23-CS-3 Sustainability, Engineering and the Environment · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
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.
Risk = likelihood × consequence. A coal plant emits continuously, so the likelihood of exposure is high and the consequence per exposure low–medium (chronic, population-wide harm). A nuclear plant releases little normally, so the likelihood of a harmful release is low but the consequence of a major accident would be high. Coal is thus a high-likelihood/low-consequence chronic risk (causing more routine harm), while nuclear is a low-likelihood/high-consequence catastrophic risk.
LD50: the dose lethal to 50% of a test population—a measure of acute toxicity. Hazard: something with the potential to cause harm. Exposure: contact between a person and a hazard (the amount, frequency, and duration of contact). Threshold: the dose below which no adverse effect occurs (for non-carcinogens). Uncertainty factor (UF): a safety factor (e.g. 10× for extrapolating from animals to humans, and another for sensitive subpopulations) applied when deriving a reference dose to account for data gaps and variability. Environmental justice: the principle that environmental burdens and benefits should be distributed fairly, so that no group—especially disadvantaged communities—bears a disproportionate share of pollution or risk.
At the source: recognize the chemistry—iron-sulfide scale reacts with sulfuric acid to release H₂S—and eliminate or substitute the hazard by using a different cleaning method or chemistry that does not liberate H₂S, or pre-treating/neutralizing the scale so the reaction cannot occur. Along the path: conduct the acid cleaning under local exhaust ventilation or in an enclosure with gas scrubbing, with continuous H₂S atmospheric monitoring, so any released gas is captured and detected before it reaches workers. At the worker: inform, instruct, and train the workers about the H₂S hazard (the specific failure here) and provide appropriate respiratory protection (supplied air/SCBA) and an emergency procedure. Best action: the source control—anticipating the reaction and preventing H₂S generation—is best, because it removes the hazard entirely; however, this case turned on a basic information/training failure (the workers were never told they might be exposed to H₂S), so ensuring hazard communication was the critical missing safeguard. Ideally source control is combined with proper hazard information, since PPE and monitoring alone did not prevent the exposure.
Work backward from the target risk. Required lifetime-averaged dose:
LADD relates to concentration by $\text{LADD} = \dfrac{C\cdot IR}{BW}\cdot\dfrac{EF\cdot ED}{AT}$. The paper gives no lifetime, so the standard cancer averaging time of a 70-year lifetime is assumed (AT = 365 × 70 days):
An arsenic concentration of about 8 µg/L would give this woman a 10⁻⁴ lifetime cancer risk—notably close to the 10 µg/L drinking-water standard.
The paper says "as described in part c", but the exposure is described in part (d), so that is the exposure used here: the same woman, 2.0 L/day, at the concentration found in (d). The chronic daily dose during exposure at C = 8.1×10⁻³ mg/L:
Since $HQ \approx 1.08 > 1$, the exposure slightly exceeds the reference dose and is not considered safe for non-carcinogenic (nerve-damage) effects. (If the dose is instead averaged over the whole exposure period, multiplying by 350/365 as in strict RAGS practice, HQ = 1.04, still greater than 1, so the verdict is the same.) Notably, at the concentration giving a 10⁻⁴ cancer risk, the non-cancer hazard is also marginally exceeded—both endpoints signal concern at this level.
