Dealing with the "Big Dig"

"The Big Dig" -- or more formally, Boston's Central Artery/Tunnel Project (CA/T) -- is a 13-year, $15-billion endeavor that is one of the largest, most technically difficult and environmentally challenging projects undertaken in the United States.

The project includes two main elements. The first element was to extend the Massachusetts Turnpike from its current terminus south of downtown Boston, under Boston Harbor to Logan Airport. The second element was to replace Interstate 93 through downtown Boston with a tunnel through the heart of the city. Keeping the City of Boston open for business as well as ensuring strict environmental compliance was of vital importance.

The Big Dig's environmental review process began in 1982 to meet requirements under both the National Environmental Policy Act (NEPA) and the Massachusetts Environmental Policy Act (MEPA). The first final environmental impact statement/report (FEIS/R) was approved in 1985. Again, to ensure environmental compliance, a supplemental FEIS/R was submitted and approved for the South Boston Haul/Bypass Road in 1990.

Performing an Extensive Environmental Review
Prior to commencing the excavation, contractor Bechtel/Parsons Brinckerhoff (B/PB) and ROW (right-of-way) Assessment, and remediation services consultant Camp Dresser and McGee, performed the extensive job of characterizing soil and groundwater conditions. The testing began in 1989 and has continued in parallel with the design and construction of the CA/T project, which is slated for completion in 2005.

As part of the testing, the assessment, remediation and materials disposal team reviewed the site's history. While the Boston waterfront today consists of residential and commercial properties, 100 years ago it was used exclusively for industrial purposes. Based on the historical information gathered, field investigators inspected specific areas where the possibility of soil or groundwater contamination existed. The team began discussions with Baker Tanks, a rental containment equipment supplier, regarding its ability to develop a customized solution drawing from its extensive equipment line of tanks, pumps, roll-off boxes and filtration equipment that would allow water containment and remediation onsite.

To determine contamination levels, if any, holes were drilled -- typically boring every 200 feet across the property's alignment, spacing the holes 30 feet to 50 feet apart. The borings would drill through layers of waterfront landfill and into the harbor clay to identify the soil contents. To test the groundwater, monitoring wells were installed in up to 20 percent of the borings. This monitoring was done in advance of excavation to avoid the contamination of the vicinity, an incident that can easily occur any time the groundwater table is depressed.

A Well-Earned Moniker
The Ted Williams Tunnel is a 1.6-mile tunnel (of which 0.75 miles lie underwater) that was built using a dozen steel tube sections -- each longer than a football field -- that were sunk into a trench on the Boston Harbor floor and connected together. To create the landside tunnel, the ground was excavated, trenches were dug and the tunnel was built from the bottom up. When the construction was completed, the earth was replaced around the tunnel to bury the tunnel structure and the surface was then restored.

The previously unheard of amount of digging and dumping of dirt not only earned the CA/T project its nickname The Big Dig, but it also presented the contractors with one of the development's biggest environmental challenges: how do you safely and appropriately handle more than 15 million cubic yards of dirt and any accompanying groundwater?

Concentrating on the construction of the Ted Williams Tunnel, more than one million cubic yards of clean, dredged clay has been deposited at the Massachusetts Bay Disposal Site (MBDS), under the supervision of the U.S. Army Corps of Engineers. Approximately 435,000 cubic yards of contaminated or hazardous soils were contained in 400 Baker Tanks' roll-off boxes and sent to properly licensed landfill facilities, mostly located outside of the State of Massachusetts.

Controlling Area Aquifers
From the beginning, one of the environmental concerns facing the Big Dig builders was the Ted Williams Tunnel's proximity to Logan Airport. When the project started, the airport had been in existence for more than 40 years. Over time, the area had been exposed to a significant amount of jet fuel and aviation gasoline. Given the site's proximity to Boston Harbor, Baker Tanks was consulted regarding its customized solutions for containing contaminated water, thereby allowing the builders to remain in compliance with environmental regulations.

The builders created two aquifers: a shallow one at a depth of 10 feet to 15 feet below the ground's surface; and a deeper aquifer placed 80 feet to 100 feet below the surface. In cut-and-cover trench construction, such as the type performed on the Ted Williams Tunnel, it is necessary to control both aquifers since the shallow aquifer has the greatest possibility of becoming contaminated and the deeper one will destroy the bottom of the excavation if not properly handled. To manage this, the water was pumped out of, or around, the excavation to depress the groundwater table. In some places, the builders were able to drop the groundwater table 20 feet in the vicinity of the tunnels to keep the water flows sufficiently low.

Complying with Strict Water Discharge Regulations
The discharge of water into Boston Harbor from CA/T's construction dewatering activities is regulated by the project's National Pollutant Discharge Elimination System (NPDES) permit under the Clean Water Act. In addition to monitoring, the permit establishes a stringent set of performance standards and so-called "Best Management Practices" that must be met before water can be discharged into the harbor.

Quarterly monitoring is also required for all discharges, as well as a monthly review to record total suspended solids (TSS) and flow (gallons). The discharge limit for total petroleum hydrocarbons is 5 milligrams per liter (no more than 5 milligrams of hydrocarbons in one liter of water, or 5 parts per million). In addition to sampling and laboratory analysis, the CA/T's NPDES permit requires visual inspection of discharges, which cannot have floating solids, visible oil sheen or foam in other than trace amounts, and cannot discolor the receiving waters. This visual inspection must be performed daily and includes an examination of the condition of all pollution-control devices, such as sorbent booms and baffles. Under the permit, treatment systems must also be designed to remove known or suspected pollutants, and be sized for a 10-year storm if they are receiving flows that may be affected by precipitation.

In compliance with the above requirements, the CA/T's contractors were allowed to presumptively discharge the water from the Ted Williams Tunnel's deep aquifer with periodic monitoring. However, in handling groundwater from the shallow aquifer, the contractors were required to test all water before it was discharged. In the cases where hydrocarbons were found, additional steps had to be taken to treat and clean the water.

Remediating Hydrocarbon Contamination
The amount of hydrocarbons found on the Ted Williams Tunnel site varied from parts per million -- only visible by the water's oily sheen -- to floating product, which in some cases consisted of two or three feet of jet fuel resting on the groundwater's surface. Due to the sheer volume of water and space constraints onsite, standard remediation methods were not an option; the size of available filtration units were much too large to operate within the site's tight spaces. The only other alternative would have been to install a series of small filters -- a cost-prohibitive solution given the project's enormous scope.

To treat the contaminated water, Baker Tanks and Camp Dresser and McGee developed a custom filtration unit that would address quickly and efficiently the large hydrocarbon contaminant remediation. The process involved a 21,000-gallon Baker EZ Access tank and a 10,000-gallon Baker EZ Clean tank fitted with a series of baffles that enabled the tanks to control the stream of water from the aquifers. The tank's baffles allowed the water to rise and flow over each baffle in the tank. This separated the hydrocarbons from the groundwater while settling heavier solids, which allowed the non-contaminated water to be pumped into Boston's Harbor.

After this process was completed, CA/T contractors implemented a sump and floating pump system to better isolate the contaminated and non-contaminated water for treatment. While a floating pump skimmed the oily residue off the surface of the water into one tank, another pump in the deeper part of the sump extracted mostly clean water into a second separation tank.

In compliance with the project's NPDES permit, the proprietary hinged deck-lid panels on the storage tanks allowed the remediation team to perform daily visual inspections of the pollution-control devices. The tanks were chosen specifically because of their unique hinged-lid capabilities. Moreover, instead of testing at the bottom of the tank, valves were installed at six locations on the tanks at every two feet. Lines attached to the valves directed where the contaminated and non-contaminated water would be stored for proper discharge or further testing and treatment.

Completed in 1995, the Ted Williams Tunnel was CA/T's first project milestone.

By renting the necessary storage tanks and customizing them rather than purchasing filtration devices, the solution developed by the project management team realized a cost savings of nearly $500,000 for the Massachusetts Transit Authority. This proven remediation process serves as a benchmark for subsequent construction projects to cost-effectively realize regulatory treatment requirements.

This article originally appeared in the 10/01/2003 issue of Environmental Protection.

About the Author

Michael N. Wolfe, PhD, is an associate professor at the University of Houston - Clear Lake in Clear Lake, Texas.

Featured Webinar