Processes > Dissolution and precipitation

Defining insoluble Organic-Nitrogen in variably saturated flow model

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adityain2003:
Hello everyone,

I have integrated PhreeqcRM into a variably saturated flow model. I am trying to define moles of organic-Nitrogen (insoluble) into every cell of model as a solid substance and then perform kinetics to convert some portion of organic-Nitrogen into soluble Nitrate. I have performed the kinetics operations in the past but I am seek knowledge for the organic-Nitrogen part.

Thank you!

dlparkhurst:
Sounds like you need to define KINETICS and RATES in your input file, and distribute the KINETICS definition to the appropriate cells in the model (InitialKinetics2Module). The number of moles of insoluble nitrogen would be the number of moles of "M" in the KINETICS definition. The RATES definition determines how fast the insoluble nitrogen is converted to reactive nitrogen. You will have to decide on the stoichiometry of the reaction, but releasing the insoluble nitrogen to solution might have -formula NH3 +1, where the 1 mole of NH3 would be released for each SAVEd mole in RATES; one mole of insoluble nitrogen will be decreased for each mole SAVEd moles, such that the value of M will decrease for the kinetic reactant.

adityain2003:
Thank you Parkhurst for quick reply.

I am working with a finite element mesh with variable volume, so the moles in each cell will be different. Is there a way that I can convey the initial moles to PhreeqcRM?

dlparkhurst:
You usually do not use the element volume with PhreeqcRM, rather a representative volume is used for each cell that is usually 1 liter. The concentration units for the transport calculations are set with the method SetUnitsSolution, mol/L. The volume of water for each cell for PhreeqcRM is calculated as the representative volume times the porosity times the saturation. SetConcentrations transfers concentration data to PhreeqcRM, which is converted to moles with the water volume within PhreeqcRM. After reactions (RunCells), concentration data is returned with GetConcentrations. PhreeqcRM usually does not know the total volume of a model element; you will have to do the calculations for mass of element in the element based on concentrations and element volume.

Reactants in the PHREEQC input file are defined in moles. The methods SetUnitsSurface, SetUnitsExchange, and others define how the number of moles in the initial PHREEQC run are translated to moles in the cells of the model.

adityain2003:
Thankyou Parkhurst,

Actually, I meant to ask that if there is any way that I can assign total moles of a species (say organic Nitrogen) for all nodes in the mesh by passing an array in PhreeqcRM. Then these moles of species can undergo kinetics.

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