A.21 Grasse River, Massena, NY

A.21.1 Summary

Environment:

River

Scale:

Pilot

Contaminants of Concern:

PCBs

A.21.2 Site Description

The site is located on the Grasse River in Massena, NY. Alcoa, Inc. began a pilot study in 2001 to evaluate subaqueous cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment. as a potential remedial alternative for addressing polychlorinated biphenyls (PCBs) in sediment and biota of the lower seven miles of the Grasse River near Massena, NY. The pilot study examined various capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment. materials and application techniques in a seven-acre study area. Data collected over the following year demonstrated that the cap had remained intact and relatively unchanged and was functioning as designed. Spring 2003 monitoring results, however, indicated a loss of cap material and underlying sediment in the study area. Investigations found that these changes were caused by a severe ice jam that formed directly over the cap.

CSM summary: Modeling indicated that scour of the cap material, underlying sediment, and sediment outside the study area was caused by the turbulence and high velocity of water flow below the ice. The turbulence and high water velocity resulted from an increase in water stage upstream of the ice jam, a reduced cross section below the jam, and the roughness of the ice jam. Sonar imagery and underwater videography supported the finding that scour resulted from hydraulic forces below the toe of the ice jam rather than physical contact between the ice and sediment.

The extent and magnitude of sediment disturbance caused by the ice scour event was characterized by examining changes in sediment elevation and type relative to pre-ice-jam conditions. Comparisons indicated that scour ranged in depth from 0.4 to 5.0 ft and occurred in about 15% of the river bottom in the uppermost 1.8 miles of the Lower Grasse River. Part of this area included the cap demonstration area, and some of the deepest erosion occurred in an area that contained a 24-inch thick sand/topsoil cap covering approximately 1.2 acres. Much of the material that eroded in this area was deposited immediately downstream, where a 4.6-foot increase in sediment elevation was noted.

Redistribution of sediments and PCBs during the 2003 ice jam and scour did not significantly affect average PCB concentrations in sediment, water, and fish, suggesting that potential PCB exposure in the river did not change significantly. Surface sediment PCB concentrations in the scour area, however, were higher and more variable than before capping, averaging 13 ppm instead of 8 ppm. This increase is attributed to exposure of deeper sediments typically containing higher PCB concentrations. Surface sediment PCB concentrations decreased in areas subject to deposition, as evidenced by a three-fold reduction immediately downstream of the study area.

A review of historical records and physical evidence such as tree scarring indicated that possibly six ice jam events have occurred in the Lower Grasse River over the past 40 years. Analysis of high-resolution and stratigraphic cores suggested that ice jam-related scouring occurred in the Lower Grasse River four times over the same period or about once each decade. Results of this and other investigative work to date indicate that ice jams, and resulting scour associated with severe ice jams, are limited to the upper 1.8 miles of the Lower Grasse River and current proposed remedial plans have included additional armoring of a cap in this area.

A.21.3 References

Beckingham, B. & U. Ghosh, 2011, Field Reduction of PCB Bioavailability with Activated Carbon Amendment to River Sediments, ES&T, 45(24): 10567-74.

Record of Decision, Grasse River Superfund Site (a.k.a. Alcoa Aggregation Site). http://www.srmtenv.org/web_docs/Superfund/alcoawest/2013/2013-April-Grasse-ROD-Full.pdf.

Publication Date: August 2014

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