Environmental Engineering Reference
In-Depth Information
1979). The relative size of Ca and Na is such that two Na ions require more space than
one Ca ion; hence the replacement of Ca with Na causes an increase in the dimension of
the crystal lattice, resulting in a decrease in permeability. This can cause degradation in
the agricultural productivity of soils. The sodium adsorption ratio (SAR) is a measure of
the capacity of a given irrigation wastewater to induce sodic soil conditions (DWAF
1996b). More information with respect to the application of SAR as a control parameter
is given in Chapter 10.
An essential plant nutrient, K is abundant in many soils and is readily available for
plant uptake in large quantities, second to nitrogen. In soils, K exists as water soluble,
exchangeable, non-exchangeable (fixed) and mineral potassium. Due to its positive
charge, K is readily bound by negatively charged clays with a high cation exchange
capacity and water holding capacity. This retards the K movement down the soil profile.
Thus the movement of K can be a few centimeters from the point of application (Foth
1984). K is the most abundant cation in plants and is taken in larger quantities than any
other cations. The fixed form of K is found in the interlayer spaces of soil minerals.
However, for sandy soils with a low cation exchange capacity, leaching can be as high as
90% of the added K, after 150 kg/ha were added followed by an irrigation of 400 mm
(Harter et.al. 2002). A similar study showed that following 40 years of K application, no
K accumulated in the top 75 cm of a sandy soil, due to leaching. There are no reported
adverse effects of K on plants and the environment.
2.3
Pollutants transport in groundwater
In considering transport mechanisms in groundwater, a distinction is made between the
transport mechanism of groundwater and the solutes. At very low concentrations, the
dissolved phase of the solute may assume the same transport mechanism as the
groundwater, but at higher concentrations the pollutants can sometimes move faster than
the groundwater (Domenico & Schwartz 1997). Examples are the highly soluble nitrates
and conservative chloride solute. In many cases, as the solute moves through the porous
media, physical and chemical interactions between the solute and the porous media affect
the transportation process of the pollutant. Thus, transportation of pollutants in
groundwater is more complex than the transportation of the ordinary groundwater.
Different processes that include advection, dispersion, diffusion, adsorption and decay
influence pollutants transport in groundwater. These processes can act in combination or
separately to influence the transport process. Physical factors such as moisture content
and water balance in the unsaturated zone, together with the hydraulic gradient and the
water balance in the saturated zone, influence the pollutants transport. Wright (1999)
concurs with these factors, and notes some geological properties, such as the adsorption
capacity and hydraulic conductivity of soils, as additional factors. The two major
transport mechanisms of pollutants in groundwater are advection and dispersion.
Advection is defined as the flow of water due to a hydraulic gradient between two
points. The velocity of flow is proportional to the hydraulic gradient. Advective flow is
determined by factors such as the terrain slope and soil permeability, and is typically in
the order of cm/d. It is described by the Darcy's equation (9.3):
(9.3)
