Mikvaos 1:3-4

Mikvaos 1:3

If ritually-unclean water fell into a pool and a ritually-clean person drank from it, he is rendered unclean. If unclean water fell into it and then water was drawn from it using a clean utensil, it is rendered unclean. If unclean water fell into a pool and then a loaf of trumah fell into it, if he rinsed the loaf in it, it is rendered unclean; if he didn’t, it remains clean. Rabbi Shimon says that the loaf is rendered unclean regardless of whether or not he rinsed it in the pool.

Mikvaos 1:4

If a corpse fell into a pool, or if a ritually-unclean person walked through it, and then a ritually-clean person drank from it, he remains clean (because pool water is only susceptible to impurity when it's detached). The same rule applies to the water in pools, cisterns, ditches and caverns, to rainwater ponds whose flow has ceased and to mikvahs smaller than 40 seah (approximately 140 gallons). These are all ritually-clean when it’s raining; when the rain stops, those near a city or a road are ruled unclean, while those far away remain clean unless most people pass by them.