We should begin with the regulations connected with a traditional miqveh which are clear. It should be at least three cubits long, a cubit wide, and a cubit deep and contain forty seahs of water(Er. 4b; Yoma 31a; Shulhan Arukh, Yoreh Deah 201.1). In other words, a space which contains between 171 and 191 gallons of water would be sufficient.
The water must be from a natural source. It may be form a spring, a lake or a river which has been fed by a natural spring in accordance with a statement in Leviticus (11.36):"Nevertheless a fountain or a cistern wherein is a gathering of water shall be clean." The opening word of that statement has been interpreted to be restrictive(akh) according to tradition(Hul 84a; Sifra to Lev. 11.36). Rain water is also appropriate as is water melted from ice or snow(M. Miq. 7.1; Yad Hil. Migvaot 3.1 ff; Shulhan Arukh, Yoreh Deah 201.2; 201.30).
It is clear from the rabbinic sources that the only usable liquid is water(Shulhan Arukh, Yoreh Deah 201.23) and that it must be still water(Sifra to Lev. 11.36; Rashi to Shab. 65b, to Nid. 67a; Tos. to Hag. 11a; Yad Hil. Migvaot 10.16; Shulhan Arukh, Yoreh Deah 201.2). The water which enters the miqveh may not be drawn or poured into it(Smag Positive Commandment#248; Tos. to B.B. 66b, to Pes. 17b; Shulhan Arukh Yoreh Deah 201.3). It must enter through a system of pipes not subject to uncleanliness; this excludes pipes of metal, wood or clay unless specifically treated to turn them into"vessels"(Rosh Miq. 5.12; Yad Hil. Miq. 5.5; Shulhan Arukh Yoreh Deah 201.34; Hatam Sofer Responsa Yoreh Deah 199). The miqveh itself must be constructed in the ground or be located in a building which is built into the ground. The migveh may not consist of a tub(B.B. 66b; Tos. to Pes. 17b; Shulhan Arukh, Yoreh Deah 201.6).