A containment system collects dense non-aqueous phase liquid contaminants from a closed industrial site
An innovative public/private partnership launched by Northern Utilities is helping to revitalize the city of Lewiston, Maine, and deal with the legacy of pollution from one of the city's old manufacturing sites.
Starting in the late 1950s, lower production costs elsewhere led to the closure of many of Lewiston’s textile mills, which were the city’s economic base and once produced one quarter of American textiles. Another city fixture, the Lewiston Gas Light Co., operated a manufactured gas plant (MGP) from 1854 to 1962. The site was decommissioned, and most of the gas plant structures demolished and removed or abandoned in place.
Today the property is the location of one of Northern Utilities’ natural gas distribution regulator stations, as well as a service corridor used for staging and storage associated with Northern’s construction and maintenance work.
Some of the site challenges included:
• Androscoggin River wetland boundaries and adjacent private property.
• Existing water, sewer, active gas utilities, and commuter traffic flows.
• Ice and current scouring during flood-stage river levels.
• Precipitation and high water events.
• MACTEC’s design specified that the contractor complete the trench installation and conduct work at the top of the 20-foot high riverbank slope by mid-March 2007 when snow melt and resulting high-water conditions inundated the collection trench location.
Extending 600 feet along the Androscoggin River entirely within the 100-year floodplain, the site had a severely eroded riverbank containing solidified coal tar and other MGP residuals, all from decades of former gas manufacturing operations. A potential public health hazard and environmental concern, the unsightly riverbank was the source of offensive odors that often affected the nearby residential area.
Investigations identified underground coal tar at the riverbank in the form of what environmental scientists term "dense non-aqueous phase liquid," or DNAPL. Heavier than water, the DNAPL had sunk into the subsurface to the top of the continuous clay layer forming the lower portion of the groundwater column, where it threatened to migrate to the river.
Designing the Containment System
The remediation project spearheaded by MACTEC Engineering and Consulting was conducted under the Maine Department of Environmental Protection’s (DEP) Voluntary Response Action Program. A containment system was designed to meet Northern’s objectives:
- Create a public riverside park to support Lewiston’s Southern Gateway redevelopment project.
- Eliminate odors.
- Prevent exposure to the residuals.
- Prevent migration of the residual wastes to the Androscoggin River.
The remediation team engineered a passive coal tar-collection trench using the difference in specific gravity between the DNAPL and groundwater to induce the coal tar to enter the trench. To prevent contact with the residual wastes, to minimize erosion of the residuals into the river, and to control odors, a multi-layer cap system was designed for the steep embankment. The cap system consisted of a gas-collection piping layer, geomembrane odor barrier, synthetic stormwater draining layer, and a topsoil-filled geocell stabilization layer.
The design incorporated a vapor collection system beneath a vegetative blanket anchored by a stone mattress to contain environmental contaminants, reduce erosion, and culminate in a safe and attractive public park for beneficial site reuse. All excavated waste materials were pretreated on-site and transported to an asphalt-blending facility for beneficial reuse.
The trench was sloped to pumps where DNAPL was collected for off-site disposal. This eliminated the need for costly long-term operation and maintenance normally associated with active groundwater pumping. A groundwater pump-and-treatment system would have cost at least another $1 million over the life of the collection system.
The trench was positioned 600 feet along and below the river's surface, at the base of a 25-foot high, 45-degree slope to intercept a greater extent of the DNAPL plume rather than an alternative trench positioned at the crest of the slope. This reduced the volume of silt to be excavated by nearly one-half and increased the area of DNAPL collection. The trench was excavated into the upper surface of the clay-confining layer on which the coal tar DNAPL set. Sheet-pile cofferdam walls were advanced on both sides of the trench. As a result, the excavation occurred in water-saturated soil, without the need for dewatering, a process of separating and disposing water mixed with the excavated soil that would have added tens of thousands of dollars to the project's cost.
A new hydraulic analysis method evaluated the flow of DNAPL into a trench and defined minimum width and permeability needed to collect DNAPL. Results from previous research on typical size ranges of DNAPL droplets moving through granular soil typical of the type present at Lewiston were integrated into the analysis. In addition, the design calculated the specific gravity of the on-site DNAPL using Stokes Law, the equation that calculates the force needed to move a small sphere (in this case, DNAPL) through a continuous fluid (groundwater) to predict the settling velocity of the droplets within the trench.
The assessed settling velocity, horizontal groundwater velocity, and observed thickness of DNAPL layers determined the optimum trench width, allowing oil droplets entering the trench to settle into a sump within the confining clay layer forming the trench bottom: five feet wide. Groundwater flow equations for horizontal trenches were modified to model DNAPL flow within the trench to the collection sumps.
According to the U.S. Environmental Protection Agency, a non-aqueous phase liquid (NAPL) is a term used to describe the physical and chemical differences between a hydrocarbon liquid and water that result in a physical interface between a mixture of the two liquids. The interface is a physical dividing surface between the bulk phases of the two liquids, but compounds found in the NAPL are not prevented from solubilizing into the groundwater.
Immiscibility typically is determined based on the visual observation of a physical interface in a water-hydrocarbon mixture. Numerous methods, however, can be used to quantify the physical and chemical properties of hydrocarbon liquids.
NAPLs have typically been divided into two general categories, dense (DNAPL) and light (LNAPL). These terms describe the specific gravity, or weight of the NAPLs relative to water. Correspondingly, DNAPLs, have a specific gravity greater than water. Conversely, LNAPLs have a specific gravity less than water.
The project is an important step in revitalizing the Southern Gateway to the city’s downtown. Reclamation of the site is expected to serve as a catalyst for brownfield redevelopment of adjacent mill buildings into residential housing and retail and office space.
Some of the keys benefits of the project are:
•Enhancing public health and the environment by intercepting DNAPL flowing to the Androscoggin River and preventing potential contact with coal tar residuals.
•Stabilizing 2.5 acres of steep riverbank containing debris that previously posed a physical hazard enhances public safety.
•Transforming 2.5 acres of formerly blighted industrial area into a waterfront public park.
•Encouraging brownfield redevelopment of 550,000 square feet in adjacent mill buildings.
• Improving access to the waterfront by recreational users.
• Enhancing the view of the riverbank from the river by boaters and by residents on
the opposite side.
• Infusing more than $3 million into the local economy by employing over 45 local and regional consultants, constructors, and vendors on the project.
Looking to the Future
Throughout design and construction, reports were generated and meetings were held with stakeholders, which included the Maine DEP, city of Lewiston, and Northern Utilities. The public was kept informed via regular meetings and newspaper articles that tracked the progress of the restoration.
"It’s fitting that a site with such a rich industrial history in our city is now a great place for our residents to enjoy," said Lincoln Jeffers, Lewiston’s assistant to the city administrator. "For too many years the Androscoggin River was used by upstream mills as an industrial sewer. ... A former industrial site has been turned into a community asset."