I've reused brine before. For example if I'm cooking 2 batches of pork chops, I'll brine a batch on day one, then cook that batch the next day, put a new batch in the brine, then the next day cook the 2nd batch and ditch the brine. I wouldn't try to keep it for a week, or brine pork after chicken or anything like that. If you have any doubts as to the safety though you might as well pitch it, a batch of brine is dirt cheap.