MGP DNAPL Mobility Assessment in Support of Reactive Cap Remedy Design



MGP DNAPL Mobility Assessment in Support of Reactive Cap Remedy Design

Presenter:  Sean  Carroll (Haley & Aldrich, Inc.)
Authors:  Sean M. Carroll (Haley & Aldrich, Inc.),  Marc B. Okin (NiSource, Inc.)


The setting of this project is a former MGP Site located in an historic, urban area adjacent to a flashy river with steep, eroding riverbanks.  A combined sewer has exacerbated erosion in a portion of the project area.  Coal tar has been detected in the river sediments, and causes an ebullition-driven sheen during warm weather, at times of low water.  Site characterization data indicate the coal tar migrated to the River from the MGP source area via subsurface sandy and gravelly layers.  An earlier ISS remediation solidified the bulk of the coal tar on the former MGP Site and a DNAPL cut-off trench was installed at the top of the riverbank; together these actions cut off the source of coal tar migration to the River.  However the thin, tar-impacted sandy seams remain within the riverbank itself.      As the river sediment remedy design efforts began, we sought to answer the following questions:

 1.  How much DNAPL remains within the sandy/gravelly seams adjacent to the River?
2.  What portion, if any, of the DNAPL may migrate to the River in the future?
3.  What is the appropriate approach – dredge, or cap?
4.  What sorption capacity is required for the selected capping remedy?
5.  Given the flashy nature of the River, when is the best time to install the remedy?

This presentation will summarize the results of a DNAPL extent and mobility assessment using a variety of tools including TarGOST®, pore fluid saturation and mobility testing, and 2-D and 3-D visualization tools.  The results provided an understanding of the relationship of the tar-impacted seams to the river, an estimate of the DNAPL mass remaining in the riverbank subsurface, demonstrated the limited potential for mobility, and provided the data needed for remedy design.


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