October 10, 2017
It is well-known that agricultural sources of nitrogen are the primary cause of nitrate-contaminated well water, aquifers, and surface waters in the Mississippi River Basin watershed. Further, agricultural sources of nitrogen are the primary cause for the hypoxic zone at the mouth of the Mississippi River, known as the “Dead Zone,” that occupies an average of 5,300 square miles each year. This year’s Dead Zone measures 8,776 square miles, the largest since size assessments began in 1985.
In Iowa, about two-thirds of land is used for annual row-crop agriculture. Some of the most intensively cropped lands occur in north-central Iowa where annual crops of corn and soybean dominate agricultural production. In this area, known as the Des Moines Lobe, the landscape is approximately level and natural drainage is poorly developed. To facilitate and enhance crop production in this naturally swampy area, subsurface drainage systems were installed to remove excess water. The water that drains from these systems, and the quasi-government drainage districts that manage and maintain them, presents a major challenge to the Mississippi River Basin with respect to nitrate contamination. 
This report looks at the history of drainage districts in Iowa, most notably in North Central Iowa, and the impact of drainage districts on water quality in Iowa and the Mississippi River Basin. This report deals specifically with drainage occurring within and from drainage districts, and not with drainage as it occurs naturally. This report also deals specifically with nitrate, and does not address phosphorus. It concludes with a set of policy recommendations.
A Historical Overview of Drainage Districts in Iowa
The Upper Mississippi River Basin, which includes Iowa, is one of the most productive farming regions in the world with fertile soil and abundant rainfall. The United States Department of Agriculture 2012 Census of Agriculture illustrates just how much agriculture dominates Iowa’s land. The state of Iowa covers an area of approximately 36 million acres. Of that area, nearly 31 million acres is farmland, with corn and soybean cropland accounting for about 25 million farmland acres. Additional acreage is consumed by pasture or grazing lands, woodlands, and permanent rangeland. Agriculture dominates the Iowa landscape and urban land use takes up only 1.9 million acres, approximately 5 percent of the state’s total land area. 
Iowa was not, however, always so flush with agricultural bounty. At the time of statehood in 1846, Iowa was a land rich in wetlands. Most of these wetlands were located in the Des Moines Lobe (Figure 1). These wetlands impeded settlement by making travel physically difficult and, because of the flies and mosquitoes, extremely unpleasant and unhealthy. Wetlands were feared by early settlers because they were associated with diseases, including malaria. 
Figure 1. Landforms of Iowa
A series of Swamp Acts passed by the United States Congress in 1849, 1850 and 1860 gave Iowa and 14 other states all the swamp and overflowed land lying within their boundaries for the purpose of reclaiming and draining them for public improvement. Drainage of wetlands began in the 1870s and 1880s and the number of reported deaths due to malaria was significantly reduced by the 1890s. In addition to reducing disease incidence, drained land resulted in more land that could be planted in annual row crops and in significantly higher crop yields, which more than paid for the cost of installing drainage mechanisms. 
Land owners who initially acquired some of this swampland tried to drain it using ditches, but this proved impractical in many cases because there was often no place to outlet these ditches that did not result in the flooding of neighbors’ land. In the interest of public health and to maximize the agricultural potential of the land it became obvious that the application of eminent domain would have to be expanded.
The Iowa Legislature first adopted statutes describing and defining a drainage district in about 1890. The Iowa Constitution was amended in 1908 specifically providing drainage districts with the authority necessary to carry out the purposes of drainage districts as provided by statute. Drainage districts have the right of eminent domain to acquire lands for the public purpose of establishing and maintaining drainage district facilities. 
According to the Iowa Drainage Law Manual, the following process is used to establish a drainage district.
Two or more landowners can petition to establish a drainage district by filing a petition with the county auditor’s office and the board of supervisors in the county where the district is [to be] located. The basic purpose is to provide facilities for draining the excess water in a watershed area. All lands within the watershed area that are benefited by the drainage facilities are included in the drainage district and are assessed for drainage taxes accordingly.
When a drainage district is first established, the [county] board of supervisors serves as trustee for the district. After the district has been legally established, the landowners in the district may petition the county auditor to call for a special election to elect three trustees from the membership of the landowners in the district. If the trustees election is completed, the three trustees take over the administration of the drainage district and the supervisors are relieved of further responsibility. The trustees must, of course, follow the exact same statutes that the supervisors follow under Chapter 468 of the Code of Iowa.
Drainage districts thus have the right to require even unwilling landowners to allow access by the drainage district through eminent domain and to levy taxes, which provides the legal authority and funding mechanism for implementing large-scale drainage systems that are capable of draining entire catchments.
Today, an estimated 85 to 90 percent of native wetlands in Iowa have been drained and that land now is mostly planted with corn and soybeans. The Iowa Drainage District Association estimates that more than 9 million acres of row crop farmland are drained, which represents more than one-third of all row crop farmland in Iowa. Of the 3,700 drainage districts in Iowa, 61 percent are located in the Des Moines Lobe. The drainage districts within the Des Moines Lobe are the largest source of artificially drained water in the state (Figure 2).
Figure 2. Drainage Districts in Iowa Show Dominance on the Des Moines Lobe
How Drainage Systems and Drainage Districts Work
Draining of permanent and seasonal surface water from the landscape in lakes, ponds, wetlands or potholes requires dredging ditches and installing subsurface piping (known as mains or county tiles) to connect closed depressions to natural streams. Groundwater is drained from fields with the uniform placement of field tiles that lead to a main drain (see Figure 3 below). Field tiles can be made from clay, concrete, cement, aluminum, iron, steel or plastic. Plastic tile is the most commonly used material for subsurface drains in the United States.
Figure 3. Subsurface Drainage System Used for Tiling Cropland
Source: Ontario Ministry of Agriculture, Food, and Rural Affairs
Field tiles remove groundwater, lowering the local water table and quickly removing water from saturated soil. This process allows more water to flow down through the soil, keeping soil from becoming waterlogged and improving plant growth. Water from field tiles rapidly flows to buried mains and open ditches maintained by drainage districts. Performing exactly as designed and intended, drainage systems substantially increase the amount of water discharged into natural streams. Repeated tillage of closed depressions increases infiltration rates by breaking up the clay and organic-rich soil layer that sealed the bottom of most depressions, contributing to the effectiveness of these systems at removing water.
One of the difficulties of draining agricultural lands is the impact drainage could have on neighboring properties. If an upland landowner installs drainage, flooding could occur on the property of the lowland landowner and inflict damage. Thus, the passage of the 13th amendment to Iowa’s Constitution qualified eminent domain rights such that drainage districts could be established. Drainage districts, therefore, provide a legally organized means for constructing and maintaining adequate drainage outlets and levees. According to Chapter 468, Section 2 of Iowa law,
“The drainage of surface waters from agricultural lands and all other lands or the protection of such lands from overflow shall be presumed to be a public benefit and conducive to the public health, convenience and welfare.”
At the time this law was passed, it seems elected officials in the state were in agreement that drainage was in the best interests of the citizenry. However, Iowa Supreme Court Chief Justice Mark Cady recently struck a different note in his dissent in the lawsuit filed by the Des Moines Water Works (DMWW) against drainage districts in three upstream counties that drain into the Raccoon River. Cady observed that while drainage districts have been granted special privileges by the state of Iowa, they do not have much in the way of obligations to downstream water users. The Chief Justice wrote:
It is abundantly clear that Iowa’s drainage district law did not originate and was not developed over time with the thought that a drainage district could be a polluter. If it had, I am convinced our law would have developed in a way that would have recognized a clear remedy.
Chief Justice Cady went on to speak of stewardship of Iowa’s lands and waters and footnotes Iowa conservationist, Aldo Leopold:
This state is blessed with fertile soil, vast expanses of teeming wilderness, and an overwhelming abundance of fresh water. The role and purpose of drainage districts in Iowa is important, but no more important than this state’s enduring role of good stewardship.
Once a drainage district is established by two or more landowners, the board of supervisors of the county in which the district is located becomes the managing board for that district. If a district chooses to do so, it can “opt out” of board of supervisors management and elect its own trustees. Most districts are managed by boards of supervisors. For drainage districts that include lands within two or more counties, district management decisions are made by both county boards of supervisors acting jointly. For ease of administration, a “lead” county is usually designated. Lands within the confines of an established drainage district can be assessed for the construction, maintenance, and repair of drainage district facilities. Assessments are based on the relative benefits received and may be spread over several years.
How Nutrients Move Through Drainage Systems
Corn and soybean cropland account for about 25 million farmland acres in Iowa. More than one-third of all cropland in Iowa is tiled for drainage. Industrial crop production requires extensive nitrogen additions to the soil. Sources of additional nitrogen available to crops include (in order of mass) inorganic fertilizer followed by atmospheric fixation, crop residue, soil organic matter, and direct atmospheric deposition, and manure.
For plants to use organic nitrogen, it must be converted to an inorganic or mineral form through a biological process called mineralization, which ultimately produces nitrate (NO3).
Once organic nitrogen is transformed into nitrate, a dissolved ion that moves only with water, plants will uptake nitrate through their root systems. Different crops use different amounts of nitrogen to grow. Nitrogen Use Efficiency (NUE) is a term used to indicate the ratio between the amount of inorganic nitrogen (i.e., synthetic fertilizers) a crop removes from a field as grain and the amount of nitrogen applied to a field. For example, the NUE of corn is calculated to range between 68 and 91 percent. This means up to one-third of the inorganic nitrogen applied to cropland may not be harvested as crop.
Nitrogen cycling in soils, crops and water involves many more sources than just inorganic fertilizer and more losses than just harvested crops (Figure 4 below). So what happens to the nitrogen that never makes it into crops? Some goes into the atmosphere, some returns to the soil as organic matter, and some leaches to groundwater as nitrate. Nitrate is a dissolved ion that moves only with water. Subsequently, nitrate that is not used by plants or sequestered in soil organic matter moves down in solution through the unsaturated soil zone to the water table. Once water infiltrates the soil it can dissolve nitrate or combine with nitrate-rich water, leaching the ion to groundwater. It is widely understood that nitrate is transported by the flow of groundwater that ultimately discharges to streams. Nitrate is rarely transported into the watershed by surface water runoff.
Figure 4. How Nitrogen Moves Through Air, Water, and Soil and is Used by Plants, Animals, and Bacteria
Source: Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Nitrogen_Cycle.jpg
The growing season for corn and soybean crops is relatively short. For most of the year, tiled cropland lacks any vegetation to consume nitrate from the soil. Thus, nitrate leaches into ground-water that is quickly drained by tiles. This nitrate-contaminated groundwater is discharged at points to streams, creeks, ponds, lakes, rivers (Figure 5).
Figure 5. Groundwater is Discharged at Points to Iowa Waters
Photos: Michael Burkart
Without tile drains, nitrate-contaminated water would be at least partially denitrified in aquifers. The systematic artificial drainage of the landscape, which has increased steadily since drainage districts were established more than a century ago, effectively short-circuits the natural flow system of water and nutrients through the soil. Watershed-scale measurements also showed that 45 percent of the average annual precipitation was drained from the soil profile through the subsurface drainage lines.
Under natural conditions — without artificial drainage — water seeping into groundwater below the lakes, wetlands, ponds, streams and prairie potholes of the Des Moines Lobe likely contained very little nitrate (less than 3 ppm) or dissolved oxygen as existing background concentrations show. Both surface and subsurface drainage are very poor on the Des Moines Lobe. This is due to the topography and the soil composition. This limited permeability produced frequent and long-term saturation of soils resulting in accumulation of large quantities of organic matter. The natural residence time of groundwater flowing through till to streams on this Iowa landform is measured in decades, centuries, or more based on data generated by Iowa State University researchers.
These natural conditions no longer exist in a majority of the Des Moines Lobe. Infrastructure developed by drainage districts substantially increased the amount of water discharged into natural streams when large areas of surface storage were eliminated from the landscape. Subsurface collector tiles that continuously drain the shallow saturated soil zone allow little opportunity for natural attenuation of nitrate in soil or along the groundwater flow path. Rapid drainage of groundwater to and through collector tiles eliminates the opportunity for the slow natural process of nitrate reduction and depletion. This results in seasonally large concentrations of nitrate that are collected by drainage district ditches and subsurface tiles and funneled into groundwater. Consequently, the infrastructure of drainage districts facilitates the conversion of widely disseminated or non-point sources of nitrate beneath fields into specific point sources of contaminant discharge. This nitrate-laden groundwater pours into streams, lakes, creeks and rivers. Prior to the installation of drainage systems, essentially all of this water would have been stored on the landscape. This stored water would provide no mechanism to transport nitrate from the watershed or to contribute to potential downstream flooding. Any nitrate in the system would have been recycled through soil and plant ecosystems or denitrified in the water table.
The destruction of natural wetlands, increase in artificial drainage, and increase in nitrogen application to fields that now almost exclusively grow corn and soybeans all have combined to increase the amount of nitrate in our water. The extensive annual crop system covering Iowa has been engineered to facilitate movement of water and nitrate from the landscape. Concentrations of nitrate frequently exceed drinking water standards and more frequently exceed background levels in both aquifers and aquitards throughout the state. 
It is important to note that while most drainage was completed in Iowa during the early 1900s, the most dramatic increases in nitrate loads to surface waters were not associated with the initial installation of drainage systems. Rather, those increases came with the major shifts in cropping systems and dramatic increases in fertilizer applications that occurred during the 1960s and 1970s. On an annual basis, agricultural sources dominate nutrient loads to surface waters in Iowa. Tile lines provide a direct route for nitrate to enter waterways via a number of specific points, and “thus tiling has been implicated as a prime contributor to nitrate pollution in the Gulf [of Mexico]”. Thus tiling in combination with industrialized farming practices has reduced capacity of the landscape to retain nitrogen inputs when compared with natural, unfarmed landscapes or with farming practices that do not rely heavily on commercial nitrogen inputs.
It is clear from years of research and observations that nitrate contamination of our waterways is a direct response to the cultivation of annual row crops and it is exacerbated by artificial drainage. The infrastructure of drainage districts, in particular, creates conditions for mass collection and rapid transport of nitrate pollution into our waterways. Drainage districts short-circuit denitrifying conditions and pipe an average of 54 pounds/acre of nitrogen into our waterways compared to 20 pounds/acre in other parts of Iowa where annual row crops similarly dominate the landscape. The nitrate concentration in drainage district water frequently exceeds the U.S. Environmental Protection Agency’s Maximum Contaminant Level (MCL) of 10 mg/L for drinking water. Some of the highest concentrations of nitrate in groundwater were found underneath drainage tiles estimated to be older than 1963, based on tritium analyses. Subsequently, to avoid serious pollution, we must find methods to decrease nitrate contamination in streams and rivers at the scale of drainage districts.
Policy Recommendation: Use Drainage Districts as a Mechanism for Change
Iowa’s Nutrient Reduction Strategy (NRS), created in 2013, seeks to reduce nitrate pollution leaving the state by nearly half. The results of a five-year study of three different tile-drained catchments in Iowa underscored the importance of working at the drainage district scale to achieve nitrate reductions necessary to meet water quality goals. Two mechanisms permit drainage district managers to act positively to improve water quality:
(1) Drainage districts should exercise their existing statutory authority; and
(2) State regulators and others should use the local expertise and outreach available from drainage districts to better target existing voluntary soil and water conservation practices to reduce nutrient loads in our waterways.
Statutory Authority of Drainage Districts
Historically drainage districts have not been held accountable for nutrient inputs into our waterways. Additionally, they have not been treated by the courts as liable for damages inflicted on property or the environment, including pollution of ground- and surface waters. Traditionally the courts have ruled that drainage districts need not use their statutory authority to implement pollution reduction measures or to mitigate damages to the environment. This was most recently demonstrated in the 2017 Iowa Supreme Court ruling dismissing the DMWW case against three county boards of supervisors for nitrate contamination in the Raccoon and Des Moines Rivers emanating from upstream drainage districts.
Under existing statutory authority, however, drainage districts have the power to level fees and they have the power of eminent domain. This is important flexibility that reinforces the purposes of these entities. Fees are necessary for the maintenance, repair, expansion, and improvement of drainage district infrastructure. Eminent domain is necessary for expansion of drainage district systems. Iowa’s aging drainage district systems are nearing 100 years old and are beginning to fail, and are estimated to be replaced by year 2050 at a cost of $6 billion. There is an opportunity here to obtain better management practices (BMPs) for existing, new and replacement drainage components to reduce nitrate in our waterways.
We suggest that drainage districts use their statutory authority to mitigate nitrate pollution discharge from drainage district infrastructure. Such mitigation actions can include, but are not limited to, requiring water quality monitoring and reporting, wetlands conservation and restoration, and bioreactor installation at discharge points to reduce nitrate loads into our waterways. Existing partnership programs, such as the Iowa Conservation Reserve Enhancement Program (CREP) and quasi-public commodity check-off organizations (Iowa Corn Growers Association or Iowa Soybean Association), also could assist in the funding and coordination of drainage infrastructure improvements for the purpose of reducing nitrate loads in drainage effluent emanating from drainage districts.
We also assert that there a broader obligation for drainage districts to address water quality issues under the existing statutory mandate that drainage “shall be presumed to be a public benefit and conducive to the public health, convenience and welfare.” Public health and welfare can and should be interpreted to mean keeping our waterways free of nitrate pollution. Nitrate pollution is a public health issue most notably illustrated in the DMWW lawsuit that sought to reduce the contamination of source water to more easily meet the U.S. EPA MCL standard of 10 mg/L of nitrate. Thus, drainage districts can and should utilize their existing statutory powers to mitigate nitrate pollution flowing from their systems for the benefit and well-being of the public.
Because of the quasi-government organization of drainage districts, the possibility already exists to leverage the eminent domain and tax levying rights of drainage districts for the mitigation of water quality impacts within districts and to progress toward one of Iowa’s NRS goals. Drainage districts have the ability to raise funds to develop mitigation actions at the source or at discharge points that could include their eminent domain rights to acquire and manage wetlands and other conservation areas below their intermediate or final discharge points. If members of any drainage district object to their association using its powers for the good of the environment, any of the associations could ask for additional authority from the Iowa Legislature. While they may have the powers now, explicitly stating that drainage districts have an obligation to reduce nutrient pollution would not be inappropriate.
An incentive for drainage districts to act is that they discharge water and contaminants at point sources making them potentially vulnerable to future re-interpretations of the Clean Water Act. Tile line discharge points are quantifiable and easily observed sources of water outflow. Drainage district-scale policies to reduce nitrate contamination entering public waters should be based on principles that either (1) increase the presence of plants during more months of the year (thereby reducing contamination at the source); or (2) remove contamination off site. Generally, addressing contamination at the source is both socially and economically more effective than cleaning up pollution offsite, once it enters public waters. Further, implementing mitigation at the drainage district level allows for local buy-in and tailoring mitigation methods specific to the needs of a drainage district.
Existing Voluntary Soil and Water Conservation Practices
The second mechanism through which drainage districts have the ability to improve water quality is the continued reliance on voluntary conservation practices to reduce nutrient loads entering our waterways; however, these measures have yet to be proven effective for achieving Iowa’s NRS nutrient reduction goals. Organizing and coordinating conservation measures at the scale of a drainage district — or among several adjoining districts — potentially can improve the outcomes of voluntary efforts using the existing quasi-public mechanism of drainage districts. Additionally, as drainage infrastructure ages and requires maintenance and replacement, the Iowa Department of Natural Resources permitting process should include requirements to improve the environmental quality of discharge water. Project engineers should confer with IDNR on cost-effective mitigation as part of system renovations.
Addressing water quality issues at the scale of drainage districts has the capacity to substantially reduce Iowa’s contribution to nitrate pollution of the Mississippi River Basin. And if we follow the logic of Chief Justice Cady, drainage districts may have an existing responsibility to address water quality for the public good.
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 Chapter 468, Section 2 of Iowa law.
Sarah Garvin joined the Iowa Policy Project as a research associate in June 2017 to work in the areas of energy and environmental policy. Sarah holds an M.A. degree in Marine Affairs and Policy from the Rosenstiel School of Marine and Atmospheric Science at the University of Miami, and a bachelor of science degree in biology from Le Moyne College. She served as an outreach intern and then a natural resource specialist for five years at the National Oceanic and Atmospheric Administration (NOAA). She later operated a small business in Muscatine with her husband.
Michael Burkart had a 34-year research career with the U.S. Department of Agriculture and U.S. Geological Survey before joining the Iowa State University Department of Geological and Atmospheric Sciences as Affiliate Associate Professor (retired). His research interests include quantitative techniques to assess and estimate water quality responses to agricultural activities at a regional scale. Most recently he has studied how to reduce agricultural nitrogen loads to streams and develop methods to allocate source-loads of nutrients that may impair water resources. He has also worked to develop nutrient criteria for lakes and streams in Iowa.
David Osterberg is professor emeritus in the Department of Occupational and Environmental Health at the University of Iowa and a former state representative known for his passion for energy and environmental issues. David holds an M.S. in water resources management and another in agricultural economics from the University of Wisconsin-Madison. As co-founder of the Iowa Policy Project, he served as director for the first 12 years of the organization and remains as IPP’s lead staff researcher on issues affecting policy on energy and the environment.
We gratefully acknowledge the support of the McKnight Foundation and the Fred and Charlotte Hubbell Foundation. Views expressed are solely the perspective of the authors and the Iowa Policy Project.