Page Banner  United States Department of Agriculture
Agricultural Research Service  Laboratory Investigation of Anion
Exclusion Effects on Nitrate Mobility in Unsaturated Soil
headline bar  Summary:  Nitrate derived from fertilizers applied
at the surface of farm fields can leach downwards and
contaminate underlying aquifers.  Percolating water containing
nitrate can also be intercepted by shallow subsurface drainage
pipe systems and then discharged offsite into streams, rivers,
and lakes; in turn potentially having an adverse impact on
surface water quality at local, regional, and national scales. 
Therefore, solving environmental problems associated with
nitrate (NO3-) requires a better understanding of how NO3- moves
through the soil profile under unsaturated flow conditions.  To
a large extent, NO3- mobility in soil is governed by
electrostatic processes, particularly anion exclusion.  Nitrate
ions have a negative charge (i.e. nitrate is an anion), and
anion exclusion processes occur when mineral/organic