Question 1 of 7: PCE Spill – Migration Pathways and Site Assessment

Question A-1: PCE Spill – Migration Pathways and Site Assessment (20 marks)

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.

Roughly 1,000 L of PCE (10% of the release, since sorbent recovery captured ~90%) remains available to migrate. PCE is a dense non-aqueous phase liquid (DNAPL) — density ~1.62 g/mL, well above water — and this single fact governs every pathway below.

Migration pathways. (1) Overland flow: free-phase and dissolved PCE on the gravel surface can be mobilized by precipitation and follow the lot's slope directly toward the ditch, the fastest and most exam-relevant pathway given the short 20 m distance. (2) Vertical infiltration through the vadose zone: gravel fill over silty loam gives moderate-to-low permeability but is not impermeable; PCE will infiltrate downward under gravity, partly volatilizing and partly sorbing to the loam's organic/clay fraction along the way. (3) DNAPL sinking past the water table: unlike a fuel LNAPL, any PCE that reaches the saturated zone continues to sink under its own density, pooling on the first low-permeability lens or bedrock surface and creating a persistent, difficult-to-locate source zone below the water table — this is the pathway most likely to be missed if the investigation stops at the water table. (4) Vapour-phase diffusion: PCE is volatile; vapours can migrate laterally through the vadose zone air-filled pore space independent of the liquid front, raising a vapour-intrusion concern for any nearby structures. (5) Preferential pathways: the gravel lot itself, buried utility trenches/bedding at a chemical transfer station, and any nearby storm drains are all higher-permeability conduits that can short-circuit the "expected" diffuse infiltration pattern and route PCE toward the ditch faster than the silty loam matrix alone would suggest.

Risk of reaching the ditch. The risk is assessed as moderate-to-high in the short term via the overland-flow pathway (short distance, favourable slope, an uncontained gravel surface) and low-to-moderate but persistent via the subsurface pathway (silty loam retards but does not stop infiltration, and any preferential pathway such as a utility trench bypasses the retardation the soil type would otherwise provide). The ditch should be treated as a credible receptor requiring immediate confirmation sampling, not a theoretical one.

Sampling and assessment. Because this is a known, confirmed release (not a suspected historical one), the investigation should proceed directly to a targeted Phase II / detailed site investigation rather than a desktop Phase I — a Phase I ESA (CSA Z768) is for establishing whether contamination is likely, which is already established here. Sampling should include: (a) a source-to-ditch soil transect along the slope (surface and shallow subsurface, biased toward the observed flow path), (b) soil borings at and downgradient of the release point advanced through the vadose zone to the water table (and beyond, given the DNAPL-sinking concern), with continuous PID field screening to locate the sorbed/pooled mass, (c) ditch sediment and surface-water grab samples to confirm whether the ditch has already been impacted, and (d) soil-vapour samples if any occupied structure lies within the vapour-intrusion screening distance. Samples are required — both to delineate the lateral and vertical extent of the residual 1,000 L and to confirm (or rule out) that the ditch has been impacted, which drives any reporting obligation under the applicable provincial contaminated-sites regime (e.g., BC's Environmental Management Act / Contaminated Sites Regulation spill-reporting requirements).