18-Env-B4 Site Assessment and Remediation · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Davis & Cornwell, Introduction to Environmental Engineering, 6th ed.; Freeze & Cherry, Groundwater, 1979; Fetter, Contaminant Hydrogeology, 2nd ed.; LaGrega, Buckingham & Evans, Hazardous Waste Management, 2nd ed.; Suthersan, Remediation Engineering: Design Concepts, 2nd ed.; Leeson & Hinchee (AFCEE), Principles and Practices of Bioventing, 1997; CSA Z768/Z769 (Phase I/II ESA); BC Environmental Management Act & Contaminated Sites Regulation.
The paper instructs candidates to answer any THREE of the FIVE questions in Section A and any TWO of the THREE questions in Section B. All eight questions are answered in full below, since this solution set is used as a complete study resource.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Gasoline is a light non-aqueous-phase liquid (LNAPL, specific gravity < 1) and a multi-component mixture, so it does not migrate or behave as a single substance once it leaves the tank — four distinct fate processes act on it simultaneously from the moment of release.
Immediately after release through a corrosion hole or failed joint, gasoline moves downward under gravity and capillary suction through the unsaturated (vadose) zone. Silty loam has fine, well-connected pores, so a meaningful fraction of the product is retained as residual (immobile) saturation along the way — typically several percent of the pore volume — while the remainder continues to advance. With the tank bottom at 2.5 m bgs and the water table roughly 1.5 m below that (≈ 4.0 m bgs), the product has about 1.5 m of vadose-zone travel before it reaches the capillary fringe. Because LNAPL is less dense than water, it cannot easily displace water in the saturated zone below; instead it spreads laterally across the capillary fringe, forming a floating “pancake” that can locally depress and mound the water table if enough volume has accumulated — a real possibility given the estimated 6,500 L loss.
From this smear zone, three parallel processes redistribute the gasoline. The lighter, more volatile fractions (including the BTEX aromatics) partition into soil vapour and diffuse upward through the vadose zone, creating a vapour-intrusion risk to any building foundation in their path — and because the station is connected to all municipal services, sewer and water-line trenches through this zone are preferential pathways that can carry vapour (and dissolved-phase contaminant) laterally well beyond the immediate source area, so the utility corridors specifically need to be investigated. The water-soluble fractions dissolve into infiltrating precipitation and into groundwater at the LNAPL/water interface, forming a dissolved-phase BTEX plume that migrates with the ambient groundwater flow direction and threatens any downgradient receptor, including the municipal water supply if it is drawn from the same aquifer. Simultaneously, the more hydrophobic compounds sorb onto the silty loam's organic matter and clay-silt fraction, retarding their own migration relative to groundwater and effectively immobilizing a portion of the mass close to the source — this sorbed mass becomes a long-term, slowly-desorbing source that will keep re-charging the dissolved plume for years unless it is specifically addressed.
Over time, aerobic biodegradation at the fringe of the dissolved plume (where oxygen is available) and evaporative weathering of the residual/free product both act to reduce the mass and shift its chemical signature, which is why natural attenuation is a legitimate long-term component of the eventual remedy, but not a substitute for delineating and, where recoverable, removing the free-product lens itself.
