Sustainable remediation of LNAPL in fractured basalt

Since late 2007, URS Australia (URS), on behalf of a major oil company, has managed the continuing investigation and remediation of a petroleum fuels pipeline release site in Victoria. Sustainability principles and measures have been a major feature of the remediation. The release was initially believed to be short term and consisted of primarily unleaded petrol, jet fuel, and diesel fuel from a multi-product pipeline. The pipeline was promptly repaired and contaminated fill soil from the immediate vicinity of the release was removed and disposed of off site. One of the most significant challenges at this site is determining the extent and techniques for removal of light non-aqueous phase liquids (LNAPL) within regulatory mandates and time lines.

Remediation approach for LNAPL in fractured basalt

LNAPL is difficult to remove from fractured basalt because of geologic heterogeneity, poor recoverability using well systems, and high-residual capillary pressures in small fractures. For that reason, remediation of such sites tends to be empirically driven, with observed LNAPL recoverability and stability of extent being primary indicators of success. Current worldwide literature suggests that this type of remediation site is likely to have limited success in recovering a majority of the original release volume. Thus, one potential end point for LNAPL in fractured basalt in Victoria is Clean Up to the Extent Practicable (CUTEP), in accordance with overall regulatory requirements of EPA Victoria including native groundwater quality maintenance. Currently, the goal of the LNAPL remediation program at this site is demonstration of CUTEP, with objectives including:

• Maximise mass recovery of LNAPL as practicable using best-available technologies;
• Assure LNAPL stability by focus on residual LNAPL recoverability at the boundary of extent;
• Implementation of Groundwater Restricted Use Zone (GRUZ) as one element of a comprehensive site management plan, to restrict impacted groundwater use;
• Long-term monitoring to demonstrate achievement of clean-up goals and continued absence of human health or environmental risk; and
• Sustainable approach to all remediation activities.

Efforts leading to the remediation activities at the site have included: developing a conceptual site model for LNAPL (and dissolved phase derived from LNAPL), recoverability testing to quantify the potential for LNAPL mass recovery, LNAPL physical and chemical characteristics from laboratory analyses, understanding the role of LNAPL thickness in wells relative to remediation effectiveness, observations of LNAPL migration and stability as determined by site monitoring, and mass recovery statistics associated with various remediation technologies applied at the site. In total, these activities follow LNAPL remediation guidance developed in recent years by the American Petroleum Institute (API), Australian Institute of Petroleum (AIP), individual oil company research and demonstration projects, and university research around the world, focussing on the development of multiple lines of evidence to support LNAPL remediation end points.

Sustainable remediation techniques and successes

Given the extent of the petroleum product release at this site, it has been necessary to apply remediation technologies at significant size and capacity applications. For example, the LNAPL extraction and recovery well field currently consists of more than 60 individual wells, and the site monitoring network comprises approximately 100 total wells.

Typical LNAPL extraction well field.

Active remediation technologies include liquid LNAPL recovery and re-refining, as well as hydrocarbon vapour extraction and on-site catalytic oxidation. Other technologies employed at the site have included multiphase extraction (MPE), oil/water separation, groundwater treatment to remove volatiles, and on-site groundwater re-injection.

Recovered LNAPL storage awaiting offset re-refining.

Considering the necessity of large-scale remediation technology applications to achieve regulatory mandates and timelines for clean up at the site, remediation efforts have been focussed on sustainable remediation technologies and approaches. Sustainable remediation principles that have been adopted at this site include the following:

• Low-energy (solar-powered) LNAPL skimming technology use at selected sites;
• Passive LNAPL skimming technology use to recover residual LNAPL as remediation end points are approached;
• Recovery of LNAPL with re-refining and return to product specifications for beneficial re-use;
• Continuous tracking of electrical energy use at the site with a goal of minimising external power use;
• Carbon emission estimates to understand overall remediation sustainability;
• Net benefit analysis to determine when LNAPL remediation is no longer sustainable or economic, relative to use of resources and alternative environmental impacts;
• Natural mass loss estimation to determine sustainable long-term prospects for restoration of groundwater beneficial uses; and
• Development of long term site management plans incorporating sustainable property management approaches.

Solar-powered LNAPL skimming unit.

The results of remediation, to date, at the site include the following:

• LNAPL behaviour in fractured basalt at the site is similar to that at other sites with similar geology, including difficult remediation prospects for residual LNAPL using high-energy remediation technologies.
• LNAPL recovery has been most effective using soil vapour extraction (SVE) and vacuum-enhanced (VE) skimming technologies, versus multiphase extraction (MPE) that requires more complex oil/water separation technologies, groundwater treatment, and groundwater re-injection on site.
• Vacuum extraction of hydrocarbon light ends from LNAPL has significantly decreased the flux of light end components (eg, benzene) to adjacent groundwater, allowing for much more rapid natural attenuation of light ends dissolved in groundwater at the site.
• Recovered liquid LNAPL using skimming technology is easily re-refined and sold as commercial-grade petroleum products.
• Energy use (primarily for hydrocarbon vapour catalytic oxidation) has become proportionately greater per unit of recovered LNAPL as active remediation end points are reached, using empirical data from a large recovery well and monitoring network.
• As a result of decreasing LNAPL recovery efficiency and non-sustainable external energy use, active remediation has been stopped at the site in favour of passive remediation techniques, with the approval of regulatory authorities.
• LNAPL driving head from the original pipeline release source has dissipated with time after release, using sustainable remediation technologies, stabilising the remaining residual LNAPL and shrinking the extent of LNAPL impacts to soil and groundwater.
• Long-term property management and continued demonstration of no risks to human health or the environment are current drivers for site management and restoration.

As a result of the extensive sustainable remediation efforts at this site, CUTEP for LNAPL is expected to be achieved soon, after more than four years of sustainable remediation efforts.

URS Australia Pty Ltd

www.ap.urscorp.com/

*William Smith is a certified senior project manager for URS. He has been a US registered professional engineer for more than 30 years, practising in chemical and environmental engineering specialties, including 16 years with URS. His experience spans the full range of environmental consulting including planning, site investigation and remediation, risk assessment, process and engineering design, construction contracting, and facility operations and maintenance. In addition to his consulting career, Smith has also been employed as military petroleum fuels officer, chemical process engineer for DuPont Corporation and Dow Chemical Company, and US regulatory agency administrator with the Louisiana Department of Environmental Quality.