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Dead zone phenomenon revealed: Why are Mississippi River's nitrogen concentrations putting marine life at risk?
Pollution in the Mississippi River is growing, directly affecting the life that lives in the water and the people who rely on the river for food and recreation. One of the main factors causing pollution is excessive nitrate (NO−3) caused by chemical waste and agricultural runoff. The water body covers about 40% of the 48 states in the United States, and within this area, 7 of the 10 major agricultural product-producing states are located here.
The impact of nitrogen cycle
In the environment, nitrogen undergoes the nitrogen cycle, which converts it into different forms, including nitrogen gas (N2), nitrate (NO3−), and ammonia (NH3). This process involves different steps such as immobilization, deamination, nitrification and denitrification. The environmental impact of nitrogen does not depend solely on its form during the nitrogen cycle, but rather on the overall concentration of nitrogen in its various forms.
When certain forms of nitrogen are present in excess, negative environmental impacts can ensue. The U.S. Environmental Protection Agency sets a maximum nitrate concentration of 10 mg/L in drinking water and surface water. Once nitrate is excessive, it may create a so-called "dead zone." The low oxygen concentration in such areas can cause suffocation and death of aquatic organisms. Such dead zones are particularly evident at the mouth of the Mississippi River.
According to NOAA projections, the Gulf's dead zone will reach approximately 5,898 square miles in 2016, with nitrate flows reaching 146,000 metric tons.
Since the early 20th century, nitrate concentrations have increased significantly, by a factor of 2 to 5. Of this, more than 52% of nitrogen concentration comes from soybean and corn production.
Measurement method of nitrate concentration
In order to effectively monitor the changes and effects of nitrate concentration, accurate concentration measurement is required. The "weighted regression based on time, flow and season" (WRTDS) method is used to estimate concentration. The formula for this method is as follows:
ln(c) = β0 + β1*t + β2*ln(Q) + β3*sin(2πt) + β4*cos(2πt) + ε
Among them, c is the nitrate concentration, β0, β1, β2, β3< /code> and β4 are fitting coefficients, t is time, Q is average daily traffic, ε for unexpected variability from other sources.
This calibration curve is generated daily and compared to the previous day's curve, but this process is affected by diurnal variations in water flow.
Trend analysis
Land use patterns in the Mississippi River Basin have significantly reduced the amount of nitrogen available in the soil, with most of it found in the form of nitrates. These nitrates seep into groundwater through agricultural practices such as the use of drainage systems, and eventually flow into surface water. Nitrate concentrations in some areas are approaching legal drinking water limits, and the EPA has declared fish in the river unfit for consumption.
The study looked at data from 1980 to 2010 and showed that nitrate concentrations in the Mississippi River observed in the upper reaches of the old river outlet channel remained stable but increased by 12% between 2000 and 2010.
The source of this nitrogen remains unknown, and the reasons why nitrate concentrations rise in some areas and fall in others are not yet understood. However, studies show that concentrations in autumn and winter are generally higher than in spring and summer. This overall upward trend has led scientists to believe that the dead zone at the mouth of the Mississippi River is expanding.
Continuous solutions
One possible way to reduce nitrate concentrations is to rebuild the associated ecological environment. Grass and crop buffers and forest buffers have all been shown to be effective in reducing excess nitrate accumulation. Nitrogen taken away by plants can reduce nitrate concentrations in the water, but rapidly disappearing wetlands are a natural barrier.
In order to reduce nitrate concentrations by at least 40%, 22,000 square kilometers of wetlands need to be reconstructed. This task is extremely difficult to achieve because future needs are 65 times the total amount of wetland restoration in the past 10 years.
Changing agricultural practices could also be a solution to the nitrate problem. For example, strip tillage and winter cover crops can effectively reduce nitrate leakage.
Facing worsening water quality problems, how can humans rebalance our agricultural practices and environmental protection?
