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Phreeqc Alkalinity Question

Dr. Parkhurst,

I have a question regarding how PhreeqC calculates alkalinity.  I am running
a simple speciation calculation on a low pH mine water sample.  When I enter
the solution composition without indicating a value for alkalinity, Phreeqc
calculates a negative alkalinity (-2.551e-3 eq/kg).  If I assign a
concentration of zero alkalinity, the alkalinity result is the same.  If
however I specify a nominal amount of alkalinity (0.0001 mg/L as CaCO3),
Phreeqc adds a significant amount of total carbon to the solution (5.85
mol/kg).  The carbon is essentially all present as H2CO3 with a minor amount
of HCO3- (2.431e-3 mol).

Is a negative value for alkalinity valid?  Conceptually is this equivalent
to the amount of acidity associated with the sample?   This water quality is
to be mixed with other waters to simulate the water quality of a pit lake.
Will inclusion of a negative alkalinity make the results invalid? 

Thank-you for your time,

Cheryl Ross

PhreeqC input file
	SOLUTION	6					
#	SOLUTION	surficial run-in to the pit from the escombrera
	Temp	10					
	pH	2.81					
	pe	5.1		# assume 300 mV			
	redox	pe					
	units	mg/kgw					
	density	1					
	Alkalinity	0.0	as CaCO3				
	Cl	5.26				
	S	2000	as SO4 			
	Co	1.60				
	U 	0.0790	as U			
	Ni	2.70				
	Na	48.0				
	K	9.8				
	Ca	283				
	Mg	194				
	Al	22				
	Fe	71.7				
	Mn	29.4				
	Cu	0.800				
	Zn	7.1				

Cheryl Ross, M.Sc. 
Project Hydrogeochemist
Golder Associates
18300 NE Union Hill Road., Suite 200
Redmond, WA  98052
Office:  425-883-0777 (ext. 2208)
Fax:     425-882-5498

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