Speak to Dalia who complains that she cannot grow plants because the water is polluted and sets you off to find the polluter.
You can find the trail of pollution by inspecting the nearby waterfall where melon rinds occasionally float down, Climb up and pursue the rinds as they float down to the Eastern corner of the town.
Her name is Calyban and she's sitting in the western corner eating Hydromelons and carelessly throwing the rinds into the water. She asks for 10 wildberries which grow in the snowy areas of the Gerudo Highlands. Collect them and give them to her.
Return to Dalia for your Hydromelon reward.
Dalia then tells you she is planting a Wildberry patch and to return and see her.
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