Dye Volatilization Options in EEMS 12.1
When simulating chemical fate and transport, volatile chemicals at the air-water interface frequently off-gas from the water into the atmosphere. The Chemical Fate module in EEMS is designed to work primarily in conjunction with the Sediment module, as the toxic substances sorb to sediments. In cases where sediment data is sparse or missing altogether, using a dye class as a proxy for the dissolved chemicals is recommended. To help users more accurately simulate this case, a volatilization component is required in the dye proxy. This blog describes how EEMS 12.1 has been enhanced to simulate dye volatilization.
Volatilization is the movement of chemicals across the air-water interface as the dissolved neutral concentration attempts to equilibrate with the gas phase concentration. Equilibrium occurs when the partial pressure exerted by the chemical in solution equals the partial pressure of the chemical in the overlying atmosphere. The rate of exchange is proportional to the gradient between the dissolved concentration and the concentration in the overlying atmosphere and the conductivity across the interface of the two fluids. The conductivity is influenced by both chemical properties (molecular weight, Henry’s Law constant) and environmental conditions at the air-water interface (turbulence controlled by wind speed, current velocity, and water depth).
For a flowing system, the transfer coefficients are controlled by flow-induced turbulence. The film liquid phase transfer coefficient (KL, m/s) is then computed using the Covar method (Figure 1), in which the equation used varies with the velocity and depth of the cell. For cells with a depth less than 0.61 m (2 feet), the Owens formula (Eq. 1) is used to calculate. For segments with cell depth greater than 0.61 m and 4.08 the O’Connor-Dobbins formula (Eq. 2-1) is used. In all other cases, the Churchill formula (Eq. 3) is used to calculate the volatilization rate.
Eq. 1
K_L = \left( \frac{D_w u}{D} \right)^{0.5}
Eq. 2-1
Eq. 2-2
K_L = \frac{5.049}{86400} \frac{u^{0.969}}{D^{0.673}}
Eq. 3
Where:
U is the depth-averaged velocity of the water (m/s), D is the depth (m), Dw is the diffusivity of the chemical in water (m2/s) calculated by Eq.2-2, and MW is the molecular weight.
For more quiescent conditions, such as lakes or ponds, the transfer coefficients are controlled by wind-induced turbulence. For these systems, the liquid film transfer coefficient (KL) is computed using either the O’Connor equations or Mackay & Yeun (1983). The user interface for configuring these options is shown in Figure 2.
Once the liquid file transfer coefficient (KL) is computed, the volatilization transfer rate (conductivity, Kv) is calculated based on the two-layer resistance model by Whitman et al. to incorporate the gas phase resistance. The final conductivity is then adjusted for water temperature.
Although many calculations are involved in determining volatilization, most are performed internally using a small data set. Volatilization data specifications are summarized in Table 1 below. Not all of the constants are required. Volatilization is only active for the surface layer. It can be observed from Figure 1 that there are two temperature coefficients. The main form uses the temperature coefficient for the decay options. Only the temperature coefficient in the Volatilization Parameters form is used for volatilization.
Table 1. Dye Volatilzation Input Data
| Description | Notation | Range | Units |
|---|---|---|---|
| Measured or calibrated conductance | 𝐾𝑣 | 0.6 — 25 | 𝑚/𝑑𝑎𝑦 |
| Henry’s Law Constant | 𝐻 | 10−7 — 10−1 | 𝑎𝑡𝑚−𝑚3/𝑚𝑜𝑙𝑒 |
| Concentration of chemical in atmosphere | 𝐶𝑎 | 0 — 1000 | 𝜇𝑔/𝐿 |
| Molecular weight | 𝑀𝑤 | 10 — 103 | 𝑔/𝑚𝑜𝑙𝑒 |
| Reaeration coefficient (conductance of oxygen) | 𝐾𝑎 | 0.6 — 25 | 𝑚/𝑑𝑎𝑦 |
| Experimentally measured ratio of volatilization to reaeration | 𝑘𝑣𝑜 | 0 — 1 | |
| Current velocity | 𝑢𝑥 | 0.2 | 𝑚/𝑠 |
| Water depth | 𝐷 | 0.1 — 10 | 𝑚 |
| Water temperature | 𝑇 | 4 — 30 | ∘C |
| Wind speed 10m above surface | 𝑊10 | 0 — 20 | 𝑚/𝑠 |
As described, EEMS now supports the simulation of chemical fate without the requirement to use the sediment transport module. This gives the user considerable flexibility when sufficient sediment data is unavailable.
Do you want to try these options for yourself? You can start by downloading EEMS, activating in the free demo mode, and then running a demonstration model. To see these features in action, head over to our YouTube page.
Please get in touch with us if you have comments or questions. For more information on EFDC+ capabilities, contact the DSI team today.
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References
Covar, A. P. (1976). Selecting the proper reaeration coefficient for use in water quality models. In Proceedings of the Conference on Environmental Modeling and Simulation. Cincinnati, OH, EPA-600/9-76-016, Environmental Protection Agency, Washington, DC, pp. 340–3
DSI LLC. 2024. EFDC+ Theory, Version 12. Published by DSI LLC, Edmonds WA.
Mackay, D. and A. T. Yeun (1983). Mass transfer coefficient correlations for volatilization of organic solutes from water. Environmental Science & Technology 17(4), 211–217
