(defendant, a real estate developer, purchased known contaminated tract of undeveloped wetlands and court holds that defendant liable for cleanup costs of neighboring properties even though defendant sold tract before plaintiff sought reimbursement for clean-up cost of adjacent properties contaminated with PCBs; defendant had cleaned the wetland tract it purchased and then sold it before plaintiff reacted, but plaintiff had informed defendant that if defendant was going to clean-up own tract it had to clean-up adjacent tracts because wetland was source of contamination; plaintiff then hired company to clean-up adjacent tracts and billed defendant for the cost; court holds that "owner and operator" status under CERCLA determined at time clean-up costs are incurred (rather than when reimbursement lawsuit filed) for purposes of CERCLA liability).