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.
Because A-3 established that this release left both a vadose-zone source (residual/sorbed and free-phase gasoline above the water table) and a dissolved BTEX plume in groundwater, the ideal remedy has to treat both compartments — and treat the source first, because leaving it in place will keep re-charging any plume management effort indefinitely.
The first stage is free-product recovery. If a measurable LNAPL thickness is confirmed at the capillary fringe, dual-phase extraction (simultaneous vacuum-enhanced liquid and vapour recovery) is preferred over passive skimming here specifically because silty loam's lower permeability limits how fast product can flow to a well under gravity alone; applying vacuum overcomes that limitation. The second, concurrent stage targets the remaining vadose-zone source: soil vapour extraction (SVE) removes the volatile BTEX fraction, and because SVE typically leaves the heavier, less-volatile residual behind, it is paired with bioventing — injecting a low-rate air flow to sustain aerobic biodegradation of what SVE cannot volatilize (the same technique examined quantitatively in Question B-2). SVE well spacing needs to be tighter than it would be in a clean sand, again because of the silty loam's lower air permeability.
For the saturated zone, a hydraulic containment/pump-and-treat system is warranted immediately given that municipal groundwater is drawn from this aquifer (A-3) — the priority is capturing the plume before it reaches any water-supply well, not necessarily achieving full aquifer restoration with pump-and-treat alone (a technology well documented to plateau once the readily-mobile dissolved mass is removed). Once the plume's advance is controlled and the source has been substantially reduced by the vadose-zone measures above, the trailing edge of contamination is a good candidate for monitored natural attenuation (MNA), since BTEX is well-documented to biodegrade aerobically and anaerobically; MNA is backed by a long-term monitoring well network with clearly defined performance and contingency triggers, not adopted as a default.
Because the site is connected to municipal services, utility trenches identified in A-3 as preferential pathways are specifically assessed and, where they cross the plume, sealed or otherwise managed; and if any future occupied structure is planned on or near the source area, a vapour-intrusion mitigation system (sub-slab depressurization or an equivalent barrier) is included as a standing risk-management measure. The overall remedy is sequenced — product recovery and vadose-zone treatment first, hydraulic plume containment throughout, MNA as the polishing step — and driven toward regulatory closure under the BC Contaminated Sites Regulation, with a Certificate of Compliance (or an approved risk-managed closure with an institutional control) as the end point.
