Ask Mr. Smarty Plants

I don't know where your well is and what groundwater source it pulls from, but the readings for pH in the Barton Creek watershed groundwater for 2007 ranged from slightly acidic (6.63) to slightly alkaline (7.55). I suspect other groundwater sources in the Austin area have a similar range. You can check your watershed data yourself with the Water Quality Protection Database Query. To do so, select "Conventional/Nutrients" from the Parameter Types, your watershed, and choose a single year (e.g., 2007) to avoid having a huge file to download. It is true that the pH of rainwater may be slightly acidic because the raindrops pick up CO2 out of the atmosphere as they fall, but I can't think that it would be necessary to acidify your well water for your plants if they are Central Texas natives since they are well-adapted to our alkaline soils and water. If you are watering plants that aren't native to Central Texas but are, for instance, native to East Texas, such as Rhododendron canescens (mountain azalea) or Rhododendron viscosum (swamp azalea) that require acidic soils, your best solution is to grow them in containers (either above ground or buried in the ground) with added supplements (e.g., peat) to make the soil more acid. Central Texas natives don't need that. You could, however, mix your rainwater with your well water before you use it if you want to go to that trouble. This should make your well water slightly less alkaline.

If you don't already have a copy, you might like to download a copy of The Texas Manual on Rainwater Harvesting