Environmental Protection Agency investigators detected the mercury over the weekend. Scene coordinator Kathy Parker estimated it was less than half a cup but it's a hazard if it is spread in the air and breathed.
The Yakima Herald-Republic reports about 8,000 square feet of the parking lot were paved with fresh asphalt to contain the contamination.
The device was described as a small plastic water bottle tightly wrapped with tape. Police said there had been a serious of blasts Thursday night from devices made with household chemicals.