18-Env-B4 Site Assessment and Remediation · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2019 — 18-Env-B4: Site Assessment and Remediation (3 hours, open book). Instructions: answer FOUR of the FIVE Section A questions and ONE of the TWO Section B questions; all eight questions (A-1–A-5, B-1, B-2) are answered in full below as a complete study resource. Each question is worth 20 marks.
Reference texts: Nyer, E.K., Practical Techniques for Groundwater and Soil Remediation; Fetter, C.W., Contaminant Hydrogeology (2nd ed.); Leeson, A. & Hinchee, R.E. (1997), Soil Bioventing: Principles and Practice; CSA Z768-01, Phase I Environmental Site Assessment; British Columbia Contaminated Sites Regulation (Environmental Management Act); Ontario Regulation 153/04 (Records of Site Condition); Canadian Council of Ministers of the Environment (CCME), National Classification System for Contaminated Sites and Canada-Wide Standard for Petroleum Hydrocarbons in Soil; Karickhoff, S.W. (1981), organic-carbon partitioning correlations.
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.
Implementation. Bioventing is implemented in four stages. (1) Site characterization and feasibility: confirm the impact is in the vadose (unsaturated) zone, that soil permeability supports air movement, and that indigenous hydrocarbon-degrading microbial populations are present and not nutrient/moisture-limited. (2) Pilot/respiration testing: a short-term air-injection or -extraction pilot test at a candidate well measures the in-situ oxygen utilization rate (O₂ depletion) and CO₂ generation rate at several radial monitoring points, which sizes both the radius of influence of a single well and confirms biodegradation (not just physical venting) is occurring. (3) Full-scale design: vertical vent wells are installed on a spacing set by the pilot-test radius of influence, screened across the impacted interval; a low-capacity regenerative blower supplies air at a rate set deliberately LOW relative to soil vapour extraction (SVE) — enough to maintain aerobic conditions (typically targeting several percent O₂ in the pore space) without stripping so much vapour that volatilization, rather than biodegradation, becomes the dominant removal mechanism. (4) Operation and monitoring: periodic O₂/CO₂ monitoring at multiple points confirms the aerobic zone is being maintained and biodegradation is progressing, with periodic soil sampling to track hydrocarbon concentration against the clean-up standard, continuing until the target is met.
Why it is effective. Bioventing delivers oxygen — the rate-limiting ingredient for aerobic biodegradation — directly to the indigenous microbial population already present in the impacted soil, rather than relying on natural diffusion, which is far too slow to meet a clean-up timeline. Because the air-flow rate is deliberately kept low, the dominant removal mechanism is in-situ biological destruction of the hydrocarbons (permanent, on-site mineralization to CO₂ and water) rather than physical mass transfer of contaminant to vapour requiring separate off-gas treatment, as SVE typically requires. This makes bioventing comparatively low-cost and low-energy, minimally disruptive to the site (no excavation, minimal infrastructure), and effective on the weathered, less-volatile fractions of a hydrocarbon plume that SVE alone struggles to remove once the more volatile fractions are depleted — a good fit for reducing a hydrocarbon-impacted soil below a numeric clean-up standard without the cost and disruption of excavation.
