TS-CHEM Example Applications – Commingled Plume Analysis

In the course of performing site investigations, environmental professionals are often tasked with identifying sources of contamination (e.g., areas where contaminants may have been discharged into site soils and may have potentially migrated to groundwater) and delineating the nature and extent of impacted groundwater. In many cases, releases of the same chemical from multiple operational area sources may form downgradient plume zones that commingle, making it difficult to distinguish which source (or sources) may be responsible for impacted groundwater. Additionally, commingling of a site groundwater plume (or plumes) with plumes from off-site sources may further complicate the determination of source contribution and responsibility for cleanup. 

Although evaluating which source (or sources) of contamination at a site (or sites) may be associated with impacted groundwater when commingled plume conditions are present can be difficult, there are resources and tools that can assist environmental professionals with demonstrating that commingled plumes are present and assessing the relative contribution from individual plumes to the commingled plume. For example, New Jersey’s Commingled Plume Technical Guidance Document (which can be accessed HERE) describes several types of commingled plume scenarios that may be encountered, how to investigate them, and how to develop lines of evidence to evaluate commingled plume conditions and select appropriate remedial measures. 

One of the lines of evidence identified in the New Jersey Commingled Plume Guidance Document is solute transport modeling. With TS-CHEM, site investigators can quickly and easily perform solute transport modeling analyses to evaluate commingled plume conditions at a site and establish a clear line of evidence that demonstrates likely contribution from each source area identified, and which plume(s) may contribute to known and potential impacts to downgradient receptors. 

This blog post will cover the fourth Example Application in the TS-CHEM Example Application series: Commingled Plume Analysis. To follow along and review the model files, you can download this example application HERE.

Overview

In this scenario, investigators have identified areas of concern associated with historical releases of chlorinated solvents from degreasers at two separate nearby industrial sites, resulting in the release of Trichloroethylene (TCE) to groundwater. The plumes associated with these releases have apparently commingled, resulting in a plume that extends downgradient from the sites. A well search has identified a potential receptor well 3,000 feet downgradient from one of the sites. Initial site investigations have revealed the following information:

• 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).

 

Figure 2. The C v t chart in TS-CHEM displaying commingled plume TCE concentrations at the receptor well (located 3,000 ft downgradient from Source 1).

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.

Figure 3. The C v t chart in TS-CHEM displaying TCE concentrations associated with plumes from Source 1 and Source 2  at the receptor well (located 3,000 ft downgradient from Source 1).

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 6. The C v t chart in TS-CHEM displaying commingled plume TCE concentrations at the stream (located 5,000 ft downgradient from Source 1).

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).

Analysis 3: Examining A Higher Plume Degradation Rate

Oftentimes, regulatory agencies prescribe longer half-lives for constituents for the purposes of risk evaluations. In many cases, however, half-lives of contaminants like TCE may be shorter than the default degradation rates typically prescribed by regulatory agencies. In the analyses performed thus far, a TCE half-life of 10 years was assumed.  For this analysis, we want to examine TCE impacts at both the receptor well and the stream if the TCE half-life of the plume is shortened to 4.5 years.

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.  

Figure 9. The C v t chart in TS-CHEM displaying TCE concentrations associated with plumes from Source 1 and Source 2 at the receptor well and stream (assuming a 4.5-year TCE half-life).

Conclusion

The presence of commingled plume conditions at sites can be challenging for environmental professionals responsible for investigating and remediating impacted groundwater, making it difficult to determine the extent to which sources may contributing to the commingled plume, and in turn, who may be responsible for cleaning them up. In this exercise, we demonstrated how TS-CHEM can be used to perform quick and easy analyses that allow site investigators to not only quantify the impacts of the commingled plume (e.g., impacts to downgradient receptors), but also evaluate the individual contributions from each source. This line of evidence (along with other lines of evidence developed by the site investigator) can be used to determine which party (or parties) may be responsible for impacted groundwater, and in turn, who should pay for the associated investigation and cleanup.