Estimation of TS-CHEM Source Terms Using the Newly Released Soil Infiltration and Leaching Tool (SILT)

TS-CHEM is pleased to announce the release of the “Soil Infiltration and Leaching Tool” (SILT), a  Microsoft Excel-based spreadsheet utility that allows users to simulate one-dimensional contaminant fate and transport processes though the vadose zone in a simple, user-friendly environment.  The SILT utility is a perfect companion to TS-CHEM, allowing site investigators to evaluate potential impacts to groundwater associated with contaminated soil, and estimate contaminant source terms that can then be incorporated into TS-CHEM and used to estimate the extent and duration of resulting contaminant plumes.  

Figure 1: Vadose Zone Transport Conceptual Model for SILT

SILT simulates contaminant transport from the soil source, through the vadose zone, to a groundwater contaminant source zone in three basic steps:

1.  The initial source zone water concentration, located in the Source Zone (of user-specified vertical thickness), is calculated after partitioning of the user-specified Source Zone Total Soil concentration among the solid-, air- and dissolved phases

2.  The dissolved phase contaminant source is transported vertically downward through the Vadose Zone, being acted upon by dispersion and degradation processes, until it reaches the  groundwater table, producing a leachate breakthrough curve

3.  At the bottom of the vadose zone, leachate (at the calculated flux rate and concentration) is delivered to a mixing zone at the top of the water table. When the infiltrating soil water reaches the water table, it is mixed with groundwater and the contaminant is diluted (using a Dilution Attenuation Factor (DAF)), resulting in a lower concentration. The diluted concentration data can then be used for comparison to published groundwater standards, and/or utilized to calculate source terms for groundwater plume models (such as TS-CHEM).

Figure 2: Groundwater Source Term Conceptual Model for SILT.

SILT’s Interface is set up in an intuitive manner, with a series of worksheets that allow for a streamlined, step-by-step workflow.  These worksheets are described below.

•  Step 1 – Source Zone – In this worksheet, the user can specify fate and transport parameters for the contaminant of concern in the soil source zone, as well as the amount of time soil leachate concentrations will be calculated for, and the nature of the source (e.g., constant source, decaying source, or variable source concentrations), allowing the user to estimate soil source concentrations through time.

Figure 3: Source Concentration Input Table in SILT “Source Zone” tab.

•       Steps 2 and 3 – Vadose Zone and Mixing Zone – Using the soil source zone concentrations estimated in Step 1, the Vadose Zone and Mixing Zone worksheet allows the user to input properties of the vadose zone soils (e.g., distance from bottom of source to water table, water content, and seepage velocity) and the contaminant as it migrates downward through the vadose zone (e.g., decay rates in water and soil) to estimate leachate concentrations at the bottom of the soil column.

Figure 4: Worksheet layout for the “2 Vadose & 3 MixingZone” Tab – Step 2

      The leachate concentrations estimated as part of Step 2, along with a DAF value (which can be specified by the user or calculated using multiple methods) are then used to estimate dissolved-phase concentrations in mixing zone groundwater beneath the soil source zone.

 

Figure 5: Worksheet layout for the “2 Vadose & 3 MixingZone” Tab – Step 3

• Step 4 – Plume Model Source Steps – In this worksheet, the user can discretize the diluted mixing zone groundwater concentration data estimated in Step 3 into a specific number of source concentration steps (each with its own duration), which can then be incorporated into a groundwater contaminant plume model (such as TS-CHEM) as a source term.

Figure 6: Worksheet layout for the 4 Plume Source Steps Tab.

SILT also incorporates a number of features that facilitate model input, and allow for easy export of model results.  For example, SILT includes a “unit conversion” feature, which allows the user to enter a variety of commonly used units for most input parameters, which are automatically converted by the model before running model calculations. This feature prevents unnecessary confusion related to units and ensures model output is consistent and accurate, while also allowing greater flexibility.  Additionally, users can export time series charts of model output with the click of a button (including modeled source concentrations, leachate concentrations, diluted groundwater concentrations in the mixing zone, and plume model source steps). Model generated time step source concentrations can also be exported to a comma-delimited .csv file, allowing for easy incorporation into groundwater plume models as a source term.

SILT includes a detailed User Guide which provides a comprehensive overview of the SILT utility, step-by-step instructions for all model inputs, and thorough descriptions of the calculations that are performed in SILT. And, if you’re not sure where to start, SILT comes packaged with a built-in example soil leaching model that simulates migration of TCE through the vadose zone, and is based on vadose zone fate and transport parameters presented in Sanders (1995). Simply click the “Sanders 1995 Example Model” button on the “1 Source Zone” tab, and all associated input parameters will be populated in the model worksheets. 

The SILT utility can be downloaded for FREE from the TS-CHEM website (https://www.transportstudio.com/SILT/).

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. 

TS-CHEM Solution Library - AT123D-AT

The TS-CHEM program provides an easy-to-use software environment in which to analyze contaminant plume transport, and includes a comprehensive library of more than 30 different analytical solutions to the advection-dispersion equation. Each solution incorporates different capabilities, including how it represents the contaminant source and how the plume interacts with the aquifer. For this post in the Solution Library series, we will be focusing on the AT123D-AT family of models published by Dan Burnell in 2012 which represents an updated version of the original AT123D model suite developed by G.T. Yeh at the Oak Ridge National Laboratory.

What is AT123D?

The original AT123D was a suite of analytical model solutions that are used to simulate one-, two- and three-dimensional transport in groundwater. The program is complex, and allows for many different source configurations, including

• patch sources
• line sources
• point sources
• volume sources

In contrast to many analytical plume models that represent the source using a first-type specified concentration, AT123D represents the source as a second-type specified mass flux boundary condition. This means, for example, if you know the nitrogen load from a residential dwelling to a septic system (i.e. number of residents times average daily nitrogen load per person) you can represent that source in AT123D as milligrams of nitrogen per day instead of trying to estimate a septic source concentration value. The primary difference between AT123D and AT123D-AT is in how the solver arrives at a solution to the advection-dispersion equation. AT123D-AT incorporates a Romberg numerical integration scheme that works to prevent oscillations, speed convergence, and improve accuracy for a wide range of input parameter combinations.

A conceptualization of the many possible specified mass flux source geometries available in AT123D-AT is shown in Figure 1 below, and can also be found in the Model Selection Tool in TS-CHEM:

Figure 1 - AT123D-AT Source Geometries

TS-CHEM was developed with usability in mind, so the numerous AT123D-AT solutions available to the user (differing aquifer geometries, source geometries, and source types) have been broken up into six “versions” (see table below). These pre-configured versions allow the user to select the type of AT123D-AT model that will best represent their site (aquifer and source), and to specify the desired parameter input values to represent site-specific properties, using model types that closely match conditions simulated by other analytical ADE solutions included in ­TS-CHEM’s Solution Library. This last consideration makes it easier to compare different plume transport solutions (e.g. a first-type source vs a second-type source) by selecting similar type models (e.g. with aquifer boundaries or without aquifer boundaries) from the TS-CHEM Solution Library.

The following nomenclature is used between each version to help the user quickly determine which version they are looking for: Infinite boundary (I), Finite boundary (F), Constant mass release rate (C), Instantaneous release source (I), Time-variable (transient) mass flux (T):

Model Version	Aquifer Boundary	Source Type	Analagous TS-CHEM models
AT123D-AT IC	Infinite unbounded	Constant mass flux	3DADE-3 or 3DADE-4 (patch source)
AT123D-AT II	Infinite unbounded	Initial mass instantaneous release	3DADE-5 or 3DADE-6 (volume source)
AT123D-AT IT	Infinite unbounded	Constant specified concentration	ATRANS4 (with concentration stepping)
AT123D-AT FC	Finite bounded	Constant mass flux	ATRANS1
AT123D-AT FI	Finite bounded	Initial mass instantaneous release	3DADE-5 or 3DADE-6 (volume source)
AT123D-AT FT	Finite bounded	Constant specified concentration	ATRANS4 (with concentration stepping)

What are the differences between the AT123D-AT models?

The two defining factors that highlight the differences between the different AT123D-AT models are 1) whether the aquifer extent is assumed to be infinite (as is the case for many analytical transport solutions) or finite (bounded horizontally or vertically), and 2) how the source concentration or flux is applied at the model boundary over time (continuous, instantaneous pulse release, or time-varying). The three different types of source concentration fluxes are conceptualized in Figure 2 below:

Figure 2 - AT123D-AT Model Source Types

As you can see in Figure 2, there are three distinct source types. The source type that is most appropriate to use in a particular application is dependent on the known conditions at a site, and/or the known or assumed conditions of the source.

What applications are the AT123D models best suited for?

As discussed in previous blog posts, TS-CHEM can assist environmental professionals with the development of conceptual site models (CSMs) that characterize the extent and behavior of groundwater contaminant plumes. The AT123D-AT models are highly flexible, able to provide users with the ability to evaluate solute fate and transport in one-, two- or three-dimensions. In its fundamental form, AT123D-AT is solving the 3D ADE equation in a 1D (X-direction only) aquifer flow field, but 2D plume transport and even 1D “column” transport can be set up using the bounded aquifer settings. i.e. the AT123D-AT “F” models (FC, FI, and FT) can be used to represent sites where the aquifer thickness and/or width is known or is believed to be bounded (finite aquifer boundary). This type of AT123D-AT model is similar to the ATRANS family of models which include an upper (water table) and lower (aquifer base) no-mass-flux boundary.

But the AT123D-AT models are not all limited to bounded aquifer conditions. The AT123D-AT “I” models (FI, II, and IT) can be used to represent sites where there is no limitation on aquifer thickness or width (infinite aquifer boundary). This type of AT123D-AT model is similar to the 3DADE family of models, which do not impose any finite boundaries on the aquifer.

Further, the investigator can use a variety of patch source geometries to best represent the conditions at their site. For example, if an investigator has information on source mass flux at the downgradient edge of a source area, they may use a patch source. Or, if a large regional scale model is being considered, the user may want to utilize a point source to track the general plume behavior over time.

When choosing between source types, a user may choose to use a constant mass release source AT123D-AT model If only a single source concentration is known, and/or the investigator wished to perform a conservative analysis in which the source does not deplete through time. If the user wanted to investigate the plume behavior in a case where a slug of solute is introduced into the groundwater, they can use an instantaneous release source type. If information is available on the changing history (both increases and decreases) of source flux over time, the transient source type can be used.

Similar to the ATRANS1 model, the AT123D-AT FC model is useful for simulating scenarios that have aquifers of a finite extent that can be represented by a continuous mass flux. This type of analysis may be useful for evaluating a conservative maximum plume extent, or when the plume becomes stable (Figure 3):

Figure 3 - AT123D-AT FC solution showing maximum plume extent for a stable benzene plume with a constant source flux of 0.001 ld/d at 200ft, 600ft and 1000ft from the source.

The AT123D-AT FI model is useful for simulating scenarios where a slug of contaminant is assumed to have rapidly entered the groundwater. This type of model is sometimes used in an emergency spill response analysis if an investigator wants to simulate a conservative condition where a “slug” of contaminant enters the groundwater instantaneously and then is allowed to flush away from the spill area toward a possible receptor location (Figure 4):

Figure 4 - AT123D-AT FI solution showing plume concentrations up to 4000 days after an instantaneous mass input of 22.05 lbs of benzene introduced to groundwater. Concentrations are shown at 200ft, 600ft and 1000ft from the source. 

Similar to the ATRANS4 model, the AT123D-AT FT model allows the user to define transient source behavior. Unlike ATRANS4, the AT123D-AT FT model uses a second-type boundary condition and is therefore defined by time-flux pairs (instead of time-concentration pairs). The transient source condition allows the user great flexibility in how they define the time series of their source and can enable investigators to simulate complex scenarios where there may be intermittent single sources, multiple sources that overlap at different times, or even source termination that would result from remedial actions.

An example of a time-variable source flux is shown in Figure 5 below for a hypothetical release from an underground storage tank (UST). The benzene source progressively decreases until it eventually reaches zero after 2000 days. This might be indicative of a UST that leaks over time, or it could be indicative of a UST removal at around 500 days with a small amount of product remaining after removal. In either case, the benzene concentration is reduced over time through degradation/natural attenuation and flushing.

Figure 5 - AT123D-AT FT solution showing benzene plume concentrations entering groundwater from a hypothetical underground storage tank at 200ft, 600ft and 1000ft from the source.

To summarize: the AT123D-AT family of models available in the TS-CHEM Solution Library allow for a very flexible representation of the source over space and through time and can be used for a variety of environmental scenarios and conditions. The feature that sets the AT123D-AT models apart is that they are the only models in the TS-CHEM solution library that utilize a second-type (specified mass flux) source boundary condition, allowing for users to directly input an estimate of source mass flux for their site. AT123D-AT also differs from AT123D in that it incorporates enhanced solver capabilities that reduce solve times and increase solution accuracy. The six AT123D-AT model types included in TS-CHEM allow a user to easily identify which model is best for their site or application. These models can assume either a finite or infinite aquifer boundary and a multitude of source geometries and source release types.

To learn more about TS-CHEM, or to download a FREE DEMO VERSION of the software, visit the TS-CHEM Website today!

Darcy’s Law Groundwater Velocity and Plume Arrival Time

In his 1999 article titled “On Misuse of the Simplest Transport Model”¹ Ernesto Baca explained why it is not a good idea to calculate the rate of travel of a contaminant plume front, or its time of arrival at a receptor location, using the Darcy’s Law groundwater velocity. The missing factor is dispersion.

Baca noted that the Darcy’s Law velocity equation: 

V = Ki/n

only accounts for advection of the dissolved contaminant. He stated that, by measuring the plume from the source location to the far extent of the tip of the plume, one is implicitly considering both advection and dispersion. Therefore, for a relatively mobile dissolved contaminant being transported by seeping groundwater, the larger the dispersive effects, the more your estimate of arrival time will be off.

The toe of the contaminant plume will arrive earlier (i.e. in less time) that you would predict based on a Darcy’s Law groundwater velocity estimate.

The process is depicted in this video:

The video file is available for download. The video presents an analysis of a relatively mobile (slightly adsorbed and retarded) chemical, is for illustrative purposes only, and does not represent McLane Environmental’s analysis of, nor conclusions regarding, any particular real site or plume.

¹ Baca, E. 1999. On the Misuse of the Simplest Transport Model. Groundwater v 37, no 4, Jul-Aug 1999.

TS-CHEM Example Applications – Natural Source Flushing with MNA

 Monitored Natural Attenuation (MNA) is a remediation method that relies on natural processes to decrease contamination in the soil or groundwater. It is a popular method of remediation since it tends to involve less equipment and labor and therefore, less cleanup costs. 

Scientists typically monitor the contamination at a site to ensure that it is attenuating properly and within a reasonable time period. According to New Jersey’s MNA guidance, the applicability of MNA must be demonstrated by lines of evidence directly or indirectly indicate that natural attenuation processes are occurring. With TS-CHEM, it is simple to establish a clear line of evidence that demonstrates natural attenuation is occurring at your site.

This blog post will cover the second Example Application in the TS-CHEM Example Application series: Natural Source Flushing with MNA. To follow along and review the model files, you can download this example application HERE.

Overview

In this scenario, there has been a gasoline release from the dispenser island at a service station. While the leak is repaired shortly after the release, regulators and residents are worried about the release of benzene to the ground considering a residential development is located 1,200 feet to the east where shallow domestic wells are located. An initial on-site investigation reveals the following information:

• Aquifer material = medium sand; some gravel; little silt

• Hydraulic gradient to the east = 0.003 ft/ft

• Source area (MW-4) benzene concentration = 3,000 ug/L

Figure 1. Site map showing the distance from the benzene source at the dispenser island to the residential area.

Concerned by cleanup costs, the responsible party would prefer to remediate the contaminated area using MNA if possible. For MNA to be applicable, the responsible party needs to demonstrate that benzene is being flushed away sufficiently so as not to impact the domestic wells 1,200 feet away. For this example project, the benzene plume boundary is set to 5 ug/L since the USEPA drinking water standard for benzene is 5 ug/L.

Setting Up the Model

Since the leak from the dispenser island has been repaired, there is no constant source of contamination. Because of this, BIOSCREEN-AT is a good model solution for this analysis because its vertical patch source (in a semi-infinite aquifer bounded at the water table)  represents an exponentially decaying source concentration. This model solution will allow you to see changes in maximum plume extent from an early phase of growth, and then through subsequent plume decrease as the sources flushes away. In TS-CHEM, the following model parameters should be set:

• Hydraulic gradient = 0.003 ft/t

• Hydraulic conductivity = 60 ft/d

• Effective porosity = 0.25

• Source width = 10 ft

• Source depth = 2 ft

• Initial Benzene source concentration = 3,000 ug/L

• Initial estimate of source flushing half life = 4 years

Analysis 1: Examining Source Decrease Through Flushing

First, model observation points should be set downgradient from the dispenser at 100 ft (MW-1), 600 ft (mid-way to the neighborhood), and 1000 ft (approaching the neighborhood).

Figure 2. Site map showing model observation points located in between the dispenser island and the residential area.

These observation points should give insight into the levels of benzene at these locations over time and whether the benzene plume is decaying at a sufficient rate. After running the model for ten years, the C v t plots reveal that the plume reaches a peak around 1.5 years on the station property but continues to advance towards the neighborhood. The observation points located farther downgradient from the source show the arrival and growth (increasing concentrations) of the plume until the reduction of the source causes concentrations to decline in these areas.

Figure 3. 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 (red).

The contour plot shows that the plume reaches its maximum extent of about 875 feet after 7 years and does not extend into the residential development. The calculated plume at the 7-year time point encompasses an area of  88,350 ft².

Figure 4. TS-CHEM's contour chart indicates that the maximum plume extent (bound at 5 ug/L) does not reach the residential area.

The contour plot set to year 10 shows that the extent of the benzene plume shrinks by about 150 feet. The plume will continue to shrink slowly over many years until it decreases below the 5 ug/L standard. Due to this slow rate of decay, a decision may be made to either move to evaluating an active remedy scenario in which the source is removed, or possibly to evaluate a scenario with a more rapid rate of source depletion. That source depletion Analysis 2 is presented below.

Figure 5. TS-CHEM's contour chart showing a reduction in plume extent from year 7 to year 10.

Analysis 2: Examining a Higher Source Degradation Rate

Let’s say that the responsible party continues to investigate the source area of the contamination and develops information that supports a Conceptual Site Model in which the source is depleting more rapidly. Instead of the 4-year half-life employed in the previous analysis, let’s change the source decay rate to a two year half-life, and observe the effects. After running the model with this new degradation rate, the C v t plot shows a faster depletion of benzene overall as well as a faster concentration decline.

Figure 6. The C v t chart in TS-CHEM displaying benzene concentrations at the three set observation points.

The contour chart shows that after 6 years, the benzene plume (with a boundary of 5 ug/L) extends about 810 feet with an area of 68,150 ft2 before receding, which is 22% smaller than the maximum plume extent in the previous analysis. By year 10, the plume boundary recedes to about 220 feet.

Figure 7. The contour chart in TS-CHEM set to year 6 highlighting its shorter plume extent compared to the previous analysis.

The smaller plume area and quick recession of the plume may support the case for MNA at this site. Further source zone characterization may be needed to support the use of the faster degradation rate, however.

Analysis 3: Examining A Higher Source and 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 benzene are shorter than the default degradation rates typically prescribed by regulatory agencies. In this analysis, let’s increase the degradation rate of the plume (in addition to the increased degradation rate of the source in the last analysis) to observe the contamination extent in the case of a rapidly depleting source and plume. Instead of a 2-year half-life, let’s change the benzene degradation rate to a 0.5 year half-life.

The C v t plot shows that the maximum concentration observed at MW-1 is less than the maximum concentrations observed in the previous two analyses. Also, the plume decays before reaching the next two observation points at 600 and 1000 feet.

Figure 8. The TS-CHEM C v t chart showing benzene concentrations in the three set observation points. In this analysis, benzene isn't detected at the latter two observation points.

The benzene plume extends to about 350 feet after 2 years before quickly shrinking in areal extent. The plume area at year 2 is 26,150 ft², which is 61% smaller than the maximum plume extent in the first analysis. After year 10, the plume only extends to about 110 feet due to the groundwater flushing of the source, as well as the higher degradation occurring within the plume itself.

Figure 9. A greatly reduced benzene plume seen in TS-CHEM's contour chart following the increase of plume and source degradation rates.

This analysis - - especially using the more rapid benzene plume degradation rate - - shows that MNA could be a good potential remedial option for this site if acceptable information can be generated to support the degradation rates of the source and the plume. 

Conclusion

MNA is an attractive remedial option for parties looking to reduce cleanup costs, however, it must first be demonstrated that your site is a viable candidate for MNA per state and federal guidance. This often includes using data-driven solutions to form lines of evidence as to why your site is suitable for MNA. TS-CHEM provides a simple tool to evaluate the growth and decay of a contaminant plume, which allows for the evaluation of potential impacts to receptor locations downgradient from the source in the case the source concentration decreases through time due to flushing. This type of analysis can help determine a site’s suitability for MNA as a remedy.

Monitoring Well Placement and Plume Behavior

 

Groundwater monitoring wells are often placed in, around, and downgradient from a dissolved contaminant plume for a variety of purposes including:

• Monitoring plume dynamics (concentration changes) in different areas of the plume following a chemical release as the plume grows
• Bounding the plume in the horizontal and vertical for regulatory compliance and remedy planning purposes
• Evaluating plume stability as a criterion for considering a Monitored Natural Attenuation (MNA) remedy
• Projecting plume reduction over time as the result of a source remediation and/or MNA remedy
• Proper placement of downgradient sentinel wells to protect potential receptors

 

Planning the layout of a groundwater network is highly source and aquifer specific, with well location distances and anticipated contaminant arrival times being dependent on groundwater velocity and source concentration. Placement and interpretation of concentration patterns are also related to the source type; i.e. whether the source is constant, slowly declining (flushing), or suddenly removed (excavation).

 

Simple analytical contaminant fate and transport modeling can be very helpful in identifying optimal locations for monitoring wells, and in interpreting sampling results obtained from those wells. The following examples illustrate the application of TS-CHEM for the design and analysis of monitoring well networks for differing source and aquifer conditions.

 

Example 1 – Constant Source: Plume Growth and Stabilization

 

For the case where a chemical spill creates a continuing constant concentration source to groundwater, a dissolved contaminant plume will form, grow, and eventually stabilize (stop growing) (see time series in Figure 1). Monitoring wells placed down the axis of the plume can show this progression as concentrations rise through time and then sequentially level off at each downgradient location (Fig 2 and Fig 3).

Figure 1. Plume growth and stabilization caused by the slow constant dissolution of a vinyl chloride source. Monitoring wells are located at the following distances from the source: MW-1  400 ft; MW-2  600 ft; MW-3  700 ft; MW-4  875 ft. The placement of MW-4 coincides with the farthest downgradient extent of the plume after it has stabilized about 7.5 years after the initial release.

 

Figure 2. The spreading plume reaches each downgradient monitoring well in turn, causing the vinyl chloride concentration at that location to rise. After some period of time following first impact at a well location, the plume stabilizes in that area as indicated by the leveling off of monitoring well concentration.

 

Figure 3. Zooming in on the lower concentrations measured at MW-3 and MW-4 shows that these downgradient wells exhibit the same concentration versus time pattern as the wells in the central area of the plume.

 

Example 2 – Instantaneous Source: Plume Drift and Dissipation

 

For the case where a small chemical spill enters groundwater and dissolves - - creating an initial slug source in a small localized area - - a dissolved contaminant plume will leave that area and drift downgradient with groundwater seepage, spreading and dissipating as it does so (see time series in Figure 4). Monitoring wells placed down the axis of the plume can show this progressive arrival at, passing through, and leaving each monitoring well area (Fig 5 and Fig 6).

 

Figure 4. Plume behavior resulting from initial dissolution of a vinyl chloride source, with subsequent downgradient drift. Monitoring wells are located at the following distances from the source: MW-1  400 ft; MW-2  600 ft; MW-3  700 ft; MW-4  875 ft. The plume initially expands as it drifts, but attenuation (dispersion and degradation) gradually reduce the plume concentrations and plume area, and eventually cause it to disappear.

 

Figure 5. The drifting and spreading plume reaches each downgradient monitoring well in turn, causing the vinyl chloride concentration at that location to rise, peak, and then decline. Notice that the velocity of the plume peak does not match the groundwater Darcy velocity, nor the retarded groundwater Darcy velocity.

 

Figure 6. Zooming in on the lower concentrations measured at MW-3 and MW-4 shows that these downgradient wells exhibit the same concentration versus time pattern as the wells in the central area of the plume.

 

For the ideal case (often described in textbooks) the downgradient movement of the plume peak (even as that peak concentration declines with time) would occur at the same rate as the calculated Darcy’s Law groundwater pore velocity, which is 146 ft/yr for the model depicted above. If the chemical of interest were affected by adsorption and retardation, one might expect the plume peak to move at the rate of the groundwater velocity divided by the retardation rate. For this vinyl chloride model, that rate is 146 ft/yr / 1.113 = 131.1 ft/yr.

 

Instead, the plots above demonstrate that the plume peak is moves downgradient from the source at a faster apparent rate of approximately 155 ft/yr - - almost 20 % faster than the retarded groundwater velocity; and faster even than the average groundwater linear pore velocity of 146 ft/yr. This is caused by the nonlinear interaction of all of the processes acting on the vinyl chloride plume: groundwater seepage velocity, retardation, dispersion, and degradation. Thus, a model can be very useful in developing a more accurate estimate of the rate of movement of a detached (from the source) contaminant plume than simple velocity calculations would provide.

 

To learn more about TS-CHEM and how it can be used to assist with the estimation of the extent and movement of contaminant plumes, or to download a FREE DEMO of the software, visit the TS-CHEM Website today!