# Alkalinity in Water

If you add Alum - $\ce{Al2(SO4)3}$, to a clay water sample which contains $\ce{Ca(HCO3)2}$ - due to the water's alkalinity, the pH of this water sample would drop as Alum drops the pH levels. How would you counteract this imbalance. As in, what could you add to the sample to keep the pH the same as it was before the addition of Alum.