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Nutrient concentrations are going down in several major Iowa rivers.
Recent analyses show declining trend lines for nitrates in both the Raccoon and Iowa rivers, and a third analysis shows declines in both nitrates and phosphorus in the Upper Iowa River.
Those results bolster hopes that the state's voluntary nutrient reduction strategy eventually will succeed in cutting by 45 percent the volume of nutrients flowing from Iowa.
Those downward-trending rivers, however, also constitute but a fraction of the nutrient-laced water leaving the state.
“It's not a cause of celebration that we are done. It's an impetus to keep doing what we've been doing, only a lot more of it,” said Susan Heathcote, water programs manager for the Iowa Environmental Council.
While the results are encouraging, “we still have a lot of work to do. That's my take-home message,” said Mary Skopec, a research geologist with the Iowa Geological and Water Survey.
Skopec said downward nutrient trends have also been documented on the Des Moines River downstream of Des Moines and on the South Skunk River below Ames. Many rivers are showing flat levels of nutrient concentrations and some, like the Cedar River at Cedar Rapids, are on “a bit of an upward trend.”
Despite nutrient reductions in some rivers, nitrate concentration increased 12 percent from 2000 to 2012 at the mouth of the Mississippi River, where excessive nutrients have created a large zone of low oxygen uninhabitable by most aquatic life, according to a U.S. Geological Survey report issued last year.
That same USGS report found that flow-normalized nitrate concentrations decreased 11 percent from 1980 to 2010 at Wapello near the mouth of the Iowa River.
A recent analysis by Northeast Iowa Resource Conservation and Development found that flow-normalized nitrate concentration declined from about 5.4 mg per liter in 2004 to about 5.1 mg per liter in 2012, based on samples collected near the Upper Iowa River's mouth south of New Albin. Samples collected at the same site showed that phosphate concentration declined from about 0.13 mg per liter to about 0.10 mg per liter during the same period.
A 2013 Iowa Farm Bureau analysis of Raccoon River data recorded at Des Moines Water Works showed a decline in nitrate concentration from 9 parts per million in 2006 to 5 ppm in 2013.
A separate study released earlier this year by the Iowa Soybean Association found that Raccoon River nitrate levels have trended downward at a rate of 0.28 milligrams per year since 1999, despite a significant increase in corn acreage in the area where the samples were collected.
Nitrate levels in the Raccoon River, an important source of drinking water for Des Moines and other cities, drew considerable public attention last year when they spiked to record levels after heavy spring rains flushed nitrates that has been unused during the preceding year's drought.
The downward trend on the Raccoon was “a little bit surprising but also quite encouraging,” said Roger Wolf, environmental programs and service director with the Iowa Soybean Association.
Wolf said the study also factored in conservation practices and fertilizer management in areas where the samples were collected.
“I feel good about gaining some understanding of how rivers respond to conservation practices. We can't afford to do it on a hope and a prayer,” he said.
Rick Robinson, environmental policy advisor for the Iowa Farm Bureau Federation, said the data show “something has changed in recent times.”
Robinson said he would argue that “resources in farm bill conservation programs are having an effect.”
Heathcote noted nitrate loss is highest during peak river flows, which tend to dilute concentration levels while greatly increasing the overall nitrate load.
Reducing concentration levels helps meet the 10 mg per liter drinking water standard, but heavy nitrate loads continue to worsen the Gulf of Mexico dead zone, she said.
The Upper Iowa river's improving water quality, which is also reflected in increasing clarity, is primarily the result of landowners voluntarily implementing conservation practices in the watershed, said Lora Friest, executive director of Northeast Iowa Resource Conservation and Development and a former Upper Iowa watershed coordinator.
Friest said landowners have been increasing implementation of conservation practices in sub-watersheds of the Upper Iowa's cold-water tributaries. The results of those practices, she said, also can be measured in the proliferation of naturally reproducing trout in many of those tributaries.
The Upper Iowa “has responded to all the care that people have put into it,” providing “an example that good environmental stewardship can make a difference,” she said.
Farmers have become more efficient in their use of fertilizer, and more of them are adopting conservation practices such as no-till, filter strips and cover crops, Iowa Agriculture Secretary Bill Northey said.
“It's logical to think there has been a positive response,” he said.
Northey said data showing nutrient reductions in some rivers was not widely available when state agriculture and environmental officials collaborated with Iowa State University scientists to formulate the nutrient reduction strategy.
“It's hard to know for sure” that increased adoption of conservation practices is causing nutrient levels to fall, “but I do believe that farmers are making better sustainability decisions,” and Jim Gillespie, soil conservation director for the Department of Agriculture and Land Stewardship.
Recent progress should be regarded as encouragement to keep going rather than an excuse to back off, both Northey and Gillespie said.
Skopec said rivers with downward trending nutrient concentrations could be responding to conservation practices.
“In the larger context, there are lags in the system, and what we did in the past five years might not necessarily show up right away,” she said.