In Zechariah, Jesus is the source of the Fountain opened for Sin and Uncleanness

“In that day a fountain shall be opened for the house of David and for the inhabitants of Jerusalem, for sin and for uncleanness.” Zech. 13:1
“In that day” in this book seems to refer to the Day of the LORD at the end of time. This gives the Jews hope that they will be forgiven for their sin of crucifying the Messiah.
The fountain was the atoning death of the pierced One. Jesus’ side was pierced while on the cross. “But one of the soldiers pierced His side with a spear, and immediately blood and water came out.” John 19:34 The water is for cleansing and the blood for covering (atonemnent).
The beneficiaries are the house of David and the inhabitants of Jerusalem, or Jews.
The purpose of this fountain is for cleansing and purification.
Another alternative is that the fountain refers to the gospel, which began to be preached in Jerusalem, then flowed out from that city.

Tags: atonemnent, fountain, purification