AD&D 2E Spell Idea:

Acidify Water 5th Level (Alteration) 

Comp - V,S

Range - 120 Yards

Duration - 4 rounds +1 round/2 lvls of the caster

Casting Time - 5

AoE - Special (Same as Raise/Lower Water)

Saving Throw - Neg.

By means of casting this spell, the caster turns a body of water into a body of burning acid. The acid is a foul smelling, sickly green liquid that, for other spell purposes, is also considered to be water as well as poison (This way, a Transmute Water to Dust or a Part Water may affect acidified water). Should someone come into significant contact with the acidified water, they will take damage as if they were hit by a Melf’s Acid Arrow. The acid has no attack or damage bonus, but it inflicts 2d4 points of acid damage (with saving throws for items on the target). (There is no splash damage.) For every three levels that the caster has achieved, the add lasts for another round, unless somehow neutralized, inflicting another 2d4 points of damage each round. So at 9th-10th level, the acid lasts four rounds, at 12th-13th level, the acid lasts for five rounds, etc. The water also stays acid for this duration. So if a pool of water is turned into acid by a 9th level caster, the pool will remain acid for four rounds. 

The water is acidified within a square area whose sides are 10 feet long per caster level. Thus, a 9th-level caster affects a volume up to 18' x 90' x 90', and so on, a 10th level caster affects a volume up to 20’ x 100’ x 100’ and so on. 

The minimum amount of water that may be acidified must be concentrated in at least a 1’ x 10’ x 10’ cube. After that, the water may then be distributed in any way, and still be acid. So this spell may target any body of water or other states of water, but this does exclude water in trace amounts, such as water in the human body or condensation. 

Examples of targets include; A storm cloud raining down, thereby acidifying the rain coming down for the duration of the spell. A pool or moat of water, thus creating a toxic river warding off intruders. Any sort of fog, whether it’s origin is natural or from a spell. Snow, creating a solid cover on the ground of acid.