23-Chem-B2 Environmental Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — May 2018. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.
Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.
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.
Unlike organic contaminants, heavy metals (e.g. mercury) cannot be destroyed — they can only be extracted, immobilized, or their mobility reduced. In-situ chemical stabilization/solidification (S/S) is a standard engineering method: reagent (e.g. Portland cement, or a sulfur-polymer/chelating reagent specifically for mercury) is injected and mixed through the contaminated zone to bind the metal into a low-permeability, low-leachability solid matrix, left in place.
| Step | Engineering description |
|---|---|
| 1. Site characterization and delineation | Define the vertical/lateral extent and concentration profile of contamination via soil borings/geoprobes, so the reagent-injection grid and dosing target only the contaminated volume (minimizing cost and reagent volume). |
| 2. Reagent selection, dosing and delivery design | Bench-scale treatability testing sets the reagent type and dose (e.g. % cement or chelating agent by dry soil mass) needed to meet the target leachate standard (e.g. TCLP); delivery is via auger-mixing, jet-grouting, or direct injection depending on soil permeability and depth. |
| 3. In-situ mixing/curing and verification | Reagent is homogeneously mixed through the treatment zone (auger or jet mixing to design depth), allowed to cure to develop the target strength/permeability, then confirmation cores are taken and leach-tested to verify the stabilized zone meets the regulatory criterion. |
| Aspect | Physical-chemical (e.g. soil washing/solvent extraction/thermal desorption) | Biological (e.g. bioventing/landfarming/composting) |
|---|---|---|
| Treatment time | Advantage: fast — typically days to weeks, since it is a physical separation or chemical-extraction process not limited by microbial growth rates. | Disadvantage: slow — typically months to years, limited by microbial degradation kinetics and seasonal temperature effects on biological activity. |
| Applicability/cost | Disadvantage: energy- and reagent-intensive (solvent/thermal energy costs), and may generate a secondary waste stream (spent solvent, residual sludge) requiring its own disposal. | Advantage: low capital/energy cost and no secondary hazardous waste stream — petroleum hydrocarbons are mineralized to CO2/H2O/biomass in place, well suited to large, dilute-contamination sites where speed is not critical. |
For a program comparing concentrations across seasons (e.g. a water or air contaminant sampled in winter vs. summer), the goal is to isolate the true seasonal signal from sampling/measurement noise:
| Source of error | Remedy |
|---|---|
| Temporal/diurnal variability being mistaken for seasonal variability — a single grab sample per season can be dominated by time-of-day effects (e.g. traffic-peak air concentrations, or diurnal stratification in a water body) rather than the true seasonal trend. | Use a statistically designed composite or multiple replicate samples spread across each 24-hour period (and across multiple days within each season) so the seasonal mean is not confounded with diurnal variance. |
| Sampling/handling bias — inconsistent sample collection depth/location, container material, or holding time between the two seasons introduces a systematic bias that mimics a seasonal difference that isn't real. | Use identical, documented standard operating procedures (fixed sampling location/depth, container type, preservation and maximum holding time) for every sampling event in both seasons, verified by field blanks. |
| Analytical/instrument drift — a change in instrument calibration, analyst, or laboratory between the two seasonal sampling rounds introduces a measurement bias unrelated to the environment. | Recalibrate against certified reference standards before each sampling round, run duplicate/spike samples and method blanks with every batch, and use the same laboratory/method for both seasons so any inter-round difference is attributable to the environment, not the measurement system. |
