18-Env-B4 Site Assessment and Remediation · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams; December 2017 — 04-Env-B4 / Site Assessment and Remediation. 3 hours duration; open-book exam (Casio or Sharp approved calculator only). The paper is split into Section A (five questions, candidates asked to answer four) and Section B (two questions, candidates asked to answer one), each question worth 20 marks. All seven required questions plus the second Section B option are solved below for completeness — eight questions in total.
Reference texts. Suthersan & Payne, Remediation Engineering: Design Concepts (CRC Press); Freeze & Cherry, Groundwater; Schwarzenbach, Gschwend & Imboden, Environmental Organic Chemistry; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); American Petroleum Institute (API) publications on fuel-release site assessment and UST modelling; ASTM E1527 Standard Practice for Phase I Environmental Site Assessments and ASTM E1903 Standard Practice for Phase II ESA; Ontario Reg. 153/04 under the Environmental Protection Act (Record of Site Condition regime) and O.Reg. 406/19 (excess soil management); Transportation of Dangerous Goods Act/Regulations (Canada); CCME Canadian Environmental Quality Guidelines.
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.
Two site features drive this assessment: the taxiway grades directly toward the grass infield, giving surface flow a defined, short (10 m) route to the ditch before any subsurface process even begins; and loamy sand is a relatively coarse, well-drained texture, so once kerosene leaves the paved surface it infiltrates quickly rather than pooling. Kerosene (a middle-distillate fuel, roughly C₉–C₁₆) is denser in the light-non-aqueous sense — it floats on water (LNAPL) — but is less volatile and less soluble than gasoline, so its dominant transport mode here is advective flow along the ground surface and infiltration, not vapour-phase spread.
Migration pathways — fuel. (1) Overland flow along the taxiway's drainage grade directly toward the infield and ditch — the fastest pathway and the one of most immediate concern given the short 10 m distance. (2) Vertical infiltration into the loamy sand at the release point and along the flow path, forming a residual/pooled LNAPL body that can continue to migrate vertically toward the water table if the volume and soil permeability allow, and laterally by capillary spreading once it reaches a lower-permeability lens or the capillary fringe. (3) Vapour-phase migration in the unsaturated zone immediately around the pooled product, though limited in extent given kerosene's lower volatility relative to gasoline. (4) Once/if any fuel reaches groundwater, dissolved-phase transport of the more soluble aromatic fraction (BTEX) with the ambient flow direction.
Migration pathways — fire-suppression materials. Water and firefighting foam add their own volume to the surface-flow pathway, effectively increasing the total liquid volume moving toward the ditch and diluting/emulsifying some of the fuel, which can make it more mobile in the aqueous phase even as it reduces free-product volume. Class B aqueous film-forming foam (AFFF) historically contains per- and polyfluoroalkyl substances (PFAS), which are highly water-soluble, chemically persistent, and not effectively retained by loamy sand or captured by a hydrocarbon sorbent — PFAS is therefore a second, independent contaminant pathway to the ditch and to groundwater that must not be conflated with the kerosene assessment. The chemical sorbent, by contrast, is applied specifically to immobilize free-phase product at the surface and, if promptly recovered, reduces (rather than adds to) the overland pathway.
Risk rating to the drainage ditch. Given (i) a defined, short (10 m) overland drainage path directly connecting the release point to the ditch, (ii) a coarse, permeable soil that does little to retard surface flow, and (iii) a large release volume (20,000 L), the surface-water pathway risk is rated HIGH in the absence of mitigation, and remains at least moderate even after sorbent/foam response, because sorbent addresses free product but does not reliably intercept the dissolved/emulsified fraction carried by the water and foam runoff. The rating should be confirmed, not assumed, by the sampling program below; a fast, effective first-responder sorbent boom placed between the spill and the ditch is the single factor most likely to move the actual outcome from "high" toward "moderate."
| Element | Assessment |
|---|---|
| Fuel pathways | Overland flow to ditch; vertical infiltration/LNAPL pooling; limited vapour phase; dissolved BTEX if groundwater reached |
| Foam/sorbent pathway | PFAS in AFFF — independent, highly mobile pathway to ditch/groundwater; sorbent reduces free-product mobility if recovered promptly |
| Ditch risk rating | High (unmitigated) to moderate (post-response), pending confirmatory sampling |
| Sampling type | Biased grid + step-out along the drainage path, staged depths, TPH/BTEX/PAH + PFAS, field PID screening, ditch sediment/water |
Sampling program. Use a biased grid centred on the truck/tank rupture point with step-out sampling points following the drainage grade toward the ditch, at three depth horizons: surface/near-surface (0–0.3 m, captures the free-phase pool and sorbent-treated zone), an intermediate vadose-zone interval, and, if warranted by field screening, a sample near the water table. A transect of at least three points between the taxiway and the ditch delineates the overland/shallow-infiltration pathway, with a confirmation sample at the ditch bank and both a sediment and a surface-water grab sample in the ditch itself (upstream/background and downstream of the inflow point). Field photoionization-detector (PID) headspace screening triages the fixed laboratory sample set. Laboratory analysis: TPH (kerosene/diesel range organics), BTEX, and PAHs at every location; PFAS specifically at the foam-application footprint and in the ditch water/sediment, since this analyte suite is easily missed if the assessment is scoped only around "the fuel."
