Quest related dirt mouds don't get revealed by standing near them anymore but you still can use a shovel on that spot.
Fort Joy Beach
Overview
X227, Y287. You will find a level 1 Zombie (100XP) inside that grave.
X235, Y254. Just south of the zombie grave between two rocks is a decent bow buried.
X291 Y363. Behind the abandoned shipwreck near the turtles.
X131, Y238. Hidden behind a rock after crocodiles. Has some random loot.
Fort Joy Ghetto
X205, Y95. Buddy will tell you that he buried a key here.
This is the key to enter the Hound Master's quarters in prison.
X500, Y555. This hatch leads to Withermoore's Forgotten Cell
and is found in the cave with the elves.
If you play hide and seek with Mody he will also reveal it to you.
X263, Y91. Hidden entrance to the Underground Passage into the Fire Slugs Cavern.
It's located right next to Migo.
X189, Y561.This treasure is burried is inside of the Underground Passage.
Hollow Marches
X400, Y267. If you keep running east after you met Windego there a small island on the seaward side
you can dig up a treasure with some decent loot.
X375, Y248. A buried skeletton! right next to Windego.
X376, Y100. Contains some genric loot
X496, Y90. Contains some pretty good loot. +2 str helm, +2 finesse gloves, +2 int ring.
X495, Y76. Right next to it is another dirt mound.
Good chance at finding level 6 epic gear.
X561, Y242. a locked supply chest.