Arkansas poultry farmer Jeff Marley grows 240,000 birds a year on his farm outside Fayetteville. But it’s because of city dwellers in Tulsa, OK, that he rarely spreads manure on his pastureland.

Marley sells most of the 1,500 tons of manure his birds generate via a litter bank. He doesn’t store it on his property even for a few days, he said. As soon as the hauler shows up and loads his truck, the manure is gone.

“I spend more time managing my litter than I do my chickens. There is no comparison in terms of what we did and what we do today,” Marley said. “We used to pile the litter. We didn’t care where we piled it. We would never consider doing that today.”

Nearly 15 years after the Tulsa Metropolitan Water Authority sued six poultry companies and one small Arkansas city for contaminating Tulsa’s water supply with phosphorus from manure, most of the waste never touches the land in the watershed that feeds Tulsa’s lakes. Instead, it goes to Kansas, Missouri and other areas not near the watershed, called the Eucha-Spavinaw for its two lakes. Marley’s farm is in a neighboring watershed, but he still has the manure hauled out. Why spread it at home when it’s so valuable elsewhere?

The Tulsa lawsuit was just one of several in which an urban entity has blamed agriculture for polluting its water supply. In 2013, environmental groups in Washington State sued Cow Palace, an 11,000-head industrial dairy, claiming that its manure lagoons were polluting the water supply of the Yakima Valley. In 2015, Des Moines Water Works sued three rural Iowa counties, alleging that the drainage tiles used by the counties’ agricultural drainage districts were a conduit for nitrates into the Raccoon River, and ultimately, Des Moines’ water supply. Other urban regions, such as Toledo, are trying to determine the best way to tackle agriculture pollution that has made its water unfit to drink.

Nitrate, phosphorus from fertilizers, and sediment pollute drinking water and surface water in the Chesapeake Bay watershed, too. It’s an issue that officials along the Susquehanna and the Potomac rivers are watching closely. Both rivers supply many cities with drinking water and have agriculture-intensive areas. No entity has filed a lawsuit over nitrates as in Des Moines, but many water plant managers and regulators are monitoring the Iowa case.

The Potomac River, for example, supplies drinking water to nearly 5 million people, many of them Washington, DC, residents. The utilities in the area have formed the Potomac River Basin Drinking Water Source Protection Partnership, which meets regularly to communicate about keeping the water safe. It’s working pretty well, said Curtis Dalpra, communications manager for the Interstate Commission on the Potomac River Basin. The Susquehanna River Basin Commission is forming a similar partnership.

The urban cities and counties are pushing back on agriculture so their ratepayers and taxpayers don’t have to bear the enormous cleanup costs. The Clean Water Act and the Safe Drinking Water Act require cities and water authorities to maintain safe standards for clean water; if the entities can’t meet those, they need to increase treatment, and ultimately, raise rates to pay for that. Agriculture, meanwhile, is exempt from many of the requirements under the Clean Water Act, and much of what farmers do to clean up their polluted runoff is voluntary. The government offers incentives, such as money for cover crops and technical assistance to install bioreactors and buffers. But federal and state regulators can’t force a farmer to undertake those measures.

Under the Chesapeake Bay Total Maximum Daily Load, there are goals for pollution reduction from each sector. The agricultural sector is the largest source of Chesapeake Bay pollution, and large reductions are needed. But, if agriculture doesn’t deliver, the federal and state authorities will turn to the urban sectors and seek greater pollution reductions from the sources they can regulate, including sewage treatment plants and stormwater systems.

“It’s absolutely true — if you look at the math in the Chesapeake Bay, agriculture is a major pollutant source, and unless we improve our ability to reduce agricultural pollution, the burden will be placed on the urban sector,” said Chesapeake Bay Commission Executive Director Ann Swanson. “Make no mistake, the urban sector is not off the hook, but we need to do more than just improve the urban sector to get the job done.”

Many sewage plants have already been upgraded, and at a certain point, efforts to reduce their pollution will reach a point of diminishing returns. The cost to remove additional pounds of nitrogen from a sewage-treatment plant that is already highly effective will be much higher for each additional pound than the cost to remove nitrogen from agricultural land.