That's really nice brewing water. SO4 is actually 57 ppm. To treat the alkalinity add 1 mL of 88% lactic acid per gallon, or alternately, add 0.78 mL of 85% Phosphoric Acid per gallon, or 10.5 mL of 10% Phosphoric Acid per gallon.
I should preface that the acid addition advice I listed above is intended for lighter colored beers. Add progressively less for progressively darker beers. For the likes of a big SG and highly robust Russian Imperial Stout you may not need to add any acid.