Can building wetlands reduce dangerous high nitrate levels and thus provide clean, safe drinking water for thousands of people?
But, when it comes to ensuring clean water, not all wetlands are created equal.
Biologists know how to restore great wetlands to draw in ducks and shorebirds. Restoring wetlands to also help people may require a different approach.
That’s the focus of an intensive research effort conducted by Nature Conservancy scientists on the Mackinaw River watershed in central Illinois. The wetlands—while providing wildlife habitat and healthier rivers—are being designed and tested to provide safe drinking water for the 90,000 residents of Bloomington, Illinois and surrounding communities where the town’s primary reservoir has had a history of high nitrate levels.
You don’t want high nitrate levels in your drinking water, particularly if you have infants. High nitrates can cause all sorts of nasty problems, including a condition called methemoglobinemia (more commonly known as “blue baby syndrome”) that can inhibit oxygen flow in the bloodstream of infants.
Nitrate levels in some Illinois waters rank among the highest in the country. This is due to agriculture—the endless fields of corn and soybeans you simply can’t miss if you drive around the state.
These fields are heavily fertilized, and fertilizer is high in ammonia, which easily converts to nitrates when exposed to water. When it rains, those nitrates ends up in rivers and groundwater—and eventually reservoirs.
When nitrate levels are elevated in drinking water, as has happened in Bloomington, officials have basically two long-term options:
- Install an expensive treatment facility that costs millions to maintain, requires staffing and generates its own toxic waste that can’t be easily disposed.
- Use a watershed conservation approach, including building wetlands that not only purify water but provide wildlife habitat and healthier streams and rivers.
A simple choice, right?
But when it comes to removing contaminants like nitrates, not just any wetland will do.
That’s where the Conservancy’s research program comes in. Scientists are designing and testing the effectiveness of different wetland designs in removing nitrates from the drinking water.
Conventional wisdom is that any wetland is good, and more wetlands are even better. That is true, to a point—but not when it comes to removing nitrates from the water.
To understand why means confronting an agricultural practice known as tiling.
Miles of Tiles
Central Illinois fields are known for their deep, dark topsoil. But historically, these fields were also wet and boggy places. Those conditions make for a poor cornfield, so farmers created drainage ditches and tiles.
Originally tiles were clay pipes that were placed underground in fields to allow for drainage. Today, tiling is made of perforated plastic piping that drains huge swaths of farmland. Literally millions of miles of tiling run underneath Illinois.
“When it rains, tiles transport high amounts of nitrates, phosphates and other pollutants from the field directly into the river system,” says Maria Lemke, aquatic ecologist for the Conservancy. “They eventually end up in places like drinking water reservoirs, and the Gulf of Mexico. These nitrates and phosphates are one of the leading contributors to the Gulf’s dead zone.”
The elaborate tile system, located two to four feet underneath the field surface, effectively drains it – shooting all those nitrates and phosphates directly into the river.
“We know wetlands act as filters, and we believed they could reduce nitrate levels less expensively than a treatment plant,” says Lemke. “Wetland plants take up the nitrogen, and microbes convert it into harmless nitrogen gas, released into the air.”
The typical way to construct these types of wetlands is to build them between the field and the river. One problem: Underground tiling runs underneath the wetland, bypassing the wetland’s benefits while still depositing water directly in the river. Thus, you can build hundreds of wetlands and not achieve the goal of cleaner water.
The Conservancy is currently designing projects where the tiles run into wetlands before seeping into the river. Does it work?
Designing Wetlands for People
The Conservancy is currently testing wetland design in different areas of the Mackinaw Watershed. One of the research projects is at a seventh-generation property known as the Franklin Research and Demonstration Farm. At the farm, three replicate wetland complexes were constructed, each in turn with three interconnected wetland cells resembling square ponds.
The inlets and outlets of each wetland cell are closely monitored to determine the size of wetland that is needed relative to the area drained by tiles in order to effectively remove nitrates.
The minimal size necessary to achieve results is important. Corn prices are high. Farmers want to lose the minimal amount of crop land.
Each wetland cell represents 3 percent of the tile-drained area in the adjacent field. As tile water flows into the first cell and subsequent two cells of each wetland complex, monitoring results represent wetland efficiencies for cells representing 3, 6 and 9 percent of the tile-drained area.
Water is tested as it flows from the field tiles into the first wetland cell and then is tested as it flows out of each subsequent cell. An automatic sampler collects samples according to tile flow and the rate of rising water entering the wetland, providing an accurate picture of fluctuating nitrate levels.
Research has found a wetland size of 5-6% of the drainage area of a tiled field removes close to 50% of the nitrates. Larger wetlands offer diminishing returns.
“The 9% wetland is not getting much more bang for your buck,” says Dave Kovacic, University of Illinois professor and a key research partner on the effort.
“These efforts are all voluntary, and farmers are very conscious about how much land they have to take out of production,” adds Lemke. “Farmers want to see success on the ground. Farmers are watching how these wetlands work. It’s important for these projects to work.”
The wetlands also provide wildlife habitat. As we walk towards them, a heron lifts off; frogs croak from the edges. But wetland design is being driven by research on how the wetlands will best remove nitrates.
If enough could be strategically placed in tiled fields, wetlands would work in lowering nitrate levels on a watershed level. Location is important. While 12 million acres of Illinois is tiled, not every field has tiles. Placing wetlands in the best areas in the watershed, according to soil type and drainage, will yield the best results for water quality.
With tiles located underground, just finding them can be exceedingly difficult. The Conservancy is working with researchers from the University of Illinois and Illinois State University to develop methods to determine the location of tiles (I’ll explore this effort in a future blog).
But even if tile in fields could be mapped, establishing sufficient wetlands will require substantial outreach and policy in addition to the cost, ongoing research and hard work of constructing them.
Is it worth the expense and effort? To answer that, I visited the City of Bloomington water treatment plan.
The Cost of Clean Water
Rick Twait, director of the water treatment facility for the City of Bloomington, agreed to show me the steps necessary to ensure the water that comes out of your tap is clean, safe and drinkable.
He’s responsible for a sprawling facility that cleans about 11.5 million gallons of water per day. He’s rightfully proud of his city’s treatment process, a dizzying array of filters, chemicals and storage tanks, all designed to remove pathogens and algae from water.
But even these extensive processes can’t remove nitrates. The city currently draws water from two reservoirs, one of which regularly exceeds nitrate levels established by the Safe Drinking Water Act. Right now, the approach has been to simply switch to the other reservoir, but that is not a long-term strategy.
“We’re very concerned about droughts,” says Twait. “We are so close to being over the limit. Nitrates are just one issue we have to deal with, but it’s a big one.”
For Twait, the choice is clear when it comes to an ion treatment plan or the wetlands approach.
“We want to avoid an engineered solution if we can,” he says. “It’s expensive. It requires constant staffing, even though we may only exceed nitrate levels once every few years. The ion plant creates its own toxic waste that would be expensive to dispose. The wetlands approach, in contrast, does a lot of good things. It has benefits even beyond clean water.”
But it does face challenges. The research wetlands at the farm show that the wetlands reduce nitrates significantly. The next step would be doing this at a watershed scale, difficult when agricultural nitrate reduction is currently voluntary.
“We know wetlands work,” says Lemke. “But what will it take to see water quality improvements on a watershed scale? Our research is showing us what we need to do, where we need to work and how we need to design wetlands. Now we need a mechanism to convince farmers and the public.”
Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.
(Photo: Test wetland in the Mackinaw River watershed. Credit: Matt Miller/TNC)