For 51 years, a small federal program has been paying scientists to keep American waterways healthy. It's called Sea Grant — part of the National Oceanic and Atmospheric Administration — and President Donald Trump"s proposed budget for next year would eliminate it.

Sea Grant funds support more than 800 projects around the country. To find out what would be lost if it's defunded, let's start in a yard next to the Severn River in Maryland. It's my yard. There's a white tube sticking up from the ground. The ubiquitous icon of suburbia — the septic tank — lies underneath.

A lot of septic tanks break down, often invisibly, and what's inside pollutes waterways. Even properly functioning tanks leak a little. When they do, some of the sewage inside seeps invisibly downhill to, in my case, the Severn River. Once pollutants are in the river, it's really hard to tell where they came from. Is it from fertilizer? Livestock waste? From burning coal or gasoline? Sewage treatment plants? Or from the ubiquitous septic fields in suburban America?

There are some people who think they can track down the septic-tank portion of what are referred to as nonpoint sources of pollution — the pollution that comes from diffuse sources via snowmelt or rain runoff, say, rather than from, factories or sewage treatment plants. You might think of these trackers as "septic detectives."

"Septic detectives?" asks Lora Harris. "I haven't heard that one." She prefers to call herself a coastal ecologist; she's an associate professor at the University of Maryland's Center for Environmental Science. But the day I meet her in southern Maryland, she is, in fact, chasing septic waste along a wooded gully behind a housing development. She's looking for a particular kind of pollution — nitrogen.

"A lot of it comes out of toilets," she says, "a lot of it comes out of farms, and a lot of it comes falling out of the sky from the U.S. combustion of fossil fuels. This particular project is focused on what's coming out of the toilets."

We bushwhack through underbrush to find a small stream, just a few inches deep, trickling through the gully. Empty beer cans and candy wrappers suggest it might be a neighborhood hangout. Harris and colleague Andrew Heyes start filling bottles with stream water, being careful not to stir up the muck at the bottom.

Harris explains that nitrogen in a river or bay is fine in the right amount. But it's a nutrient for algae, and too much nitrogen leads to algal blooms. When these big blooms of algae die, their decomposition sucks oxygen out of the water in a chemical reaction, creating low-oxygen conditions that harm or kill marine life — a dead zone.