidity to the shroud, provide a means to bolt the shroud  134  to the slotted shroud tubes  133 , and provide support to permit the shroud  134  to penetrate the subsurface bottom  145 . The seal shroud  134  provides a form into which a sealing tube  135 , shown in FIG. 12, is deployed and also functions as a protective cover to prevent damage to the sealing tube by external debris, such as floating logs. The sealing tube  135  is comprised of a heavy flexible material, preferably rubberized, that expands and contracts if pressurized or depressurized. 
     FIG. 12 presents a cross sectional view of the sealing tube  135  deployed and pressurized. Pressurization of said tube is accomplished by filling the tube with water and, if necessary, pressurizing the tube with air. FIG. 12 shows a cross section of the sealing tube cap  136 , which is located at the top end of the sealing tube and into which a water-fill tube  137  and air-fill fitting  139  are fabricated. The water-fill tube  137  provides the means to introduce water into the sealing tube in order to fill and pressurize it. A water-fill tube cap  138  is available to seal the tube after it is filled. The air-fill fitting  139 , shown in FIG. 12, is used as a means to introduce compressed air into the sealing tube  135 . Introducing compressed air into the sealing tube  135  provides the means to adjust the tube pressure to whatever value is necessary to tighten the seal. This additional pressurization is accomplished by closing the water-fill tube cap  138  and introducing compressed air through the air-fill fitting  139 . Total pressure levels of less than 10 pounds per square inch are suitable for such an application. 
     The combination of deployed shroud and sealing tube system comprise the hydropneumatic seal, which is capable of substantially reducing flow through the vertically aligned sheet piles deployed in a water column with a pressure differential between the outer and inner zones. The introduction of either water, air or both provides the means to pressurize the sealing tube  135 , which expands and penetrates into the sheet pile&#39;s knuckle joint thereby cutting off the flow of water through said joint. The pressure differential between said inner and outer zones provides additional means for the shroud  134  to force and hold the pressurized sealing tube  135  into the seam between the two sheets. 
     FIG. 12A presents a profile view of a vertically aligned sheet pile  103  extending into the bottom of a waterway  145  with the shroud  134  and sealing tube  135  extending the length of sheet. When such a system is deployed, seepage from the outer zone to the inner zone is limited for the most part to that which permeates through the subsurface soil, under the sheet pile. 
     The subject sheet pile sealing system can readily be deployed, demobilized, and redeployed using the same sheet piles and sealing system without any modifications to the existing sheet piles or hydropneumatic sealing system. 
     Deployment, as previously outlined, requires attachment of the shroud  134  to the shroud guides  131  that are welded to the sheets to provide the means to slide the shroud down the length of the sheet. The sealing tube can readily be deployed by inserting the bottom of the sealing tube into the deployed drape and pushing down on the water-fill tube  137 . This water-fill tube  137  is constructed from a rigid material, such as PVC plastic, and extends the length of the sealing tube  135 , as shown in FIG.  12 A. The sealing tube  135  readily slides down the length of the shroud, penetrating the bottom sediment  145 . The bottom of the sealing tube is sealed with two metal brackets located on each side of the bottom of the sealing tube  135  bolted together. The metal brackets located at the bottom of the sealing tube provide rigid support to the tube that enables its penetration into the bottom sediment  145 . One bracket  140  is shown in FIG.  12 A. The introduction of water through the water-fill tube  137 , during deployment, can be used to assist deploying in the sealing tube by overcoming the buoyant forces that may resist deployment. 
     Demobilization of the sealing tube  135 , shroud  134  and sheet piles  103 , respectively, involves injecting air through the air-fill fitting  139 , to force water back up the water-fill tube  137 , where it can be collected, raising the sealing tube  135  and shroud  134 , and lifting the sheet piles  103 . The entire contaminated sediment remediation vessel  101  can then be moved to another location where the vertical barrier wall can once again be redeployed. 
     While FIGS. 9,  10 , and  11  depict a flat sheet pile section, a U-shaped interlocking mechanism, and a clip and H-beam falsework pile arrangement, the presentation of such configurations is not intended to limit the fact that alternative sheet pile sections, interlocking mechanisms, and falsework arrangements could be employed to establish the vertical barrier wall and to permit deployment of the hydropneumatic seal in the manner outlined above without departing from the objectives and essence of the invention. 
     To treat the water pumped from the control zone, the inventors have supplemented the dredging process with a membrane water treatment system, adapted for deployment on the contaminated sediment remediation or a separate water treatment vessel. Those who are versed in the art of membrane filtration design will recognize that complete rejection of all micron-sized and submicron sized particles (for example, greater than 0.1 micron) can readily be attained by using such membrane filtration technology. The referenced water treatment system is capable of treating soluble contaminants, if needed, utilizing microfiltration or ultrafiltration membrane technology or alternative commercial methods for soluble contaminant removal such as chemical precipitation for soluble metals or activated carbon treatment for soluble organics or reverse osmosis. Since most contaminants (particularly organics) are tightly bound to free particulate matter, microfiltration should, in most cases, be capable of effecting adequate treatment. 
     FIGS. 13,  14 ,  15 , and  16  show design views through a separate mobile water treatment vessel containing a low pressure hollow fiber membrane treatment system. FIG. 13 shows the main deck plan view of such a vessel  151  and includes an equipment room  152 , a main deck enclosure  153  to house the membrane module racks (four shown)  154 , and tank hatch covers  155  that cover the equalization and pretreatment tanks, located below deck, as shown in FIGS. 14,  15 , and  16 . Also shown in FIG. 13 is a hydraulic crane  156  used to remove the hatch covers and to lift and offload solids collection containers from the vessel. 
     FIG. 14 shows a plan view of the hold level, highlighting the location of the equalization and pretreatment tanks  157  (nine shown) and fuel tank  158 . Also shown in FIG. 14 is the location of a solids thickening tank  159  that is used to thicken settled solids, collected from the pretreatment membrane treatment systems, a solids stabilization tank  161  where solids are mixed with a cementitious stabilization reagent (such as Portland cement) to improve its handleability, and the anchor chain locker  162 . 
     FIG. 15 shows a profile view of the vessel depicting the aforementioned equipment room  152 , the membrane module racks  154 , the equalization and pretreatment tanks  157 , and equalization and pretreatment tank hatch covers  155 . Also depicted in FIG. 15 are a tug push notch  163  and an anchor windlass  164 . 
     FIG. 16 shows a section through the water treatment vessel further highlighting the main deck enclosure  153 , the membrane module racks  154  located below deck, and the equalization and pretreatment tanks  157 . 
     The design layout shown in FIGS. 13 through 16 can be readily modified to accommodate the installation of an immersed pressurized membrane system or supplementary treatment operations. Its presentation is in no way intended to limit the scope of the application to either low pressure or immersed membrane systems and is intended only to illustrate additional detail associated with the barge mounted treatment vessel concept. 
     An isometric view in FIG. 17 shows an integrated dredging system, which includes a water treatment vessel  151 , a contaminated sediment remediation vessel  101 , an excavator barge  171  with a supporting hydraulic excavator  172 , a solids stabilization and transportation vessel  173  with a hopper screening plant  174  and containment ramps  175  (to connect the contaminated sediment remediation vessel  101  with the excavation barge  171 , and the excavation barge  171  with the solids vessel  173 ) to prevent spillage of sediment during the excavation process. Also shown in FIG. 17 on the solids stabilization and transportation vessel  173  are storage containers  176 , and a conveying system  177 . Screened sediments, from the hopper screening plant  174 , and a cementitious stabilizing reagent (such as Portland cement), which is stored in a silo  178 , are introduced into the conveying system  177 , mixed (using screw conveyors) and transported (using flite conveyors) for distribution to the storage containers  176  through the conveying  177  and hopper distribution system  179 . 
     While FIG. 17 depicts the introduction of both the stabilizing agent and the screened contaminated sediments into a conveyor to effect a mixing process, an alternative mixing approach makes use of a blade mixer attached to a telescoping crane with a pump to inject and mix the stabilizing agent with the contaminated sediments after the contaminated sediments are distributed to the storage containers  176  onboard the solids stabilization and transportation vessel  173 . Such an approach would permit the mixing process to occur while the solids stabilization and transportation vessel is in transit to the land-based transfer station. 
     The multiple vessel arrangement and the equipment shown in FIG. 17 depict one potential vessel and equipment configuration for dredging operations. Its presentation is in no way intended to limit the manner in which vessels can be deployed at a cleanup site, which vessel might support a specific piece of equipment or operation, nor the type of dredging equipment that may be employed to remove the sediment, since this will, in most cases, be dictated by the size of channel or other site-specific issues. For example, where channel space is limited, the vessels could be configured linearly or the excavator could be deployed on the contaminated sediment remediation vessel  101 , eliminating the need for an excavator barge  171 . It may also be desirable in certain instances to utilize a crane and bucket as opposed to a hydraulic excavator as the preferred equipment in the excavation process. The presentation shown is also not intended to limit the preferred number of each vessel located at a particular dredge site. For example, it may be desirable to locate the water treatment process directly on each contaminated sediment remediation vessel  101 , or deploy one water treatment vessel  151  to support multiple contaminated sediment remediation vessels  101  in order to reduce the number of vessels required for the dredging operation. Deploying several contaminated sediment remediation vessels  101  is advantageous, from an operational viewpoint, since it provides the means for the excavator barge  171  to move from one control zone  102 , located within each contaminated sediment vessel  101 , to the next control zone  102  without slowing the excavation process (waiting to redeploy the contaminated sediment remediation vessel  101 ). Neither is the depiction of one control zone  102  within a given contaminated sediment remediation vessel  101  intended to limit the number of control zones  102  that might be established on one vessel. For example, it might be advantageous to have several control zones  102  on one large contaminated sediment remediation vessel  101 . 
     While the above description emphasizes equipment used in a dredging operation, it should be apparent that where in-situ treatment or conditioning of the bottom sediments is the desired activity, the excavator barge or solids vessel would be unnecessary. In such an operation these vessels would be replaced by a means to introduce and mix the additives with the bottom sediments. 
     FIGS. 18,  19  and  20 , respectively depict a plan, profile and sectional view of a reagent delivery and sediment mixing system for the in-situ treatment of contaminated sediments within a control zone  102 , inside the contaminated sediment remediation vessel  101 . Establishing the low pressure control zone prevents both the release of contaminants that may be suspended or released during the mixing operation and the loss of reagent from the control zone. FIGS. 18 and 19 show a plan and profile view of a separate vessel  191  supporting a hydraulic excavator  192  and a slurry and pumping system  193  used to deliver stored reagents  194  to the sediment surface  145 . Delivery of the reagents to the sediment is provided through hoses and piping  195 . The reagent is uniformly distributed into the subsurface sediment through a sediment agitation and mixing device, shown in FIG. 20 as a horizontal auger  196 , located at the end of the excavator. The horizontal auger is lowered to the bottom surface by the hydraulic excavator to simultaneously mix and deliver the reagent. Churning of the bottom sediments by the auger provides the necessary mixing over the cross section of the control zone. Reagent distribution is effected by a manifold system  197  used to distribute the reagent along the section of the auger  196 . The horizontal auger  196  shown in FIG. 20 is one possible sediment agitation and mixing device. Other methods of agitating the sediment in order to mix the sediment with the reagent can be used. These could include but are not limited to vertical augers, plows, vibratory mixers or jet type mixers that would inject the reagent into the subsurface sediment. A mobile water treatment vessel  151  provides the pressure control and water treatment necessary to maintain the control zone  101  and treat the control zone water. 
     FIG. 21 shows a plurality contaminated sediment remediation vessels  101  and a plurality of control zones  102  deployed along a water body with a plurality of water treatment systems  254  shown on the deck  251  of a plurality of floating contaminated sediment remediation vessels  101 . 
     In summary, in a preferred embodiment for the secure dredging embodiment shown in FIGS. 1 through 20 and  12 A, operation of the overall system involves the following activities: 
     1. positioning of the contaminated sediment remediation vessel  101  over the zone  102  to be dredged; 
     2. lowering and inserting of sheet piles  103 , attached to the inner hull  121 , into the bottom  145  of the waterway and driving them into the subsurface formation with the driver, such as the vibrating hammer  146 , if necessary; 
     3. for each sheet pile, lowering the impermeable shroud  134  located on the external side of the sheet pile  103 , inserting the sealing tube into the shroud  134  and pressurizing the sealing tube  135 ; 
     4. initiating flow into the water treatment system  151  to induce a negative pressure gradient into the control zone  102 ; 
     5. initiating and completing a mechanical dredging operation to remove contaminated sediments from the control zone  102 ; 
     6. continuing to pump and treat control zone water after the completion of the excavation to maintain the negative pressure gradient, and to reduce suspended solids and/or soluble contaminants in the water column within the control zone to specified levels; 
     7. permitting the solids to settle for an extended period (e.g., 6 to 24 hours) after dredging of the control zone  102  is completed; 
     8. vacuum dredging the bottom  145  of the control zone  102  using a cutterless vacuum head to remove residual lightweight sediment (fluff) that has settled, and directing the vacuumed slurry to the water treatment system  151 ; 
     9. monitoring the water column and bottom sediments to ensure adequate cleanup; 
     10. withdrawing the shroud  134  and sealing tube  135 , and raising the sheet piles  103 ; 
     11. relocating the contaminated sediment remediation vessel  101  to the next area to be dredged using mooring winches and spuds, 
     12. redeploying the vertical barrier and hydropneumatic sealing system in order to reestablish the new control zone, 
     13. processing the contaminated dredge solids on a solids stabilization and transportation barge  173  involving conveying and mixing the finer-grained dredge muds with a cementitious stabilizing reagent and distributing the mixed materials to storage containers  176 , using a conveying  177  and hopper distribution system  179 ; and 
     14. covering storage containers  176  and securely transporting stabilized muds to an off-loading facility. 
     In summary, the preferred embodiment for an in-situ treatment or conditioning operation involves the following activities: 
     1. positioning of the contaminated sediment remediation vessel  101  over the control zone  102  to be treated or conditioned; 
     2. lowering and inserting of sheet piles  103 , attached to the inner hull  121 , into the bottom  145  of the waterway and driving them into the subsurface formation with the driver, such as the vibrating hammer  146 , if necessary; 
     3. for each sheet pile, lowering the impermeable shroud  134  located on the external side of the sheet pile  103 , inserting the sealing tube  135  into the shroud  134  and pressurizing the sealing tube  135 ; 
     4. initiating flow into the water treatment system  151  to induce a negative pressure gradient into the control zone  102 ; 
     5. introducing by means of a pumping and delivery system, and mixing treatment or conditioning reagents with the bottom sediments using a horizontal auger  196  or alternative sediment agitation and mixing device; 
     6. continuing to pump and treat control zone water after the introduction of reagents to reduce suspended solids loadings and/or soluble contaminants in the water column to specified levels; 
     7. maintaining the control zone in place, and continuing to mix, if necessary, until the desired treatment or conditioning of the sediments are achieved; 
     8. redeploying the vessel to the next location. 
     Although the aforementioned particular embodiments are shown and described herein, it is understood that various other modifications may be made without departing from the scope of the invention, as noted in the appended claims.