Allan Armbrecht is using a natural “drain cleaner” to purify tile water. The Colo, IA, farmer and his neighbor installed a 10-acre wetland below several existing tile outlets. The wetland reduces nitrates flowing from 1,100 acres of cropland, keeping pollutants out of the nearby Skunk River. It also provides wildlife habitat; and all without lowering drainage efficiency.

Armbrecht’s wetland – one of more than 50 strategically located wetlands built through the Iowa Conservation Reserve Enhancement Program (CREP) – is part of a drive to improve drainage water quality in Iowa’s heavily tiled midsection. These wetlands can cut the nitrates moving from farm fields into rivers by half, long term, says Matt Lechtenberg, CREP field support coordinator.

“I’ve always been interested in conservation,” says Armbrecht, who grows corn and soybeans in Story County, IA, and also sells seeds and farm chemicals. He started ridge-tilling in the early 1980s and went to no-till 10 years ago. He uses Nu-Till planter attachments to manage residue and apply starter fertilizer. He sidedresses nitrogen (N), and does spring and fall stalk-nitrate tests to fine-tune N rates. Over the past 20 years, he’s tiled most of his cropland and installed terraces on erodible terrain.

Yet even the most careful farmers, like Armbrecht, can’t avoid some nutrient losses from row-crop land, says William Crumpton, Iowa State University (ISU) wetlands ecologist. “You have to let water drain to grow crops, and when water moves, nitrates are carried with it.”

Nutrient-laden farm drainage water is one source of poor water quality in rivers and lakes, and of oxygen deprivation in coastal waters, says Donald Hey, Illinois wetlands scientist and environmental engineer who served on the EPA’s Gulf of Mexico Hypoxia Assessment Plan committee. This environmental damage is “an external cost” of agriculture, which is now being shouldered by society, says Hey, a leading proponent of using wetlands to clean up water pollution and manage flooding.

Farm drainage water can also contaminate drinking water, ISU’s Crumpton says. “Nitrate concentrations discharged from row-crop land typically exceed drinking-water standards.”

The EPA is calling for a 45% cut in the amount of N and phosphorus (P) entering the Gulf of Mexico from the Mississippi and Ohio River watersheds. And states are being asked to adopt N and P water-quality standards. For corn and soybean growers, the proposedstandards “will be very difficult to meet just by how you manage in-field practices,” Crumpton says.

Iowa is leading the effortto use wetlands to purify ag drainage water.

These wetlands convert nitrates to harmless nitrogen gas through natural biological processes. They also help to store P-loaded silt. The wetlands can be placed below existing tile outlets, or integrated into the design of a modern subsurface-drainage system, says CREP’s Lechtenberg.

The Corn Belt already has many thousands of acres of restored wetlands, which provide excellent wildlife habitat. But those wetlands usually aren’t located in the right places to improve tile-water quality, Crumpton says. “Most of the nitrates come through subsurface flow.” CREP wetlands, by contrast, are carefully positioned so as to treat both surface runoff and tile water from agricultural watersheds of 500-3,000 acres.

Allan Armbrecht’s 10-acre wetland, installed in 2009, spans a drainage ditch. It’s surrounded by a 35-acre grass buffer. “This area of the farm has a very high water table, and I couldn’t drain it,” Armbrecht says. “Years ago it was permanent pasture.” Most seasons, he was able to crop only about 10 acres of the site. About 15% of Iowa CREP wetlands are located on marginal cropland or pastureland, Lechtenberg says.

At Armbrecht’s site, an earthen berm was built across a drainage ditch to impound tile water and create a permanent wetland. The wetland outlet consists of a broad-crested weir, which is designed to protect drainage rights on adjacent lands. Even during periods of flooding in both 2009 and 2010, “there was no problem with water backing up” into fields above the wetland. The dam functioned the way it was supposed to,” Armbrecht says.

Below the weir, a rip-rap plunge pool dissipates the falling water’s energy. A draw-down structure in the embankment can be raised and lowered to manage vegetation in the wetland. Plants growing there include cattails, bulrushes, sedges and arrowheads. The 35-acre buffer was seeded with native, warm-season grasses and forbs. The site is a haven for waterfowl, pheasants, frogs and other wildlife, Armbrecht says. Landowners retain ownership of CREP wetlands and are free to hunt there, or lease them out to others.

The cost to build Armbrecht’s wetland and grass buffer – about $140,000 – was paid for through a federal-state partnership, Lechtenberg says. Armbrecht and his neighbor shared a one-time, up-front payment of about $15,000 to enter into a 30-year CREP easement. Armbrecht will also receive 15 years of annual rental payments from USDA at 150% of the average soil rental rate. “We combine CRP and easement payments to equal the agricultural market value for the property,” Lechtenberg says.

ISU began monitoring Armbrecht’s wetland last summer, sampling inflow and outflow every six hours. “The wetland removed about 40% of nitrates,” Crumpton says, “including during a flood period.” That’s close to the long-term average nutrient reductions for all of Iowa’s CREP wetlands, he says. “In dry years, there’s a larger reduction; in flood years, it’s smaller.”

The effectiveness of wetlands also varies with size, Crumpton says. “The larger the wetland, the more effective it is” at cleaning up drainage water. CREP wetlands range in size from 0.5% to 2% of the watershed area. However, the economic optimum size is not yet clear, Crumpton says. Iowa CREP wetlands, which average 0.8% of the drainage area, remove nitrates for about 23¢/lb., Lechtenberg says.

Iowa has just over 700 acresof nutrient-removal wetlands, treating drainage water from 86,000 tiled acres. Meanwhile, there are six million acres of tiled cropland in Iowa alone – and more than 50 million tiled acres in the Midwest. Crumpton estimates that it would take from 500,000 to 725,000 acres of wetlands to reduce nitrate contamination in the Upper Mississippi River Basin by 30%.

For now, Iowa’s nutrient-removal wetlands are attracting “lots of interest from other states” in the Mississippi River Basin, Lechtenberg says.“We’re putting them in, monitoring them, getting the word out and showing people the good things they can do.”