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!

TS-CHEM Version 2023-1 Now Available!

 New Features in TS-CHEM v2023-1

We are proud to announce the release of TS-CHEM version 2023-1! There are a host of new features, with key updates including:

•  A new color spectrum control for contour charts
• The addition of a plume boundary specification with contour plots using the log scale
• Refined unit conversion factors for model setup parameters
• An enforced QA check on the model data’s save and reload process
• Minor bug fixes

The new changes to contour plots allow for more flexibility than ever when visualizing your modeled plume! A brief overview of these new features is included below.

 

A COLORFUL (HALF-) LIFE

Contour plots in TS-CHEM just got more colorful! In v2023-1 a color spectrum control has been added for contour charts. This allows the user to fine-tune their color scale with up to five colored contour intervals!

In addition to allowing for custom-colored contour intervals, if the user wants to define their concentrations as “high” or “low”, just adjust the Gradient Style to Two Colors and refine the plot.

The new version of TS-CHEM can even represent a single color in style! With the One Color Transparency option the concentrations become more transparent the lower they get, emphasizing where the high concentrations occur in your plume.

Transparency is not just limited to the one-color setting, either! As seen in the figures above, uniform transparency can be applied to the contour plots using any of the color spectrums the user defines. This can be especially useful when combined with the Map Overlay feature!

PUT A LIMIT ON YOUR LOG PLOTS

The logarithmic scale (“log” scale) is an important and useful tool when visualizing contaminant plume data. It allows for large differences in concentration to be expressed in an understandable way. For instance, here is a contour plot of a contaminant plume with a linear scale:

We can see that our concentrations go from 100 – 1000 ug/L, with contours every 100 feet. What if we wanted to see what was going on with the lower concentrations less than 100 ug/L? We can either add a lot more contours or use a logarithmic scale:

We can now easily see our concentrations ranging from 1 to 1000 ug/L with only 10 contour lines. And, in TS-CHEM v2023-1, we can now add a boundary to this log scale plot. For example, we are interested in what the plume looks like between 10 and 1000 ug/L. We can set a plume boundary equal to 10 ug/L and update our plot to reflect this new data range:

These new contour plot controls provide additional tools that can assist the user in producing customized report- or presentation-ready graphics from your TS-CHEM plume transport modeling analyses.

To learn more about the new features of TS-CHEM v2023-1, or to download a FREE DEMO VERSION of the software, visit the TS-CHEM Website today!

State and Federal Guidance Spotlight - Use of TS-CHEM for Estimation of NJDEP Classification Exception Areas

 Introduction

There are a number of state and federal regulatory documents that prescribe the use of solute transport models to support site investigation and remediation activities, along with guidelines on the types of analyses that need to be performed, models that should be utilized, and in some instances, specific input parameters that must be incorporated into analyses. Common applications of solute transport models in state and federal guidance documents include analyses to estimate the expected extent and duration of groundwater plumes, whether sensitive receptors may be impacted, and as a line of evidence to support Monitored Natural Attenuation (MNA) evaluations.  In this first post in the “State and Federal Guidance Spotlight” series, we take a look at the New Jersey Department of Environmental Protection’s (NJDEP’s) Classification Exception Area Guidance, and how TS-CHEM can be used to estimate plume extent and duration to support the delineation of Classification Exception Areas for sites where impacts to groundwater may be present.

Overview of NJDEP CEA Guidance

In the state of New Jersey, groundwaters of the state are classified according to a combination of natural characteristics and actual or potential uses, and groundwater quality standards (GWQS) have been established for these classification areas to ensure that the characteristics and/or actual and potential uses are protected. In instances where GWQS may not be met in a particular area (e.g., an area where impacted groundwater may be present as a result of a discharge of contaminants into the subsurface), the NJDEP requires the establishment of a Classification Exception Area (CEA), which provides notice that the constituent standards for a given aquifer classification are not (or will not) be met over a particular localized area, and that designated aquifer uses in the affected area are suspended for the duration of CEA term.   

CEAs have three main components, including 1) delineation of the horizontal and vertical boundaries of the affected exception area; 2) identification of all groundwater constituents of concern (COCs) to which the exception applies; and 3) an estimate of the longevity of the CEA (i.e., the duration in which COCs will remain above GWQS within the exception area). In the event that the designated use of groundwater within the CEA includes potable use, the NJDEP will identify the CEA as a Well Restriction Area (WRA), which functions as an institutional control by which potable use restriction can be effected (though the NJDEP will not typically prohibit the installation of wells in WRAs).

Appendix A of the NJDEP CEA Guidance provides an overview of the methods that may be used for CEA delineation, and in particular, how to estimate the amount of time required for COCs to reach the GWQS, and the distance in which COCs are anticipated to migrate. With regard to the latter, as long as a sufficient amount of sampling data have been collected, the NJDEP recommends that a “best-fit” methodology to estimate COC attenuation rates (such as the methodology described in the 2003 USEPA issue paper “Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies”), which can be used to calculate the anticipated time to reach the GWQS.  With regard to the estimation of the maximum distance a plume is expected to travel, the NJDEP identifies several different numerical models that may be used, as well as a more simple analytical solution, which is often utilized by environmental practitioners responsible for the delineation of CEAs at sites in the state of New Jersey.  As discussed in a recent TS-CHEM Blog Post, however,  a 1-dimensional analysis of contaminant transport (such as the one described in Appendix A of the NJDEP CEA Guidance) often greatly overestimates plume extent, resulting in an overly-conservative assessment.  Fortunately, TS-CHEM provides a library of analytical and semi-analytical solute transport modeling solutions that can be used to estimate both the length and duration of groundwater plumes more accurately, without the steep learning curve typically associated with more sophisticated numerical models.

CEA Delineation Using TS-CHEM

As noted above, TS-CHEM, with a library of more than 30 analytical solutions, is perfectly suited to analyze the length and duration of groundwater plumes to assist with CEA development.  The library of solutions allows for the selection of model that is best suited to conditions at a particular site.  For example, at a site where source concentrations appear to be attenuating exponentially, a model like BIOSCREEN-AT may be a good fit.  Or, if source concentrations are variable over time (e.g., as a result of site remediation activities), a model like ATRANS4 may provide the best solution.  And since these models are semi-analytical solutions that analyze the transport of COCs in three dimensions, they are not overly conservative, and as such, will not result in an estimated plume length (and CEA delineation) that is much larger than it is realistically likely to be.

In addition to allowing for the selection of a model solution that best fits conditions at a site, TS-CHEM’s built-in charting and mapping tools can be used to generate all of the necessary output for documenting your analyses (as required when submitting a CEA application), including:

• Charts showing concentration vs. distance and concentration vs. time
• Plots showing plume extents through time
• Mapping tools that allow for the overlay on an interactive digital map of the maximum plume extent in the direction of groundwater flow; or the mapping of plume extent through time to support an application at some point in time for a reduction of the CEA area

Figure 1 - Use of TS-CHEM mapping tool to overlay depiction of maximum plume extent in the direction of groundwater flow

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

TS-CHEM Example Applications – Receptor Impact Assessment

TS-CHEM is not only simple and easy to use, but it can be applied to a number of different common  situations when it comes to impacted  groundwater, making it the perfect tool for environmental professionals who perform groundwater investigation and remediation activities. To show how TS-CHEM can be used to evaluate a number of common groundwater issues, we have created a series of Example Applications.  In this blog post, we introduce the first Example Application in the series, which demonstrates how TS-CHEM can be used to model the length of a contaminant plume (in this case a benzene plume) in order to determine if it will impact domestic wells at a nearby residential development. You can download the model files and accompanying overview slide deck for the example application described below HERE.

 

Overview

In Example Application 1, there has been a small leak beneath a dispenser island at a gasoline station. This leak has led to a release of benzene into the groundwater below the site with the source concentrated near monitoring well MW-4. A residential development is located 1200 ft (1/4 mile) away from the leak location, and there is concern as to whether domestic wells may be impacted by benzene.  TS-CHEM can be utilized to determine whether the dissolved benzene plume from the dispenser release will reach the domestic wells above the drinking water standard of 5 ug/L.

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

Setting up The Model

Considering the benzene release recently occurred and no remediation has taken place yet, we want to treat the contamination source as constant source rather than a decaying or transient source. Therefore, ATRANS1 is the perfect model solution for this scenario since it is a continuous and constant source model, and it will allow for a conservative evaluation of the maximum extent of the benzene plume.

Once we’ve chosen ATRANS1 as our model solution, we need to input all of the necessary model parameters that have been measured or estimated from a preliminary investigation of the station, as shown below:

• 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

 

Analysis 1

First, we set two model observation points downgradient from the source at 600 ft and 1000 ft away. This will allow us to view the rise and stabilization of the benzene plume at these locations. Next, we run the model for a duration of 10 years. Once the model is finished running, we can display the concentration versus time (C v t) chart:

Fig 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 (red).

The dark blue line shows that benzene concentrations at MW-1 where groundwater leaves the station property stabilize at around 76 ug/L, while our observation points we set at 600 ft and 1000 ft show benzene concentrations stabilizing around 11 ug/L and 3 ug/L, respectively. This chart shows how the stabilized concentration of benzene decreases as you move further away from the gas station.

Now, let’s take a look at our contour chart set to model year 10:

Fig 3. TS-CHEM’s contour chart indicates that the maximum plume extent (bound at 5 ug/L) does not reach the residential area.

In the chart above, we’ve set the plume boundary to 5 ug/L, which in this example, is the applicable drinking water standard for benzene. Notice how the maximum extent of the plume does not reach the neighborhood under these model conditions. In this case, one might conclude that based on the results of the analysis, the benzene contamination at the gas station does not impact receptors (domestic wells).

Analysis 2

As noted above, the initial analyses performed using ATRANS 1 indicate that groundwater concentrations are not likely to reach downgradient receptor wells above 5 ug/L. However, let’s say that local regulations impose a different standard, and we must now evaluate a drinking water standard for benzene of 1 ug/L. Accordingly, we will need to perform an additional analysis to evaluate whether downgradient receptor wells may be impacted at concentrations above 1 ug/L.

Since nothing at our site has changed (including aquifer characteristics, source concentrations, and source size), we just have to adjust our outer plume contour from 5 ug/L to 1 ug/L. Once we’ve changed the plume contour, we can re-examine the contour chart at model year ten:

Fig 4. TS-CHEM’s contour chart indicates that the benzene plume, with a boundary of 1 ug/L, will impact the residential area.

As shown in Figure 4 above, the plume visibly extends much further to the east than it did in Analysis 1 (Figure 3), and therefore, our conclusion has changed; under this more stringent drinking water standard, the benzene spill at the gas station may impact downgradient receptors. But as noted above, these analyses are very conservative.  What if more reasonable inputs are used?

Analysis 3

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 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.

As shown in Figure 5 above, the C v t chart shows that benzene stabilizes around 35 ug/L at MW-1 near the station property boundary, which is about half the concentration observed in the previous analyses. Additionally, the benzene barely registers at our downgradient observation points. Now let’s look at the contour chart to examine the maximum extent of the plume using this higher degradation rate.

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?

In the case of a leak or spill of hazardous substances, government agencies may require some form of receptor impact evaluation in order to protect people and ecological receptors from exposure to contamination. TS-CHEM is a quick and easy, and science-base, way for any environmental professional to assess potential risks to receptors, evaluate the effect of changes in input parameters, and support decisions regarding the possible need for remedial actions. With over 30 solution models to choose from and a library of the most commonly modeled constituents, you can select an appropriate model and quickly (and easily) perform a receptor impact evaluation at your site!

To learn more about TS-CHEM, or to download a FREE DEMO of the software, visit the TS-CHEM Website! If you would like to see what else TS-CHEM can do, check out our other Example Applications!

1-D vs 3-D Transport Analysis of Contaminant Plume Extent

 A 1-D analysis of contaminant transport often greatly overestimates plume extent, resulting in an overly-conservative assessment.

A common question related to contaminant plumes: “How far downgradient will the plume extend?”

We may want to know this for several reasons:

•       Will the plume reach a receptor at some distance from the source?

•       How far should I place monitoring wells to measure the plume?

•       And in some states (e.g. NJ) how far should I draw the boundary of the groundwater Classification Exception Area (CEA).

Possible Methods

To analyze plume extent from the source over time, we can perform screening analyses using models ranging from simple analytic 1-D solutions, to 3-D analytic and semi-analytic solutions. We could also use more costly and sophisticated 3-D numerical fate and transport models - - but to examine the difference between a 1-D model and a 3-D model, analytical solutions are perfectly capable analysis tools.

1-D Plume Transport Modeling

For many decades, from the 1970s through present, investigators and regulators have often explored the movement of a dissolved chemical plume extending downgradient from a steadily-releasing source using some form of mathematical solution to the 1-D transport equation (e.g. Ogata and Banks 1961; Bear 1972 & 1979). The often cited Ogata and Banks 1961 solution is a simple representation of advective-dispersive transport that does not incorporate the processes of adsorption (retardation) nor degradation. The Bear 1972 & 1979 solutions are more useful because they incorporate the effects of dispersion, retardation, and degradation.

1-D Example Application 

To illustrate the plume length that a 1-D model would calculate, a simple example was developed in which the Bear 1-D transport solution was used to calculate the location of the 5 ug/L plume boundary of a benzene plume 5880 days (16 years) after release (Fig 1). Source benzene concentration was assumed to remain constant at 1000 ug/L; groundwater velocity was set to 1 ft/d and retardation factor was set to 2; 1-D aquifer dispersivity was 50 ft; and a conservative benzene plume half-life of 2 years was applied. 

Fig 1. 1-D calculation shows plume extends 2900 ft from the source after 5880 days.

3-D Plume Transport Modeling

Another approach often employed by investigators or regulators is to begin with the full 3-D transport equation and apply that 3-D solution to calculate concentration along the centerline of the plume to determine the extent of the plume in the downgradient direction.

One of the widely known solutions to the 3-D transport equations was developed by Domenico in 1987. This solution forms the basis for a number of models used by regulatory agencies to estimate contaminant plume movement:

•     BIOSCREEN developed by USEPA (see USEPA website for BIOSCREEN v1.4)

•     BIOCHLOR (USEPA) v 2.2

•     Quick Domenico model described by PADEP in certain of its regulatory documents (see PADEP website Quick Domenico spreadsheet model)

3-D Example Application 

So, we can now solve the same plume transport problem illustrated in Fig 1, but we will instead use the 3-D Domenico solution model (instead of 1-D). Transverse dispersivity is set to 1/10^th the longitudinal dispersivity and vertical dispersivity is set to 1/1000^th the longitudinal dispersivity (these parameters were not present in the 1-D model). The 3-D transport solution calculates a much shorter plume length; 1280 ft versus the 1-D length of 2900 ft (Fig 2).

Fig 2. 3-D calculation shows plume extends 1280 ft from the source after 5880 days.

Newer 3-D Methods

Analytical

In recent years minor discrepancies have been reported between the Domenico solution and more rigorous solutions to the 3-D transport equations (see for example (Guyonnet and Neville 2004; West et al 2007; Srinivasan et al 2007; Karanovic et al 2007; Devlin et al 2012). The discrepancies occur primarily along the centerline axis of the plume; this means that errors may be introduced when attempting to estimate the plume length (i.e. how far downgradient from the source contamination may extend).

Several investigators have modified the original Domenico 3-D transport solution to attempt to mitigate the errors caused by the original formulation. For example:

•     BIOSCREEN (USEPA) was updated to BIOSCREEN-AT (Karanovic & Neville 2007)

•     Srinivasan, Clement & Lee (2007) published an updated version of the Domenico solution

Semi-Analytical

The 1-D and 3-D solutions we have examined to this point are analytic solutions. Certain simplifications are made in the formulation of the transport differential equation that allows it to be solved in closed form - - i.e. the solution does not contain an integral term; the algebraic equation can be solved in a spreadsheet

There is a class of more rigorous solutions that are not simplified and still contain an integral term; and because of that, they are more accurate. These solutions are typically solved in a simple program that employs a numerical integration routine to arrive at the calculated concentrations.

Examples of these semi-analytical transport models include:

•       ATRANS

•       BIOSCREEN-AT (Domenico 1987 solution modified by Karanovic et al 2007)

•       3DADE (USDA 1994)

•       N3DADE (USDA 1997)

•       AT123D-AT (Yeh 1984 solution modified by Burnell et al 2012)

These solutions have been assembled into a unified user interface in TS-CHEM. They provide a means of calculating more accurate estimates of contaminant plume extent for environmental assessments.

To summarize: Contaminant plume analyses based on 1-D models are likely to greatly overestimate plume extent. This may result in an overly-conservative assessment that causes concern, or results in actions, related to impacts that are not likely to occur. More accurate evaluations of plume extent can be calculated using 3-D contaminant transport model. TS-CHEM provides a library of over 30 analytical 3D plume transport solutions for making these types of evaluations.

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

 

TS-CHEM Solution Library - ATRANS

The TS-CHEM program includes a comprehensive library of more than 30 different analytical solutions, each with different capabilities, including how they represent contaminant sources. For this first post in the Solution Library series, we will be focusing on the ATRANS family of models developed by Chris Neville at SS Papadopulos & Associates. In fact, the ATRANS1 model is included in the DEMO version of TS-CHEM, so you can try it out for yourself at any time!

 

What is ATRANS?

ATRANS is a suite of analytical model solutions that are used to simulate three-dimensional advective-dispersive transport from a patch source along the inflow boundary of an aquifer, as show in the conceptual model below:

Figure 1 - ATRANS conceptual model

TS-CHEM includes all four ATRANS models: ATRANS1, ATRANS2, ATRANS3 and ATRANS4. All ATRANS models are based on the following key assumptions:

• Finite aquifer bounds (semi-finite in the x-direction, finite in the z-direction)
•  This essentially places no-mass-flux boundaries on the upper water-table boundary and the lower aquifer base boundary
• Uniform one-dimensional flow along the x-axis
• First-order reaction kinetics (e.g. biodegradation)
• Chemical sorption onto the aquifer material
• Rectangular patch source areas with user-specified concentration

What is the difference between the ATRANS models in the ATRANS package?

The main difference between the different ATRANS models is in how they handle the source concentrations over time. Taking from the Model Features Table located in Appendix D of the TS-CHEM User Guide we can see that in the ATRANS models the source can either be constant, decaying, or transient (time varying):

Solution Model	Source vs Time
	Constant Source	Decaying Source	Transient Source
ATRANS1	X
ATRANS2		X
ATRANS3			X
ATRANS4			X

With this table in mind, we can take a visual look at how these sources are represented with figures of concentration over time from the ATRANS user manual and the Model Selection Tool in TS-CHEM:

Figure 2 - ATRANS model inputs: source concentrations over time

You may have noticed that ATRANS3 and ATRANS4 both handle transient (time-variable) source concentrations. The difference between these two models is in how the transient concentration data is introduced to the model. ATRANS3 asks the user to input a series of time-concentration pairs that define the source concentration history (for example from measured values at certain points in time.  The software then creates discrete time steps with histograms that mimic the continuous data, with the specified concentration point at the center of each histogram bar. ATRANS4 asks the user to input a series of time-concentration pairs, but for this model the user is specifying the concentration level at the start of a histogram bar that remains in effect until the next starting time and concentration is specified (for example from historical knowledge of the starts of spills or releases at a site source at certain points in time).

What applications are the ATRANS models best suited for?

As discussed in the previous blog post “TS-CHEM – The Swiss-Army Knife of Solute Transport Modeling” environmental professionals are often tasked with developing conceptual site models (CSMs) that characterize the extent and behavior of groundwater contaminant plumes, which the ATRANS models can assist with. In particular, the ATRANS models are useful for representing sites where the aquifer thickness is known or is believed to be bounded at a finite depth by an impermeable base (finite aquifer boundary)and where the investigator has information on concentration at the downgradient edge of a source area (patch source). If only a single source concentration is known, and/or the investigator wished to perform a conservative analysis, a constant source ATRANS1 model can be applied. If the source is understood to be flushing and depleting through time, an exponentially decaying ATRANS2 source model can be applied. And if information is available on the changing history (both increases and decreases) of source concentration with time, then an ATRANS3 or ATRANS4 model can be applied, as described further below.

 

The ATRANS1 model is useful for simulating scenarios that can be represented by constant concentration sources. For instance, it can be used to support remedial design by evaluating a conservative maximum plume extent and when the plume becomes stable. It can also be used for regulatory compliance by conservatively simulating potential receptor well impacts, or to assist with the delineation of groundwater Classification Exception Areas (CEAs).

Figure 3 - ATRANS1 solution showing maximum plume extent and stability for a benzene plume with a constant source at 200ft, 600ft and 1000ft from the source

The ATRANS2 model is useful for simulating scenarios that can be represented by an exponential decay in the source concentration and is one of three models in the TS-CHEM library that can do so (the others being BIOSCREEN-AT and BIOSCREEN-AT NI). For example, if an environmental professional wanted to simulate a scenario where a benzene plume from a small spill source that is flushing and degrading with time, they could use TS-CHEM and ATRANS2 with a decaying source to see how a Monitored Natural Attenuation (MNA) remedy would reduce plume concentrations over a ten-year period, particularly at locations close to the source:

Figure 4 - ATRANS2 solution showing benzene plume concentrations after natural source flushing and MNA at 200ft, 600ft and 1000ft from the source

The ATRANS3 and ATRANS4 models are two of only four solutions in the TS-CHEM library that can account for transient source concentrations (the others being AT123D-AT FT and AT123D-AT IT which differ from the ATRANS models in that AT123D-AT models employ mass flux specified sources in unbounded aquifers). The ability to have transient source concentrations allows for the simulation of intermittent single sources, multiple sources that occur at different (or overlapping) times, and even termination of a source as would result from a source removal remedy. In the example below, ATRANS4 was used to simulate effects on a benzene plume as a result of remedial activities, where the source was ceased after 180 days (to account for source removal as part of active remediation) and MNA was able to reduce the concentration of the plume to below 5 ug/L in the entire plume after just 2.5 years.

Figure 5 - ATRANS4 solution showing benzene plume concentrations after source remediation and MNA at 200ft, 600ft and 1000ft from the source

To summarize: the ATRANS family of models, which are built-in as part of the TS-CHEM solution library, allow for flexible representation of the source through time. The models assume a finite aquifer boundary, so they are ideal for bounded aquifer models. The choice of model will depend primarily on how the concentration of the simulated source changes over time, as ATRANS models allow for sources that are constant, exponentially decaying, or even time-variable. The ATRANS models allow environmental professionals to evaluate plume characteristics for a variety of groundwater plume transport scenarios.

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