Question 5 of 5: Conceptual Site Models, Bioremediation Suitability, and Managing Residual Gasoline

Question 5: Conceptual Site Models, Bioremediation Suitability, and Managing Residual Gasoline (equal value)

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.

Part (a) — three principal components of a conceptual site model (CSM). (1) Source characterization — the nature, location, mass, and history of the contaminant release(s): what was spilled, when and for how long, whether NAPL is still present, and its physical/chemical properties (solubility, density relative to water, volatility, degradability). (2) Geologic and hydrogeologic setting — the stratigraphy, soil/aquifer properties (porosity, hydraulic conductivity, organic carbon content), the direction and rate of groundwater flow, recharge/discharge relationships, and the presence of preferential pathways (fractures, utility corridors, coarse lenses) that control how contaminants move once released. (3) Contaminant fate, transport, and exposure pathways — the plume's current extent and expected future migration (advection, dispersion, sorption, degradation), and the complete exposure pathway(s) linking the source to potential human or ecological receptors (drinking-water wells, vapour intrusion into buildings, surface-water discharge). A CSM ties these three together into a single evolving picture that identifies what data are still needed and where risk actually lies, and is revised as new site data (borehole logs, monitoring results) come in.

Part (b) — bioremediation suitability and optimum conditions. In-situ bioremediation is best suited to contaminants that are biologically degradable and reasonably bioavailable — petroleum hydrocarbons (BTEX, straight-chain and lightly-branched alkanes), and, under the right redox conditions, many chlorinated solvents via reductive dechlorination or co-metabolism. It is poorly suited to highly recalcitrant or non-biodegradable contaminants (heavy metals, which cannot be destroyed biologically, only immobilized or bioaccumulated/removed; and to a lesser extent highly chlorinated, highly branched, or very high-molecular-weight compounds that resist microbial attack). Optimum conditions for in-situ bioremediation generally require: (i) an indigenous (or introduced/acclimated) microbial population capable of degrading the target compound; (ii) an adequate and continuously-resupplied electron acceptor (oxygen for aerobic degradation of most petroleum hydrocarbons, or nitrate/sulfate/Fe(III) for anaerobic pathways); (iii) sufficient nutrients (nitrogen, phosphorus) in bioavailable form; (iv) a favourable temperature and pH range for microbial activity (roughly neutral pH, above-freezing temperatures); and (v) sufficient hydraulic conductivity/permeability for amendments (oxygen, nutrients) to actually reach the contaminated zone — a tight, low-permeability aquitard starves the treatment zone regardless of how favourable the biochemistry is.

Part (c)(i) — problems from leaving residual (post-recovery) gasoline in place. The residually-saturated fraction (gasoline trapped by capillary forces in soil pores after free-product recovery, roughly 60,000 L here) is not immobile forever: it acts as a long-term, slowly-depleting source that continuously feeds several ongoing problems. (1) It continues to dissolve into passing groundwater, sustaining a dissolved-phase plume that can persist for decades (as the Q2(d)-style mass balance shows, only a tiny fraction of NAPL mass dissolves per pore-volume flush, so the source can outlast any reasonable monitoring period). (2) Volatile components partition into soil gas and can migrate to enclosed spaces, creating a vapour-intrusion risk to any current or future buildings above or downgradient of the residual zone. (3) The residual NAPL itself represents an ongoing regulatory/liability exposure and constrains future land use/redevelopment (excavation, foundation work, or utility trenching through the residual zone can re-mobilize NAPL or expose workers). (4) Natural attenuation processes (dissolution, volatilization, biodegradation) are slow relative to the mass present, so "walking away" typically means the source and its downgradient plume remain a de facto contamination problem for a very long time unless actively managed.

Part (c)(ii) — feasible alternatives for removing the remaining residual volume. Because residual saturation, by definition, cannot be recovered as free product by pumping alone, further reduction requires methods that either mobilize it or destroy/remove it in place: (1) Soil vapour extraction (SVE) — applies a vacuum to the vadose zone to volatilize and remove the more volatile gasoline components, effective in permeable, unsaturated soils. (2) Air sparging (often paired with SVE) — injects air below the water table to strip volatile compounds from the saturated zone and also stimulates aerobic biodegradation of the residual mass. (3) Enhanced in-situ bioremediation — oxygen/nutrient injection (or slow-release oxygen compounds) to accelerate natural biodegradation of the residual hydrocarbons, well suited to this contaminant class per part (b). (4) Surfactant- or cosolvent-flushing — injects a surfactant/cosolvent solution to lower interfacial tension and mobilize residual NAPL toward extraction wells, at the cost of added complexity and the need to manage the flushing fluid itself. (5) Excavation of the residual-zone soil where it is shallow enough and site access permits, followed by ex-situ treatment or disposal — often the fastest and most certain option but limited by depth, footprint, and cost, exactly the trade-off flagged in Q2's CSM/remediation-selection logic. (6) Monitored natural attenuation (MNA) as a long-term polishing step once active measures have reduced the mass loading, appropriate only where the residual is not an immediate risk to receptors and attenuation can be demonstrated and monitored. In practice, an industrial-scale spill of this size is typically addressed with a combination — SVE/air sparging or bioremediation to reduce the bulk of the residual mass, with MNA for the long tail.