Formatted Title:

Incremental Investigation Steps to Evaluate Sediment Remediation at a Typical MGP Site

Background/Objectives:

The purpose of this presentation is to describe the incremental approach for evaluating sediment remediation requirements at a typical MGP site. A case may be made to leave some affected sediment in place if it is located beneath the biologically active zone and immobile.

The mechanisms by which contaminants in deeper sediment may mobilize involve various migration pathways, including ebullition, sediment migration, groundwater transport, and NAPL migration. Each of these mechanisms could result in further migration, deposition, and exposure to MGP affected sediments. Therefore, the consideration of each of these potential pathways is vital in determining the appropriate remediation approach.

This presentation will describe the typical investigation steps that would be implemented to evaluate these potential migration pathways. The focus of this presentation will be on the optimal order of each investigation to maximize efficiency and cost effectiveness.

Approach/Activities:

Several of these investigations are substantially more expensive than others, and therefore should be initiated after other less expensive investigations/evaluations have been completed. The optimal order of investigations is:

Ebullition facilitated NAPL flux
Sediment stability
Porewater bioavailability
Groundwater migration and transport through the sediment matrix
Advective NAPL mobility.

An evaluation of ebullition facilitated NAPL flux is a cost-effective first step, because it is limited to a visual survey of bubbles and/or sheens on the water surface when conditions at the site are most conducive to gas ebullition.

Sediment transport modeling to evaluate the potential for sediment scour is the next appropriate step because it is generally less expensive than subsequent evaluations and is a critical recontamination mechanism.

Next, the potential for the migration of dissolved organic contaminants through porewater should be evaluated through a porewater bioavailability and mobility evaluation. The porewater analysis is less costly than analysis of the undisturbed cores for advective NAPL mobility, so it is advantageous to rule out this pathway first.

Once other potential mechanisms for transport of the deeper affected sediment have been ruled out, a NAPL mobility investigation should be performed to evaluate if NAPL has the potential to migrate from the sediment to the surface water. Due to the cost and time required to perform the NAPL migration investigation, this investigation should be delayed until the other pathways described above are confirmed to be incomplete.

Results/Lessons Learned:

While the incremental approach described in this abstract is not prescriptive for all MGP sites with impacted sediment, the information provided is useful for utilities, consultants, and regulators to apply at their own sites to perform these evaluations in an optimal order.

Ryan Hoffman

Affiliation

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(Group 2, Poster Board #45) Incremental Investigation Steps to Evaluate Sediment Remediation at a Typical MGP Site