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!