This spell makes dirty, contaminated water clean and pure, suitable for consumption. Up to one cubic foot per level of the druid casting the spell can be thus purified. All affected water must be within a 10-foot cubic volume.
The reverse of the spell, contaminate water, works in exactly the same manner, and even holy/unholy water can be spoiled by its effects.
Notes: Uncommon for druids; otherwise, unknown (1st Ed. PHB). Most other priests have purify food & drink.