Objection 1. It seems that plain water is not necessary for Baptism. For the water which we have is not plain water; as appears especially in sea-water, in which there is a considerable proportion of the earthly element, as the Philosopher shows (Meteor. ii). Yet this water may be used for Baptism. Therefore plain and pure water is not necessary for Baptism.
Objection 2. Further, in the solemn celebration of Baptism, chrism is poured into the water. But this seems to take away the purity and plainness of the water. Therefore pure and plain water is not necessary for Baptism.
Objection 3. Further, the water that flowed from the side of Christ hanging on the cross was a figure of Baptism, as stated above (3, ad 3). But that water, seemingly, was not pure, because the elements do not exist actually in a mixed body, such as Christ's. Therefore it seems that pure or plain water is not necessary for Baptism.
Objection 4. Further, lye does not seem to be pure water, for it has the properties of heating and drying, which are contrary to those of water. Nevertheless it seems that lye can be used for Baptism; for the water of the Baths can be so used, which has filtered through a sulphurous vein, just as lye percolates through ashes. Therefore it seems that plain water is not necessary for Baptism.
Objection 5. Further, rose-water is distilled from roses, just as chemical waters are distilled from certain bodies. But seemingly, such like waters may be used in Baptism; just as rain-water, which is distilled from vapors. Since, therefore, such waters are not pure and plain water, it seems that pure and plain water is not necessary for Baptism.
On the contrary, The proper matter of Baptism is water, as stated above (Article 3). But plain water alone has the nature of water. Therefore pure plain water is necessary for Baptism.
I answer that, Water may cease to be pure or plain water in two ways: first, by being mixed with another body; secondly, by alteration. And each of these may happen in a twofold manner; artificially and naturally. Now art fails in the operation of nature: because nature gives the substantial form, which art cannot give; for whatever form is given by art is accidental; except perchance when art applies a proper agent to its proper matter, as fire to a combustible; in which manner animals are produced from certain things by way of putrefaction.
Whatever artificial change, then, takes place in the water, whether by mixture or by alteration, the water's nature is not changed. Consequently such water can be used for Baptism: unless perhaps such a small quantity of water be mixed artificially with a body that the compound is something other than water; thus mud is earth rather than water, and diluted wine is wine rather than water.
But if the change be natural, sometimes it destroys the nature of the water; and this is when by a natural process water enters into the substance of a mixed body: thus water changed into the juice of the grape is wine, wherefore it has not the nature of water. Sometimes, however, there may be a natural change of the water, without destruction of species: and this, both by alteration, as we may see in the case of water heated by the sun; and by mixture, as when the water of a river has become muddy by being mixed with particles of earth.
We must therefore say that any water may be used for Baptism, no matter how much it may be changed, as long as the species of water is not destroyed; but if the species of water be destroyed, it cannot be used for Baptism.
Reply to Objection 1. The change in sea-water and in other waters which we have to hand, is not so great as to destroy the species of water. And therefore such waters may be used for Baptism.
Reply to Objection 2. Chrism does not destroy the nature of the water by being mixed with it: just as neither is water changed wherein meat and the like are boiled: except the substance boiled be so dissolved that the liquor be of a nature foreign to water; in this we may be guided by the specific gravity [spissitudine]. If, however, from the liquor thus thickened plain water be strained, it can be used for Baptism: just as water strained from mud, although mud cannot be used for baptizing.
Reply to Objection 3. The water which flowed from the side of Christ hanging on the cross, was not the phlegmatic humor, as some have supposed. For a liquid of this kind cannot be used for Baptism, as neither can the blood of an animal, or wine, or any liquid extracted from plants. It was pure water gushing forth miraculously like the blood from a dead body, to prove the reality of our Lord's body, and confute the error of the Manichees: water, which is one of the four elements, showing Christ's body to be composed of the four elements; blood, proving that it was composed of the four humors.
Reply to Objection 4. Baptism may be conferred with lye and the waters of Sulphur Baths: because such like waters are not incorporated, artificially or naturally, with certain mixed bodies, and suffer only a certain alteration by passing through certain bodies.
Reply to Objection 5. Rose-water is a liquid distilled from roses: consequently it cannot be used for Baptism. For the same reason chemical waters cannot be used, as neither can wine. Nor does the comparison hold with rain-water, which for the most part is formed by the condensing of vapors, themselves formed from water, and contains a minimum of the liquid matter from mixed bodies; which liquid matter by the force of nature, which is stronger than art, is transformed in this process of condensation into real water, a result which cannot be produced artificially. Consequently rain-water retains no properties of any mixed body; which cannot be said of rose-water or chemical waters.
Source: Summa Theologica (New Advent)