Denny,
If you dump Muriatic acid onto gravel you'll end up with calcium chloride, water, and carbon dioxide and it won't matter much how far you dilute it. This is probably the best disposal method if you have to dispose of some. Similarly phosphoric acid will react to give you calcium phosphate, water, and carbon dioxide although it may be a little slower. Dumping it onto soil is another story, likely will lower the pH for a "LOT" of ground water.
Dumping it like this, it's not likely that it will ever evaporate enough water to concentrate itself. If you put your diluted Muriatic in a container and tried to evaporate water to concentrate it you will only get to about 20% HCl since HCl and water form a maximum boiling azeotrope. Don't try this at home, believe me, boiling hydrochloric acid is NASTY stuff.
Are you sure it wasn't sulfuric acid you added to lead acid batteries?
