A sampling program was initiated to examine the depth distribution of the sand cap and Reserved channel dredged sediment within the first in-channel borrow pit constructed as part of the Boston Harbor Navigation Improvement Project (BHNIP). The purpose of this program is to document the long term changes and properties of in-channel disposal of contaminated marine sediments using a sand cap to isolate the contaminates from the surrounding marine environment.

On August 13, 1997, a group of researchers with the M.I.T. Sea Grant College Program conducted the first sampling effort. The group was comprised of Gene Gallagher (professor at U. Mass), David Shull (U. Mass graduate student), Sanjay Pahuja (M.I.T. graduate student), Scott FitzGerald (M.I.T. graduate student), and Dan Regan (SCUBA diver). All sampling operations were conducted off a 26 foot marine head pump out vessel equipped with a hand held differential GPS unit, VHF radio, and thru-hull depth sounder. Please see Figure 1 for the location of the disposal cell and sampling locations. All sampling data were logged and entering into a GIS file for future use. See Table 1 for a partial listing of this data.

We intended to sample the sand cap along the edge of the cell to determine the depth distribution of cap and Reserved channel dredged material. The edge of the cell was chosen because it was assumed that the cap would have a minimum thickness in this location. This was important because it is assumed that it would be difficult to drive a gravity corer of the type available through 3+ feet of sand.

Several attempts were made to collect a sample using a gravity-corer equipped with a 4 foot sampling tube. The locations of these coring attempts are labeled LC 1 and LC 2 on Figure 1. During the first attempt (LC 1) the core was dropped from just below the water surface in an attempt to give the core significant velocity to penetrate the sand cap. However, the divers found that the core had plunged sideways into approximately 3 feet of fine-grained, porous, organic-rich, black mud. Inspection by divers indicated no definite sediment-water interface and that the organic-rich mud layer was greater than 4 or 5 feet thick. Attempts to retrieve the core from this material resulted in the loss of the sampling tube from the corer, presumably due to the torque and suction from the mud. A new tube was secured in the corer and a second attempt was made to collect a sample (LC 2). The sampling tube was lowered gently into the sediments and then capped at the bottom by the SCUBA divers as it was brought to the surface. Once on board the vessel attempts were made to remove the sampling tube from the corer. However, the tube had been attached too tightly to the corer and attempts to remove the tube resulted in the loss of the sample tube cap. As a result, the contents of the tube emptied out into the vessel. (See pictures)

While attempts were being made to remove the sampling tube from the corer, a surface sample and a deep sample of the bottom material were collected by SCUBA divers using small 12" sampling tubes. The deep sample was taken approximately 4 feet below the estimated sediment-water interface. The locations of these samples are labeled SC 1 and SC 2 on Figure 1. The material is primarily composed of silt or clay-sized particles with porosity ranging from 81 to 85 percent. However, we also noted the presence of larger sand grains. Results of a grain size analysis are presented in Figure 2 and Figure 3. This analysis consisted of both a sieve and pipette analysis without a hydrogen peroxide treatment.

We also attempted to collect a gravity-core sample closer to the center of the disposal cell (LC 3). The corer was retrieved empty because the divers had finished and were not available to cap the bottom of the sampling tube. However, some sand grains and clay were found adhering to the coring device, suggesting that the material towards center of the cell has a different composition than the material in the southern portion of the cell. There was no evidence of the black, fine grained organic material on the corer.

These observations suggest that the sand cap does not uniformly cover the dredged material within the cell. However, we do not know the origin of the fine material found on top of the cell. Possible sources include slumping of the sides of the pit, accumulation of ambient sediment from the channel, or the redistribution of Reserved channel dredged material from the disposal mound.

Before more disposal cells are capped, it is imperative that the cap material in this disposal cell be located to determine the effectiveness of capping methods currently being employed. We propose to conduct another sampling trip to collect samples using coring devices which will effectively sample mud and can located a mud-sand interface. We intend to document the spatial distribution of different sediment types located in the cell using analysis of sediment grain-size, organic-carbon content and possibly heavy-metal concentrations.

Table 1. Sampling Log Data

Figure 1. MIT Sampling Locations

Click on Image for a larger picture. (and better quality)

Figure 2. Grain Size Analysis of Sediment at Sediment-Water Interface

Figure 3. Grain Size Analysis of Sediment 4' Below Sediment-Water Interface