When grower Brad Goehring looks across his rows of grapes, he can’t help but see a pool of murky water that breaks the rhythm of his vines, which otherwise stretch steadily into the Sierra foothills. The pool is relatively small, maybe half an acre, but vital, according to environmental regulators. They say it helps to clean the runoff from Goehring’s fields and provides a home for critters such as marsh birds. And by law, it can’t be disrupted, which is what makes this mini wetland a headache for Goehring.