For pH < 9 you can consider all the alkalinity to be caused by bicarbonate. At pH 9 about 5% of it is caused by bicarbonate and hydroxyl ions come into play as well. To compute alkalinity divide the bicarbonate by 61 and multiply by 50. To be precise add 2.5. This should be done beacause it is alkalinity that is measured and bicarbonate calculated from it. Even distilled water has an alkalinty of 2.5 or so. Your lab probably didn't take that into account when they calculated bicarbonate and none of the spreadsheets do when they calculate alkalinity from bicarbonate so you can probably safely ignore that nuance.
Example: Bicarbonate 122 mg/L, pH 7. Alkalinity = 50*122/61 + 2.5 = 102.5. The actual alkalinity for this case (assuming the bicarb is all calcium bicabonate) is 101.8.
Example: Bicarbonate 112.2, pH 9. Alkalinity = 50* 112.2/61 + 2.5 = 94.5. The actual alkalinity in this case is 100.
Now where the pH is high like this the report should include carbonate ion as well so the report should read:
Bicarbonate pH 9, bicarbonate 112.2, carbonate 3.6. Add twice the carbonate divided by 60 before multiplying by 50 i.e.
alkalinity = 50*(112.2/61 + 2*3.6/60) + 2.5 = 100.5
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