|Fig 1. Site map showing the distance from the benzene source at the dispenser island to the residential area.|
Setting up The Model
- Hydraulic gradient = 0.003 ft/ft
- Hydraulic conductivity = 60 ft/d
- Effective porosity = 0.25
- Benzene source concentration = 3,000 ug/L
- Source width (width of dispenser island) = 10 ft
- Source depth (smear zone thickness beneath water table) = 2 ft
2. The C v t chart in TS-CHEM displaying benzene concentrations near the source
at MW-1 (dark blue), 600 ft from the source (aqua), and 1000 ft from the source
3. TS-CHEM’s contour chart indicates that the maximum plume extent (bound at 5
ug/L) does not reach the residential area.|
|Fig 4. TS-CHEM’s contour chart indicates that the benzene plume, with a boundary of 1 ug/L, will impact the residential area.|
Analysis 3Oftentimes, regulatory agencies prescribe longer half lives for constituents for the purposes of risk evaluations. In many cases, however, half-lives of contaminants like benzene are shorter than the default degradation rates typically prescribed by regulatory agencies. In our last two analyses, we used the default degradation rate of 9.58E-04 d-1 (half life = 2 years). Let’s change the degradation rate to a more realistic value of 3.8E-03 d-1 (half life = 0.5 years). After running the model again, there is a noticeable difference in the concentration and length of the benzene plume.
|Fig 5. The C v t chart in TS-CHEM shows that benzene concentrations stabilize at a much lower level when the degradation rate is increased.|
|Fig 6. The contour chart demonstrates the effect of increasing the degradation rate of benzene. The plume, while still bound at the more protective 1 ug/L, no longer impacts the residential area.|
Review of contour charts for each model year indicates that the benzene plume stabilizes in less than 4 years, which is about half the time it took the plume to stabilize in the previous analyses. Also, the plume, bound at 1 ug/L, reaches a maximum length of approximately 580 ft, and thus, does not impact the neighborhood domestic wells. The length of the benzene plume observed in this analysis is consistent with benzene plume lengths seen in many literature studies such as API 1998 and Connor et al 2014.
Why is this important?