18-Env-B3 Contaminant Transport · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 04-Env-B3 / Contaminant Transport. 3 hours duration; closed-book exam (any non-communicating calculator permitted). Five problems are printed, each worth 25 marks; per the exam’s own Note 3, only the first four as they appear in the answer book constitute a complete marked paper, and Note 5 states that the sub-parts (a)–(d) of each problem can be treated independently. All five problems are solved below for completeness.
Reference texts. Freeze & Cherry, Groundwater; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd 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.
Plume rise, Δh, is the additional height a stack’s exhaust plume rises above the physical top of the stack, before it levels off and begins travelling essentially horizontally with the wind. It occurs because the exiting gas carries its own upward momentum (from its exit velocity) and is usually buoyant (hotter, and therefore less dense, than the surrounding ambient air). The effective stack (release) height used in dispersion calculations and models is therefore H = hs + Δh, the physical stack height plus the plume rise, not the physical stack height alone.
The greatest impacts on plume rise come from: (1) the stack exit velocity / volumetric flow rate of the gas, which drives momentum-dominated rise immediately at the stack tip; (2) buoyancy — the temperature (density) difference between the stack gas and the ambient air, and the associated heat emission rate, which dominates rise once the plume is a short distance downwind; and (3) wind speed and atmospheric stability, both of which work against rise — a higher wind speed tilts and dilutes the plume faster, and a more stable atmosphere resists the vertical displacement, so both reduce the final plume rise achieved.
Given. CO air-quality standard = 9.0 ppm (by volume, 8-h average); conditions 1 atm, 25 °C (298.15 K); molar mass of CO, MW = 28 g/mol.
Find. The equivalent concentration expressed as (i) a percentage by volume; (ii) mg/m³.
Approach. ppm(v/v) converts directly to %v/v by dividing by 10,000; converting to a mass concentration uses the ideal-gas molar volume evaluated at the stated temperature and pressure (not the 22.4 L/mol STP value, which applies at 0 °C).
| Quantity | Value |
|---|---|
| CO standard, %v/v | 9.0×10−4% (0.0009%) |
| CO standard, mg/m³ (25 °C, 1 atm) | 10.3 mg/m³ |
Biodegradation is the microbially-mediated breakdown of an organic contaminant into simpler compounds by naturally occurring bacteria (and, to a lesser extent, fungi), which use the contaminant as a carbon and/or energy source (metabolism) or transform it incidentally while metabolising another substrate (co-metabolism). Under aerobic conditions, biodegradation ultimately converts the organic compound to CO2, water, and new microbial biomass; under anaerobic conditions, it proceeds via alternate electron acceptors — sequentially nitrate, manganese/iron oxides, sulphate, and finally CO2 itself (methanogenesis) — producing CH4/CO2 and reduced by-products instead. In groundwater specifically, biodegradation is typically slower than in surface water because dissolved-oxygen concentrations and microbial biomass/diversity are both lower in the subsurface; a common field signature downgradient of a contaminant plume is sequential redox zonation, as each electron acceptor in turn is depleted closest to the source and the next one takes over further downgradient.
