Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 250

All of the above effects may be direct or indirect, via secondary contamination
or food chain. The long-term ecological impacts of increased rates of agricultural
nitrogen and phosphorus input will depend on the levels to which these nutri-
ents accumulate in various nonagricultural ecosystems. These levels are uncertain
because of the complexities of the global biogeochemistry of nitrogen and phospho-
rus. These nutrients accumulate in a variety of forms in many different sinks (such
as arable soil organic matter, atmospheric nitrous oxide, groundwater, freshwater,
and marine ecosystems and their sediments) after agricultural application.
7.4 Solute Transport Processes in Soil
For minimizing the pollution of soil, water and the environment, we should know
the processes involved in the transport, the ability of soils to transmit chemicals in
soil solution, and techniques to remove them from the solution.
7.4.1 Transport of Solute Through Soil
Transport processes in soils, particularly the movement of water and solutes, play
a vital role in the provision of suitable conditions for plant growth, and in the
replenishment and quality of groundwater supplies. Our understanding of these pro-
cesses has developed largely from experimental studies on simple uniform porous
materials, such as sands, that have led to the widespread acceptance of Darcy's
law and Richards' equation for soil-water flow and of the dispersion equation for
solute movement. These equations imply that soils can be considered as continua
and effectively assume that the water and solutes pervade the whole volume rather
than being contained in the complex network of pores in which the velocity of flow
and the solute concentration vary from point to point. They adequately describe the
macroscopic water and solute movement in simple porous materials. However, field
soils are commonly more complex, often having a bimodal pore structure with a
network of macropores separating aggregates of soil particles that form regions of
micropores. The transport behavior in these aggregated soils, especially when unsat-
urated, can be very different from that in the simple porous materials in which the
theory has been developed.
The way in which water and solutes move in aggregated soils depends on the
mode of saturation of the pore space that is made up of the micropore region
within the aggregates and the macropores surrounding them. When both regions
are saturated, a hydraulic head gradient causes water to flow preferentially in the
macropores with little flow within the aggregates, so that movement of solutes into
or out of the aggregates is mainly by diffusion caused by the difference between the
solute concentrations of the water in the two regions. The movement of water in the
micropore region within the aggregates can be considered to behave as if in a con-
tinuum and can be described by Darcy's law. With water moving into the aggregates