Agricultural Research Service (ARS) scientists are finding ways to stem the flow of nitrates that are washed out of crop fields into regional surface and ground water sources.
These nitrates primarily come from nitrogen fertilizers that
are not taken up by crops. After the nitrates are flushed out of the soil, they
flow into subsurface tile drains that channel excess water away from crop
But these underground drains can facilitate the eventual
passage of nitrate-laden runoff into the Gulf of Mexico, the Chesapeake Bay and
other water bodies. When the runoff enters these areas, it can intensify the
development of oxygen-deficient "dead zones," a condition called
ARS research leader Patrick Hunt, agricultural engineer
Kenneth Stone, and soil scientist Matias Vanotti developed a process for
denitrifying nitrate-laden runoff in subsurface drains before the runoff
reaches sensitive aquatic ecosystems downstream. They cultured and encapsulated
denitrifying bacteria in polymer gels, and verified their denitrification
rates. The resulting product was called "immobilized denitrification
sludge," or IDS.
They then devised a bioreactor by placing the IDS into a
small reactor cylinder. The team tested a bioreactor in the field, where
nitrate concentrations in runoff averaged 7.8 milligrams per liter.
Hunt and ARS environmental engineer Kyoung Ro determined
that the hydraulic retention time (HRT) – how long the field drainage water
remained in the bioreactor – was crucial in the denitrification process. With a
one-hour HRT, 50 percent of the nitrogen was removed from the runoff. When the
HRT was increased to more than 8 hours, the nitrate removal efficiency
approached 100 percent.
The team concluded that the daily nitrate removal rate of a one-cubic-meter
bioreactor would be approximately 94 grams per square meter of nitrate from
field runoff. This is significantly higher than removal rates reported for
in-stream wetlands, treatment wetlands or wood-based bioreactors.
Reducing Nitrate Discharge
February 13, 2009