Novel Approaches for Chlorobenzene Plume Delineation at a Nearshore Sediment-Bedrock Interface to Support CSM Development

Presented at Battelle Sediments, January 29, 2025, Tampa, Florida.

Background/Objectives 

Understanding contaminant flux to a nearshore sediment environment is important for source control and in-water sediment remedy design.  In Portland Harbor, a chlorobenzene plume was delineated in upland groundwater and neighboring sediments.  Pure-phase chlorobenzene is a dense non-aqueous phase liquid.  The initial conceptual site model (CSM) assumed dilute chlorobenzene in groundwater was migrating into the nearshore sediment environment.  Pre-remedial design investigations identified chlorobenzene at the base of the nearshore sediment on top of basalt bedrock.  This discovery required further investigation to ascertain whether chlorobenzene had penetrated to basalt interflow groundwater zones.  Additional investigation revealed that dissolved-phase chlorobenzene penetrates the bedrock at the sediment-basalt bedrock interface in weathered- or fractured-bedrock zones in some areas.  In other areas, chlorobenzene is contained on top of the basalt bedrock surface with no evidence of bedrock penetration. 

Approach/Activities 

Because the preliminary chlorobenzene CSM was incomplete, flexibility was required for subsequent phases of pre-remedial design investigation and investigations were redirected. Modifications included (1) delineating the lateral extent of chlorobenzene in sediment into the river channel, (2) conducting basalt coring, utilizing barge-mounted sonic drilling, to delineate the vertical extent of chlorobenzene, (3) modifying sonic drilling methods, utilizing water as the drilling fluid, to obtain rock core samples to better understand basalt bedrock quality/weathering/mineralization/fracture intervals, and (4) utilizing temporary pre-packed monitoring wells to collect groundwater samples, measure hydraulic head, conduct slug tests, and collect single-well recovery tests to better understand groundwater and contaminant flux from the basalt.

Results/Lessons Learned 

We present on the iterative investigation process that utilized newly collected sediment, bedrock solids, and groundwater chemistry data, and hydraulic parameter data to assess data gaps and refine the chlorobenzene CSM.  Drilling and sampling techniques were modified to support CSM revisions and sediment remedial design.  Lessons learned from drilling and installing temporary basalt bedrock monitoring wells in a nearshore environment will be shared.