The same may be said of Detroit, just north of Lake Erie, or Toronto, on the shore of Lake Ontario, or Knoxville, where the Holston and French Broad rivers join to form the Tennessee.
Many sources are hundreds of miles from the watershed, and even further from the Bay.
That is a finding that officials admit will raise tough issues if pollution reductions to benefit the Chesapeake are sought from those areas. Both are also forms of nitrogen which, along with phosphorus, are targets of a 40 percent nutrient reduction goal that the Bay states are trying to achieve by the turn of the century. Evidence in recent years has suggested that roughly a quarter of the nitrogen that ultimately reaches the Bay is the result of air pollution. Dennis, who is on assignment to the EPA's Atmospheric Research and Exposure Assessment Laboratory in North Carolina, has been leading efforts to define the Bay's airshed. Efforts that would reduce nitrogen emissions have been blocked in court, backed away from by the EPA, opposed in the states and threatened in the Congress.