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weird phreeqc question

Hi David,

Something weird is going on with my phreeqc simulations.  Biodegradation will 
work if coupled with dispersion, sorption, and/or gas phase.  It will NOT 
work if I take those processes out.  The error I'm getting is that there are 
negative moles in solution after transport step 9.  How can there be negative 
moles in solution if I have the line "If Benzene <= 0 then goto 200?"  I've 
changed the 0 to 1e-30, 1e-10, etc.  and it seems that phreeqc isn't even 
reading that line.  Any suggestions?

Attached is my input file (not deck!!):)
Thanks a lot,
Margaret Dodds
SOLUTION 0 Pulse solution leachate	#solution injected
	units		ppb
	pH		7.0
	density	1.00
	temp		10.0
	Benzene	13.4
	Benzdp	0.0
SOLUTION 1-10	Background solution initially filling column
	units	ppb
	pH 7.0
	density 1.00
	temp	10.0
	-file			grid2.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	Benzene Benzdp
	-gases	Benzene(g)
#SURFACE 1-128
#	Surfa_sH2O	20.9	300	20.9
#GAS_PHASE 1-128
#	-fixed_volume
#	-volume	1.0
#	Benzene(g)	0.0
#	CO2(g)		0.000316
#	N2(g)			0.78
#	O2(g)			0.2	

	-formula	Benzene	-1.0	Benzdp	1.0
#	-tol	1.0E-25
	-M	1.71E-7
	-M0	1.71E-7
	10	Benzene = TOT("Benzene")
	20	if (Benzene < 0)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
TRANSPORT Pulsing of Solution 0
	-cells				10
	-shifts				10
	-time_step				4.93E5
	-flow_direction			forward
	-boundary_conditions		flux	flux
	-length				10*1
	-dispersivities			10*0.1
	-correct_disp			true
	-diffusion_coefficient	0.3e-9
	-stagnant				0
	-thermal_diffusion		1
	-initial_time			0
	-print_cells			10
	-print_frequency		1
	-punch_cells			1-10
	-punch_frequency		1

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