|MadSci Network: Earth Sciences|
Early observations of the Martian surface indicate acidic surface lakes/oceans in the areas sampled because of the relatively uniform ratio of the concentrations of phosophorous, sulfur, and iron. However, I don't know of any observations that would clue us into why the lakes/oceans were acidic. Speculatively speaking, one cause may have been due to extreme volcanic activity in an oxidizing atmosphere. On Earth, volcanic eruptions release vast amounts of sulfur into the atmosphere. SO2, and its relatives (collectively SOx), can be oxidized in the atmosphere to form sulfuric/sulfurous acid. Precipitation can then deposit this acidic product into the oceans. As for terraforming Mars... while we do not know for certain that there is active volcanism on Mars (although some volcanic cones suggest a possibility), compared with the past, current volcanic activity is surely diminished. The acid already in the soil has likely undergone reactions with the soil and underlying rocks. We cannot be sure (without further study) whether or not a return of surface waters to the Martian surface will result in the same acidic oceans as previously indicated.
Try the links in the MadSci Library for more information on Earth Sciences.