All radioactive elements decay. Tritium has a half life of about 12½ years and it turns into ordinary hydrogen. If they keep releasing tritium at the same rate for a long time, it will reach a maximum concentration in about 25 years (or maybe less, depending on how accurate my fuzzy math is). Once it reaches that point, it will decay as fast as it’s released.
It’s also worth noting that if they want to release the tritium at a constant rate, they’ll have to gradually increase the rate at which they release the contaminated water, because the tritium is already decaying in storage.