with water depths that range from 42 m to 74 m. The regulated and monitored deposition 

 of dredged material has been occurring at PDS since 1977, with an average aimual disposal 

 volume of approximately 99,000 m' (Morris 1996). However, usage of this region as a 

 disposal site dates back to 1947, as material was disposed over a 17.7 km^ irregularly 

 shaped area of seafloor surrounding the current PDS boundaries (Figure 1-4). 



The depositional environment of PDS, especially within the deeper fine-grained 

 basins, indicates that volumes of dredged material can be placed without movement of 

 these deposited sediments beyond the disposal site boundaries. The demonstration project 

 was planned to take advantage of the irregular topography of the PDS. Strategic placement 

 of a disposal buoy within the center of a basin surrounded by natural ridges would serve to 

 contain the initial UDM deposit within the confines of the basin. The basin feamres would 

 minimize any lateral spread of the UDM deposit, and aid in the complete and efficient 

 isolation of the project material with a similar volume of CDM. 



A capping project conducted at PDS from October 1991 through July 1992 set a 

 precedent, as all other DAMOS sediment capping operations were conducted at disposal 

 sites with water depths of approximately 20 m. The 1991-92 capping project consisted of a 

 13,300 m^ UDM deposit of silt and clay, capped with 37,800 m^ of CDM consisting of 

 sand and silt (Wiley 1996). Comprehensive analysis of the sediments collected over the 

 surface of the capped mound showed chemical concentrations corresponding to the levels 

 detected in the CDM before dredging operations conraienced. However, analysis of 

 sediment-profile photographs revealed a heterogeneous mixture of sand and silt 

 components in the project capping material which confounded the physical differentiation 

 between the UDM and CDM layers. As a result, insufficient data were gathered during 

 the 1991 and 1992 monitoring cruises to unequivocally determine the behavior of the 

 dredged material at deeper sites, and the ability to form distinct UDM and CDM layers 

 without mixing (Wiley 1996). 



A joint effort between NAE, EPA, and the Casco Bay Program formulated a closely 

 monitored capping demonstration project at PDS to gather more data on the behavior of 

 dredged material at deeper water containment disposal sites. A small dredging project 

 (estimated barge volume of 86,000 m^) from the Royal River in Yarmouth was used to 

 examine the feasibility of capping at PDS. The Royal River project was selected due to the 

 distinctive sediment characteristics within the esmary, and the presence of sediments 

 deemed suitable for unconfined open water disposal. 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



