From my readings: It seems that we are continually losing H to space when water molecules break open which translates to losing light water while keeping heavy water so, slowly shifting our water balance to heavier water.(assuming that deuterium would be les likely to escape into space) and then It seems that we are continuously gaining water from comets and asteroids. Certainly nothing to compare to the LHB, but none the less, ongoing. then Are we at equilibrium? Are we neither gaining nor losing net earthbound water? Or, are we losing more than we are gaining, or the other way round? Is our percentage of heavy water slowly increasing? Your thoughts?