Mikvaos, Chapter 1, Mishnah 4. Our chapter is working its way up through the grades of water, weighing what each grade can and cannot do. One principle has already been fixed: rain that has gathered in a hollow or a ditch in the earth cannot receive tumah at all. Bring a source of tumah into contact with it and nothing happens to it, since it lies where it fell and was never lifted out of its place in the ground. Our mishnah puts that principle to work, and then introduces a qualification of an entirely different character.
A corpse in the water
The wording of the mishnah is "nafal letochan", if something fell into such water, meaning rain lying in the earth in a quantity smaller than forty se'ah, and what fell in was a "meis", a corpse, or even a kezayis of one. The same holds, says the mishnah, "o shehalakh bahen hatamei", where a man in a state of tumah waded through the pool. In neither case does the water pick up tumah. It remains bound to the earth and was never ne'ekar, never separated from the place where it settled, so it also has nothing to pass on to anyone else. Hence "shosah tahor, tahor": someone who is tahor may drink from it and his taharah is untouched.
Which waters are included
The mishnah then enumerates the kinds of ground water to which this applies, so that we know the scope of the rule. "Ehad may geva'im", whether it is water standing in hollows and puddles, "may be'eros", water in pits dug into the ground, "may sihin", water gathered in irrigation ditches, "may me'aros", water lying in caves and in the crevices of the rock, "o may tamtziyos shepasku", or seepage water that trickles up from the ground once the rains have stopped. In every one of these cases the water is tahor, for it was never uprooted from the ground, and its natural condition is one of taharah.
The seasons of the year
At this point the mishnah turns to an altogether different consideration. The question is no longer what the water is by nature, but whether we ought to worry that human beings have tampered with it. It speaks of "may tamtziyos shepasku", trickling seepage water whose flow has run out, together with "mikva'os" that hold less than "arba'im se'ah", forty se'ah. Regarding such water the ruling is "bishe'as hageshamim, hakol tahor": throughout the rainy months it is all tahor. At that time of year we entertain no suspicion that some tamei person drank there and spat back in, or handled the water in some other way, because rain is falling everywhere and few people are out on the roads.
"Pasku hageshamim", once the rains have stopped, the picture changes. "Hakrovim la'ir veladerekh, teme'im", those collections of water near a town or near a roadway are treated as tamei, because we are apprehensive that a tamei person drank from them, or that a tamei vessel was dipped in to draw water, or the like. "Veharehokin, tehorin", but those situated at a distance from settlement and from the main roads remain tahor, since it is unlikely that anyone came upon them.
A boundary is then set even on that leniency: "ad sheyelekhu rov benei adam." When the season has moved on far enough that the majority of people are once again out and about, the far off pools forfeit their assumed taharah too. From that point we have to reckon with the possibility that someone tamei made his way there as well and did something to the water that rendered it tamei.
The mishnah therefore gives us two distinct measures. Water resting in the ground is inherently immune to tumah, whatever falls into it or walks through it. What can undo its taharah is not the tumah itself but human traffic: proximity to people, and the time of year when people are on the move.