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Because buffers
increase water infiltration, concern has been expressed that leaching
of pesticides and nitrate might be increased, possibly to shallow
groundwater. When examining this possibility, it is important to
consider the properties of pollutants normally present in field
runoff. Because nitrate is water soluble and not adsorbed to soil
particles, it quickly moves off the soil surface and into the soil
with rainfall. In most settings runoff contains little nitrate.
As discussed previously, nitrate is carried to surface water primarily
be subsurface flow. Similarly, weakly adsorbed pesticides (which
would have the greatest leaching risk) often are not detected at
significant concentrations in runoff, as they quickly move into
the soil. Pesticides detected in runoff are primarily strongly adsorbed
compounds attached to suspended sediment and moderately adsorbed
compounds both adsorbed to sediment and dissolved in water.
Strongly adsorbed
pesticides have very low leaching potential due to adsorption to
soil. Moderately adsorbed pesticides can sometimes leach below the
root zone in small concentrations. However, quantities leaching
are normally as much as 1,000 times less than quantities carried
off fields by runoff. Parts per million concentrations of some products
can be detected in runoff at the field edge, while concentrations
detected in shallow groundwater are often only a few parts per billion,
if detected at all. Because of landscape positions of many buffers
near streams, pesticides or nitrate leaching into buffers would
likely be carried by subsurface flow to streams. Cycling runoff
through buffer soil prior to discharge to streams by subsurface
flow is much better than allowing surface runoff to directly enter
streams. Pesticides can be adsorbed and degraded and nitrate taken
up by plants or denitrified within buffers.
Because of the relatively
low concentrations of pesticide trapped in buffers, leaching risk
from buffers should be much less than leaching risk from source
fields. For example, in an Iowa study, atrazine concentrations in
a source corn field were 4,800 ppb in the surface 2 cm of soil after
the first runoff event of the season (Fawcett et al., 1995). Atrazine
concentrations in the buffer strip were 750 ppb. Use of BMPs to
reduce pesticide runoff from source fields not only reduces pesticide
loads ultimately reaching surface water, but also reduces loads
trapped by buffers.
Conservation buffers
have been shown to cause degradation of pesticides and to attenuate
pesticide concentrations in subsurface water flow. In Iowa (Schultz
et al., 1997) atrazine concentrations in soil water 2 feet below
a corn field were 13 ppb. Atrazine concentrations beneath an adjacent
grass and woody vegetation buffer were only 0.2 ppb. In a Georgia
study (Lowrance et al., 1997) no atrazine was detected in shallow
groundwater beneath a 3-zone buffer for the first two years of the
study. In the third year of the study a large rain event soon after
herbicide application resulted in atrazine detections in monitoring
wells 6.6 foot deep. A concentration of 6 ppb was detected at the
field edge. At the downslope edge of a 26 foot-wide grass strip
adjacent to the field, atrazine concentrations declined to 2 ppb.
At the downslope edge of the tree strip at the stream edge, atrazine
was detected at only 0.2 ppb.
Considering the
relatively small load of pesticide intercepted by buffers compared
to that applied to crop fields, and the adsorption and degradation
of pesticides by soil and vegetation in buffers, increased leaching
of pesticides does not appear to be a significant risk from conservation
buffers.
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