Unexpected average age/ residence time question

[Adam Haynes]

Hi,

I am calculating different residence times throughout my domain. Below is an example of calculations over 50 years. I was expecting a plateau but am getting a cyclic pattern, and I am not sure why/if this is correct. It should be noted that this is a tidal system, and I am observing the upper 10 cm of sediment. 

 To get an average age I have been using the suggested Tracer_Age [M] model/Tracer [M] model from the theory guide:

"Output is given in terms of  α and C from which the mean age A can be obtained as A
= α/C."

I also used the initial concentrations from the theory guide:

    Tracer     1.e-8        F
    Tracer_Age 1.e-16       F

However, I used total aqueous component instead of free ion and am unsure how that impacts this calculation. 

Thanks! 

[Hammond, Glenn E]

Peter,

 

Any thoughts on this?

 

Glenn

 

From: 'Adam Haynes' via pflotran-users <pflotra...@googlegroups.com>
Date: Tuesday, March 4, 2025 at 9:53 AM
To: pflotran-users <pflotra...@googlegroups.com>
Subject: [pflotran-users: 8310] Unexpected average age/ residence time question

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[Hammond, Glenn E]

Adam,

 

A couple questions/comments:

 

1. If the boundary condition is cyclic (tidal), why would you expect a plateau?
2. Can you set up a scenario without the tidal fluctuation (steady state boundary condition) and obtain the expected behavior?
3. Total and free ion concentrations are the same when there are no secondary aqueous complexes (SECONDARY_SPECIES).

 

Glenn

 

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[Adam Haynes]

Turns out it was an input error!

[Adam Haynes]

Glenn,

I expect a plateau/short-term behavior because I am averaging the yearly ages using the same annual inputs. Over enough simulation years, the system should reach some quasi-steady-state pattern. However, given the cyclic (tidal) boundary condition, I might expect an oscillating steady state rather than a constant plateau. For more context, these calculations come from the top of my domain in a salt marsh system, where these cells are flushed from 'inundation' on tidal time scales and get seasonal flushing from terrestrial groundwater inputs. The variation in average age would more likely be a few months on a year-to-years scale rather than 1-5 years over a 20-year cycle, for which I can not come up with an explanation.

 
After revisiting my inputs, I got the attached result, which I more or less expected (shorter residence time and more stability). 

Additionally, I am trying to run the simulation for 100 years to determine if this is a true 'steady-state,' but I keep getting this error:

  Setting up TRAN Realization
  Finished setting up TRAN Realization

ERROR: For flow condition "evapotranspiration_flow_conditio" dataset "rate", the number of times is excessive for synchronization with waypoints.

Stopping!
WARNING! There are options you set that were not used!
WARNING! could be spelling mistake, etc!

I am assuming this is happening because my ET flow conditions (RATE SCALED_VOLUMETRIC_RATE VOLUME/SOURCE_SINK) are at daily steps vs my other flow conditions (LIQUID_PRESSURE SEEPAGE/BOUNDARY_CONDITION), which are 1800 sec. However, I don't know how to fix the issue. Any ideas?

Thanks for the response!

Adam

[Hammond, Glenn E]

Adam,

 

The issue is that your datasets are very large. You can remove the error message, but the simulation may run out of memory or take too long to initialize. A workaround is to use a CYCLIC dataset.

 

Glenn

 

From: 'Adam Haynes' via pflotran-users <pflotra...@googlegroups.com>
Date: Thursday, March 13, 2025 at 2:33 PM
To: pflotran-users <pflotra...@googlegroups.com>
Subject: Re: [pflotran-users: 8325] Unexpected average age/ residence time question

Glenn,

 

I expect a plateau/short-term behavior because I am averaging the yearly ages using the same annual inputs. Over enough simulation years, the system should reach some quasi-steady-state pattern. However, given the cyclic (tidal) boundary condition, I might expect an oscillating steady state rather than a constant plateau. For more context, these calculations come from the top of my domain in a salt marsh system, where these cells are flushed from 'inundation' on tidal time scales and get seasonal flushing from terrestrial groundwater inputs. The variation in average age would more likely be a few months on a year-to-years scale rather than 1-5 years over a 20-year cycle, for which I can not come up with an explanation.

 
After revisiting my inputs, I got the attached result, which I more or less expected (shorter residence time and more stability). 

To view this discussion visit https://groups.google.com/d/msgid/pflotran-users/91629784-900d-4158-bb80-6728629793den%40googlegroups.com.