Saltwater corrodes firefighting equipment and may harm ecosystems, especially those like the chaparral shrublands around Los Angeles that aren’t normally exposed to seawater. Gardeners know that small amounts of salt – added, say, as fertilizer – does not harm plants, but excessive salts can stress and kill plants.
I’m guessing it’s the volume of water that is the area of an acre times a foot deep.
Freedom units. Equal to 3.2 million big gulps ;)