It depends on the geological profile of the site itself.
If the body is buried in an area where radioactive isotopes occurr naturally in groundwater, then the age of the bones, which have been absorbing these isotopes slowly over the years, can be dated.
That is how some items, such as carvings in ivory, have been dated in eastern Europe.
Ping! This, an old dead guy, will be of interest to you.
What kind of variance is there in the rate of absorption? Wouldn't the amount of current in the water, the amount of surface area exposure, the size and quantity of the pours on the bone and such have to be accurately estimated to know it?