The Minnesota Environmental Quality board today gave the green light to a new north/south runway at Twin Cities International Airport. The Board unanimously approved an environmental impact statement for the runway, over the objections of the city of Richfield. But Richfield officials said the decision was a partial victory. Minnesota Public Radio's Bill Catlin reports. Richfield's primary complaint has been that the Metropolitan Airports commission environmental impact statement gives short shrift to problems caused by low frequency noise ... the dull Rumbling from jets that can cause vibration in walls windows and doors. Richfield residents packed the basement hearing room in the State office building, hefting signs tha