Nagog Pond is one of the Commonwealth’s “Great Ponds” (greater than 20 acres), thus it is controlled by the State, not by any particular Town. In 1884 the State legislature laid out the rights of Acton, Concord, and Littleton to the withdrawal of water from Nagog Pond.
The complete original Chapter is available at this link in PDF form
Here is section 10, where Acton’s rights are outlined:
Section 10. Nothing contained in this act shall prevent the town of Acton nor the town of Littleton from taking the waters of said Nagog Pond whenever said towns may require the same for similar purposes, and in case of such taking by either of said towns or both of them, if from any reason the supply of water in said pond shall not be more than sufficient for the needs of the inhabitants of the towns of Acton and Littleton, then the needs of the inhabitants of said towns shall be first supplied ; and if either of said towns of Acton or Littleton shall hereafter be authorized to take and shall take the waters of said Nagog Pond or any part thereof which the town of Concord may have taken under this act, said town so taking shall pay to said Concord a just and proportionate part of whatever sums the said town of Concord shall have paid or become liable to pay for water damages to any persons or corporations for the taking of water rights from said pond or the outlet thereof, to be ascertained, if the parties shall fail to agree, by three commissioners to be appointed upon the application of either party by the supreme judicial court ; the report of said commissioners made after hearing the parties, and returned to and accepted by said court shall be final between the said parties.