Where do you live? Any pics? A good species ID is one of the most crucial pieces of information when dealing with WC animals; this way you can oftentimes get a more concise answer for care requirements. I would also recommend possibly putting the crab back; as wild-caught animals tend not to do well in captivity; and from your questions, it may be advisable to do a bit more preliminary research. As for your questions, depending on the species, it would most likely need a paludarium-type setup; and an air pump would, unfortunately, not negate this need. I've kept vampire, fiddler, red-clawed, and a few species of East Asian freshwater crab; they all preferred at least 4 inches of fine, yielding substrate (used sand for all except for the fiddler crabs; they were given silty mud), some sort of rockwork to climb up on and to hide under (used ohko/dragonstone for all of my setups; most inert porous rocks should work, though), around a 50/50 land-water ratio (75-25 for the fiddlers, but that's probably more unique to them; and the EA FW crabs seemed to prefer more water; if I were to keep them again, I would give them approx. a 30-70 ratio), at least 3 inches of water (my EA FW crabs got 6 inches), some sort of filtration system (as is standard for most, if not all, aquatic or semi-aquatic organisms excluding plants; might also add that since your crab was riverine, more flow would probably be required), amongst other things. They're really not very hard to keep once you get the hang of it. If you decide to keep the crab, good luck! Hopefully this may help in some way.