As mentioned above, the Mishna (17b) discusses the unique case of a water hole in the public thoroughfare that can be surrounded by four right-angled walls in each corner – referred to by the Mishna as Deyomadin – in order to allow access to the water for travelers and their cattle.
Rabbi Yitzhak bar Ada argues that this leniency – the ability to carry within that rectangular space – is not permitted for all, but is the exclusive benefit of the Olei Regalim – the Jews who are traveling to Jerusalem for Pesach, Shavuot or Sukkot – to fulfill the commandment of visiting the Temple on these holidays.
The Jerusalem Talmud brings a dispute among the amoraim on this question. One opinion agrees with Rabbi Yitzhak bar Ada that these walls can only be used as an eiruv by olei regalim. A second opinion argues that the special leniency was approved by the Sages with the olei regalim in mind, but once it was adopted, the ruling works for all, and anyone can use the water in these wells. The third opinion argues that the ruling was made with the olei regalim in mind, but during the times of year when people are oleh regel, anyone – even those not coming to Jerusalem – can benefit from them.
What is clear is that according to all, this method of fencing off the area of the well or water-hole with four deyomadin is related to the needs of olei regalim. In other words, the walls are so poorly designated that it was only the desire to assist people involved in this mitzva that led the Sages to permit their use. Since the olei regalim invariably brought with them animals for sacrifices in the Temple, there was a desperate need to make water as readily accessible as possible. During the times of year that the masses are commanded to travel to the Temple in Jerusalem, the only available water is in wells or cisterns that collected rain water.