1. Drew A. Carey, Ken Hickey; CoastalVision Christopher Wright; CR Environmental Joe Germano; Germano & Associates Steven Wolf, Tom Fredette; USACE How the oldest capped mound survived the ages: Integrating multibeam acoustic imaging with optical imaging to read the record of Brenton Reef Disposal Site

2. Oldest capped mound: Brenton Reef Mound The Brenton Reef Disposal Site mound (BRDS) was constructed from 1967 to 1971 as part of the Providence River Navigation Improvement Project. The project has the distinction of initiating the practice of capping harbor sediment with outer channel sediments to isolate harbor sediments from the marine environment. The BRDS was the location for the first known use of sediment capping in New England when sediments from the project were sequentially dredged and point-dumped. Because the dredging began in the upper Providence River and proceeded down the channel, the finest sediments with the greatest concentration of contaminants were disposed first and sequentially covered with coarser and less contaminated sediments from farther down the navigation channel. The project also provided the impetus to begin scientific monitoring of dredged material disposal activities in the northeastern United States

3. What has happened to this old mound in open water? The long-term status of the sediments deposited over forty years ago in 30 m of open ocean has interest for dredged material management programs worldwide. Two surveys were conducted in September 2007 and August 2009. The objectives of the surveys were to characterize physical, chemical, and biological conditions sufficiently to determine the stability of the disposal mound and whether contaminated sediments were isolated from the environment by the site’s intentional, but not engineered, cap. The monitoring investigation also sought to gather and analyze data from this 40- year-old disposal site that could potentially provide insights beneficial to current and future dredged material placement practices.

4. Development of Mound 21 % complete 58 % complete 88 % complete 1968 1970 1969

5. Conceptual model A conceptual model was developed to provide a framework for interpretation. The mound was expected to contain a complex sedimentary structure with a trend of finer, more organic-rich and contaminated inner harbor material at the bottom of the mound and coarser, low-organic and low-contaminant-burden outer harbor material on the top of the mound. Because the mound was not created in a rigorously engineered manner, we expected to see interlayering of the fine and coarse layers within the mound rather than a discrete boundary between the harbor material and a cap layer. However, we expected to find that a layer of less contaminated, coarser sediments covered the bathymetrically defined mound. The surface of the mound and margin was expected to have evidence of deposition and reworking of both dredged materials and ambient sediments.

6. Disposal Area Monitoring System: DAMOS DAMOS has been monitoring dredged material disposal in the Sound since 1978 and an extensive knowledge base has allowed critical evaluation of placement and capping techniques and physical, chemical and biological responses to placement of dredged material on the seafloor. The program utilizes a formal tiered monitoring protocol to assess potential effects of dredged material placement and guide management actions. Standard Survey Techniques: Sequential bathymetric surveys to characterize the height and spread of discrete dredged material deposits or mounds. Sediment profile imaging (SPI) surveys to evaluate benthic habitat conditions and recovery from disturbance. Innovative Survey Techniques: Plan view imaging Side-scan sonar Towed video Sediment coring Grab sampling for biology and bulk chemistry

7. Survey activities Sediment cores Sediment Profile Imagery Bathymetry

8. Acoustic Backscatter patterns over surface of mound show clear evidence of sediment reworking: Braided and flame structures Wave forms Reworked impact craters

9. Acoustic Backscatter patterns off mound show evidence of less sediment reworking: Sand ribbons Intact impact craters

10. Sediment textures typical of surface of mound Silt on margin of moundCoarse sand over silt on mound top Thick coarse sand on mound top

11. Sediment textures typical of surface of mound

12. Transect across mound surface

13. Location of sediment cores on depth difference between 1939 and 2007

14. Long cores in center of mound reached buried silt layers with elevated PAHs and metals Complex interbedded horizons with coarse gravels, oyster shells, silty clay and sands Cores off mound also had buried discrete horizons of silt with elevated concentrations of metals

15. Lower horizons of cores have elevated metal and PAH concentrations compared to upper horizons and reference cores. Exception is Silver but values are below Sediment quality guidelines for all but one sample.

16. Conclusions The mound has persisted as a stable feature on the seafloor for over four decades. It is clear that despite evidence of surface sediment modification from sediment transport across the surface of the mound, the mass of material placed on the seafloor from 1968 to 1973 has remained in place. Variable lag deposits composed of well-sorted sand, gravels, shells, and poorly sorted mixtures with patches of compacted silt now cover the surface of the mound. These lag deposits apparently become mobile during extreme storm events but appear to protect the mass of the mound from large-scale erosion. The complex patterns of sediment types appear related to the episodic nature of the movement with deposition of silt and biological reworking between storm events. The biological community on the mound surface has developed in response to the episodic reworking and mixture of coarse and fine sediments. The concentrations of metals and PAHs in the surface of the mound are similar to reference area concentrations.

17. Conclusions The investigation supported a conceptual model of inner mound sediments (dredged from the Upper Providence River and likely to be deemed unsuitable for unconfined open water placement) effectively sequestered from the overlying water column based on the following three lines of evidence: Physical – The mound has remained virtually the same size and shape as surveyed in 1978 and the surface of the mound is armored with coarse sands and shells that resist strong erosive forces. Biological – The biological community on the surface of the mound and surrounding area is in equilibrium with the sediment grain size and dynamics of the benthic habitats. There is no evidence of adverse impact on biological resources from contact with unsuitable sediments. Chemical – The bulk sediment chemistry results from the surface of the mound (0-20 cm) were similar to the results from off mound reference areas. The results from most sediment samples collected below the surface of the mound (> 20 cm depth) were similar to results from dredged material that would be deemed suitable for open water placement. Cores were unlikely long enough to reach and document the higher concentrations of contaminants that likely exist within the mound.