Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6g. LNAPL Sites: Understanding and Managing Risks

Formatted Title:

Methodologies for Mobility Assessment and LNAPL Partitioning

Background/Objectives:

Light non-aqueous phase liquid (LNAPL) in subsurface environment presents challenges due to the soil heterogeneity of the environment and the diversity of conditions existing on the management of environmental liabilities, with its removal being limited at sites that have low hydraulic conductivity and transmissivity, with low efficiency of conventional remedial methods, therefore requiring permanence and coexistence with the product on site subsurface.

In this situation, it is necessary to evaluate potential risks to human health of on-site and off-site receptors, The main objective of this work is to propose two testing methodologies for the determination and evaluation, of the product retention curve in soil and partitioning tests for the dissolved and vapor phases. It is expected that the results to support the defense of the application of these methods, on evaluating the existence of a real risk to human health and the remedial/management action.

Approach/Activities:

This work proposes two methodologies. The first involves determining the fluid retention curve in the soil, while the second focuses on assessing the potential partitioning of LNAPL into groundwater and soil vapor to determine the maximum concentrations of product compounds in the free phase.

For the first methodology, a combination of two soil fluid extraction methods is proposed. The first method involves using a Büchner funnel, which operates at lower pressures, and the second involves using a Richard's chamber, which operates at higher pressures. To perform these methods, undisturbed soil samples must be collected to ensure the natural structure of the soil. These undisturbed samples should be subdivided into 6 test specimens and saturated with the contaminat proposed in the study. Subsequently, the test specimens are divided between the Büchner funnel and the Richard's chamber methodologies, where predetermined pressures are applied for a set period. After this process, the amount of fluid remaining in each sample is determined by mass difference, and the values are then calculated and plotted on the retention curve. The curve is divided into two axes, with the x-axis graduated in kPa and the y-axis graduated in the percentage of fluid present in the sample after pressure. This curve illustrates the fluid's behavior as the applied pressure on the soil increases or decreases.

For the second methodology, the sample is prepared for partitioning testing in a container (contact chamber) containing product and water. The sample is agitated and left to rest for a predetermined period for the collection and analysis of Semi-Volatile Organic Compounds (SVOC) (SW-846-USEPA 8270E chromatographic analysis), Total Petroleum Hydrocarbons (TPH) (SW-846-USEPA 8015C chromatographic analysis), and Volatile Organic Compounds (VOC) (SW-846-USEPA 8260D chromatographic analysis). Sampling should be performed with a peristaltic pump with reduced flow to remove water from the container without carrying the product.

Sampling for the partitioning test of the product into the gas phase should be conducted in two stages. The first stage is for determining the stabilization time and the maximum concentration of volatile compound partitioning into the gas phase in a closed system. After each incubation period, samples should be collected and injected into a gas chromatograph-mass spectrometer (USEPA 8260D) for qualitative and quantitative evaluation of VOC.

After obtaining the stabilization time and maximum concentration, the second stage of the test is performed using the USEPA TO-15 analytical methodology for the analysis of volatile compounds and the USEPA TO-15/MADEP APH for TPH analysis. Both sample analyses require the use of evacuated containers and closed systems. The stabilization period obtained in the previous stage is awaited before sampling the air. The sample analysis is conducted using a gas chromatograph coupled with a mass spectrometer (GC/MS)

Results/Lessons Learned:

In general, the results of all these tests will provide evidence for assessing potential risks to local and surrounding receptors in the area of interest for the relevant scenarios. These results will help determine the actual mobility of the product in the physical environment and potentially support the argument that the contaminant may not move over time given the physical and chemical conditions in the study area. This will also help to define the real partitioning of the residual phase into the vapor phase and the actual partitioned concentration. When compared with intervention values and maximum acceptable concentrations obtained through human health risk assessment, these results will indicate the actual risk to receptors and exposure pathways, providing the basis for future control, containment, or remediation actions. Therefore, it is also possible to state that the study can be used to consider less invasive and more sustainable measures, such as maintaining the retained phase in the environment, subject to natural biodegradation processes and periodic monitoring.

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Maiara Pillon

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