There is an aquifer.
Since I don't have a link handy I will state IMO
Mojave Region Groundwater | USGS
Neither is within or under the Mojave Preserve.
I'm curious though. Why the concern about underground water in Mojave NP? Afton Canyon is the closest above ground area; Newberry Springs has a spring (outside the NP), and as I mentioned, scientists are logging evidence of tiny springs within the preserve (very rare, remote, barely usable for humans). The tiny springs seem to support insect and small plant populations and not much else.
Are you thinking Barbara could have wandered 15-20 miles? Or do you think she purposefully strode off on a 15-20 mile hike to a place where she thought she could sustain herself (without food) for a while?
Unless she had a topo map committed to memory, she'd end up having to climb the granite hills - not the sandstone boulders nearer the RV, but the Granite Hills. You can only see the front of them in the picture, but they are about 2000 feet above the desert floor and go on for some miles. Then, unless she knew about and could find the defile that forms a pass toward Afton Canyon, she'd have to climb a couple more sets of granite formations, sharp, hot and tall.
It's really unforgiving terrain and like all theories that posit that Barbara wandered off into deep desert, I think that means she was not in her right mind, either due to heat exhaustion or some other reason. If her heat exhaustion was so severe that she was in fact completely delirious, she didn't stay on her feet much longer than that. It's true that people hide their symptoms of heat exhaustion, but she'd have had to go in the opposite direction from the RV. If you do think she headed up and into the Granite Hills and beyond, I'd say this is an attempt at self-harm (unless she was in fact delirious).
I think it's more likely she got to the RV, couldn't find the key, had to pee, and went down HH Road to find a spot. I am not sure that's even a likely scenario (or why she wouldn't then come back to the RV, because RT should have been there by then).
If there are pictures of her from about 2 pm and she looks smiling and healthy, it's really unlike she was delirious by 2:30 pm (about the time RT says he went back to the RV). It is possible she walked off, angry or irritated, with no clear purpose and got lost. But I don't think she could have gotten completely outside the Mojave NP and anywhere near standing on top of an aquifer (and I don't see how being near an aquifer is relevant to her situation even if she was).