פירוש על מקואות 1:3

IMPURE WATER FALLS: We learn here that, that of Pooled Water can receive impurity on account of impure fluid which falls into it (the Pooled Water)

If unclean water fell into it and a clean person drank of it, he becomes unclean.
If unclean water fell into it and then water was drawn from it in a clean vessel, it becomes unclean.
If unclean water fell into it and a loaf of terumah fell in: If he washed [his hands] in it, it becomes unclean; But if he did not wash [his hands] in it, it is clean.
Rabbi Shimon says: it becomes unclean whether he washed in it or whether he did not wash in it.
The first three sections of today's mishnah are the same as the first two mishnayot, except in today's case, the water in the pool was defiled because someone poured unclean water into the pool. In the last section of today's mishnah, Rabbi Shimon disagrees with the laws in section three (concerning the loaf) all first three mishnayot.
Section four: Rabbi Shimon holds that the water was not fully absorbed into the loaf. There will be at least a drop that remains on the loaf. This drop will defile the entire loaf. The same would be true in the situations in the previous two mishnayot.

RABBI SHIMON SAYS WHETHER HE RINSES OR WHETHER HE DOES NOT RINSE: R' Shimon holds in all cases, whether the impurity comes as a result of a person, whether by a vessel, or whether by fluid, in all cases he holds that whether (afterward) he rinsed to morsel or whether he did not rinse the morsel, the morsel is impure. The reasoning of R' Shimon is [as follows] - I say that the impure droplet of water which fell in from the mouth of the impure person or from the vessel or from the (impure) fluid, (that droplet) will remain on the morsel once all the other water has fallen of of it, and it (the morsel) will be impure. The halacha is not in accordance with R' Shimon.