The Wood Street Bridge connects the city of New Bedford with the town of Acushnet. Hot Spot "A", which is contiguous with the shoreline at the Aerovox plant, covers 1.72 acres and is 1,202 feet south of this bridge. Area "B" encompasses 2.6 acres of sediment off the New Bedford shoreline and is situated 2,090 feet south of the Wood Street Bridge. Areas "C" and "D" are located 2,300 feet south of the Wood Street Bridge and encompass 0.2 and 0.4 acres, respectively. During the design of the remediation for the "hot-spot" area, further sampling was conducted to more accurately define those areas with sediment PCB levels greater than 4,000 ppm. Six Hot-Spot areas were delineated. Hot Spots "A" and "B" were essentially divided generating two additional "hot-spot" areas. Wetlands are located in the region encompassing the "hot-spots" on the Acushnet shoreline, immediately east of the Aerovox plant.

The Acushnet River estuary which contains the "hot-spot" areas is a 230 acre region that extends from the Wood Street Bridge to the Coggeshall Street Bridge, 1.6 miles south. A wetland area is situated in the estuary on the Acushnet shoreline approximately 2,500 feet south of wetlands area I.

The discharge limits for the treated water are defined in a permit which is set up like an NPDES permit, with maximum instantaneous limits and monthly average limits for both PCBs and metals (cadmium, chromium, copper, and lead). In order to set water discharge limits that would meet the "no degradation in the river" requirements, EPA established background water levels prior to dredging by means of a monitoring program. It's interesting to note that the water discharge was not required to satisfy ambient water quality criteria (AWQC) since the hot spot dredging program was categorized as an interim measure (e.g., hot spot levels may only be reduced to 4,000 ppm from an initial 4,000 - 200,000 ppm range).

Reportedly, PCB discharge limits were consistently met but copper limits (monthly average) sometimes proved difficult to meet.

• Volatilization of PCBs aggravated at this site due to (1) exposed hot spot areas at low tide and (2) oil slick forming on water during dredging.
  
  
• Air sampling locations: ten (four near CDF; six near hot spot)
  
  
• Air samples are collected (filtered) over 8-24 hour period; 72-hour lab turnaround
  
  
• Massachusetts Ambient Air Limit: 0.5 ng/m3 (1 ng = 10-9 gm)
  
  
• Average background near hot spot: 40 ng/m3
  
  
• Average background represents 8 x 10-5 increased lifetime carcinogenic risk
  
  
• Shut down Level: (NIOSH, 40 hr/wk, 50 yrs): 1000 ng/m3
  
  
• Action Level (one-half NIOSH): 500 ng/m3
  
  
• Notice Level: set at average of four previous background levels plus 30 ng/m3 (represents increased carcinogenic risk of 0.3 x 10-6)

Some results from the first 3 months of dredging, quoted from Reference E-8:

"The notice level was initially 76 ng/m3 at the start of dredging in April and was increased to 172 ng/m3 as of June 24, 1994."

"During the first 3 full days of dredging the "notice level" was exceeded on each occasion with averages of 252, 133, and 134 ng/m3 being measured with a reading of 825 ng/m3 at the sampler closest to the dredge. This resulted in modifications to the dredging procedures. Since that time, the notice level was slightly exceeded on only 4 occasions. The monthly averages for the dredging area are shown below:

- Month/Year: April 1994; Range (ng/m3): 54-252; Average (ng/m3): 143
- Month/Year: May 1994; Range (ng/m3): 21-163; Average (ng/m3): 69
- Month/Year: June 1994; Range (ng/m3): 62-158; Average (ng/m3): 101"

"Monitoring in 1994 at the CDF did not result in modifications to operations although it highlighted the need for the floating cover on CDF Cell #1. On several occasions, readings of 1,800 ng/m3 (24 hour sample) have been recorded at an individual sampler during periods when the cover was partially removed for maintenance."

Water Monitoring During Remediation:

Resuspension Monitoring (Source: References A-126)

• Four sample locations in river/estuary: at WWTP discharge; 200' from dredge; at Coggeshall St. bridge; at Hurricane Barrier
  
  
• Five liter water sample; expose to biota in lab (toxicity test)
  
  
• Dissolved/suspended PCBs analyzed on 24 hr. turnaround; compared to pre-determined background
  
  
• Criterion: produce no increased impact at Coggeshall St. bridge
  
  
• Initial frequency was every day for first 21 days; no increased impact observed (i.e., none discernible separate from overall impact of estuary - wide contamination); frequency reduced to once or twice per week; is periodically increased when moving into more highly-contaminated areas
  
  
• Two factors limit observable effects from resuspension, namely:
  
  - Tide is incoming for 50% of the dredging period; and
  
  - Operating with high suction rate and slow cutter rotation.
  
  
• Discharge from WWTP: Refer to summary under "Water Discharge Limit"

References E-8 and E-47 present data on resuspension during the hot spot dredging. Results collected at the Coggeshall St. Bridge are presented. The bridge is one mile downstream from the southernmost of the hot spots. The two references present data that show that the average PCB flux measured at the bridge during dredging ranged from 0.12 to 0.45 kg/day (0.26 to 0.99 lb/day), a factor of 2-10 higher than pre-dredging.

Achieved the less than 4,000 ppm PCBs target based on a relatively few post-dredging sediment verification samples collected in dredged areas. Fifteen final composite samples were used to document post dredge PCB levels in the 5 acres of hotspot areas. The 15 samples ranged from 67 to 2,068 ppm PCBs with a median of 707 ppm.

Restoration and Post-Monitoring:

Reference A-334 (1997, circa) describes pre-and post-hot spot dredging monitoring as follows:

"Prior to initial remedial activities, a comprehensive long-term monitoring program was developed to assess the effectiveness of dredging at this site, both spatially and temporally. Pre-remedial baseline sediment sampling consisted of quantifying a suite of exposure indicators (PCBs, metals, sediment toxicity), habitat indicators (TOC, AVS, grain size) and ecological indicators (benthic community indices) at each of 72 stations along a gradient from the severely contaminated upper estuary to Buzzards Bay. Recently, the first phase of remediation was completed, dredging of a five acre "Hot Spot" and the full suite of indicators were measured again. Pre- and post-dredging comparisons were made for each indicator and are presented in GIS format. This approach will be used during each remedial phase, as well as after completion of all remedial activities, to assess ecological recovery at this Superfund site."

"Sediments were collected before any remedial activities (1993) to establish baseline conditions. First post-remedial sampling was conducted immediately after initial "Hot Spot" dredging (1995).

Three of the fmdings in Reference A-334 are quoted below:

• (Based on before and after samples at 72 stations). . . "A significant PCB contamination gradient exists from the upper to lower harbor. While any major redistribution of contaminated sediments from the Hot Spot dredging was confined to the immediate vicinity of remedial activities, there is evidence that low molecular weight PCBs migrated greater distances. This newly redeposited material is different in composition from either Aroclor 1242 or 1254, the predominant Aroclors at the Hot Spot, and may be the by-product of dechlorination occurring in the Hot Spot sediments."
  
  
• "Increased contaminant concentrations (e.g., PCBs, Cu) in 1995 were observed primarily in the upper harbor.
  - Corresponding increases in short-term acute sediment toxicity also were measured in the upper Harbor."
  
  
• "Smaller increases in 1995 contaminant concentrations and sediment toxicity occurred in lower harbor depositional areas."

The Upper Harbor is about 1.5 miles long and occupies roughly 230 acres. As presented in Reference A-334, the mean total PCB concentration in the Upper Harbor (24 sampling stations) was 94 ppm in 1993 (pre-dredging) and 124 ppm in 1995 (post-dredging). Sediment samples were grabs of the top two centimeters. This overall mean average PCB surface concentration increased 32% in the Upper Harbor following dredging.

In Reference C-271, results of air, soil, and produce samples collected up-and down-wind from the hot spots both pre (1992) and during dredging (fall 1994) are reported. One of the findings is quoted below.

"A significant increase in PCB concentrations was found in samples of tomatoes grown during a summer of harbor dredging compared to those from the pre-dredge period. Downwind of the harbor (site A), PCB concentrations in tomatoes grown during dredging exceed the concentrations in pre-dredging samples by a factor of 6."

Site-Specific Difficulties:

Dredging limited to a 4 to 6 hour daytime window on either side of high tide, to ensure sufficient water depth to float the dredge.

Four to six hours of dredging at a suction rate of 2,100 gpm would "max-out" the WWTP for 24 hours. Even without the dredging being tide-limited, this factor would have limited the hours of daily dredging.

Excessive PCB volatilization to air caused numerous exceedances of PCB-in-air limit -- resulting in numerous temporary shutdowns of dredging.

As described in Reference E-8, dredging operations brought a layer of PCB oils to the surface, causing elevated PCB-in-air levels. As a result, the following dredging modifications were made:

• the dredge's swing speed was reduced to the slowest speed the operator could maintain;
  
  
• the dredge was also stopped at times and used to pull oil off the surface;
  
  
• a shroud was fabricated at the site and installed over the cutterhead to catch oil as it was released, allowing it to be pulled into the dredge;
  
  
• 3 swings were made in an attempt to remove a 1.5 foot lift of material prior to advancing the dredge; and
  
  
• the silt curtains were removed because they appeared to be contributing to the oil problem by their continuous disturbance of the bottom in the varying tidal and weather conditions.

• Sediment:
  
• Water:
  
  
• Fish: