William Collier steers his tractor across MD Route 312 in Henderson, MD, following a well-worn path through acres of high corn and green soybeans. He idles the engine when he reaches his destination: a square cover near a ditch, sitting among deep weeds.

It doesn't look like much, but the technology underneath the cover is something worth seeing. It helps to reduce the nitrogen-laden runoff that enters the ditch, which protects streams and rivers. In dry times, it also helps the farmer conserve his water. In wet times, and during storms, it protects the crops from floods.

Called a ditch water-control system, the technology is simple to manage. Several boards block water in the ditch from entering the drain. When the farmer needs to drain some water off the fields so it doesn't accumulate, he removes as many boards as he needs to. When he wants to raise the water table, he puts them back. The aim, though, is to keep it at a stable level.

The ditches reduce nitrogen in two ways. First, less water coming off the farm fields into the ditch means less water to trickle, or rush, into streams that eventually lead to the rivers of the Chesapeake. Second, the technology slows the water down. This gives the nitrogen time to convert from nitrate to a nitrogen gas, which dissipates, harmlessly, into the air. That process is called denitrification. Denitrification is a natural chemical process in the soil, and scientists have long been looking for technology to encourage more of it.

The state of Maryland believes so strongly in the power of these drain systems that it helps farmers finance them. About two years ago, the Maryland Department of Agriculture began picking up 87.5 percent of the tab for the drain systems, which cost between $5,000 and $12,000. Since then, the department has financed about 45 drain systems. Like Collier's, many of them are in Caroline County.

John Rhoderick, the department's administrator for resource conservation operations, said he is hoping those 45 systems are just the beginning.

"There are very few best management practices that we have that are end-of-pipe technology, and that is what this is," he said. "We're actively promoting it. It's one of our major practices."

The structures work so well, researchers say, because they restore the land to its original hydrological state. Hundreds of years ago, Maryland's Eastern Shore was a marshy, low-lying peninsula with limited acres for farming. Ditching the soils in the 1700s created dependable, dry farmland. More ditches came in the 1930s, when the United States was in the grips of a malaria epidemic.

The ditches protected the land from floods and lowered the water table. But they had downsides. Farmers needed the water, and couldn't retrieve it once it ran off the farm fields and into the ditches. And the Chesapeake Bay could do without the nitrogen, phosphorus and sediment the water carried as it hit the ditch and flowed into nearby streams and creeks. Plus, the ditches drained away wetlands, which function as the Bay's kidneys by filtering pollution.

That runoff contributes to pollution loads from agriculture that delivers far more nitrogen and phosphorus to the Bay than urban and suburban sources, despite major investments in buffers and cover crops. But ditch management systems may be able to improve those numbers.

Tom Fisher, an aquatic and nutrient expert at the University of Maryland Center for Environmental Science, conducted the first studies of the ditches at Collier's farm a few winters ago. He found 10–15 milligrams per liter of nitrate in the groundwater under the fields. As he moved closer to the ditch, the levels dropped to 5 milligrams per liter. When he tested at the ditch, it was almost zero.

"It's very clear, what that drainage control process does is encourage denitrification," Fisher said. "It was very effective at that site. I think the data is very convincing."

Fisher discovered Collier's ditches by accident, while he and his graduate students were conducting other experiments under a contract with the U.S. Department of Agriculture. Collier installed his first ditch structure in 1996, because he thought it would help his crops. Fisher immediately recognized the nitrogen-reduction potential, and added the structures to the scope of his study.

Fisher did caution that the ditch story is only in its beginning stages. More testing would be a benefit, he said, as the technology wouldn't work as well in sandier soils. Rhoderick's staff is also working with the University of Maryland's Joshua McGrath to make available an add-on filter. The filter would be placed just before the water control gate to reduce the amount of phosphorus that reaches waterways.

Greg McCarty, the USDA soil scientist in Beltsville, MD, who oversaw Fisher's work, said he wasn't surprised at the result.

"Anytime you're restoring some of the original hydrology of the system, that's going to help retain nutrients," he said. "A lot of these areas that have been drained by ditches were former wetlands. So, by restoring some of that hydrology, you're allowing some of that wetland function to be restored, and wetlands, by nature, remove nitrogen from water."

But what might be surprising to some farmers is the way Collier has financed the last few structures he's added to the farm. The money came from Drew Koslow, the Choptank Riverkeeper.

Riverkeepers and farmers on the Eastern Shore have long regarded each other warily. The Shore is home to many poultry farmers who grow chickens for large integrators. Nationally, Riverkeepers are known for taking on pollution from hog farms in North Carolina, and many farmers feared that activism could spread to their back yards. That fear appeared founded when the Riverkeeper organization and the Assateague Coastkeeper filed a lawsuit against Perdue and a Berlin chicken farmers, Alan and Kristin Hudson, accusing him of violating the Clean Water Act.

Collier doesn't think many of his neighbors would welcome Riverkeepers on their farms. But Koslow put his money where his mouth was, coming up with a $48,000 grant from PEPCO that was administered by the Chesapeake Bay Foundation and Forest Trends to help Collier pay for several more structures. Koslow worked with Rhoderick and Caroline County's soil conservation district to cost-share the funds. The grant paid for two structures outright; he then worked with the cost-share program to pay for Collier's share of the third structure.

Koslow's involvement dovetails with the philosophy of his employer, the Mid-Shore Riverkeeper Conservancy, which was formed in 2008. Tim Junkin, a longtime Washington attorney and author, founded the group to focus on cleaning up the rivers where he fished and swam as a teen.

"When we go to farmers, we always promise that we are here to work with you and that we will not take the information and use it to sucker-punch you," Junkin said. "Our M.O. is always to approach the farmer and solve the problem."

The organization is working with several farmers in similar collaborative efforts along the Choptank, Wye and Miles rivers, which have their own Riverkeepers. Koslow will be monitoring Collier's ditches for the next five years. He hopes to stay in the neighborhood.

He may get his wish. The Eastern Shore has 852 miles of public drainage. There are 101 so-called "tax ditches" owned by associations composed of several property owners along that stretch of drainage. Those owners no longer receive much state support to maintain the ditches, so the cost-share program could be helpful.

"We want to do more projects like this," Koslow said. "If you look at the economics of reducing nitrate, it's way cheaper to get a pound of nitrate off of agriculture land than anywhere else."