The Mystery Polluter is a side quest in The Legend of Zelda: Breath of the Wild. It is a quest given to Link by Dalia in Gerudo Town in the Gerudo Desert. It is one of the side quests related to and needed to be completed for the side quest "The Thunder Helm".
When spoken to, Dalia will express dismay that even though she has been working to make the soil fertile so she can start a garden, someone keeps sending refuse down the irrigation system.
Following the trail of melon rinds back up to their source will lead Link to Calyban, a Gerudo sitting up on the wall beside the water that flows throughout Gerudo Town. She has been eating Hydromelons and throwing the rinds into the water, which is what is polluting Dalia's garden.
Calyban will demand Link bring her ten wildberries in exchange for her ceasing to throw melon rinds down the irrigation system. Upon tasting the wildberries, Calyban realizes she has been selfish and vows to go help Dalia with her garden.
When Link goes to speak to Dalia again, she will tell him that some Gerudo woman she had never met before came to help her clean up the garbage, and even gave her a hydromelon. She will give the hydromelon to him to thank him for helping her resolve the problem. Dalia says the Gerudo woman also gave her some wildberries, so her garden will be a wildberry plot now, and that Link can come and visit whenever he likes. Dalia can be found at night sleeping in her mother's fruit stand tent. You cannot talk to her until morning, when she will run to the patch of dirt she has been trying to cultivate a garden inside of. This patch of dirt is to the right of the tavern.