What is a dead zone?

As the name implies, “Dead Zone” is an area with no living beings. When a dead zone occurs within a waterbody, that zone has no aquatic life. Dead zones can be found around the world. They are a problem for coastal waters, bays, and lakes. The Clean Water Act was passed in 1972, and is now being celebrated for its 50th anniversary. The Act aims to reduce pollution which causes Dead Zones and other environmental hazards.

These sensitive environments receive nutrients, sediments, and debris that flow downstream from upstream sources. In the United States, a dead zone occurs every summer in the Gulf of Mexico. Excess nutrients from farm fields and industry along the Mississippi River (Minnesota to Louisiana) flow into the Mississippi River Basin.

Over the past five years, the average size of the Mississippi River Basin dead zone is 5,380 square miles and was measured as 6,334 square miles in 2021.

Dead Zones are typically caused by excess nutrients in the water, such as nitrogen and phosphorus. These nutrients cause an overgrowth of algae in a short period, commonly called “algae bloom.” As these microscopic “plants” grow, they consume oxygen in the water. And their multiplying population blocks sunlight for underwater plants. In addition, when the algae die, they sink to the bottom. As they decompose, more oxygen is consumed.

grass waterway in the middle of farm field
A grass waterway in Illinois. This practice grows a perennial grass along a slope. Water naturally moves into the ditch, where the flow is slowed. The grass helps catch sediments and nutrients. Notice how clean the runoff water is. Credit: Janith Chandrasoma

This little- to no-oxygen environment makes it impossible for aquatic life to survive. Larger organisms who require oxygen begin to die in large numbers creating a dead zone.

Also, these algal blooms contaminate drinking water and cause illnesses to animals and humans by releasing toxins.

Solving the problem of dead zones

Scientists who have been studying the Mississippi River Basin dead zone have worked together to create an action plan. The goal is to reduce excess nitrogen and phosphorus carried into the Mississippi River Basin. Federal and state agencies, universities, and other non-profit organizations work together to identify how each state can reduce pollution from wastewater treatment plants, factories, agriculture practices, and stormwater.

Edge-of Field Practices

Nitrogen and phosphorus are essential for agriculture. Without those, there will not be enough food to support the growing population. Famers provide nitrogen and phosphorus to their crops through fertilizer and animal manure. Because nitrogen is highly water-soluble and phosphorus attaches to the soil, these elements are highly vulnerable to moving away from the source with rainfall and runoff.

“Edge-of-field” practices are located at the edge of the farm field. They work to reduce the amount of nitrogen and phosphorus going to local water bodies such as streams, rivers, ditches, and lakes.

saturated buffer with underground tile drains leading to drainage outlets
A saturated buffer that has underground pipes (not shown), called tile drains, which lead to drainage outlets. Shown, outlets in Central Illinois. Drainage tile water slowly moves through the soil to the ditch or streams. During this movement, soil microbes convert nitrate in tile water to harmless nitrogen gas. Credit: Janith Chandrasoma

Vegetated Buffers

A vegetated buffer is a section of land between a farm field and a water body (stream, ditch, or a small lake) that has natural or established vegetation. Filter strips and riparian buffers are types of vegetated buffers. They slow stormwater runoff and reduce soil and stream bank erosion. Prairie strips are also a vegetated buffer that protects soil from runoff water, improves soil water recharge, and provides habitat for pollinators and wildlife.

Grassed Waterway

Grassed waterways are a section of farmland that grows grasses or other non-crop vegetation. They are graded (sloped) channels (man-made or natural) planted with perennial grass or appropriate vegetation. The growing grasses and vegetation slow down the runoff water and trap sediments. Grassed waterways are an excellent solution for gully erosion and stop phosphorus movement with sediment. Some consider grass waterways as an in-field practice.

Saturated buffers

Saturated buffers are a conversion of a riparian buffer. Saturated buffers can treat tile drainage water to remove nitrogen while removing other nutrients from runoff water. Midwest farmland has drainage systems to drain excess water, which allows for planting earlier during wet springs. These drainage outlets go through the buffers and discharge to nearby ditches or streams. Saturated buffers carry part of that tile drainage through the buffer by a perforated distribution pipe (underground, of course). The tile water slowly moves to the ditch or streams through the soil, and microbes in the soil convert nitrate in tile water to harmless nitrogen gas.

Denitrifying Woodchip Bioreactors

Woodchip bioreactors are also engineered to remove nitrogen from tile drainage water. Like saturated buffers, the tile drainage is diverted into a woodchip-filled trench that lets the tile water move through the woodchip media. The microbes naturally live on soil which also presents in woodchips (and tile drainage water) convert nitrate in tile water to nitrogen gas as in saturated buffers. Practices like woodchip bioreactors need to have a tile drainage outlet to be installed. (Click here see a video about denitrifying woodchip bioreactors.)

woodchip bioreactor and construction equipment with large pit of mulch surrounded by tarp. corn field and sky in background
Woodchips are used to fill the trench where drain pipes flow water from farm fields. The woodchips and soil microbes reduce the amount of nitrogen in the water, which then runs out into nearby waterways. Credit: Janith Chandrasoma

Drainage water management

Drainage water management reduces the annual tile flow volume through the outlet. Reducing tile flow volume (tile drainage) also reduces the nitrogen and phosphorus that move out from the field. A water control structure installed intercepting the outlet enables the control of flow volume leaving the field. Like woodchip bioreactors, drainage water management needs to have a tile drainage outlet to be installed.

Constructed Wetlands

Constructed wetlands are an artificially engineered environment that reduces nitrogen, phosphorus, other chemicals, and sediments from farm fields. They are designed in a way to intercept tile drainage and field runoff to prevent nutrients from moving downstream. They also provide habitat for wildlife, pollinators, and some aquatic species.

Two-stage ditches

Two-stage ditches are designed to carry drainage from a field and modified by adding a vegetated floodplain bench. The vegetated benches reduce the flow speed downstream and increase sediment deposition. The growing vegetation use some nutrients for their growth.

Answered by Janith Chandrasoma, University of Illinois Urbana-Champaign. To learn more about the lab and research happening where Janith works, visit here.

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