This is a scenario for many of the faithful, especially when getting access to a priest or deacon is not foreseeable within the moment needed. First of all, we have to say that there is no current liturgical law that deals with this question. However, the general idea with blessings is that “things that are blessed retain their blessing so long as they remain whole, unbroken, integral, and are recognizable as what they were when they were blessed”. This means that, for instance, an altar loses its blessing when its mensa (the top of the altar) is broken.
The only document that gives us a kind of response is the document on the Rite of Baptism that was used before the liturgical reforms of the 1970s, there we have this provision, if “the baptismal water has so diminished that it is foreseen it will not suffice, unblessed water may be added even repeatedly, but in lesser quantity [ minore tamen copia] than the blessed each time this is done.” (“De Sacramento Baptismi Rite Administrando,” in The Roman Ritual, 1950). From this, the understanding is that whatever quantity of water is added to the holy water, the majority of the water that remains must be that which received the blessing. That is, the water being added must not be more than the holy water.
This theological principle actually follows the logic of the teaching of St. Thomas Aquinas regarding adding water to the Precious Blood (during the purification of the holy vessels by the priest at Holy Mass): “if the liquid of any kind whatsoever added be so much in quantity as to permeate the whole of the consecrated wine, and be mixed with it throughout, the result would be something numerically distinct, and the blood of Christ will remain there no longer.” Conversely, “if the quantity of the liquid added be so slight as not to permeate throughout,” the Precious Blood remains (STh., III q.77 a.8 resp). Inversely, it follows thus that if a small amount of holy water is added to a large container of unblessed water, the contents of the container do not thereby become holy water.
From the aforementioned, it seems that the practice of adding unblessed water to holy water is envisaged by the Church in the context of the baptismal water which has as its primary goal—that for giving the gift of salvation in the conferral of Holy Baptism—but maybe not for general application. We are inclined to this view because, it is important to note that the Pre-Vatican II blessing of baptismal water (outside of Easter and Pentecost), was a more extensive process than our current practice including the praying of the Seven Penitential Psalms and the Litany of the Saints, the Lord’s Prayer, the Apostles Creed, an exorcism over the water, breathing upon the water, and the pouring of the Oil of Catechumens and the Sacred Chrism in the water in the form of a Cross, both individually and then simultaneously. It is precisely due to these many requirements for the baptismal water that the document cited above notes, if “the baptismal water has so diminished that it is foreseen it will not suffice, unblessed water may be added even repeatedly, but in lesser quantity than the blessed each time this is done.”
So with no current liturgical law on the issue, and inferring from the only document that gives us an indication (albeit, Pre-Vatican II), what is the response to adding unblessed water to holy water which is not for the purpose of Baptism (like a faithful who wants to “top-up the holy water” he got from his priest)? Though the challenge is more pastoral than liturgical, in the end, we have to say that while there is a possibility that some unblessed water might be added to holy water and it maintains its reality as holy water, it is best to simply return to your priest at the parish whenever there is need of more holy water.
