The bubbles are probably natural occurring...natural gases from the ground gathering in the water. Bubbles are more noticeable when the outside temperature is higher then the ground temperature. Cold water holds more gases, then that of water at a higher temperature. Means, you are adding cold water to warmer water causing the gases to release the cooler water. These are harmless unless it is Radon gas or Methane gas. In large amounts can be hazardous. If you are concerned, have your water tested for what type of gases that are present. Another reason for the bubbles, your well water could be at a lower water level causing the pump to draw extra air or gases from your water supply chamber.
