About Mississippi tributary nitrate concentrations
Nutrients (fertilizers) applied to croplands in
the Mississippi River drainage basin often make their way into rivers
and streams, and ultimately are discharged into the Gulk of Mexico.
Fertilizers in the Gulf of Mexico increase the production of marine
algae, thereby decreasing the oxygen content of coastal waterways.
Massive algal blooms following wet season runoffs can cause severe
hypoxia, threatening a wide variety of marine organisms, including
commercial fisheries.
Compounds of nitrogen (such as nitrates) are one
of the most important fertilizers used on croplands in the central
United States. As a result, about 1 million metric tons of nitrate
are carried into the Gulf of Mexico every year by streams draining
the Mississippi River basin. This value of nitrate flux is about 3
times higher than the flux 30 years ago, corresponding to increasing
use of fertilizers to boost or maintain crop yields.
In order to quantitatively evaluate the effect
of agricultural practices on water quality, Goolsby et al (1999) made
a comprehensive analysis of 42 sub-basins within the Mississippi-Atchafalaya
drainage system. Individual interior basins range in size from 7000
km2 to 237,000 km2. Each basin was measured
for the percentage of land in row crops (corn, soybeans and/or sorghum),
which ranged from less than 0.1% to 74% (see data table). The concentration
of nitrate in river water discharging from the "outlet"
of each of these basins was also measured repeatedly (from 40 to 300
times), and the mean nitrate concentration was calculated (see data
table). Concentrations are reported in milligrams of nitrate per liter
of water (mg/L), which is essentially equivalent to parts per million
(ppm). Concentrations range from near zero to almost 7 mg/L.
The graph shows a scatterplot of % cropland versus
nitrate concentration. There is a clear overall positive correlation
between these two values, suggesting that agricultural practices are,
indeed, influencing the runoff of nitrates. One could try fitting
both a linear and exponential model to these data and comparing the
two. Must the best fit regression pass through the origin? What is
the maximum possible value for nitrate concentration, as predicted
by forward extrapolation of a regression model?
Interior basin # 24, the Kaskaskia River basin
in Illinois, has an anomalously low nitrate concentration. One explanation
is that algal blooms in the reservoirs behind two large dams on the
Kaskaskia strip out nitrates, which are incorporated into bottom sediments
when the algae die. Should more dams be built to trap nitrates? Interior
basin #35, the St. Francis basin in Arkansas, is mostly planted in
soybeans, a nitrogen fixer, which requires very little nitrogen from
fertilizers.
Source: Goolsby D. A. and 7 others (1999), Flux and Sources
of Nutrients in the Mississippi-Atchafalaya River Basin: Topic 3 Report
for the Integrated Assessment of Hypoxia in the Gulf of Mexico; NOAA
Coastal Ocean Program Decision Analysis Series # 17, 130 pp.
NOAA's Coastal Ocean Program can be found at: http://www.cop.noaa.gov
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