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ECO LO GIC A L HEALT H
In 2015, the Gulf of Mexico dead zone covered approximately 6,475 square miles.
Nitrate
Overall nitrate pollution to the river has increased substantially. Excess nitrate threatens human health and aquatic life, and is a primary contributor to the Gulf of Mexico “dead zone.”
Nitrate concentrations in the river increased by 44% from 1976-2014.
Description and impacts. Nitrate is an important form of nitrogen for plant life, but too much in waters can be harmful to fish and other aquatic life.1 Human activities can greatly increase nitrate levels, which are typically low in undisturbed landscapes.2 Excess nitrate can quickly enter surface waters and groundwater, where it presents three primary challenges: • Human health. Nitrate levels above drinking water standards (10 parts per million) can pose human health risks, including the potentially fatal “blue baby syndrome” in infants.3 • Aquatic life. Excess nitrate in surface waters interferes with the healthy growth and development of aquatic life.4 • Hypoxia and the Gulf dead zone. Further downstream, surplus nitrate contributes to the “dead zone” in the Gulf of Mexico, where excess nitrogen feeds massive algae blooms each year. When the algae die and sink, their decomposition robs bottom water of oxygen (a condition called “hypoxia”), suffocating marine life that is unable to escape.5 Sources. In Minnesota, cropland is the dominant source of nitrogen, contributing 72% of the load to the Mississippi River:6
The river meets drinking water standards for nitrate, but Minnesota lacks standards to protect aquatic life in the river.
• 48% of nitrogen comes from farm drainage systems. • 21% is from cropland runoff leaching to groundwater and moving underground until it reaches streams. • 3% is from cropland surface runoff.7, 8 Other sources include atmospheric deposition, wastewater, forests and urban runoff (Figure 1).9 Total nitrogen loads to the river are influenced by flow; large volumes of water can move more nitrogen through the river system.
Figure 1. Sources of nitrogen to the Mississippi River in Minnesota in average conditions
cropland drainage 48%
cropland groundwater 21% cropland runoff 3% forest 5%
urban runoff 1%
point sources 12%
atmospheric deposition 8% septic 2%
Source: Minnesota Pollution Control Agency, Nitrogen in Minnesota Surface Waters