A BETTER PATHWAY TO CLOSURE
By Colin Plank, CPG, and Gene McLinn, PG
Many towns operated manufactured gas plants to generate gas used for lighting, heating and cooking. More than 3,000 shuttered sites remain, many with underground tanks that have leaked dense coal tar. As communities remediate sites, one of the biggest challenge has been figuring out where contamination has spread. Until now.
HOW WE DID IT
Burns & McDonnell conducted the bench-scale experiments in 6-gallon, glass-panel tanks that had been modified to include a wetting reservoir behind the model stratigraphy. Common stratigraphic scenarios were built using sediment in three grain sizes:
- silt (~0.02 mm to 0.062 mm)
- coarse to very coarse sands (0.5 mm to 2.0 mm)
- fine to coarse gravel (2.5 mm to 4.0 mm)
Once constructed, the wetting reservoir was filled with water, saturating the stratigraphy from the bottom up. While some of the stratigraphy was displaced during the wetting process, upward migration of silts was minimal.
After a brief period of equilibration, the water table was held static, and laboratory-grade coal tar with a kinematic viscosity of 500 cSk at 70°F and density of ~1.2 g/cm3 was inserted into the top inch of the site stratigraphy. A volume of NAPL was allowed to flow downward into the saturated stratigraphy.
Time-lapse photography shot over a 16-hour period documented the results.
Between the mid-1800s and early 1900s, thousands of communities constructed manufactured gas plants (MGPs) to bring residents the then-new modern conveniences of heating and lighting. A century later, utility operators are struggling to achieve closure of these sites, where coal tar residuals have leaked from aging underground storage tanks into ground and surface waters.
Digging up and removing the old tanks is the easy part. The bigger challenge is understanding where the contaminants have gone. It has historically been difficult to predict where or if they have migrated. As a result, many utilities and regulators face considerable uncertainty in remediating these subsurface contaminants.
To set and achieve remediation goals, they must first define the extent of contamination, estimate the volume of subsurface tar present, and evaluate potential Non-Aqueous Phase Liquid (NAPL) plume migration pathways and/or potential groundwater impacts, both on and off the MGP site. Understanding and conceptualizing the architecture of site porosity and permeability are key to making effective interpretations of subsurface conditions. Traditionally, that has required costly and often inadequate soil boring and water testing.
TAKING A PREDICTIVE APPROACH USING BENCH-SCALE DEMONSTRATIONS
But there is a faster, more economical alternative. By studying site geology and applying what is already known about how sedimentary processes have acted in similar geomorphic settings, we can draw a powerful predictive picture of subsurface conditions.
Although each site has unique characteristics, MGP owners can greatly reduce remediation uncertainty by understanding the stratigraphic characteristics of common depositional settings. Many former MGPs, for example, are located near rivers, estuaries and coastlines on sites chosen for the ready supply of water needed for the coal gas manufacturing process and transportation of raw materials. The sedimentary deposits in these environments are well understood in terms of typical dimensions, geometry, lateral and vertical grain size trends and stratigraphic connectivity.
To guide the remediation of MGP sites, we took a nod from the petroleum industry, which has studied and conceptualized multiphase fluid behavior for decades. In this case, environmental scientists conducted a series of bench-scale experiments to identify the impact of fluvial stratigraphic characteristics on plume geometry and subsurface migration.