Letter to the editor: Snapshot shows high nitrate levels despite heavy flow


To the editor:

On Oct. 10 volunteers from the Raccoon River Watershed Association monitored water quality at 36 sites throughout the Raccoon River Watershed and pulled samples for lab analysis at 26 of them. Lab analysis was completed by Des Moines Water Works and the data compiled by Lynette Seigley of the IOWATER program.

Before we talk results, let’s look at some context. Checking the Des Moines, Carroll, Perry and Storm Lake precipitation reports, the last notable precipitation had occurred Sept. 29. Precipitation recorded on that date ranged from .35″ to .78″. (Carroll recorded .01″ Oct. 6, but obviously that’s not enough to have any impact).

Despite the lack of precipitation for nearly two weeks prior, water levels were high. According to Seigley’s flow tables, using U.S. Geological Survey data for the snapshot date, the North Raccoon River at Sac City showed a flow of 397 cubic feet per second (cfs), which is 144 percent of the average for that date based on 55 years of records at the site.

The station on the North Raccoon River at Jefferson recorded a flow of 1,050 cfs, or 256 percent of average flow based on a 73-year record.

I also checked the meter on the South Raccoon at Redfield to verify whether the same held true in that part of the watershed. The flow there was 518 cfs, which is an increase of 350 percent over the average flow for that date based on a 75-year record.

Let’s look at some ag numbers for the sake of context in that department as well. According to the Oct. 13 U.S. Department of Agriculture Iowa Crop Progress Report, by Oct. 11 the statewide corn crop was 96 percent mature, and 29 percent of the corn crop and 65 percent of the soybean crop had been harvested.

Topsoil moisture was rated 81 percent adequate and 6 percent surplus and subsoil moisture at 83 percent adequate and 8 percent surplus. I found this interesting. Given the high surface water flows, I though the surplus number would be higher. The report also notes that “there were reports of some manure and fertilizer being spread.”

Bear in mind this is all statewide information, not watershed specific.

So let’s look at the big, controversial topic: nitrate. Before I break out those numbers, I would remind readers the U.S. Environmental Protection Agency has set the safe drinking water standard for nitrate at a maximum of 10 milligrams per liter (mg/l). The Des Moines Water Works either switches sources or activates its nitrate removal facility when levels reach 9 mg/l to allow for a margin of safety.

Lab analysis for nitrate was completed for 26 sites. The 50th percentile level was 14.07 mg/l. The mean (average) level was 15.5 mg/l. The minimum level recorded was 8.72 mg/l, located on the South Raccoon about 2.5 miles southwest of Redfield before the confluence with the Middle Raccoon River. The only other lab-analyzed sample below 10 mg/l was site D16 on Sugar Creek in southwest West Des Moines.

The highest nitrate level recorded was 30.71 mg/l, recorded at on Hardin Creek at Highway 30, just northeast of Jefferson. A close second with 30.65 was site DD 34B, a drainage ditch feeding into Cedar Creek 3.5 miles northeast of Sac City. Third highest was 20.47 on site CA7 on Camp Creek about 4 miles northwest of Lake City.

Of the sites with no lab analysis and for which we have only the field test strip results, all except the two Black Hawk Lake sites recorded either 10 or 20 mg/l as a result. The two lakes sites recorded 0 and 1 mg/l.

Unfortunately, this time we did not get any data on Elk Run Creek. Agriculture’s Clean Water Alliance (ACWA) has released their reports on their 2015 monitoring that is done from April through July, and the average nitrate level over four months of monitoring was 35.4 mg/l on Elk Creek Run.

Interestingly, Outlet Creek was very close to the middle of the pack, with 14.04 mg/l. This is much lower than previous recorded results (39.79 mg/l on Nov. 2, 2013, 34 mg/l on July 21, 2007, and 59 mg/l on July 22, 2006). Whether the decrease is due to dilution because of a higher base flow or to renovations of the wastewater treatment plant (or both) isn’t clear from one set of data, but hopefully the WWTP improvements are having a positive effect.

The Raccoon River Watershed Association volunteers also have lab data for other parameters, such as bacteria, phosphorus, etc., but given that nitrate is the hot button issue for water quality at the moment, I have concentrated on those results here.

Steve Witmer, vice president, Raccoon River Watershed Association


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