The water from your tap is highly oxygenated - about 8 ppm O2. It takes 2 moles of SO2 for each mole of O2. Based on the relative MW of O2 and SO2, one ppm of O2 will react out 4 ppm of SO2. You can neutralize an SO2 solution by diluting ti with oxygenated water (pour it down the toilet and flush a few times), or simply by agitating it and giving it O2 from the air.