> Margaret here again, from CSM. I'm still struggling with making biodegradation work in phreeqc. The decay equation I'm using is benzene(final) = benzene(initial)*e^(-rate*time). If C = C0e^-kt, then dC/dt = -kC, which is linear decay. > I'm not sure what "TOT" means (total moles of benzene in the system, moles of aqueous benzene?) TOT("Benzene") is the total molality of benzene (i.e. mol/kgw, p. 128 of manual). > and what the term "moles" represents and how it is recycled in the transport function. For a kinetic reaction, PHREEQC expects to know how many moles are reacted over a period of time given by TIME, moles = dC/dt*kgw*TIME, but normally the system contains 1 kgw, so that factor is dropped. > The way I have the RATES block set up, biodegradation is occurring to some extent. However, the 1/2 life for benzene is a fraction of a day, so after the first time step (3 years), no benzene should exist and yet it does. > I've included my input deck and database. How old are you? I have been trying to forget about card decks for many years. You don't want to go back to that. > If there's some obvious problem, can you let me know? I think your input file should look like this. Given the fast degradation rate, the long time step doesn't show you much. Note that there is -tol in the kinetics definition, that controls how accurately you want to integrate the rate along with a cutoff of 1e-15 that I've left in the rate expression. SOLUTION 0 Pulse solution leachate #solution injected units ppb pH 7.0 density 1.00 temp 10.0 Na 620000 Cl 752000 Ca 304000 Mg 300000 K 317000 Benzene 13.4 Benzdp 0.0 END SOLUTION 1-6 Background solution initially filling column units ppb pH 7.0 density 1.00 temp 25 Na 1966000 F 390 Cl 3033000 Ca 468000 Mg 528000 K 15900 END SELECTED_OUTPUT -file grid3.csv -selected_out true -high_precision false # set value for all indentifiers to follow (lines 1 - 6) -reset true -simulation true -state true -solution true -distance true -time true -distance true -step true -percent_error true -totals Cl Na Benzene Benzdp -gases Benzene(g) END SURFACE 1-6 Surfa_sH2O 0.03 300 0.03 KINETICS 1-6 Benzene -formula Benzene -1.0 Benzdp 1.0 -tol 1e-15 # -M 1.71E-7 # -M0 1.71E-7 INCREMENTAL_REACTIONS true RATES Benzene -start 10 Benzene = TOT("Benzene") 20 if (Benzene < 1e-15)then goto 200 30 rate = 2.5/(60*60*24) * Benzene #day-1 to sec-1 40 moles = rate * TIME 50 if (moles > Benzene) then moles = Benzene 200 SAVE moles -end TRANSPORT Pulsing of Solution 0 -cells 6 -shifts 6 -time_step 1.05E8 -flow_direction forward -boundary_conditions flux flux -length 6*11.0 -dispersivities 6*18.0 -correct_disp true -diffusion_coefficient 0.3e-9 -stagnant 0 -thermal_diffusion 1 -initial_time 0 -print_cells 6 -print_frequency 1 -punch_cells 1 2 3 4 5 6 -punch_frequency 1 END > If not, do you have any free time this week when we could meet for perhaps an hour and discuss the intricacies of phreeqc? My schedule is open except for Thursday morning and Friday after 3:00 pm. I'm feeling swamped right now, but I'll continue to answer your email. David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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