In an interview with KCCI, Iowa Governor Kim Reynolds blamed the weather for high nitrate levels in the Raccoon and Des Moines rivers that led to an unprecedented outdoor watering ban for the Des Moines metro and dismissed any suggestion that Iowa needs to change its policies. Milli Vanilli’s 1989 hit “Blame it on the Rain” captures the vibe perfectly, so I covered the song and made a silly video with footage from the interview. However, the situation is no laughing matter.
Let me explain the situation as simply and clearly as I can. The high nitrate levels we are seeing this spring are not a fluke, drinking water is not completely safe in many communities across Iowa, and if we stay the course with Iowa’s Nutrient Reduction Strategy we will be waiting a long time for things to get better.
You can’t have it both ways
It’s true, nitrate pollution in rivers is influenced by both rainfall and recent drought. This has been a recurring theme in my analysis of water quality data (most recently here, and most rigorously here). But the Governor and her administration want to have it both ways. When nitrate in the Cedar River dropped due to favorable weather, they took that as a sign that a voluntary approach was working and took the unusual step of trying to withdraw a pollution budget that might have placed limits on new pollution from industry. Just last fall they were arguing that the following rivers should be removed from the Impaired Waters List, based on a 10% rule that makes no sense in the context of drinking water. The table below shows the data that they’re using to make those determinations.
Regardless of whether they meet the technical threshold for impairment, as a practical matter, the Raccoon River and Des Moines River regularly have nitrate levels high enough to cause problems for drinking water supply. The Central Iowa Water Works had to run their nitrate removal facility in 2024, 2022, 2019, 2018, 2017, 2016, 2015, 2014, and 2013. (This was reported recently by KCCI). It’s the largest such facility in the world, and this year it wasn’t enough!
Nitrate levels this spring are higher than average but not a fluke
The graphs below shows how daily nitrate concentrations and discharge (streamflow) in the Raccoon River this year compares to the median for that time of year. It is unusual for wet weather and high nitrate levels to persist through late July, but nitrate does exceed 10 mg/L about half the time in May and June. If you want to understand long-term nitrate trends in the Des Moines River and Raccoon River, read Chapter 5 of the new source water report commissioned by Polk County.
The Raccoon and Des Moines Rivers are not alone in having high nitrate levels this year. Here is 22 years of weekly data from a site on the South Skunk River, just downstream of Ames. This spring, peak nitrate levels (22 mg/L) were the highest we’ve seen since 2014. Average spring nitrate levels (15.5 mg/L) were the highest we’ve seen since 2015. However, it’s only a little above the long-term average (13 mg/L, indicated with a dotted black line) and ranks 7th out of 23 years for which we have data.
None of the data I’ve seen rules out a slight improvement in water quality in this or other Iowa rivers, masked by the ups and downs of an 8-10 year cycle. However the data does rule out this year being a fluke and us not having to worry about high nitrate levels in the future!
In Iowa, it’s normal for it to rain a lot in the spring. In watersheds with a lot of tile-drained farmland, it’s normal to see high nitrate levels for several days following a rainstorm. Ames is lucky to have a buried sand-and-gravel aquifer with the right geology and chemistry to remove most of the nitrogen before it reaches wells. If we withdrew water directly from the Skunk River, it would exceed the drinking water standard every year. Many other communities in Iowa are not as fortunate in their geology.
Nitrate in Drinking Water Poses a Widespread Health Risk
The Safe Drinking Water Act was written so that we must draw a hard line between “safe” and “unsafe.” Drinking water utilities must meet a Maximum Contaminant Limit of 10 mg/L nitrate as nitrogen. Most large water systems have been able to say on the “safe” side of the line, although it can be costly to do so. However, the standard hasn’t been updated to reflect research on the link between chronic exposure to nitrate in drinking water and various cancers. And really, it’s better to think of health risks as a continuum from “more safe” to “less safe.” The Environmental Working Group created a map a few years ago showing which communities fall in the “not as safe as they could be” range for nitrate in drinking water. Last year, Iowa Environmental Council recently released a report about the health risks of nitrate in drinking water and is now hosting a series of listening sessions on cancer and the environment.
Conservation efforts have been partially offset by increased fertilizer use
Shouldn’t we expect some improvement in nitrogen levels due to the state’s nutrient reduction strategy and the conservation efforts of farmers? I’ve written another article to dig into this question but the short answer is that we can expect at most a 2% reduction in nitrogen losses over the past decade. That’s for the state as a whole, some watersheds are doing better or worse and we don’t have a good tracking system to evaluate it. Most of the progress that we can expect from cover crops and nitrification inhibitors have been offset by increases in fertilizer application rates, which apparently made economic sense to do.
I can’t really blame farmers for acting in their economic self-interest. I do think it’s fair to blame your elected officials if they can’t take drinking water safety seriously and offer better solutions. Just don’t blame it on the rain.
Before state wastewater standards went into effect in the 1960s, raw sewage could flow directly to a stream without treatment. Despite the standards, this continues in many areas today. In areas called “unsewered communities,” outdated or poorly functioning septic tanks still allow untreated wastewater into our waters. The Iowa DNR works with these communities to find funding sources and alternative treatment systems and to allow adequate time to upgrade the systems.
The Governor has announced that additional funding through the infrastructure bill that will be available to help unsewered communities upgrade their systems. Could this make a big difference for water quality in Iowa? Statewide, I’m not sure, but I’ve taken a closer look at the Iowa River Basin upstream of Marshalltown, where we know of 11 unsewered communities. Based on my first look at the data, it appears that these communities have little influence on E. coli in the Iowa River itself, but could make a difference for water quality in tributary streams like Beaver Creek in Hardin County.
There are 11 unsewered communities in the upper part of the Iowa River Basin, marked here with yellow circles with an X.
A Water Quality Improvement Plan for E. coli bacteria in the Iowa River Basin was released by Iowa DNR in 2017. As required by the Clean Water Act, these kinds of plans include a Total Maximum Daily Load (TMDL) of pollutants that a water body could handle and still meet water quality standards. Author James Hallmark compares this pollution budget to a family budget: regulated point sources are your fixed bills, non-point sources are your variable expenses, and the margin of safety is your emergency fund. I like this analogy and would add that without some understanding of where your discretionary spending is going, and a realistic strategy to reign it in, you’re probably not going to achieve your goals.
The Water Quality Improvement Plan includes a comprehensive list of E. coli sources but doesn’t single any of them out as being particularly important. It includes a list of potential solutions, but it doesn’t identify which of those would make the most difference. That’s a job for a Watershed Management Plan written with stakeholder input, apparently. However, the document is chock-full of load-duration curves, which I wrote about previously. We can use the information in these charts and tables to take the next step and begin to narrow down where and when the pollution is most serious!
In this article, I won’t pay much attention to “High Flows” and “Low Flows” because there wouldn’t be much recreational use under these conditions. I also don’t look at “mid-range” flows because there’s a bigger mix of sources influencing water quality in these conditions. A closer look at the other two categories is revealing.
If houses are discharging raw sewage directly into a stream, we’d expect to see the highest E. coli concentrations when the stream is running lower than normal, and there’s less dilution. This is indeed what we see in Beaver Creek in Hardin County, which is downstream from the unsewered community of Owasa. Beaver Creek would need a 79% reduction in E. coli load to meet the primary contact recreation standard during “Dry Conditions” and a 38% reduction during “Wet Conditions”.
If not fully treated, sewage could be a major contributor to E. coli in some tributaries of the Iowa River.
Treated sewage also has the biggest influence when streams are lower than usual. The upper reaches of the South Fork receive effluent from the small towns of Williams and Alden, which have waste stabilization lagoons. It’s likely that some bacteria makes it through the treatment process, and this would explain why E. coli is higher during “Dry Conditions” (needing a 73% reduction) than during “Wet Conditions” (needing a 30% reduction). When their permits come up for renewal, Iowa DNR could require a UV disinfection system to ensure that E. coli in effluent is no greater than 126 colonies/100mL.
The blue line is the wasteload allocation–the regulated part of the pollution budget. Even with the best available treatment, wastewater from two towns has a big influence on the South Fork during dry conditions.
In a watershed with few people and many hogs, we’d expect to see the highest E. coli concentrations when the streams are running high and runoff from fields that receive manure application is more likely. This is indeed what we see in Tipton Creek in Hardin County, a watershed containing 47(!) CAFOs, but the levels are not especially high compared to other sites in the Iowa River basin. The recreation standard is met during “Dry Conditions” and would need a 36% reduction during “Wet Conditions.” Handled correctly (applied to flat ground at the right time, and preferably incorporated into the soil), manure and the microbes it contains can be kept out of streams. Preventing loss of the nutrients in manure is a more difficult challenge—nitrate concentrations in Tipton Creek often exceed 20 mg/L!
Despite there being a lot of hogs in the Tipton Creek watershed, E. coli levels are not especially high, relative to downstream locations.
It’s not clear to me whether primary contact recreational use of these streams is a relevant or attainable goal, or whether we should be calibrating our level of concern to the secondary contact recreation criteria. Unless there’s a permit holder affected, IDNR doesn’t investigate whether there’s enough water for kayaking in Tipton Creek, or whether children play in Beaver Creek, so the designated use is presumptive and tells me nothing.
E. coli and recreation on the Iowa River is not as big a concern at Crystal Lake as it is at Steamboat Rock. Photo Credits: Ryan Adams, photojournalist
To protect fishing, paddling, and children’s play on the Iowa River itself, where and when should we focus? The Iowa River at Marshalltown needs a 60% reduction in bacteria load to meet the recreation standard during “Wet Conditions” (10-40% flow exceedance). However, it actually meets the primary contact recreation standard during “Dry Conditions” (60-90% flow exceedance). Focusing on unsewered communities in the watershed would NOT be an effective way to address this impairment.
Beaver Creek (left) has worse E. coli when it’s dry. The Iowa River near Marshalltown (right) has worse E. coli when it’s wet. If the green line is above the red line, that indicates that the E. coli geometric mean for that range of flows exceeds the standard.
Galls Creek in Hancock County has some of the worst E. coli levels measured in the basin, and would have a larger per-acre benefit to the Iowa River if standards could be met. Galls Creek has no unsewered communities but at least 20 farmsteads located along the creek that could have issues with septic systems overflowing under wet weather. The watershed has little woodland and no pasture, so land application of manure from the several CAFOs in the watershed would be most likely animal source of E. coli.
Table by Prairie Rivers of Iowa, using information from the Water Quality Improvement Plan for the Iowa River Basin
This is just a partial review of one of three HUC8s in the Iowa River Basin. There is much more to learn from further discussion with people who know the area well, or from on-site investigation. However, I hope I’ve demonstrated how we might squeeze some more insight out of the data we have, in order to make smart investments in water quality.
Nice looking rye cover crop near Nevada, IA November 2020
A big thank you to farmers who planted cover crops after this challenging year. Cover crops will hold soil and nutrients in place through the winter and early spring. That could be especially important this year.
After a drought, nitrogen that might normally have been taken up by a high-yielding crop or flushed away by rainfall remains in the soil. That leftover nitrogen could be available for next year’s crop, but only if:
A) we have a dry spring, or
B) farmers have made use of practices like cover crops or nitrification inhibitors that prevent nitrogen losses during the fallow season.
A wet year in 2013 following a dry year in 2012 (dark blue) caused nitrate concentrations in many central iowa streams to jump as much as 10 mg/L higher than usual (red). Figure from Van Metre et al. 2008
A drought in 2012 following by a wet spring in 2013 led to nitrate concentrations in excess of 20 mg/L in many rivers in Central Iowa. If we have a wet spring in 2021, we could see this happen again. As one scientific paper put it, “weather whiplash drives deterioration of water quality.”
“Weather whiplash” can also help explain the long-term trends I’ve been seeing in the South Skunk River and its tributaries: a decline in nitrate concentrations from 2005-2012, a big jump in 2013, and another decline over the past 7 years. I’ll walk you through my analysis.
Explaining nitrate concentrations in the South Skunk River
Technical details, feel free to skip: This data was collected by the City of Ames just upstream of wastewater treatment plant. The City has monitored the South Skunk River above and below its wastewater treatment plant almost every week since 2003! Flow is measured continuously at a few miles upstream USGS gage near S. 16th St. I’ve summarized nitrate concentrations and streamflow by season (Jan-Mar, Apr-Jun, Jul-Sep, Oct-Dec). At each step of the way, I apply a linear regression equation and graph the model residuals. Taken together, these three factors explain 59% of the variation. The effects of “weather whiplash” may extend beyond one year, since nitrate from some parts of the field may travel more slowly to streams via groundwater.
In a given quarter, nitrate concentrations in the South Skunk can be up to 10 mg/L higher than the long-term average, or as much as 10 mg/L lower. The following graphs show how much variation is left to explain after correcting for current weather, last year’s weather, and season.
Nitrate in the South Skunk is again declining after a sudden increase in 2013.Low-flow conditions in 2011-2013 explain unusually low nitrate concentrations.
Wet springs following dry years explain unusually high nitrate concentrations.Seasonal patterns explain some of the remaining variation.
The lowest nitrate concentrations can be explained by streamflow: when the weather is dry and tiles aren’t flowing, nitrate levels in rivers taper off to the background levels seen in groundwater.
The highest nitrate concentrations can be explained by weather over the previous 12 months: a wet period following a dry period will flush out nitrate that’s accumulated in the soil.
After that, there’s still a seasonal pattern independent of rainfall: nitrogen is most susceptible to loss in spring when soils are bare and microbial activity picks up (April-June) and least susceptible when the maturing crop is using up the available nitrogen (July-Sept).
Can some of the remaining pattern be explained by greater adoption of conservation practices in the watershed in the past 5 years? We hope so, but let’s see what happens next spring!
