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Is it leaking from the PVC joint or the threads? If it is the threads unscrew, re-tape and try again (Cement? You didn't cement the nut, did you?). If it is from the pipe, you have a couple of options.
Only real way is to start over, new bulkhead etc. With bulk heads, I usually get spares and use the extra nuts over the end where the slip is or the same with internal threads. The walls on them tend to be thinner and by using the extra nuts it prevents cracks.
You want to keep that ball valve, so probably best to "lose" the bulkhead. It looks like you can cut up against the bulkhead and reuse what remains.
If you're brave (and a little crazy) you can cut at the mid-point of the pipe and use a heat gun to pull the PVC out of both sides. That's a whole 'nother set of instructions if you're interested. Not as bad as it sounds, but not without risk of losing the fitting.