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Mitigation Overview

Throughout the planning stages for the I-93 Improvements, the impacts to wetlands were carefully evaluated.  The term wetland mitigation refers to the method of minimizing effects on wetlands during the planning and construction phases of the project.  The New Hampshire Department of Transportation (NHDOT) utilized the following approaches to minimize impacts on wetlands: 1) avoidance; 2) minimization; and 3) compensation.  While evaluating the highway improvement alternatives, avoidance of wetland impacts was a primary concern.  Due to the large scale of the project, however, all options considered involved impacts to wetlands.  Where it was not possible to avoid wetlands, impacts will be minimized by using steeper side slopes or by constructing retaining walls.  A compensatory mitigation package was developed to offset any remaining impacts to existing wetlands.  In other words, equal or greater amounts of wetlands will be created or preserved within the five communities directly impacted to replicate and preserve the functions and values lost from these impacts to the ecological systems.

Wetland Mitigation Site #31:
Pelham Road

The Selected Alternative impacts approximately seventy-seven acres of wetlands along 19.8 miles of improvements to the I-93 corridor.  For the most part, the proposed highway improvements follow the existing corridor so the bulk of the direct wetland impacts are along the edges of existing wetland systems located adjacent to I-93.  In most cases, these “edge impacts” represent a small percentage of the total acreage within larger wetland systems and therefore are not expected to significantly diminish the performance of the existing wetlands.

Selection of mitigation sites was done through a collaborative effort.  NHDOT worked with each of the five communities along the I-93 corridor to locate potential mitigation sites. As NHDOT received input and feedback from communities and federal and state regulatory agencies such as the United States Environmental Protection Agency (USEPA), numerous sites were compiled and investigated between early 2001 and the spring of 2002.  In all, more than 60 sites comprising 7,000 acres of land were researched, visited, and assessed by NHDOT. In response to this feedback, NHDOT developed a mitigation package that offsets the impacts from highway improvements.  This package includes:

Wetland Mitigation Site #16:
Sybiak Property
  • 17 sites protecting over 1,000 acres of land.  Within five of these sites, approximately 27 acres of new wetlands will be created.
  • $3 million in NHDOT funding to the New Hampshire Department of Environmental Services (NHDES) Drinking Water Supply Land Grant Program to be used to purchase property rights to aid in the protection and long-term viability of Massabesic Lake, the water supply for Manchester and portions of Derry and Londonderry. 
  • A commitment to study the feasibility of reconstructing existing culverts at Policy and Porcupine Brooks in Salem, Beaver Brook in Derry and Cohas Brook in Manchester to address wildlife crossings.  These measures will be fully evaluated as part of the final design of the Selected Alternative.  To the degree that these measures are practical, they will be constructed.
  • Development and funding of the Community Technical Assistance Program (CTAP) to assist communities in addressing the anticipated growth and development associated with the southern I-93 corridor. 
Wetland Mitigation Site #30:
Cluff Crossing

This significant land protection effort, in combination with the commitment to provide funding for planning technical assistance (CTAP) to help communities address future growth pressures, compensates for the overall project impacts.  On May 2, 2006, the NH Department of Environmental Services (DES) issued the Wetlands Permit and the 401 Water Quality Certificate.  NHDOT has obtained all required permits from the Army Corps of Engineers and continues to move forward.

View an interactive map to access information on the individual mitigation sites

For additional information on the mitigation process please see: 

Wetland Mitigation for I-93 Improvements  - Detailed information on the mitigation efforts (21 pages)

Mitigation Fact Sheet – Summary of mitigation efforts (2 pages)

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