Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

Not All Is as It Seems: Reinvestigating a Persistent Benzene Plume Using HRSC

Background/Objectives:

The site is located adjacent to an estuary in a heavy industrial complex in NSW, Australia. Investigations in 2013 found concentrations of benzene up to 800 mg/L in groundwater within 20 m of the waterway. The source of the contamination was not known but suspected to be from the loss or illegal discharge of waste solvent to the ground. Multi-phase extraction groundwater remediation was undertaken in 2015 and 2016. Biannual monitoring following remediation showed benzene concentrations initially reduced, but then rebounded with a peak concentration of 600 mg/L detected in 2022.

In response to the rebounding benzene concentrations, EPA required further investigation and remediation. Senversa was engaged to complete the further investigations, with the objectives to: a) assess the current extent of the benzene plume (which hadn’t been delineated by the prior investigations completed by others); b) refine the conceptual site model (CSM); and c) develop and implement a remedial strategy adequate and appropriate to address the remaining contamination.

Approach/Activities:

The Senversa investigations were completed across two stages, with the first stage conducted using high-resolution site characterisation (HRSC) methods utilizing a membrane interface pProbe (MIP) fitted with a hydraulic profiling tool (HPT). The MIP investigations were completed within, adjacent to, and below the known benzene plume. The locations and elevations of the MIP investigation points were surveyed to: allow direct comparison to the existing soil and groundwater quality data; assess and identify differences between the MIP responses and previous understanding of the plume extent; and guide the subsequent second stage of investigations – which consisted of installation and sampling of new monitoring wells to verify the MIP data and delineate the extent of contamination.

Results/Lessons Learned:

The MIP HRSC identified key differences in the understanding of the extent of the benzene plume at the site – including the lateral and vertical extent of the contamination. Specifically, the MIP responses showed that: the source of contamination was likely to be upgradient of the existing monitoring well network; the plume extended deeper than previously known; and the plume had advanced (within the deeper contamination zone) closer to the estuary than previously understood. The plume also appeared to exist in fill and underlying natural soils which have different hydraulic permeability characteristics. The results of the MIP were used to inform the positioning of new monitoring wells - which confirmed the data obtained from the MIP and successfully delineated the extent of contamination. The difficult conditions at the site (significant thickness of compacted slag fill and steeply sloping ground) meant the challenging and expensive process of installing permanent monitoring wells (using sonic methods) was also able to be optimized and made more cost-effective by the preceding MIP investigations.

The outcomes of the Senversa investigations identified that the previous understanding of the extent of the benzene plume was incomplete, and that the prior remedial efforts had failed since it only addressed a portion of the plume area - and importantly had not targeted the source area nor the deeper portions of the plume (in which some of the highest benzene concentrations exist). The results of the HRSC investigation have been used to update the previously inadequate CSM and are currently being utilized to develop a new remedial strategy, which is likely to include in situ oxidation (ISCO) in the source area and middle portions of the plume, and extraction and treatment at the leading edge of the plume (where injection approaches risk pushing the contamination closer the estuary).

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Michelle Agnew