Aerial surveys show that pollutants from Philadelphia and other cities along the Delaware reach more than five miles into the ocean in a murky plume, carrying "everything from nutrients" - like sewage - "to toxics to floatables," Muir said.
Philadelphia's ocean express - now believed to travel the 100 miles to the Atlantic in two weeks to two months - is dotted with litter. The junk can be swept up or down along the New Jersey or Delaware coastlines and, given the wrong winds and currents, can be washed ashore.
The plume also delivers tiny pellets of Styrofoam and plastic from the city's sewer systems. The pellets look like fish eggs, says Muir, and can choke fish and seabirds that mistake them for food.
Because the pellets come from two different kinds of city sewers, scientists are using them to help trace waste in the Schuylkill and Delaware to a sewer, and from there to a source.
The Styrofoam dots come from storm sewers, which catch water from city streets - and with it litter, including bits of food containers and coffee cups.
The plastic dots - about the size of BB pellets - come from the old combined sewers that dump raw sewage and rainwater into the rivers in a storm. The pellets are used in manufacturing and enter the combined sewers through industrial floor drains.
This winter, scientists from the EPA's national and regional offices towed nets from three small boats outside several outlets on both rivers to catch and count the two types of pellets. The EPA's 165-foot ocean research ship has towed a net along the Delaware and confirmed that pellets were reaching the main channels of the river.
Now, Muir and other scientists plan to put nets directly over several combined-sewer outlets, so they can pick through and analyze the captured waste.
All of this eventually will help the EPA propose ways to cut down on river pollution - especially the floating items that can make their way to the sea.
The new studies come at a time when the tidal part of the Delaware - the river's "estuary" between Trenton and the ocean - has been under close examination by scientific groups.
Until recently, scientists thought it took three to six months for Philadelphia's assorted pollutants to make their way to the mouth of Delaware Bay between Cape May, N.J., and Cape Henlopen, Del. A significant proportion were thought to remain in the marshes along the bay, Muir said.
But recent studies, he said, point to an "incredibly fast" trek to the ocean compared with the sluggish action in the shallower Chesapeake Bay, where pollutants spend at least half a year on the trip to sea.
The Chesapeake - long fouled by heavy concentrations of chemicals - meanders past river outlets and jagged marshlands. The Delaware, however, rushes down through a deep main channel and out through an open bay.
"If a nutrient (like sewage) or a heavy metal or pesticide gets dumped into the Delaware at Philadelphia," Muir said, "it goes down through the river and down through the bay and into the ocean. It really doesn't get hung up in the bay itself."