Formatted Title:

Positive Application of ASTM Guidance for the Occurrence and Mobility of NAPL in Sediment

Background/Objectives:

Identifying the occurrence and mobility of non-aqueous phase liquid (NAPL) in sediments is critical to developing a NAPL conceptual site model that can be used to effectively support remedial investigations. NAPL in sediments can be difficult to discern in the field due to factors such as moisture content, color, lighting, and sediment consistency. The use of various subjective terms or non-standardized nomenclature to describe NAPL occurrence adds to the challenge of data interpretation. ASTM E3281-21a provides a standardized approach for field characterization of NAPL occurrence and defines terminology for describing field observations with consistency. Information obtained through the systematic application of ASTM3281-21a is used for selecting samples for laboratory analyses to support NAPL mobility interpretations. ASTM 3282-22 provides applicable laboratory and data interpretation methods to evaluate mobility and migration of NAPL in sediments. These ASTM standards were used to guide the characterization of NAPL occurrence and mobility in sediments within an industrial waterbody.

Approach/Activities:

Investigations were conducted in 2022 and 2023 (shortly after publication of ASTM E3281-21a) to evaluate NAPL occurrence and mobility in a waterbody at a petrochemical facility. Field methods included the collection of two sediment cores at 27 locations using boat-mounted direct-push technology. One core was collected for field characterization including Unified Soil Classification System (USCS), visual classification of the occurrence of NAPL and sheen as described in ASTM E3281-21a, and photoionization detector screening. Samples for petroleum hydrocarbon analysis were collected from this core. The second core at each location was collected directly adjacent to the initial core. The core was maintained in its original vertical orientation to preserve the distribution of fluids and sediment and cut orthogonally into 9.6-inch segments. The core segments were secured vertically in a cooler and chilled with blue ice packs (cores were not frozen), then shipped overnight to a laboratory. All core segments (regardless of field screening results) were slabbed and photographed under white and ultraviolet light. Photography was used to support selection of additional sample intervals for petroleum hydrocarbon analysis and for centrifuging. Centrifuging was conducted at 25 times the acceleration of gravity (25g) for 10 hours to evaluate potential NAPL mobility, including calculation of NAPL effective hydraulic conductivity (K_n). NAPL saturation pre- and post-centrifuge, expressions of NAPL during centrifuge, and K_n were used in a weighted scoring matrix to interpret NAPL mobility within sediments tested.

Results/Lessons Learned:

ASTM methods utilized during the investigation allowed for a thorough, consistent, and effective characterization of NAPL in sediments. Sediments were typically black (Munsell color 7.5YR 2/1), non-plastic silt (to a depth of 15 feet or less) with petroleum hydrocarbons ranging from non-detect to >100,000 mg/kg. In this black, fine-grained matrix, visual observation of NAPL was difficult unless highly impacted (e.g., NAPL-saturated). Ultraviolet light photography proved to be an effective method for direct observation of NAPL occurrence. EPH fractions and associated fluorescence color (e.g., violet/blue versus white/cream) allowed for differentiation between hydrocarbon sources and past terrestrial activities. NAPL within the waterbody sediments was determined to be immobile based on the application of the ASTM methods to 48 samples (maximum K_n = 1.4 × 10^-8 cm/s). The use of a standardized approach, as outlined in the ASTM guidance, resulted in consistent field descriptions among various field personnel, and provided a reliable weight of evidence to assess NAPL occurrence and mobility within sediments at the site. ASTM methods utilized during the investigation allowed for a thorough, consistent, and effective characterization of NAPL in sediments.

Dan Canavan

Affiliation

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(Group 2, Poster Board #43) Positive Application of ASTM Guidance for the Occurrence and Mobility of NAPL in Sediment