Water Heater Cold Intake Pipe always drips from drain valve
Ben Welborn is correct.  Sediment or mineral is keeping the drain valve from seating properly if everything is closed inside especially the hot and your cold inlet valve works you could change out the valve without draining the heater. But I'm assuming since there is a sump that this is down inside a basement. Drain and replace use a 3/4 di electric nipple with a. 3/4" ball valve or pvc and remedy this problem for good