WWRC 97-05t
Fate of Nitrate and Aldicarb Under Irrigation Management
Abstract
Environmental contamination by agrochemicals
(fertilizers and pesticides) due to overuse of
irrigation water is a national concern, particularly
where flood irrigation has been the conventional
water application method. Therefore, there is a need
to assess other management approaches that can
potentially reduce leaching of agrochemicals; the use
of drip irrigation may offer such an alternative. The
objective of this study was to evaluate nitrate
(fertilizer) and aldicarb (pesticide) movement
through soils under flood and drip irrigation
practices. A transport and irrigation model,
CHAIN_IR, was utilized to assess the leaching
potential of these two solutes. With flood irrigation,
higher concentrations of both nitrate and aldicarb
were found deeper in the soil profile as compared to
drip treatment, thus, suggesting a greater leaching of
chemicals under flooding. In addition to the
downward flow, a lateral movement of both nitrate
and aldicarb was observed and the results suggested
that under drip, fertilizers and pesticides would be
available for a greater extent around the point of
application, whereas under flood irrigation, there
would be run-off loss of the agrochemicals. Nitrate
concentration at an internal point (x = 0, z = 35)
inside the soil profile increased curvilinearly with
time from 0 to 17 g/cm3 within 7.5 d, thus, indicating
leaching of the added nitrate from the surface (x = 0,
z = 0). Concentrations of aldicarb decreased
exponentially following first order kinetics with a
specific rate constant (K) of 0.25/d and half-life
period (t0.5)2.8 d. Two daughter compounds,
aldicarb sulfoxide and sulfone, were also predicted to
be formed following a logarithmically increasing
function and a zero order linear reaction (K of
0.008g/cm3/d), respectively. Correlations for all
these equations were significant at the 1% probability
level. The contour maps for soil water pressure head
(h) distribution at 0.5 d post-irrigation period,
indicated a greater percolation of the water-front
under flood irrigation as compared to the drip
irrigation, which would lead to less leaching of the
contaminants with drip practices. The overall results
of this study suggest that application of drip irrigation
is a better alternative for developing best irrigation
management practices that can diminish leaching of
agrochemicals and control overuse of water.
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