EPDM is the most water resistant of all the rubbers and absorbs a trivial amount of water, even for extended periods of time. If you're concerned because you're experiencing water loss in your pond, possible culprits could be simple evaporation or small punctures or tears in the liner itself. Evaporation increases during hot, dry weather with plenty of sunshine and is especially noticeable in shallow ponds with a large surface area. Ponds that feature a waterfall or fountain will also lose a lot of water to wind.
If you've confirmed that water losses aren't due to evaporation, it's time to check for a hole or a tear. Holes can be as tiny as a pinprick, so you'll need to use pond dye to track the leak so it can be repaired. As your supplier or look online for instructions on how use pond dye.
