Wouldn’t we expect all the ground water to have no dissolved oxygen?

Very late reply - but your question is totally fair, so I hope you don't mind:

On the face of it, you'd expect Martian groundwater to be pretty damned poor in dissolved oxygen, yes, and groundwater on Earth does get its oxygen almost entirely from the atmosphere, as you mentioned. (This would be easier on Earth than Mars due to the greater atmospheric pressure, among other things.) However:

If you've heard anything about recent discoveries of "dark oxygen" being generated on Earth's deep seafloor, you might agree with me that nature often finds a way to create chemical niches where interesting stuff happens. In the just-discovered terrestrial case, metals on the seafloor are essentially acting as batteries, zapping water and splitting the oxygen off from the hydrogen. Obviously I can't expect that this process was occurring at the Jezero Delta, but I'm cautious about saying that the groundwater there never had any dissolved oxygen, especially when we know that hot water can break down minerals and release the oxygen within.

So again, the question is a good one, but it's already been partially answered by Curiosity, which found the following on the floor of Gale Crater:

Trace amounts of the element manganese typically exist in basalt. To get a rock with as much manganese as Caribou has, the manganese needs to be concentrated somehow. The rock has to be dissolved in liquid water that also has oxygen dissolved in it.

If conditions are right, the manganese liberated from the rock can then precipitate as manganese oxide minerals. On Earth, dissolved oxygen in groundwater comes from our atmosphere. We’ve known for some time now that Mars once had vast oceans, lakes and streams. If we could peer onto Mars millions of years ago, we’d see a very wet world. Yet we didn’t think Mars ever had enough oxygen to concentrate manganese—and that’s why we thought the data from Caribou must have been an error.

In the Earth’s geological record, the appearance of high concentrations of manganese marks a major shift in our atmosphere’s composition, from relatively low oxygen abundances to the oxygen-rich atmosphere we see today. The presence of the same types of materials on Mars suggests that something similar happened there. If that’s the case, what formed that oxygen-rich environment?

Good article to read if you have the time...