- Aquifer material = medium sand; some gravel; little silt
- Hydraulic gradient to the east = 0.003 ft/ft
- Source area TCE concentrations:
- Source 1 = 10,000 µg/L
- Source 2 = 5,000 µg/L
Figure 1. Site map showing source areas, distance to downgradient receptor well.
Concerned by potential impacts to the downgradient receptor, regulators have requested that analyses be performed to determine whether TCE will impact the downgradient receptor well in exceedance of applicable the standard (in this case 5 µg/L), and if so, which site (or sites) may be responsible for impacts to the well.
Setting Up the Model
The releases of TCE at the two sites have resulted in the identification of Dense Non-Aqueous Phase Liquid (DNAPL) in the subsurface, and as such, the DNAPL source areas at the two sites are located at approximately 16 ft beneath the water table. Because of this, 3DADE-3 is a good model solution for this analysis because it is capable of representing the source areas as a vertical patch source at depth and assumes a constant concentration (which is appropriate since ongoing DNAPL sources are present beneath both sites). In TS-CHEM, the following model parameters should be set
- Hydraulic gradient = 0.003 ft/t
- Hydraulic conductivity = 80 ft/d
- Effective porosity = 0.25
- Source width = 20 ft
- Source depth = 16 ft
- Source Thickness = 4 ft
- Source 1 TCE source concentration = 10,000 µg/L
- Source 1 TCE source concentration = 5,000 µg/L
Analysis 1: Assessing Potential TCE Impacts to Downgradient Receptor Well
First, a model observation point should be set approximately 3,000 ft downgradient from Source 1 (i.e., the location of the receptor well). After running the model for approximately 40 years, the C v t plot reveals that the commingled plume first reaches the receptor well after 10 years and stabilizes after approximately 28 years (when concentrations begin to level off just above 30 µg/L) (Figure 2).
Although this analysis answers the question as to whether the commingled plume may impact the downgradient receptor well in exceedance of the applicable standard of 5 µg/L for TCE (it will), we also want to understand the extent to which each of the source areas may contribute to those impacts (including which source/plume first reaches the well, and the extent that each well contributes to TCE impacts). This analysis can easily be done in TS-CHEM by simply unchecking the “sum concentrations” option, which will display individual contributions from each plume on the C v t plot.
As shown in Figure 3, although the plume associated with Source Area 2 is the first to reach the receptor well, with the plume from Source Area 1 arriving soon after. The plot also indicates that after about 20 years, Source Area 1 is contributing about 2/3 of the TCE in the receptor well, whereas the plume associated with Source Area 1 is contributing approximately 1/3 of the TCE (once the plumes stabilize).
TS-CHEM also allows for the generation of contour plots, which in this case, indicate that the plumes from the two source areas begin to commingle after approximately two years (Figure 4), with the commingled plume reaching a maximum extent of approximately 4,000 ft after 20 years (Figure 5).
|Figure 4. TS-CHEM's contour chart showing initial commingling of plumes after two years|
|Figure 5. TS-CHEM's contour chart showing extent of commingled plume after 20 years|
Analysis 2: Evaluation of Potential TCE Impacts to Downgradient Stream
As shown in Figure 1, there is a stream located approximately 5,000 ft downgradient from Source Area 1, and regulators have expressed some concern as to whether the stream may be impacted by one or both of the TCE plumes above the standard 5 µg/L. To examine this, we can add an observation point in the location of the stream (i.e., 5,000 ft downgradient from Source 1), and then examine the C v t plot. As shown in Figure 6, the commingled plume reaches the stream after approximately 22 years and exceeds the applicable cleanup standard (5 µg/L) after approximately 30 years. But, when we examine individual plume contributions, we can see that although the plume associated with Source Area 2 reaches the stream first, the concentrations associated with that plume do not exceed the applicable cleanup standard, whereas the TCE concentrations from Source Area 1 plume do exceed the standard (Figure 7).
|Figure 7. The C v t chart in TS-CHEM displaying TCE concentrations associated with plumes from Source 1 and Source 2 at the stream (located 5,000 ft downgradient from Source 1).|
|Figure 8. TS-CHEM's contour chart showing extent of commingled plume with shorter TCE half-life after 20 years.|
The contour chart shown in Figure 8 indicates that after 20 years, the commingled plume boundary is reduced by approximately 500 ft when compared to Analysis 1 when the TCE half-life is reduced to 4.5 years. As shown in the C v t plot in Figure 9, although the commingled plume TCE concentration still exceed the applicable standard of 5 µg/L at the receptor well, concentrations do not exceed the standard at the downgradient stream.