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18-Env-B4 Site Assessment and Remediation · December 2014

Question 8 of 8: Retarded Contaminant Travel Time, Pond 1 to Pond 2

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

Notes on this paper

National Exams; December 2014 — 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); Mercer & Cohen (1990), “A review of immiscible fluids in the subsurface,” Journal of Contaminant Hydrology; Karickhoff (1981), “Semi-empirical estimation of sorption of hydrophobic pollutants on natural sediments and soils,” Chemosphere 10(8); Schwarzenbach, Gschwend & Imboden, Environmental Organic Chemistry; Freeze & Cherry, Groundwater; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); ASTM E1527 Standard Practice for Phase I Environmental Site Assessments and ASTM E1903 Standard Practice for Phase II ESA; ATSDR Toxicological Profiles for Tetrachloroethylene and Mercury; 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: Retarded Contaminant Travel Time, Pond 1 to Pond 2 (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.

Pond 1 Pond 2 Path length 700 m (sand lens) Head diff. 15 m
Two hydraulically connected ponds; TCA travels through a sand lens along a 700 m flow path under a 15 m head difference.

Given. Flow path length $L=700$ m; head difference across the path $\Delta h=15$ m (from the figure); hydraulic conductivity $K=4\times 10^{-5}$ m/s; sand porosity $n=0.32$; dry bulk density $\rho_b=2000\ \text{kg/m}^3$; $f_{oc}=0.005$; TCA $K_{ow}=317$.

Find. Travel time of TCA from Pond 1 to Pond 2.

Approach. Compute the groundwater pore (seepage) velocity from Darcy’s law, estimate the retardation factor from $K_{ow}$ via the sorption correlation, then apply the given $v_{contaminant}=v_{pore}/R$ relation to the 700 m path.

  1. Hydraulic gradient and Darcy flux. $i=\Delta h/L=15/700=0.02143$. Darcy flux $q=Ki=4\times10^{-5}\times 0.02143=8.57\times10^{-7}\ \text{m/s}$.
  2. Pore (seepage) velocity. $v_{pore}=q/n=8.57\times10^{-7}/0.32=2.68\times10^{-6}\ \text{m/s}=\boxed{0.2314\ \text{m/d}}$ (×86,400 s/d).
  3. Retardation factor. $K_{oc}=0.411K_{ow}=0.411\times317=130.3$ L/kg; $K_d=f_{oc}K_{oc}=0.005\times130.3=0.651\ \text{L/kg}$. $R=1+\dfrac{\rho_b}{n}K_d=1+\dfrac{2000}{0.32}\times\dfrac{0.651}{1000}=1+4.07=\boxed{5.07}$.
  4. Retarded contaminant velocity and travel time. $v_{contaminant}=v_{pore}/R=0.2314/5.07=0.0456\ \text{m/d}$. Travel time $t=L/v_{contaminant}=700/0.0456=15{,}340\ \text{d}=\boxed{42.0\ \text{years}}$.
TCA transport, Pond 1 → Pond 2
QuantityValue
Hydraulic gradient, $i$0.0214
Pore velocity, $v_{pore}$0.231 m/d
Retardation factor, $R$5.07
Contaminant velocity0.0456 m/d
Travel time≈ 42.0 years
Check: the 15 m dimension in the source figure is read as the head difference driving flow along the 700 m sand-lens path (the natural pairing with Darcy’s law); no other head/elevation data is given in the source.
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