While it would be best to pull new 12-2 w/ground, Here is a solution that will work. Pull a single wire from the outlet to the cold water pipe, Clamp the wire to the copper or iron pipe (not plastic) and the other end to the grounding screw on the outlet. now find a spigot outside and connect a #6 copper wire to it and drive a 6' ground rod all the way into the ground and clamp the copper wire to it.