Comprehensive Environmental Inc.Merrimack, New Hampshire

TURNPIKE WATER TREATMENT AND SPILL CONTROL PROJECT

Everett Turnpike was built next to Harris Pond, the source of drinking water for Pennichuck Water Works and the City of Nashua. To protect the water supply, a detention pond was built between busy Tinker Road and Harris Pond. The basin receives ~150 acres of stormwater, but was previously inaccessible for maintenance. Harris Pond also has limited protection from spills at this busy intersection. Pennichuck Water Works teamed with NH DES and CEI to develop an innovative design that includes pretreatment and wetlands treatment for Tinker Pond, previously known by locals as “Scum Pond”. New spill control was also put in for the Turnpike, along with a new maintenance road that will allow cleanout of the pretreatment system. The project is estimated to remove ~70 pounds/year of phosphorus from entering the water supply where it causes algae blooms among other problems. It will also prevent roughly 140,000 pounds of sediment per year from entering the water supply.

Pennichuck Water Works Project Summary Turnpike Water Treatment & Spill Control Project

Nashua, New Hampshire

Project Summary

Harris Pond is the last pond in a series of ponds along Pennichuck Brook that provide the drinking water supply for the City of Nashua and portions of other towns. The intakes for the Pennichuck Water Works treatment facility are located in Harris Pond, making it a critical location for water quality. The three major threats to the supply reservoir are:

1) spills from either local roads or from the Everett Turnpike that could cause catastrophic pollution events;

2) sediments from road sanding and erosion that can fill in water bodies and carry pollutants into the reservoir; and

3) phosphorus inflows from stormwater, which can encourage algal blooms. Although the Pennichuck treatment plant is a state-of-the-art facility, all water supplies are susceptible to algae blooms and potential taste and odor issues caused by the algae blooms.

The Everett Turnpike was constructed adjacent to Harris Pond in the 1950s, and now carries as much as 125,000 vehicles per day. A detention basin, known as Tinker Pond, was built to control runoff from parts of the Turnpike and from Tinker Road as it crosses over the Turnpike. This detention pond provided protection to Harris Pond for many years, however, over time it filled with sediments and threatened to overflow its contents to Harris Pond.

In addition to receiving runoff from some of the Turnpike, Tinker Pond also collects stormwater from a large watershed with many types of land uses that can contribute nonpoint source (NPS) pollution. The watershed is approximately 150 acres, consisting of parts of the Everett Turnpike, commercial parking lots located west of the Everett Turnpike and a large residential area located just east of the northbound on/off ramps of the Turnpike. The pond provides some detention with little treatment capabilities for contaminants associated with NPS pollution.

To address this and other watershed issues, Pennichuck Water Works funded a watershed study in 1998, which was followed by detailed subwatershed evaluations beginning in the upper watershed. Many pollution control projects in the upper watershed resulted from the watershed program and were funded by Pennichuck with assistance from NHDES. After the recommendations were implemented, Pennichuck funded a major update of the watershed plan in 2008, to include extensive pollution monitoring and watershed modeling. The results of the 2008 update identified additional steps in watershed protection, highlighting the reconstruction of Tinker Pond and spill control for the Everett Turnpike as the most critical projects. Pennichuck then sought and received additional funding from NHDES for this project, which was completed in 2010.

Major project components included:

1) spill controls for Harris Pond adjacent to the Everett Turnpike;

2) reconstruction of Tinker Pond to include a diversion structure that prevents stormwater from entering Harris Pond before treatment and a sediment collection forebay to protect from spills and stormwater from Tinker Road;

Pennichuck Water Works Project Summary Turnpike Water Treatment & Spill Control Project

Nashua, New Hampshire

3) construction of a maintenance road so that the renovated detention pond and spill controls for

Harris Pond could be maintained on a regular basis;

4) installation of maintainable pretreatment structures adjacent to Harris Pond.

BMP Design Components

The Tinker Pond BMPs consists of the construction of two precast concrete stormwater diversion structures installed at the end of two of the three inlets entering Tinker Pond. These structures are strategically placed and designed to disrupt the flow paths of the runoff entering the Pond, preventing short circuiting and increasing stormwater runoff detention time. These structures are also designed to direct and lower the velocity of runoff to drop sediment in easily accessible areas. The second part of the project is the installation of a specially designed outlet control riser structure with a shutoff valve attached within the new riser structure. This structure will prevent petroleum products and debris from migrating into Harris Pond. The shutoff valve is designed to temporarily stop all downstream flows should a hazardous material spill occur on the turnpike. The last part of the project involves the construction of a forebay structure and gabion level spreader designed to capture sediment and debris downstream of the discharge pipe on the northeastern shoreline and to evenly distribute stormwater flows, preventing erosion.

At the Harris Pond location, drainage along the Everett Turnpike discharges directly into one of the City of Nashua’s drinking water supply ponds, Harris Pond, via the Harris Pond Tributary without any treatment or spill containment capabilities. The Harris Pond Tributary collects stormwater from two different stretches of the Everett Turnpike, totaling approximately 5.2 acres of impervious roadway. One subwatershed collects from the Tinker Road overpass and this stormwater is collected through a closed network on the Everett Turnpike that eventually discharges through a 15” diameter pipe into the Harris Pond Tributary. The second larger subwatershed collects from an area just over 4 acres of impervious roadway stretching roughly 1000 feet of the Turnpike. The lower drainage area of this subwatershed collects stormwater flowing overland to a swale while the upper drainage area is collected via a closed network which eventually drains into a second swale. Both swales discharge into a small wetland area prior to flowing over a concrete spillway and into the Harris Pond Tributary.

Anticipated water quality improvements include the removal of ~70 pounds of phosphorus per year (in comparison ~5 pounds of phosphorus can produce 20,000 pounds of aquatic weeds) and the removal of 140,000 pounds of sediment annually.

Award Recognition

Comprehensive Environmental Inc. (CEI), the design engineering firm for the project, received Honorable Mention from the American Council of Engineering Companies (ACEC) of New Hampshire under its 2011 Engineering Excellence Awards program for the Turnpike Stormwater Improvements project.

2011 ACEC-NH Engineering Excellence Awards

Honorable Mention is hereby granted to:

Comprehensive Environmental, Inc

Turnpike Storm Water Improvements Awarded: May 12, 2011

Presented by the ACEC-NH Board of Directors