NAPL. Fixing. Sediments. pg 18. September Super Early Bird expires on 9/30/14! Solutions for Air, Water, Waste and Remediation

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1 September 2014 Solutions for Air, Water, Waste and Remediation > IMPROVING ACCURACY > HORMONE DISTRUPTORS > RECYCLING CUTTINGS > WEFTEC PREVIEW pg 24 pg 28 pg 34 pg 38 Fixing NAPL Sediments pg 18 Super Early Bird expires on 9/30/14!

2 COVER STORY Repairing NAPL Contamination in Sediments >> BY THOMAS R. STOLZENBURG, PH.D. AND JOHN M. RICE, PE, TRC COMPANIES INC. The very infrastructure that provided advantages for MGP facilities are now creating major headaches. Above: The site shown above is the finished NAPL trapping cap in Michigan. T he sites of former manufactured gas plants (MGPs) are often located in close proximity to canals or rivers. During the plant s useful life, waterfront property was ideal for transporting raw materials and finished products. Today, remediation and restoration of shuttered sites open opportunities for highly desirable waterside residential dwellings and public open spaces or commercial redevelopment. However, the unfortunate legacy of industrial sites is often contamination to sediments and water quality as well as aesthetic impacts and potential harm to human health. But what causes contamination to emerge from sedi-

3 Coal tar sheens are shown forming on the water surface at a former manufactured gas plant site. Above is a conceptual model of ebullition-facilitated DNAPL transport. ments and present a visible source of contamination? What is the process behind its mobilization and how can it be effectively addressed to meet modern environmental and remediation aims? Contaminated Sediments Coal tar is a complex mixture of hundreds of organic compounds. It is formed of various monocyclic and polycyclic aromatic hydrocarbons (MAHs/PAHs), including such as benzene and benzo(a)pyrene. When discharged into water in the form of a non-aqueous phase liquid (NAPL), this cocktail of contaminants can cause large rainbow sheens to spread across the water surface, where it is available to human and ecological receptors. Decades after release, the tar often remains as a liquefied residual in sediments. The distribution of NAPL within and across sediments is dependent on a number of factors. Some of it sunk or was buried in deeper layers of sediment. Some was spread downstream by high velocity flow. Some is subject to continuing redistribution by a process called ebullition. How Do DNAPLs Defy Gravity? Once NAPL enters sediments, the migration pathway is through the water column down into the sediments.

4 COVER STORY Figure 3: Schematic cap cross section showing the transport of DNAPL from the sediment upward through the transmission layer to the accumulation zone. (The cap can also contain LNAPL in the accumulation zone, as illustrated.) This cap design is covered by patent numbers US 8,419,314 and US 8,651,768 Depending on the density of the NAPL and the competence of the sediment, a dense NAPL (DNAPL) will settle deeper into the layers of sediment until it meets a lower permeability layer. Although the DNAPL has been buried in the sediment, field observations of surface oily sheens confirm that the DNAPL within the sediments continues to change water quality decades after it was deposited. A significant process causing the remobilization of residual DNAPL in sediments is ebullition. Ebullition is a wellknown and widely studied process because of its impact on greenhouse gas emissions. Microbes within the sediments are actively metabolising readily available carbon, and as they do, they generate gas commonly methane and CO 2. Methane gas bubbles rising through the sediments become coated with the strongly hydrophobic coal tar. If a bubble is large enough that its buoyancy overcomes the extra weight of the tar coating, it will continue to rise through the sediments and water column, dragging the tar all the way to the water surface. Environmental Risks The re-emergence of coal tar products at the water surface presents a significant environmental risk. The characteristic rainbow sheens and starbursts are created as the NAPL product has a lower surface tension than water, and the contamination spreads across the surface. Studies have shown that a 1 cm 3 droplet of tar can spread across a river surface over thousands of square centimeters depending on the local river conditions. Oily sheens can present an environmental risk to the aquatic environment and water quality, and a risk to human health through dermal contact. Sometimes the tar droplets on gas bubbles are too large to breach the surface. In those cases tar pendants are formed, hanging beneath the water surface. Turbulence causes the pendants to break away and deposit tar in a secondary location. This is one mechanism for coal tar to migrate downstream from its original source. Finding the NAPL Source The distribution and redistribution of NAPL in sediment is subject to current velocity, burial, migration downward due to density, and releases and remigration from ebullition. Traditional sampling methods to find tar impacts in sediments usually rely on grab or core sampling, and subsequent analysis for PAHs. Because some tar can harden over time, these methods are not effective at discerning NAPL versus hardened tar. Similarly, sophisticated subsurface methods, such as laser-induced fluorescence (e.g., TarGOST) may not be able to distinguish NAPL from hardened tar. It is an important distinction because as a liquid, NAPL poses a much greater risk of rerelease, sheen formation and direct contact than hardened tar does. For these reasons a sampling approach that identifies where liquid coal tar is both present and subject to release, regardless of depth in the sediment, is required. A corraltype sampling method has been successfully used in a wide range of aquatic settings, from small streams and ditches to large, deep, fast-flowing tidal environments. This sampling method quantifies the sheens at the water surface over a known area of the river bed and over a known time period to produce an estimate of the rate of NAPL migration. The Traditional Way Traditional solutions for remediating sediment contamination involve dredging to remove the contaminant mass in sediment, but they are not 100 percent effective in removing NAPL in sediment. Even one residual lens of NAPL left in the sediment can cause sheens to form for many years

5 after the dredging has been completed. The sheens that are formed contain hundreds of thousands of ppm of PAHs and present a clear threat to human and ecological receptors. To address the potential for ongoing migration of residual impacts, sediment caps are often placed over areas that have already been dredged. However, traditional sand caps do not prevent ebullition-driven migration of coal tar NAPL through the cap and into the overlying water column. Similarly, amended caps serve only to delay future release of NAPL and consequent sheen formation. And, because impermeable caps do not stop continuing bacterial biodegradation and gas formation, gas bubbles can build up under impermeable caps and either cause eventual uplift, resulting in a physical breach of the cap, or they migrate around the cap into the water column. A NAPL-trapping cap design offers an alternative solution to prevent NAPL migration and impact to water quality. A New Way To address all the shortcomings of traditional methods, scientists and engineers at our company engineered an approach that caps and controls contaminant release by managing the migration of gas bubbles (or ebullition). The cap creates a seal over the NAPL-contaminated area of the river or canal bed. The seal is combined with a gas venting layer to allow capture and release of naturally occurring gas bubbles emanating from the sediments. The cap allows the ebullition to occur, but the NAPLcarrying bubble mixture is channelled by the impermeable containment layer to flow through the transmission layer toward the accumulation zone. At this point the bubbles will pop and the NAPL is trapped. In most cases, the NAPL is then immobilized in a relatively inert condition. However, NAPL extraction processes can be employed to remove the accumulated NAPL, if required. Also, as illustrated in Figure 3, LNAPL can also be collected in the accumulation zone and removed as necessary. NAPL-trapping caps have been installed in three widely different settings to date, including Ann Arbor, Mich.; Bangor, Maine; and Ripon, Wis. The installation in Bangor, Maine of the former MGP site is noteworthy because construction occurred during tidal swings of more than 15 feet. It was demonstrated that precise cap geometries could be achieved under challenging flow and tidal conditions. Also, the cap was successfully constructed and sealed around an existing storm sewer discharge. In the Michigan installation, the cap was designed to integrate with the upland remedial design. This is a key feature because many MGP sites exhibit residual upland NAPL impacts that represent potentially continuing migration toward a water body. A NAPL-trapping sediment cap that addresses both in-water NAPL and potentially migrating upland residual can be a far more feasible and cost-effective remedy than attempts to hydraulically cut off upland NAPL. The Ripon site was constructed in a very small, shallow mill pond, demonstrating that these patented cap designs are adaptable to virtually any river/water body setting. The owners of these sites tout the NAPL-trapping cap solution for a variety of reasons: It is more effective in controlling the release of NAPL than dredging, which invariably is not 100 percent effective; other cap designs have finite lifespans, whereas this one is permanent; and when amortized over its lifetime, it is more cost effective. From Costly Problem to Elegant Solution In the 19th and 20th centuries, MGPs powered the United States into great growth, long before we understood the consequences of discharging coal tar residuals into water bodies. Coal tar in sediment has proven to be a more recalcitrant contaminant issue than at first realized. American ingenuity has only now devised an elegant remediation solution that is both certain and permanent. NAPL trapping cap designs are suited to a range of settings. About the Authors Thomas R. Stolzenburg, Ph.D. is a senior consultant for TRC Companies Inc. in the Madison, Wis., office. He has more than 30 years of experience in environmental consulting and five more years of experience in the research of environmental problems. John M/ Rice, P.E. is a consulting engineer and hydrologist for TRC Companies Inc. in in the Madison, Wis., office. He has more than 26 years of experience in the environment field. John provides technical expertise in surface water and groundwater hydrology, sediment and groundwater remediation. Layers of the NAPLtrapping cap under construction in Bangor, Maine. Reprinted with permission from Pollution Engineering, September 2014 issue