1. PROBLEM DEFINITION
QWASI (Quantitative Water Air Sediment Interaction) is used to
understand fate of chemicals in lakes and rivers. Normally chemicals
with significant vapor pressure are treated but here an enhanced
version of the model is used to be able to handle also metals.
Objectives:
- Summarize the QWASI model (conceptual model, input data,
principle and mechanism);
- Implement the model and compare the results for lead and PCB
with the results from the paper “Application of the QWASI fugacity
model to the dynamics of organic and inorganic chemicals in lakes”
(Mackay & Diamond, 1989).
CONCLUSION
- It is not easy to understand where the errors come from (see evaluation of
dominant processes for the air phase with PCBs as chemical);
- Very good data are needed in the input requested;
- The principle of the fugacity seems to be powerful but (i) the results of this
model should be compared with other experimental datas and (ii) the
developer of the software should consider a better trade-off between user-
friendly modelling environment and transparency of the model’s principles.
RESULTS
PRINCIPLE
The basic principle for QWASI is the use of fugacity* instead of
concentration. The advantages of this formulation is that kinetic
and equilibrium parameters adopt forms which facilitate
interpretation of the model output and assist the identification of
the dominant processes comparing the transformation/transport
parameters (D values).
Fugacity is only suitable for chemicals which have measurable
concentrations in the vapor phase. However the software is able to
manage metals (which have no significant vapor pressure) thanks to
the concept of aquivalent concentration.
*Fugacity is based on pressure and is equal to the pressure of an
ideal gas which has the same chemical potential as the real gas.
All equilibriums between the different phases are expressed as Z
values. Z values depend on type of chemical, medium and
temperature.
TRANSFORMATION
included as first order rate expressions
𝑁 = 𝐷 𝑘 𝑓𝑖 𝐷 𝑘 = 𝐷𝑖 = 𝑉𝑖 𝑍𝑖 𝑘𝑖
INTERPHASE MATERIAL TRANSFER
a volume of material physically moves the solute between phases
𝑁 = 𝐷 𝑘 = 𝐺𝑖 𝐶𝑖 𝐶𝑖 = 𝑓𝑖 𝑍𝑖
INTERPHASE DIFFUSION TRANSFER
driven by fugacity differences
𝑁 = 𝐷 𝑘(𝑓𝑖 − 𝑓𝑗) 𝐾𝑖𝑗 =
𝑍𝑖
𝑍𝑗
𝐷 𝑘 = 𝐷𝑖𝑗 = 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝐾𝑖𝑗, 𝐾 𝑇, 𝐴 𝑇 …
MECHANISM
There are three main mechanisms, (i) interphase material transfer,
(ii) transformation and (iii) interphase diffusion transfer.
INPUT DATA
Variable Symbol Unit
Partition coefficients, S-W,
A-W
KSW,
KAW
-
Fugacity capacities Zi mol/m3, Pa
Transport/transformation
parameters
Dnk m3/h
Fugacities, for A, W, S and
WI
fi mol/m3
Aquivalent Concentrations Ai mol/m3
Temperature T oC
Flows, WI, WO, R Gi m3/h
Concentrations, S, W, A,
Q, p, WI, X, WO and Y PO
Ci mol/m3
Transformation rate
constants, W and S
Ki 1/h
Mass transfer coefficients,
S-W and A-W
KT,
KV
m/h
Sediments areas, A-W and
W-S
AW, AS m2
Volumes, W and S VW, VS m3
W=water WI=water inflow S=sediment A=air
Q=aerosol p=water particles X=water particle
inflow WO=water outflow PO=particle outflow
R=Rain
STRENGTHS WEAKNESSES
D values for easy comparisons Black-Box modelling
Highlights the dominant processes Poor transparency
Works for a wide range of chemicals A lot of input data are needed
User-friendly The software gives only the steady-state solution
DOMINANT PROCESSES
(Based on D values)
LEAD PCBs
Our Res. Paper Our Res. Paper
(%)
WATER
Water to Sediment Diffusion 1.0 1.0 1.6 1.6
Sediment Deposition 95.0 94.8 77.3 78.0
Water Transformation 0.0 0.0 1.4 2.0
Volatilization 0.0 0.0 13.8 13.9
Water Outflow 3.0 3.0 5.1 5.1
Water Particle Outflow 1.0 1.2 0.8 1.0
SEDIMENT
Burial 33.0 33.1 42.2 41.4
Sediment transformation 0.0 0.0 4.3 6.1
Sediment Resuspension 65.4 65.3 41.7 40.9
Sediment-water diffusion 1.6 1.6 11.8 11.6
AIR
Wet Particle Deposition 63.7 63.8 20.7 61.0
Dry Particle Deposition 36.0 35.9 7.7 22.9
Absorption 0.0 0.0 69.7 15.7
Rain Dissolution 0.2 0.2 2.0 0.4
Authors: Karolina Härnvall and Paolo Fornaseri
Partition
Coefficients K
Steady State
Flow Rates
From
Mass Balance
Equations
For Sediment and
Water
Significat Vapour Pressure
No Significat Vapour Pressure
Rate constant
Volume V
Process
parameters
or
D values
A Values
Concentration
in water
and air
Nk=DkAk
Nk=Dkfk
Air
Water
Sediment
Chemical
Concentration
in water
and air
Flow Rates G
Concentrations
C
Material Transfer
Diffusion Transfer
Transformation
Zvaluestoexpressequilibriums
f values
LEAD
PCBs
DISCUSSION