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what types of minerals would dissolve or precipitate based on time series data of GW from a monitoring well

Kirkoff Xn

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If surface water is injected into an aquifer with a known composition (which may change with injection time) and at X distance from the injection well, groundwater data (considered a mixed surface water-native groundwater) is collected every day from a monitoring well. How is it possible to infer/know mineralogical assemblages of the aquifer that are dissolved or precipitated based on the time series of groundwater data collected from the monitoring well? Aquifer type: Carbonates with small amount of evaporites.


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I am working on data from monitoring well on an Aquifer storage and recovery project. With injection of surface water, there is a trend in freshening of the native "brackish" groundwater based on TDS values. I am wondering whether the geochemical evolution of hydrochemical facies result from simple mixing or from dissolution/precipitation of aquifer minerals. I did try to work with React by mixing both fluids ,with the surface water being the reactant and trace the evolution of mineral saturation versus mass of freshwater that is being flashed into the aquifer. It a two-layered aquifer separated by an unconfined layer(~semi confining layer). I did mixed fresh surface water with native groundwater from both aquifers(upper and lower aquifer layers). I have got different results based on mineral saturation vs Mass reacted H2O.My question is why it takes much water injected to the lower aquifer than the upper aquifer? As gypsum and anhydrite are undersaturated from Mass reacted 0(zero) does it mean that the lower decreased trend results from mixing(~dilution?). Thanks

Lower and native surface.PNG

Upper and native surface.PNG

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Thank you for providing more details. I am guessing from your post above that you are creating a flash diagram, where the basis pane is the chemical constraint for native groundwater and the reactant is the fresh surface water. Based on plots of mineral saturation indices (from two simulations with different native groundwater fluid), From your plots, I am also assuming that you have turned off precipitation to not allow any minerals from forming. 

Based on your plots alone, the more H2O is reacted, the saturation indices for each mineral falls. Please note that, this information alone is not sufficient to determine which mineral will precipitate or dissolve and if so, the quantity. The mineral SI only informs the thermodynamic tendency for a mineral to precipitate or dissolve given the current solution composition. If you are mixing a dilute fluid with your brackish groundwater, then the mineral saturations will drop as your overall fluid contains higher portion of the dilute fluid. 

I am not sure what you mean by " My question is why it takes much water injected to the lower aquifer than the upper aquifer?" If the composition of your starting fluids are different, then they should show different results as your mix in fluid. The extent of their difference would depend on their composition. Could you clarify what you mean with your question? Or provide more context on what you mean? 

Here are some suggestions that might help:

The flash configuration in React is used for mixing all proportion combinations of the fluid specified in the Basis pane and the fluid specified in the Reactants pane. You can plot x-axis (or y-axis if you like) as the mixing fraction between the two fluids. This case, you can see the various results (species concentration, mineral saturation, etc) as a function of mixing fraction and compare it to your existing dataset. This would help you get a general sense of the properties of the mixed fluid. 

If you wish to investigate mineral precipitation and dissolution with React, you would need to check the precipitation box in the Options dialog in React. Any minerals present in the system would be reported and can be plotted in your results. For more information on this, please refer to 

If you are interested in dissolution of minerals in the aquifer, you would need to include them in your simulation. Can any mineral be assumed to be in equilibrium? Are there any reactions that need to be kinetically constrained? 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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For more information on this, please refer to..., what's the reference?


Sorry for the question.. I see where I made a mistake. I put the unit of mass reacted in mol instead of kg; and you are right on the constraints I did make! My main question is to what extent mixing both fluids would precipitate minerals....Is the pattern I am seeing on the time series species data controlled by simple mixing (dilution?) or also precipitation..and to what extend the precipitation starts to impact the concentration in the mixed aqueous fluid... Dolomite would be more likely in equilibrium with the native groundwater. If I mix for example 70% surface water with 30% native groundwater, where can I see the end result of different constituents in the mixed surface water-groundwater (other than plotting them? Is it on "view results" Step #100 , aqueous species?  

And If I want to simulate freshening of groundwater around the ASR well, I think the proper process should start with surface water composition(injectate)..basis..run> pick up >reactant...reactant times..70 times(to mimic high % surface water mixing or flashed into the native water in the vicinity of the well)...

For example, I simulate mixing of 70% surface freshwater with 30% native groundwater, taking into account your recommendation(check precipitation). Y is in log..see the snapshot below. From the figure below, anhydrite and gypsum tend to dissolve more than calcite and dolomite, dolomite being the most stable(in equilibrium with the mixed fluid)...does it mean that the dissolution of both minerals(anhydrite and gypsum) and calcite is the main controlling factor of the mixing process? It is also possible that dolomite precipitated as I checked precipitation..how can I get the estimate of the dolomite that has precipitated?

And if I add minerals, there are two carbonates, calcite and dolomite,...I can swap Ca2+ with Calcite...what about dolomite? can I swap HCO3- with dolomite?






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Thanks for the additional information. I apologize for the sentence getting cutoff. I was going to refer you to section 2.3 for React examples in the GWB Reaction Modeling user guide.

You can plot various results in Gtplot on one axis vs. the mixing fraction between two fluids on the other. The mixing fraction is only available when you use the flash configuration. You can find "Mixing Fraction" in Gtplot under the Variable type. To find dissolved species concentration, you can select the "Species concentration" variable type for one of the axes. FYI, you can export all numerical data from your Gtplot. To do so, go to the "Edit" tab -> "Copy as" -> "Spreadsheet" or "Tab" depending on where you are pasting your information. See more information on Gtplot in the GWB Essentials user guide. 

I am not sure that you need to set reactant times here. Enabling the flash configuration will have React gradually remove the original fluid from the equilibrium system as it titrates in the reactant fluid. If you plot a flash diagram (i.e. with mixing fraction as one of the axis), then you can see the corresponding variable type (e.g. Minerals, Species concentration, etc) at each mixing proportion. For more information on the flash setting, please see section 3.8 "Flash Diagrams" in the GWB Reaction Modeling user guide. You may also find the "Fluid scaling and mixing" on the GWB Academy (https://academy.gwb.com/mixing_and_scaling.php) useful. If you do not enable the flash setting then you would be just titrating in amount of reactant specified into your system incrementally. In that case, you would want to make sure that the total amount of reactant to your basis is in the correct proportion. 

If any mineral precipitates in your system, you can plot it under the variable type "Minerals". If no minerals are present, then this option would not appear in Gtplot. 

Swapping is used to set your fluid in equilibrium to a mineral or gas in your basis constraint for that component. Typically this is done when you have information that allows you to make those assumptions. An example would be swapping CO2(g) fugacity for H+ to constraint the pH. You can certainly swap HCO3- for dolomite but is it in equilibrium with the fluid along with calcite for Ca++? Please see section 2 Configuring programs in the GWB Essentials user guide for more information about how the basis in GWB programs are configured. 

Hope this helps,

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After several tries , I found that reaction progress of 0 to 80% matches the time series data of all cations and anions. Now my question: I have time series of Ca, Mg, HCO3...concentrations varying with time...while in react, the simulation the reaction progress of one fluid mixing with another...while this reaction progress intuitively can translate into time, is it really "geochemically" sound to compare both data?

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