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No Turbidity Exceedances During Use of Mackworth Barriers for Seacoast Reliability Project in NH

In November 2019, Mackworth-Enviro designed, installed, and maintained operations on two bottom-sealing filter barrier turbidity systems as part of the Eversource Energy Seacoast Reliability Project (SRP). The project consisted of a 12.9-mile, 115 kV transmission project to improve the electrical reliability in the Seacoast region of New Hampshire. Three cables were buried approximately 1 mile across Little Bay north of Adams Point. The cables were installed using a jet plow along most of the route across Little Bay. Hand jetting was used to install cables close to shore where water depths were too shallow for the use of the jet plow.

The Bottom-sealing turbidity barriers were deployed on the eastern and western tidal flats during the hand-jetting process. Each barrier consisted of a surface float, a microfabric skirt that filtered out particulates, an impervious weighted bottom foot, and a series of anchors and lines for securing the float, skirt, and foot. The barriers were utilized to contain and minimize turbidity associated with the resuspension of suspended sediments from hand jetting. Turbidity is a measure of how much light penetration is reduced by suspended particles in the water.

Normandeau Associates, an environmental consulting company, conducted 28 days of continuous water quality monitoring during hand jetting. The data was typically collected: before construction activity started for the day, during the activity, and after construction stopped for the day. The in-situ water quality measurements included turbidity, dissolved oxygen, temperature, salinity, and pH.  NO exceedances of the State turbidity standards were observed over the 28-day monitoring period. Paired profile samples were taken inside and outside the turbidity barrier to examine barrier effectiveness. Samples outside the barrier were right around background levels for turbidity, typically between 2 and 5 NTU. Samples inside the barrier reported much higher turbidity, ranging up to 30 NTU. Results of the paired profile sampling clearly show the effectiveness of the turbidity barrier in this application.

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