By Todd Neeley
DTN Staff Reporter
OMAHA (DTN) -- Despite large spikes in nitrates in water during times of heavy rains following drought, overall nitrate levels have been falling in Iowa's Raccoon River watershed between 1999 and 2014, according to a study recently completed by the Iowa Soybean Association.
Des Moines Water Works has sued 10 drainage districts in the Raccoon River watershed, claiming the districts are responsible for high nitrates found in source water DMWW uses to provide drinking water to some 500,000 customers. A federal judge in Des Moines was scheduled to hear arguments in the case Monday on a motion to dismiss filed by the defendants.
The ISA study results show what many agriculture interest groups and others have been saying about the high nitrate levels in water: Spikes in nutrients have come as a result of flooding rains following drought in years such as 2012.
"Despite exceptionally high (nitrate) concentrations measured in 2013, the 1999-2014 concentration showed 39 of the 41 sites had decreasing trends," the study said.
"Examination of DMWW (Des Moines Water Works) and ACWA (Agriculture's Clean Water Alliance) data here supports the idea of large NO3-N transport following drought. The drought year of 2012 was the hottest recorded in the RRWS (Raccoon River watershed) while precipitation was much below normal. This was followed by record rainfall across Iowa during April-May 2013, and a record high NO3-N concentration of 24 mg L was reported at DMWW in May 2013.
"It is noteworthy that this extra N left behind because of drought-repressed crop yields is larger than annual river NO3-N load, even in a wet year. This illustrates the vulnerability of streams in the Mississippi River basin to climate-induced perturbations of NO3-N transport, and especially the Raccoon."
The study said nitrate concentrations and loads were at their lowest during dry years and at their highest in the wet year of 2013, following the 2012 drought. "Several studies have observed similar elevated NO3-N following drought," the ISA study said.
The group points to a 1983 study that found an accumulation of nitrate may occur in agricultural soils because of "reduced NO3-N movement and transport during dry weather regimes." Shilling and Zhang 2004, for example, found maximum Raccoon River nitrate loads "often occurred following the second year of below-normal precipitation and discharge," the ISA study said.
Although there were concerns corn-on-corn crop rotations could lead to higher nitrates runoff, the ISA study said the data doesn't support it.
"Despite warnings of increased river NO3-N from more C-C (corn-on-corn) rotations fueled by ethanol demands, increased corn area and fertilizer inputs in the RRWS did not increase river NO3-N," the study found.
Since 1999, nitrate concentrations appear to be declining at the "vast majority" of Raccoon River watershed sites, "although the ratio of river NO3-N load to applied fertilizer N has changed little since 1990."
The study says falling nitrate concentrations may be related to decreasing soybean cultivation in the watershed. Fewer acres planted to soybeans, the study said, may have "indirectly reduced" nitrate transport by shrinking land area "most vulnerable to NO3-N export, increasing denitrification in the soil profile, and decreasing water throughput in tiled fields."
"This suggests better management of this crop will improve water quality," the study said.
Jennifer Terry, environmental advocacy leader for DMWW, told DTN/The Progressive Farmer in a statement that ISA itself admits the underlying problem with nutrients runoff continues.
"The Iowa Soybean Association and Agriculture's Clean Water Alliance can talk about nitrate trends and how the nutrient reduction strategy is allegedly going to clean up our polluted waterways, but the real issue here is that there's a water quality crisis in Iowa," she said.
DMWW is finding it "increasingly hard" to provide safe drinking water to its customers, Terry said, "when our source water is polluted with nitrate moving down the river to our intakes. The data showing continued water degradation clearly demonstrates why a strictly-voluntary strategy for agricultural pollution will not succeed."
DMWW continues to oppose statewide voluntary efforts to reduce nutrients runoff, saying water pollution only can be removed through regulation.
"The Iowa Soybean Association and the state of Iowa are pinning their hopes on a nutrient reduction strategy that is doomed to fail," Terry said. "This shotgun approach has no timelines for nutrient reductions, no implementation targets, no required water quality monitoring and no sustained funding mechanism. In addition, it is optional for farmers to even participate.
"Unfortunately, we are not hearing about any fresh approaches to solving our water crisis here in Iowa, but rather simply doing more of the same to avoid regulation; it sounds like business as usual."
Todd Neeley can be reached at todd.neeley@dtn.com
Follow him on Twitter @ToddNeeleyDTN
(AG/SK)
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