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MONITORING THE EFFECTIVENESS OF CAPPING FOR ISOLATING CONTAMINATED SEDIMENTS Robert W. Morton Science Applications International Corporation ABSTRACT Disposal of contaminated sediments in the marine envi- ronment through capping with cleaner materials is ~ manage- ment option that has been used extensively during recent years, particularly in New England. Most capping projects have been restricted to quiescent, shallow waters (20-30 m) however, as a result of monitoring programs associated with these projects, a body of knowledge concerning the creation of capped disposal mounds has been developed to predict the consequences of extending such procedures to other waters. . In particular, the application of capping technology to deeper water is extremely important, because disposal site designation programs currently underway throughout the United States are predominantly aimed at water depths of 100 m or greater. Since many capping projects are considered experimental or controversial, the monitoring procedures to assess them must be carefully devised to answer specific questions re- garding the overall ability of the cap to nated materials. Therefore, the results of previous monitor- ing efforts, such as those conducted under the Disposal Area Monitoring System (DAMOS) , can provide a baseline approach for future monitoring of capping operations. This paper presents an overview of the results of capping projects conducted under DAMOS and the rationale for the existing monitoring program developed from those efforts. isolate contami- INTRODUCTION Capping of contaminated dredged material with sediment relatively free of contaminants has developed into a commonly used management technique for reducinge potential environmental impact of open-water disposal. Capping was first employed in 1977 by the New England Divi- sion of the U.S. Army Corps of Engineers (COE) at the New London Disposal Site (NLON). This project, which took place in 20 m of water in the eastern end of Long Island Sound, has led to continued appli- cation and field observations of the technique, including major capping operations at the Central Long Island Sound Disposal Site (CLIS) and the New York Mud Damp Site (EMD) in open ocean waters. 262
263 Additional studies stressing laboratory observations on the effective- ness of capping have been conducted at the COE Waterways Experiment Station. As a result of these studies, a great deal has been learned regard- ing the effectiveness of capping in the marine environment. However, during each capping operation, the fact that contaminated sediments are involved means that major issues must be addressed and fully understood to ensure that minimal adverse impacts occur as a result of the opera- tion. These issues include . thickness of the cap--related to the effectiveness of the cap- ping material in isolating the contaminants, particularly the potential for leaching of contaminants and effects of bioturba- tion; placement of the cap--related to navigation control during dis- posal to ensure coverage of contaminated sediments and to the mixing and displacement of contaminated sediment by the capping material; and stability of the cap--related to the support of the cap by typi- cal high-water content contaminated material and resistance to erosion and transport of capping material. Previous studies of capping have indicated that with careful manage- ment the operation can be successful in relatively quiescent, shallow waters. However, designation or use of new disposal sites in water depths greater than 100 m, where capping will be a management option, are currently underway in the New England region, at the Foul Area Dis- posal site (FADS) in Massachusetts Bay, in the Seattle area at the Everett Homeport Disposal Site, and potentially in the New York region at a site to be designated offshore of the mud damp site. Permits for capping of contaminated sediments at these and other disposal sites will certainly require monitoring of the disposal opera- tion and the resulting deposit. Therefore, development of a rational, practical, and meaningful monitoring approach is critical to the future application of capping technology to the management of contaminated sediments. HISTORY OF CAPPING OPERATIONS The first use of capping as a disposal management strategy occurred at the New London Disposal site (NLON) in 1977 when contaminated sedi- ments from the vicinity of dock areas were dredged first and then covered with cleaner sediments as dredging proceeded from the head of the estuary to the mouth. Capping of the contaminated sediments was assured because the mass of material used for capping was more than 30 times greater than that of the contaminated material. However, such an abundance of capping material is not always available and, for capping to become a truly feasible management strategy, procedures for capping with much less material had to be developed. The first field study of controlled capping of contaminated
264 material with more reasonable amounts of capping material took place at the Central Long Island Sound Disposal site (CLIS) in 1979 (Figure 1~. During this project, two disposal mounds were formed, each with approxi- mately 30,000 m of contaminated sediments from Stamford, Connec- ticut. These deposits were then capped, one with approximately 76,000 m3 of silt (STNH-S), and the other with 33,000 m3 of sand dredged from New Haven Harbor (STNH-N). This study produced several important conclusions, which were applied to future capping projects. Disposal of contaminated sediments must be tightly controlled. This is necessary reduce the spatial distribution of material to be capped and can be accomplished through use of taut-wire dis- posal buoys and/or precision navigation control. Disposal of capping material must be spread over a larger area. Dispersal of cap material is necessary to ensure adequate cap- ping of the margins of the contaminated deposit and is particu- larly important for silt capping material, which does not spread as evenly as sand. Silt develops a thicker cap than sand and therefore requires more cap material. Silt caps do not spread as readily during disposal; however, the greater thickness is needed because the depth of bioturbation is deeper in silt than in sand. Silt caps recolonize with fauna similar to the surrounding silt environment, sand caps with completely different species. Recol- onization of both mounds occurred as expected and impacts to sur- rounding environment were negligible. Caps are resistant to erosion. Once stabilized, both the silt and sand caps have remained essentially unchanged for more than eight years (including two hurricanes). A second study, utilizing similar sediments was conducted two years later at Cap Sites 1 and 2 (CS-1 and CS-2) with comparable results. In this study, the placement of capping material was the most significant factor affecting the isolation of contaminated sediments. In spite of efforts to distribute the cap evenly, additional disposal of silt mate- rial was required to achieve adequate coverage. Other capping operations have been successfully accomplished since 1979. . Disposal and capping in borrow pits: this approach has been suggested as an alternative for New York Harbor, but is currently on hold pending studies of the environmental significance of the borrow pits. Dredging of a depression, filling with contaminated sediments and capping with displaced material: this procedure was used successfully in Norwalk, Connecticut, but is restricted to shallow-water environments. This approach has also been pro- posed for disposal of PCB-contaminated sediments at the New Bedford Superfund site. Open-water capping at the New York Mud Dump Site: approximately 522,000 m3 of contaminated material have been successfully
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266 covered by 1.2 million m3 of clean sand in a mound that has persisted on the open shelf for seven years. Excellent management of continuous capping operations within the site requires identification of cap material prior to issuing permit for disposal of contaminated sediments. As a result of these studies, the factors affecting capping can be predicted with some accuracy, particularly the amount of material needed to create an effective cap and the controls necessary to dispose of the contaminated sediment and capping material in a stable deposit. DAMOS MONITORING PROGRAM As stated above, future capping projects are certain to require extensive monitoring programs, which will be required to evaluate the effectiveness of the capping operation and address site-specific issues. A starting point for design of such programs could be the Dis- posal Area Monitoring System (DAMOS). DAMOS has been in existence for more than ten years and has developed into a multidisciplined program that provides the COE New England Division (NED) with the necessary information to manage open-water disposal of dredged material in a scientific, rational manner. An overview of the DAMOS monitoring approach, which begins with s ite des ignation and extends through the disposal operation to post-disposal monitoring is presented in Figure 2. DAMOS has always been a flexible program designed to respond to changes in technology that result in a better understanding of the effects of dredged material disposal on the marine environment. As a result, it has been closely involved with the development, execution, and monitoring of the capping projects described above. SITE DESIGNATION (CHARACTERIZATION) PRE. DISPOSAL (~ASEUNE) DURING DISPOSAL POST DISPOSAL | tJIONITORl~G DATHY~ETRY18SCAN REMOTE CURRENTS/WAVES SEDIMENT GRAIN SIZE BIOLOGICAL CHEMICAL REUIOTSHABITAT BENTHICTYPE PRESENT BRATFISH HABITAT FISHTYPE PRESENT BULK SEDIMENT ANALYSIS BATHY~ETRY/8SCAN REMOTE HARBOR CHARACTERIZAtlON (DENSm, GS, CEOTECH) DISPOSAL CONTROL BATHY~ETRY/RE~OTS PLUME STUDIES ~USSE"IDA15Y DATHY~ETRYISSCAN REMOTE ~US5EL8IDAISY l BATHY~ETRYIRE~OTS (NEXT BEACON, THEN ANNUALLY, AUGISE~ BENTHIC BODY BURDEN COUIPOUNDS BE~CTED BASED ON WASTE CHARACTERIZATION If > ONE YEAR - RElIOTS WASTE CHARACTERIZATION BULK SEDIMENT ANALYSIS BIOASSAYS "C. REMOTE {WITHIN 2 WEEKS) REMOTE (NE" 8EABON, THEN ANNUALLY, AUGISEPT) IF RECOLONIZED: BENTHIC, eRAT, UIUSSE" BODY BURDEN _ _ IF NOT RECOLONIZED- ~ ! BULK SEDIMENT ANALYSIS FIGURE 2 Proposed integrated DAMOS monitoring/management program for dredged material disposal.
267 Through years of intense field observations, the DAMOS program been able to develop a comprehensive data base that confirms the viability of several important parameters required for capping operations: . . . . Operational feasibility: it has been shown (Figure 3) that navi- gation control and disposal operating procedures are adequate to create mounds of contaminated material and to spread sufficient cap material to effectively cover those mounds. Minimal dispersion during disposal : extensive plume tracking studies have demonstrated that most dredged material is trans- ported to the bottom during the convective descent phase of disposal (Figure 4~. This is required to ensure that, with proper navigation, contaminated materials will be contained within a reasonable area for capping and not dispersed through- out the water column or spread over a broad area of the bottom. Long-term stability of disposal mounds: repeated measurements over the past 10 years have shown that following initial rework- ing and consolidation, capped disposal mounds remain unchanged for extended periods of time. This means that disposal sites do exist where currents and wave activity have insufficient energy to erode and transport dredged material and that those areas can be considered as containment sites. These conditions are neces- sary for initial control of contaminated material prior to cap- ping and to ensure that once caps have been deposited, the cap- ping material will remain in place over the long term. Sand or silt cap material: all studies to date have indicated that either sand or silt are adequate for capping of contami- nated material, although silt caps require more material and must be spread by controlling disposal over a larger area. This conclusion is extremely important because the economic feasibil- ity of capping depends to a large extent on the availability of clean sediment and frequently sand is not common in regions where contaminated silts occur. Isolation of contaminated material: both chemical and biologi- cal monitoring have demonstrated that, given sufficient cap thickness and stability, neither bioturbation nor chemical leach- ing will expose contaminated sediments to the surrounding envi- ronment. Therefore, use of uncontaminated sediments as a cap material is a viable method for isolation. As a result of monitoring both capped and uncapped dredged material deposits for a number of years, the DAMOS program has developed and utilized an extensive array of instrumentation and procedures for evalu- ating the parameters described above. Examples of the instrumentation required for execution and monitoring of capping projects follow. . Precision navigation: navigation control on the order of 5 m is necessary to ensure correct placement of contaminated and cap- ping sediments and to accurately sample the steep gradients associated with the dredged material deposit during post- disposal monitoring.
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270 Controlled disposal of contaminated dredged material requires a taut-wire moored buoy, which must be accurately deployed. Using such a buoy, disposal can be restricted to less than a 50-m radius and the input of dredged material can be considered as a point source for subsequent capping and monitoring. On the DAMOS program, buoy deployment and sampling are con- trolled through an SAIC Integrated Navigation and Data Acquisi- tion System (INDAS), which provides computerized integration of microwave or acoustic positioning systems with environmental sen- sors and navigation displays to provide accurate ship control and data acquisition. Figure 5 presents an example of the sampling accuracy and precision attained by this system. r Precision bathymetry: replicate bathymetric surveys provide the basis for sequential monitoring of the volume and distribution of sediment at the disposal site to assess the effectiveness of capping and the long-term stability of the cap. Because of the small changes that occur as a result of erosion or consolidation, this approach requires very precise field measurement procedures and statistical analysis of replicate surveys · Sediment profile photography: the REMOTS camera has proven to be a key instrument for assessing the distribution and characteristics of near-surface sediments. In particular, the changes and conditions existing at the fringes of the mound can be examined with a resolution unattainable with acoustic measurements or conventional sampling procedures. Furthermore, this instrumentation examines small- scale effects of physical erosion and bioturbation and provides an efficient method for measuring biological parameters to evaluate the impacts of disposal and capping operations. Advanced acoustic measurements: modern acoustic instrumentation such as sidescan sonar, high resolution subbottom profilers and high-frequency plume tracking systems all provide important information on the distribution and physical properties of sediments during and after disposal. · Specialized instrumentation: development of instrumentation packages such as the Disposal Area In situ System (DAISY) provide information for addressing specific problems associated with dredged material disposal and capping. In particular, the DAISY measures near-bottom current and wave energy associated with sediment resuspension and turbidity to address the long-term stability of capped disposal mounds. Another example of specialized instrumentation is the Nuclear Density Probe which has been configured with a sediment penetration device and is used with precision bathymetry, REMOTS and subbottom profiling to determine the mass balance of sediment deposited in the capped mound.
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272 RECENT CAPPING OBS ERVATIONS The instrumentation and procedures described above have been used extensively on two major field studies recently completed at the New York Experimental Mud Dump site (EMD) and the Foul Area Disposal site (FADS) in Massachusetts Bay. The objectives of the study at the EMD were to assess the long-term (five years) stability of a sand cap deposited over contaminated sediments in the open-shelf environment; while at FADS, the short-term (several months) effects of disposal in water 90-m deep were measured to evaluate the behavior and distribution of sediments and to determine the feasibility of capping in such water depths . Experimental Mud Dump Site (EMD) At the EMD the results indicated that following disposal, a cap of approximately 1.5 to 2 m covered most of the contaminated material (Figure 6) and that this cap was essentially unchanged during the subsequent five-year period (Figure 7~. These conclusions were supported by the subbottom profile measurements, which indicated a surface deposit of more than 75 percent sand with a mean thickness of 1.5 m in the vicinity of the disposal mound (Figure 8~. Subbottom profiles across the disposal site also demonstrated that the cap was continuous. REMOTS photography supported the subbottom data, but also provided additional information. The "Benthic Process" map generated from the sediment profile photographs (Figure 9) indicated the presence of the same fine-grained, high-reflectance sand in the vicinity of the EMD. However, the photographs also showed bedforms and in some cases alter- nating layers of sand and mud suggesting that although sediment resus- pension and transport can occur on the surface of the mound, the entire region must be in equilibrium, since there has been no significant loss of cap material over time. The recolonization of the disposal mound as measured by REMOTS also indicates that physical disturbance of the sedi- ment surface occurs. State I (opportunistic) species are the redomi- nant infaunal successional stage on the disposal mound and throughout the disposal site, suggesting relatively actibottom conditions. The presence of Stage I species (on the sand cap means that bioturbation will penetrate only a few centimeters into the cap and, therefore, iso- lation of the contaminated material can be expected. On the flanks of the mound, where cap thickness is not so great and some Stage III organ- isms are present, some mixing of the contaminated sediment will occur. Foul Area Disposal S ite ~ PADS ~ Extensive disposal site designation studies have recently been con- ducted at FADS, including investigation of the potential effects of capping operations in water depths of 90 m. Disposal at FADS generally occurs from disposal scows and, occasionally, from hopper dredges;
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275 Silt/clay "puddle" VERTICAL EXAGGERATION = 86X FIGURE 8 Representative trace of acoustic impedance at the EMD. SOURCE: Parker and Valente, 1987. however, in all cases, sediment is transported to the bottom through the classical three phases of disposal: 1. the convective descent phase, during which the majority of the dredged material is transport to the bottom under the influence of gravity as a concentrated cloud of material; I. the dynamic collapse phase following impact with the bottom where the vertical momentum present during the convective des- cent phase is transferred to horizontal spreading of the mate- rial; and a. the passive dispersion phase following loss of momentum from the disposal operation, when ambient currents and turbulence deter- mine the transport and spread of material. In shallow water, cohesive sediments disposed under the influence of the above disposal phases create a distinct mound formation with thin flank deposits, while sands, or less cohesive, high-water-content sediments characteristic of the contaminated material produce a broader, more uniform deposit. At FADS, mounding of cohesive sediments was less prevalent even with cohesive sediments; however, the overall spread of material was similar. Regardless of whether the disposal operation was conducted with a hopper dredge or scow, both theoretical and observational data indicate that the majority of the dredged
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277 material was transported to the bottom at FADS as a discrete plume dur- ing the convective descent phase. When this material reached the bot- tom, the vertical momentum was transferred to horizontal momentum dur- ing the dynamic collapse phase. The overall size and thickness of the resulting deposit depended primarily on the amount of material disposed at the site and navigation control exercised during disposal effort. Recent work, completed during January 1987, using the REMOTS camera has demonstrated that the disposal of dredged material under tight con- trol at FADS resulted in a broad, low deposit spread evenly over an area similar to that covered by disposal in more shallow waters. The major difference in the deposits results from the greater spread of cohesive clumps which inhibits formation of a topographic feature (i.e., disposal mound). Figure 10 indicates the distribution of dredged material as detected by the REMOTS camera following disposal of approximately 200,000 m3 of cohesive sediment. This operation resulted in a deposit with a thin layer of dredged material extending over a circular area with a radius of 500 m; a deposit comparable to similar volumes deposited in the shallow water of Long Island Sound. The fact that disposal in deeper water results in a thin, broad deposit can have important implications for capping under such condi- tions. If careful control of the contaminated material is not exer- cised, even small amounts of material will cover the same area of the bottom as larger volumes. Consequently, it would take essentially the same amount of material to effectively cap 100,000 m3 of contaminated material as possibly 250, 000 or 500, 000 ma . Assuming the contaminated material covered an area of bottom with a 500-m radius, similar to the deposit created at FADS, then at least 1.1 x 106 m3 of material would be required to produce a deposit one meter thick extending 100 m beyond the edge of dredged material. This is not an unreasonable quantity to cover a substantial project, but would be untenable for a small contamination problem. Therefore, appropriate scheduling of small contaminated projects prior to larger uncontaminated dredging programs must be carefully considered. SCARY Extensive monitoring of capping projects throughout the New England region under the DAMOS program suggests that capping is a a feasible mitigating measure for disposal of contaminated sediments in the marine environment. However, it must be emphasized that careful control of the operation and comprehensive monitoring of the resulting deposit are required to ensure minimal impact from future projects, particularly if they are conducted in deeper water. Although capping has not been conducted at FADS, previous disposal operations at that site have demonstrated the effectiveness of disposal control in restricting the spread of material in 90 m of water This is the single most important factor in a capping operation and, if the disposal location is a containment site, capping should be feasible at those depths with sufficient material. Furthermore, the fact that caps have persisted at the Central Long Island Sound and EMD sites for five
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279 years or more suggests that containment of contaminated materials can be accomplished even in relatively high-energy environments. REFERENCES Morton, R. W. 1980. The management and monitoring of dredge spoil dis- posal and capping procedures in Central Long Island Sound. In Wastes in the Ocean, Volume 2: Dredged Material Disposal in the Ocean, Rester et al., eds. New York: Wiley and Sons. Morton, R. W. 1987. Updating the U.S. experience with aquatic capping of contaminated sediments. Presented at 13th U.S./Japan Experts Meeting on Management of Bottom Sediments Containing Toxic Sub- stances, November, 1987. U.S. Army Engineers. Morton, R. W., C. J. Lindsay, and R. C. Semonian. 1984. Use of Scien- tific Data for Management of Dredged Material Disposal in New Eng- land. Presented at Conference Dredging '84, ASCE. Morton, R. W. and R. D. Jones. 1985. The importance of accurate naviga- tion in environmental assessment programs. Sea Technology, August. Parker, J. H. and R. M. Valente. 1987. Long-Term Sand Cap Stability: New York Dredged Material Disposal Site. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Science Applications International Corporation. 1985. Disposal Area Monitoring System (DAMOS) Annual Report, 1984. DAMOS Contribution #46. New England Division, U. S. Army Engineers. Science Applications International Corporation. 1987. Monitoring Sur- veys at the Foul Area Disposal Site, February, 1987. DAMOS Contri- bution #64. New England Division, U.S. Army Engineers. LIST OF DAMOS CONTRIBUTIONS SUBMITTED TO NEW ENGLAND DIVISION U.S. ARMY CORPS OF ENGINEERS # 7 Stamford/New Haven Disposal Operation Monitoring Survey Report # 8 Management and Monitoring of Dredge Spoil and Capping Procedures in Central Long Island Sound #11 "Capping" Procedures as an Alternative Technique to Isolate Contami- nated Dredge Material in the Marine Environment #12 Precision Disposal Operations Using a Computerized Loran-C S:8ystem #17 Disposal Area Monitoring System Annual Report, 1980 #22 DAMOS Mussel Watch Program: Monitoring of the "Capping" Procedure Using Mytilus edulis at the Central Long Island Sound Disposal Site; 1980-81. #32 Summary of Disposal Monitoring Methods Used at FVP and Cap Sites #1 & #2 (2 Volumes) #33 Geotechnical Studies Associated with Capping of Black Rock Sediment #38 Results of Monitoring Studies @ Cap Sites #1, #2, and the FVP Site in Central Long Island Sound and a Classification Scheme for the Management of Capping Procedures #46 Disposal Area Monitoring System (DAMOS) Annual Report, 1984 #56 Response to Comments #57 Observations of the Effect of Hurricane Gloria on the Suspended Material Field in Eastern Long Island Sound
