||The only thing that shall [always] remain ritually clean is a mikvah of water, whether it is a [man-made] pit or a [natural] spring. Any other [water] that comes in contact with the dead bodies [of these animals] shall become unclean.
Ach mayan uvor mikveh-mayim yiheyeh tahor venogea benivlatam yitma.
||If their dead bodies fall on any edible seeds that are planted, [the seeds] remain ritually clean.
Vechi yipol minivlatam al-kol-zera zerua asher yizarea tahor hu.
||However, if water has [once] been placed on [such unplanted] seeds, and then the dead body of [any of these animals] falls on them, the [seeds] shall be unclean to you.
Vechi yutan-mayim al-zera venafal minivlatam alav tame hu lachem.
|always remain ritually clean|
Therefore, it can be used for purification. As we see in the next comment, other water would become unclean upon contact with an unclean body (Malbim; Hirsch; cf. Rashi, Pesachim 16a, s.v. Yihyeh).
See Isaiah 22:11. Here it is referred to as a 'gathering (mikveh) of water.'
|man made pit or...|
|Any other water...|
(cf. Ibn Ezra; Ramban). Thus, no water other than that in a mikvah can be used for purification.
(cf. Rashi). If they are not edible, they cannot become unclean (Yad, Tumath Okh'lin 1:11, see Leviticus 11:34).
As long as food is rooted to the ground and has not been picked, it cannot become unclean (Sifra; Rashbam, Chizzkuni; Yad, Tumath Okh'lin 2:1). Moreover, even after it is picked, it cannot become unclean until at some point it becomes wet (Rashi). This is true even if it is touched by the dead animal itself (Ramban).
Or the other liquids mentioned in Leviticus 11:34.