nitrate - Prairie Rivers of Iowa

You can lead a horse to water…

by Dan Haug | Jan 2, 2023

Prairie Rivers of Iowa is not the sort of environmental group that follows the goings on at the state capitol (that would be our friends at the Iowa Environmental Council) but the success of our watershed projects is very much affected by state and federal policy. A big part of our work is environmental education, but often “is a river still polluted and what can we do about it” is a legal and political question as much as a scientific question. I hope this tricky case study from the Cedar River will illustrate why we need more people to learn about and talk about environmental policy to make it more transparent, fair, and effective.

My New Year’s resolution for 2023 is to write fewer long articles like this one and more bite-sized lessons. For the 50th anniversary of the Clean Water Act, we’ll be sharing 50 short facts (one a week) on social media about that important and complicated law. Here are the first five:

1) The Clean Water Act (CWA) is 50 years old but it still has a big influence on how we evaluate and protect water quality in rivers and lakes.

2) The Clean Water Act is a federal law but is implemented at the state level, with oversight from the Environmental Protection Agency (EPA). In Iowa, the Department of Natural Resources (DNR) is responsible for issuing permits, setting standards, and assessing the condition of rivers and lakes.

3) The Clean Water Act requires public notice and public comment for many decisions. Staff at environmental agencies read and take seriously public comments, so it’s worth speaking up and having your voice heard.

4) The Clean Water Act also gives concerned citizens the standing to file suit if there is an ongoing violation that hasn’t been enforced, or if the Environmental Protection Agency is not fulfilling its mandatory duties.

5) Decisions by courts and federal agencies can come into conflict with state legislatures, which control the budgets for state agencies. For example, in Iowa there are over 700 river segments and lakes on the waiting list for a cleanup plan, because Department for Natural Resources doesn’t have enough staff to keep up with it.

We can sum that up with the old saying: “You can lead a horse to water, but you can’t make it drink.”

In November, the Iowa Department of Natural Resources (DNR) made the unusual decision to withdraw a cleanup plan (or TMDL) for nitrate in a part of the Cedar River that supplies drinking water to Cedar Rapids. Click here for the original plan, here for the public notice of its withdrawal, and here for the Iowa Environmental Council’s response, which provides some valuable context. TMDL stands for “Total Maximum Daily Load.” TMDLs are pollution budgets that explain where pollution is coming from and how much needs to be reduced in order to protect fisheries, drinking water, or recreation in an impaired river or lake. They are often used to set permit conditions for upstream sewage treatment plants and industrial facilities.

leading a horse to polluted water in the the Cedar River

There is a joke that TMDL stands for “Too Many D*** Lawyers.” Most state agencies ignored the part of the Clean Water Act dealing with TMDLs until a series of lawsuits by environmental groups in the 1990s. The Cedar River TMDL was actually written under a court order in 2006. The TMDL estimated that only 9% of the nitrogen in the Cedar River watershed was coming from regulated point sources of pollution like sewage treatment plants and factories. Most of the reductions would need to come from agriculture, through voluntary conservation programs. Still, the plan called for capping the pollution from point sources at the 2006 amount and not adding any more. However, it seems that the DNR did not follow the TMDL when writing permits over the next decade, and did not enforce permit violations.

One of the most surprising violations is from a drinking water treatment plant in Waverly. I don’t think of drinking water treatment as generating pollution, and maybe that’s why it was initially overlooked. The facility uses reverse osmosis, which gives you cleaner water on one side of the membrane and dirtier water on the other side. The facility has been discharging wastewater with 37.7 mg/L of nitrate into the Shell Rock River (a tributary of the Cedar). When the DNR added a permit condition that nitrate be brought down to 9.5 mg/L, the Iowa Regional Utilities Association protested, claiming that compliance would cost them $1 million. If my math is correct, bringing the facility into compliance would avoid only 5 tons of nitrogen pollution per year. The TMDL calls for a reduction of 9,999 tons per year. Enforcing this permit as written does not seem like a fair or effective way to protect water quality in the river, but I suspect there would be an easy fix if the TMDL were revised.

The Clean Water Act provides two ways to set the limits in a permit. Water quality-based effluent limits reference the pollution budget in a TMDL. They’re only for facilities that discharge to an impaired water body. Technology-based effluent limits are set statewide, based on the level of treatment that’s possible with widely available, not-too-expensive technologies. The Iowa Nutrient Reduction Strategy included new technology-based effluent limits for nitrate and phosphorus, affecting 157 municipal and industrial wastewater treatment systems. They must find a way to reduce their total nitrogen by 66% and their total phosphorus by 75% or else complete a feasibility study to show it would be cost-prohibitive to do so. Some facilities are already making the upgrades, some won’t be done until 2027. For the largest point source in the TMDL (the Waterloo sewage treatment plant), that means a reduction of some 333 tons of nitrogen a year.

Effluent from a wastewater treatment plant entering a river.

Of course, most of the nitrate reduction goal for the watershed (9,999 tons) will need to come from agriculture. We don’t know how that’s going because Iowa doesn’t have a current or complete tracking system. The most recent data I could find for cover crops by watershed is 7 years old. At that time, there were not enough acres to make a noticeable difference in water quality in the river.

Cover crops in the Cedar River watershed

Cedar River watershed map, courtesy of IIHR

Side note: The Cedar River starts in Minnesota and has several major tributaries, including the Shell Rock River, West Fork, and Winnebago. It’s a big watershed that usually gets divided into smaller chunks (i.e. there are separate watershed management authorities for the Upper, Middle, and Lower Cedar). The TMDL actually recommended prioritizing conservation in the Upper Cedar, but at some point, the focus got shifted to the Middle Cedar.

Are water quality based-effluent limits still needed? Maybe not, but the frustrating thing about this case is that we get don’t get a revised pollution budget that shows how other strategies will protect drinking water in Cedar Rapids. We don’t get a public debate over what’s not working with this law and an opportunity to change it. Instead, we get excuses for why a revised TMDL can’t be done and isn’t needed. Some of those excuses are legitimate: the chronically underfunded DNR has a lot of TMDLs to write and not enough staff to do it. Some of the excuses are flimsy: apparently, the document mishandled nitrogen units in a way that was too subtle for me to notice on the first read-through but serious enough to make the whole thing unworkable.

Another excuse—that the Cedar River is no longer impaired—seemed like a mistake at first but turned out to be technically correct on closer inspection. “No longer impaired” means that fewer than 10% of the samples collected during the last two assessment periods (2016-2018 and 2018-2020) exceeded the drinking water standard. I’ve double-checked this with another source of data and think this assessment holds up, even if we account for weather. It’s just premature. Nitrate was back up in 2022.

Well, you know what they say. You can lead a state agency to water, but they can’t make it drinkable.

(Apologies to my respected colleagues at DNR. I can’t resist a good pun!)

Peeling the Onion

by Dan Haug | Aug 3, 2022

We know that weather influences water quality in Iowa’s rivers. Last year, there was a drought and nitrate was lower than usual. This spring, it’s been wetter and nitrate is higher than usual. If you monitor for 10 years and the first 5 are a little wetter or drier than the last five, you’ll a water quality trend to go with it. Boring!

What we really want to know is how people are influencing water quality. We can get a lot closer to that answer by peeling away the obvious weather-related patterns to reveal underlying trends.

In statistics, it’s called a covariate or an explanatory variable. If there’s a relationship between your water quality metric and some other thing you’re not really interested in (i.e. streamflow), you can model that relationship to account for part of a water quality trend over time. What’s left over might be the things you’re really interested in (i.e. how water quality has been affected by changes in crop rotations, conservation practices, sewage treatment, manure management, and drainage). It’s common enough in the scientific literature (Robert Hirsch’s Weighted Regression on Time, Discharge, and Season is a good example), but should be used more often for progress tracking at the watershed scale.

To illustrate this general approach, I downloaded daily nitrate data from three stations maintained by the US Geologic Survey. The sensors at the Turkey River at Garber and the Cedar River near Palo (north of Cedar Rapids) were installed in late 2012; the sensor Raccoon River near Jefferson was installed in 2008. I wanted a high frequency dataset (to minimize sampling error) that included the episodes of “weather whiplash” in 2013 and 2022.

“Residuals” are the difference between what we predict and what we measured. In the first panel, that’s the difference between a measurement and the long-term average. In the second and third panels, we see how nitrate measurements differ from what we’d expect given flow in the stream today, and flow in the stream last year. Gray dots – daily measurements. Red dots- yearly averages. Blue dotted line – trend. If I did this right, some of the dots should get closer to the middle.

Nitrate concentrations in rivers increase as the weather gets wetter and streamflow increases… up to a point. When rivers are running very high, there’s a dilution effect and nitrate concentrations fall. Based on that relationship, we can explain high nitrate levels in the Cedar River in 2016 (a wet year) and low nitrate levels in 2021 (a dry year).

Nitrate concentrations tends to be highest on wet spring days following a dry summer and fall, as nitrate that accumulated in the soil during the drought is flushed into drainage systems or washed off the land surface and into rivers. Here I’ve calculated a moving average of flow over the previous 365 days, and compared that to nitrate concentrations during high flow or low flow conditions. Based on that relationship, we can explain high nitrate in the Cedar River on wet days in the spring of 2013 and 2022 (following a dry year) and low nitrate on wet days in the spring of 2019 (following a wet year).

relationship between nitrate and last year's flow

After making these adjustments, the downward trend in the Cedar River looks much smaller (0.53 mg/L per year, adjusted to 0.25) and is overtaken by the Turkey River (0.37 mg/L, adjusted to 0.28). The adjusted trends are statistically significant and could be attributed to conservation efforts in those watersheds.

How did I do this? For technical details, read here.

However, there’s still some weather-related patterns we haven’t accounted for. The Raccoon River near Jefferson also had a steep decline in nitrate since 2013 (1.42 mg/L per year, adjusted to 0.77 mg/L per year) but if you look at the entire record (going back to 2008), it’s part of an up-and-down cycle. I’ve seen that same pattern in the South Skunk River. The model explains some of those swings but doesn’t fully explain high nitrate in fall of 2014, spring of 2015 and spring of 2016. Perhaps the nitrogen that accumulated in the soil during the drought of 2012 took several years to flush out.

In addition to streamflow and last year’s weather (antecedent moisture is the technical term), nitrate can be explained by season, soybean acreage, and baseflow. If it’s not enough to know that water quality is improving or getting worse, and you’d also like to know why, then let’s peel that onion!

The Best Nitrogen Analogy Ever

by Dan Haug | Jun 26, 2022

Imagine the nitrogen cycle is a trust fund kid with a gambling problem.

The young man (a corn field) is very rich (has rich black soil) but the money (nitrogen) he inherited from his father (the prairie) is locked in a trust fund (soil organic matter). Only a small portion of the funds are released to him each year (mineralized) following a complicated schedule determined by the trustees (microbes in the soil). In order to maintain the lifestyle to which he has become accustomed (provide enough nitrogen to the crop for good yields), he needs supplemental income (nitrogen from commercial fertilizer or manure). His sister (a soybean field) does not need to work (apply fertilizer) because she can borrow money from her well-connected husband (symbiotic nitrogen-fixing bacteria) but she also receives payments from the trust (mineralization). She helps her brother out (corn needs less nitrogen fertilizer following soybeans) but not directly (soybeans actually use more nitrogen than they fix, so the benefits of the rotation has more to do with the behavior of the residue and disrupting corn pests).

A Richie Rich cartoon, but with nitrogen

Both siblings have a gambling (water quality) problem and are terrible poker players. Whenever they’re feeling flush with cash (when other forms of nitrogen have been converted to nitrate) they blow some of it playing cards (nitrate easily leaches out of the root zone when it rains), but the extent of the losses vary and debts aren’t always collected right away (nitrate leached out of the root zone may not immediately reach streams). They struggle with temptation more than their cousins (alfalfa and small grains) because they come from a broken home (the soil is fallow for large parts of the year) and because bills and income don’t arrive at the same time (there is a mismatch between the timing of maximum nitrogen and water availability and crop nitrogen and water use).

“”Okay, Dan, that’s very clever, but what’s your point?

Well, having compared the soil to a trust fund, I can now say “don’t confuse net worth with income.” You’ve probably heard that there 10,000 pounds per acre of nitrogen stored in a rich Iowa soil. That’s true but misleading. The amount actually released each year by decomposing organic matter (net mineralization) is only a few percent of that, comparable in size and importance to fertilizer or manure. Here’s an example nitrogen budget.

Example nitrogen budget, for Tipton Creek in Hamilton & Hardin Counties

On average and over the long-term, we know that fields and watersheds with higher nitrogen applications (taking into account both manure and commercial fertilizer) leach more nitrate into the water. On average and over the long-term, we know that that farmers can profit by reducing their application rate to the Maximum Return To Nitrogen (the point at which another pound of nitrogen does not produce a big enough yield bump to offset the fertilizer costs). Right now, with corn prices high but fertilizer prices going nuts, the MRTN is 136 pounds per acre for corn following soybeans, while in the most recent survey I could find, farmers reported applying an average of 172 pounds per acre. So there’s room to save money while improving water quality!

But having compared nitrate leaching to gambling, I can also say “don’t confuse a balance sheet problem with a cash flow problem.” In any given year, it’s always a gamble how much of the nitrogen that’s applied will be washed away and how much will be available to the crop. Maybe some farmers are passing up on an opportunity to increase their profits because they’re not comfortable with the short-term risks.

Farmers say that extra nitrogen is cheap insurance. If that’s true, maybe we need crop insurance that makes it easier to do the right thing, not a more precise calculator.

High nitrate this spring: where and why

by Dan Haug | Jun 22, 2022

The Des Moines Waterworks was forced to use their nitrate removal system for the first time in five years. Our spring snapshot found high nitrate concentrations in streams across Story County. On my way to speak at the CCE Environmental Expo in Mitchell County, I dipped a test strip in the Cedar River near Osage and measured 16 mg/L. Looking at the Iowa Water Quality Information System there’s orange (nitrate greater than 10 mg/L) across much of the state and spots of dark red (nitrate greater than 20 mg/L) in Story, Hamilton, and Hardin counties. What’s going on?

Well, differences in land use, soils, topography, and farming practices make for strong regional differences in water quality. For some streams like the North Raccoon River, this is a return to normal. For some streams, like the Cedar River, current conditions are unusual. To illustrate this, I’ve invented my own graph, which compares highest nitrate concentrations observed this spring (the blue dot) to the entire 10-20 year record (a black band showing the range, and a black square showing the median). The data comes from Iowa DNR’s Ambient Stream Monitoring Network; I will update these graphs once June data is available. A sampling of sites is shown at right, but the entire graph can be downloaded as a PDF here.

Northwest Iowa is still suffering from drought, and that means the Floyd River near Sioux City (which usually has some of the highest nitrate concentrations in the state) is barely flowing and has very low nitrate concentrations. As we saw last year, nutrient concentrations tend to be low during dry conditions except where there is a strong influence from point sources of pollution. Most of the rest of the state is back to normal, and nitrate that accumulated in the soil during two dry years is now getting flushed out. These maps are taken from the National Drought Mitigation Center at the University of Nebraska-Lincoln. I’ve drawn in the approximate location of the watersheds for the monitoring sites in my example.

“Weather whiplash in agricultural regions drives deterioration of water quality.” That’s the title and conclusion of a paper that studied previous episodes when a wet spring followed a dry summer and fall. The 2012 drought was much more severe than 2021, impacting yields so that less nitrogen was taken up by the crop and removed in the grain, and maybe that’s why nitrate in 2013 and 2014 was so much higher than it is now. I’ve compared spring highs for several sites and years, normalizing by the long-term average. It’s not clear to me whether weather whiplash increases the overall mass (load) of nitrogen that gets washed away, or just alters the timing (moving in one year what would have been parceled out over two), but high concentrations are a concern for communities like Des Moines and Cedar Rapids that get their drinking water from a river or river-influenced wells.

map showing shift out of drought in 2013

graph showing when nitrate was higher than usual for select sites

I’m procrastinating on the work I’m supposed to be doing because “Hey look! Data!” and I have to satisfy my curiosity. If you’d like to see us do more water quality analysis beyond Story County, let us know, and support us with a charitable donation so it can become work I’m supposed to be doing!

It’s Rude to Point, but…

by Dan Haug | May 27, 2022

By my calculations, over 65% percent of the nitrogen load in Ioway Creek on May 20 came from less than 1 percent of the land area in the watershed. We still don’t know why.

Revised May 31

Many people assume that fertilizer applied to turf grass is a major source of nitrogen and phosphorus pollution in Iowa. At a presentation to the Ames City Council, I was asked if a public awareness campaign aimed at lawn care professionals and homeowners would be an effective way to improve water quality in Ioway Creek. If we’re talking nitrogen, I don’t think so:

Because turfgrass covers a tiny proportion of the land in most Iowa watersheds, compared to cropland.
Because turfgrass is a perennial. Having something growing and taking up available nutrients year-round is the principle on which cover crops reduce nitrogen loss.
Because there was a study by Dr. Keith Schilling that found very low nutrient levels in shallow groundwater below six Iowa golf courses.

To that list, I can add local water quality monitoring including lab testing and sensor results from May. Nitrate in Ioway Creek and the South Skunk River were the highest we’ve seen for a few years, but while rural tributaries ranged from 12-20 mg/L of nitrate, College Creek (an urban watershed with plenty of turf grass) measured only 2.3 mg/L.

But even if turf grass in general isn’t a serious water quality problem, maybe some specific areas of turf grass are a problem. That’s what I thought after reviewing the data from our spring water quality snapshot on May 17. Volunteers found a big difference in nitrate levels between South Duff Ave and other sites in Ames. I wondered if it could be a mistake, so I went back out on May 20 with a bottle of test strips and a smartphone app that enables more precise measurements. It wasn’t a mistake (nitrate in Ioway Creek increased from 8.6 mg/L to 24 mg/L in two miles), but the results still weren’t making sense, so I kept testing and testing until I assembled the map below. By my calculations, 65% of the nitrate load in Ioway Creek that day was coming from just 1,500 acres!

The 1,500 acres includes Coldwater Golf Links, and the pattern looks like what I’d expect to see if the golf course was overapplying fertilizer. However, the golf course superintendent has informed me that fertilizer has not been applied since fall, and then only at a low rate. A volunteer tested two ponds on the course and found low levels of nitrate (1-2 mg/L).

The 1,500 acres include some developed areas north of creek drained by storm sewers, but I tested water trickling from two outfalls on May 20 and found very low nitrate levels: 0.5 mg/L and 3.1 mg/L.

The 1,500 acres acres also includes two construction sites: a flood mitigation project near South Duff Ave and an ISU recreation complex east of Jack Trice Stadium. The photo shows severe bank erosion where drainage from the ISU construction site enters the creek. An inspector with the Iowa DNR noted problems with erosion control earlier this spring on the South Duff project. However, if the nitrate spike were linked to erosion, I’d expect to see high phosphorus and low transparency.

Honestly, I’m not sure what’s going on here. It’s not a pattern we’ve seen in previous years.

erosion on Ioway Creek between Grand and Duff avenues

When interpreting this kind of data, there is a risk of jumping to conclusions and unfairly pointing fingers. In my first draft of this article, I suggested that Coldwater Golf Course was the source of the nitrate and the bank erosion. That was premature.

However, there is also a risk that we will waste time and money on the wrong solutions or the wrong areas if we don’t test water or don’t follow where the data is pointing. It’s clear from this month’s data and many other rounds of testing that water quality impacts are not uniformly distributed across the landscape.