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18-Env-B4 Site Assessment and Remediation · December 2019

Question 7 of 7: Gasoline Tank Leak – Release Mass and Bioventing Air Exchanges

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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.

Section A — answer FOUR of FIVE (all five answered)

Question B-2: Gasoline Tank Leak – Release Mass and Bioventing Air Exchanges (20 marks)

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.

Check: the paper gives a pile bulk density of 755 kg/m³ and a gravimetric water content of 25% – both taken as printed. 755 kg/m³ is on the low side for a soil pile (typical excavated/loosened soil runs 1200–1400 kg/m³), consistent with a freshly excavated, uncompacted stockpile; solved as printed.

Excavated hydrocarbon-impacted soil pile (300 m³, 800 mg/kg avg.)Perforated vent wellBlowerLow-rate air delivery → aerobic biodegradation zoneO₂ / CO₂ monitoring pointBioventing Schematic — excavated pile treatment
Bioventing of the excavated pile via a buried, perforated pipe network and a low-flow blower — the same principle as Question A-4, sized here with the pile's own O&sub2; demand.

Given.

QuantitySymbolValue
Excavated pile volumeV300 m³
Average soil concentrationCs800 mg/kg
Pile bulk densityρ755 kg/m³
Gravimetric water contentw0.25
Porosityn0.35
Gasoline formula (given)—C₈H₁₈
TemperatureT20°C (293.15 K)
Target O₂-utilization efficiencyη50%

Find. (i) The total mass of gasoline released; (ii) the number of pore-air-volume "exchanges" of air that must be delivered through the pile to meet its stoichiometric O₂ demand for aerobic biodegradation, at 50% O₂-utilization efficiency.

Approach. (i) Recover total gasoline mass from the pile's own bulk density, volume, and average concentration. (ii) Convert that mass to a stoichiometric O₂ demand via the C₈H₁₈ combustion/oxidation balance, scale up for the stated 50% utilization efficiency, convert the required O₂ mass to an air volume via the ideal gas law and air's O₂ fraction, and divide by the pile's own air-filled pore volume.

  1. Part (i) — soil mass in the pile. $$M_{soil} = \rho V = 755\times300 = 226{,}500\ \text{kg}$$
  2. Part (i) — gasoline mass. $$\boxed{m_{gas} = C_s\times M_{soil} = 800\ \text{mg/kg}\times226{,}500\ \text{kg} = 1.812\times10^{8}\ \text{mg} = 181.2\ \text{kg}}$$ Using an assumed gasoline specific gravity of 0.74 (not exam-given, flagged as illustrative only), this is equivalent to about 245 L of product.
  3. Part (ii) — theoretical O₂ demand. Complete aerobic oxidation: $$\text{C}_8\text{H}_{18} + 12.5\,\text{O}_2 \longrightarrow 8\,\text{CO}_2 + 9\,\text{H}_2\text{O}$$ With M(C₈H₁₈)=114.23 g/mol and M(O₂)=32.0 g/mol, $$\frac{m_{O_2}}{m_{gas}} = \frac{12.5\times32.0}{114.23} = 3.50\ \text{kg O}_2/\text{kg gasoline}$$ so $$O_{2,theoretical} = 3.50\times181.2 = 634.5\ \text{kg}$$
  4. Part (ii) — O₂ to be supplied at 50% utilization efficiency. Only half of the delivered O₂ is actually consumed by the biodegradation reaction, so twice the theoretical demand must be supplied: $$O_{2,supply} = 634.5/0.50 = 1268.9\ \text{kg}$$
  5. Part (ii) — convert O₂ mass to an air volume. At 20°C (293.15 K), 1 atm, molar volume $$V_m = \frac{RT}{P} = \frac{0.08206\times293.15}{1} = 24.06\ \text{L/mol}$$ Moles of O₂ needed: $$n_{O_2} = \frac{1{,}268{,}900\ \text{g}}{32.0\ \text{g/mol}} = 39{,}656\ \text{mol}$$ so O₂ volume $$= 39{,}656\times24.06 = 954{,}000\ \text{L} = 954.0\ \text{m}^3$$ Air is ~20.9% O₂ by volume, so $$V_{air} = 954.0/0.209 = 4564\ \text{m}^3$$
  6. Part (ii) — pile's own air-filled pore volume. Volumetric water content $$\theta_w = w\rho/\rho_{water} = 0.25\times755/1000 = 0.189$$ $$\theta_a = n-\theta_w = 0.35-0.189 = 0.161$$ $$V_{a,pile} = \theta_a\times V = 0.161\times300 = 48.4\ \text{m}^3$$
  7. Part (ii) — number of air exchanges. $$\boxed{N_{exchanges} = \frac{V_{air}}{V_{a,pile}} = \frac{4564}{48.4} \approx 94\ \text{pore-volume exchanges}}$$
QuantityResult
(i) Gasoline released181.2 kg (≈ 245 L at SG 0.74)
(ii) Theoretical O₂ demand634.5 kg
(ii) O₂ to be supplied (50% efficiency)1268.9 kg
(ii) Required air volume4564 m³
(ii) Pile air-filled pore volume48.4 m³
(ii) Air exchanges required≈ 94

Roughly 94 pore-volume exchanges is a large but realistic cumulative figure for a bioventing operation — it is delivered gradually by a continuously running low-flow blower over weeks to months, not as 94 discrete events, and is consistent with the deliberately low per-pass flow rate that keeps the process biological (Question A-4) rather than a physical-stripping SVE operation.

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