מלכים ב ב:כא פירוש: רש״י

To the source of the water. From where the water flowed.

And threw salt there. Now is not salt a thing that spoils water? This was a miracle within a miracle.²⁰This miracle was similar to the one performed by Moshe in Marah. After traveling through the wilderness for three days without water, the Bnei Yisroel finally came to a spring only to find the water undrinkably bitter. Moshe threw a tree whose wood was bitter into the water and the water became sweet and drinkable. See Shemos 15:22-26.—Radak