Hello Ron, great question. It appears they were buried in mass graves; trenches by the Union shortly after the battle and the area was known as the Champion Hill burial ground.

One might be able to pull 10 Union soldiers who were known to have died at Champions Hill and see if they were reinterred at Vicksburg National Cemetery after the war. If they could be located that would imply that the Union made notes of their graves after being killed at Champions Hill however it seems unlikely. As for the Confederates I would tend to believe they were forgot about and left in the mass graves at Champions Hill and Bakers Creek.

Perhaps someday someone with authority will take a serious approach to those dead left at Champions Hill but that too is unlikely. DNA might be able to determine who many of them were.