18-Env-B4 Site Assessment and Remediation · December 2015
Question 8 of 8: Bioventing a Gasoline-Contaminated Soil Pile
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams; December 2015 — 04-Env-B4 / Site Assessment and Remediation. 3 hours duration; open-book exam (any non-communicating calculator permitted). The paper is split into Section A (five questions, candidates asked to answer three) and Section B (three questions, candidates asked to answer two), each question worth 20 marks. All eight questions are solved below for completeness.
Reference texts. Suthersan & Payne, Remediation Engineering: Design Concepts (CRC Press); Freeze & Cherry, Groundwater; Schwarzenbach, Gschwend & Imboden, Environmental Organic Chemistry; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Leeson & Hinchee (1997), Soil Bioventing: Principles and Practice (AFCEE); ASTM E1527 Standard Practice for Phase I Environmental Site Assessments and ASTM E1903 Standard Practice for Phase II ESA; American Petroleum Institute (API) publications on UST release modelling; Ontario Reg. 153/04 under the Environmental Protection Act (Record of Site Condition regime); BC Environmental Management Act / Contaminated Sites Regulation.
Section A — Three of Five Questions
Question B-3: Bioventing a Gasoline-Contaminated Soil Pile (20 marks)
Find. (i) number of pore-air exchanges needed to supply the stoichiometric O&sub2; demand; (ii) mass of ammonium sulphate needed to meet the microbial nitrogen demand.
Approach. Complete aerobic mineralization of heptane, $\text{C}_7\text{H}_{16}+11\text{O}_2\rightarrow7\text{CO}_2+8\text{H}_2\text{O}$, fixes the stoichiometric O&sub2; demand per kg of gasoline; dividing by the 50% conversion efficiency gives the O&sub2; that must actually be supplied as air, which is then compared against the pile’s own air-filled pore volume to find the number of exchanges. The same combustion stoichiometry gives the carbon mass, which sets the nitrogen demand via the C:N ratio, converted to ammonium sulphate mass by its own molecular weight.
Moles of gasoline and stoichiometric O&sub2; demand. $M_{C_7H_{16}}=7(12.01)+16(1.01)=100.2\ \text{g/mol}$. Moles spilled: $n_{gas}=200{,}000\ \text{g}/100.2\ \text{g/mol}=1996\ \text{mol}$. From the balanced equation (11 mol O&sub2; per mol fuel): $n_{O_2,stoich}=11\times1996=\boxed{21{,}960\ \text{mol}}$.
O&sub2; actually to be supplied (50% conversion efficiency). $n_{O_2,supply}=n_{O_2,stoich}/0.50=\boxed{43{,}910\ \text{mol}}$.
Volume of O&sub2;, then air, at 20°C. Molar volume at 293.15 K, 1 atm: $V_m=RT/P=0.0821\times293.15=24.06\ \text{L/mol}$. $V_{O_2}=43{,}910\times24.06=1{,}056{,}300\ \text{L}=1056\ \text{m}^3$. Air is 20.9% O&sub2; by volume, so $V_{air}=1056/0.209=\boxed{5054\ \text{m}^3}$.
Air-filled pore volume of the pile. The literal soil data gives $\theta_w=w\rho_b/\rho_{water}=0.25\times1400/1000=0.350$, exactly equal to the stated porosity ($\theta_a=n-\theta_w\approx0$) — a soil with no air-filled pore space at all, which is physically incompatible with bioventing and is flagged as an exam-data inconsistency below. Using an illustrative design value $\theta_{a}\approx0.10$ (typical of a well-aerated sandy loam pile, Leeson & Hinchee 1997) instead: $V_{pore\ air}=\theta_a V=0.10\times300=\boxed{30\ \text{m}^3}$.
Number of air exchanges. $N=\dfrac{V_{air}}{V_{pore\ air}}=\dfrac{5054}{30}=\boxed{\approx169\ \text{exchanges}}$.
Check: the literal soil data (water content 25% wt at $\rho_b=1400\ \text{kg/m}^3$, $n=0.35$) gives essentially zero air-filled porosity ($\theta_a\approx0$), which cannot support the air movement bioventing requires — this is a genuine inconsistency in the exam data. Step 4 substitutes an illustrative, literature-typical design value ($\theta_a\approx0.10$, aerated sandy-loam pile, Leeson & Hinchee 1997 AFCEE Bioventing Manual) so the air-exchange calculation still returns a usable number; the true count would follow the same method once accurate field air-filled porosity is measured (e.g. via an air permeability test on the pile).
Part (ii) — ammonium sulphate demand. Carbon mass fraction of the fuel: $f_C=\dfrac{7(12.01)}{100.2}=0.839$, so total carbon spilled: $m_C=200\times0.839=\boxed{167.8\ \text{kg}}$. Stoichiometric nitrogen demand at C:N $=10{:}1$: $m_{N,stoich}=167.8/10=\boxed{16.78\ \text{kg}}$. Accounting for the same 50% microbial utilization efficiency: $m_{N,actual}=16.78/0.50=\boxed{33.56\ \text{kg}}$. Molecular weight of ammonium sulphate: $M_{(NH_4)_2SO_4}=2(14+4\times1)+32+4(16)=132\ \text{g/mol}$, of which nitrogen is $2(14)/132=0.2121$ by mass. Mass of ammonium sulphate required: $m_{AS}=\dfrac{m_{N,actual}}{0.2121}=\dfrac{33.56}{0.2121}=\boxed{\approx158\ \text{kg}}$.
