I think we can all agree that is a very risky setup! Especially since hogs have been documented TRYING to eat box turtles (going back to the 1800's here). I just read an article about the remains of a box turtle with a copperhead fang imbedded in a limb.
Substrate: Using sand as a substrate under the sole premise that it is natural is pure bunk. There is nothing natural about UTHs and 4 glass walls. The rules of the wild are usually not applicable to captivity. Aspen is the preferred substrate of MANY people. I have used sandY (not pure sand, mixed with peat moss and leaf litter) substrate for exhibits just fine, but maintaining sanitation is MUCH harder (it doesn't clump liked you'd think, and it doesn't dry out like it does in aspen). I have used the calci-sand products with a Western and had problems with the nares clogging up because of the larger grain size. Conclusion: don't let your decisions be guided by a bias towards so-called natural, go with what has worked for 1000's of others in captivity.
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"I know the human being and fish can coexist peacefully."
Governor George W. Bush, Jr.
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)