"[Minneapolis] Metro cities could be on the hook for $1 billion or more in cleanup costs in coming years as they grapple with contaminated sludge in storm-water ponds that dot the metro area."
"The main culprit: carcinogenic compounds known as PAHs, which until recently were found in common coal tar-based sealants used on driveways and parking lots. Unsafe levels of the compounds have been found in many of the metro area's 20,000 public collection ponds, which receive water from streets and parking lots after rainfall.
Testing of a handful of ponds has revealed the tip of the iceberg: Nine of 15 ponds sampled by the Minnesota Pollution Control Agency (MPCA) a few years ago had unsafe levels of the compounds.
When the city of Inver Grove Heights tested 10 ponds this year, it found two with unsafe levels. The cost to clean up just one of them, which had 7,000 cubic yards of tainted sediment, was estimated at $450,000."