Em4885 irrigation management practices to protect ground water

3 CHAPTER
2. Degradation rate - a measure of how fast the chemical breaks down into other chemicals.
The longer it takes for a chemical to break down, the more opportunity there is for
detachment and transport. Breakdown may occur through a reaction with water
(hydrolysis), through exposure to sunlight (termed photolysis), or through reactions
with microorganisms. If a chemical does leach to ground water it will last a long time
if slowly degradable. The degradation rate is reduced by low temperatures, no exposure
to sunlight, and no exposure to microorganisms (which occur mostly in the root zone).
3. Solubility - a measure of how well the chemical dissolves in water. A highly soluble
chemical can move readily with water. Thus, it may move with leaching water to ground
water or with surface runoff from irrigation or rainfall.
4. Volatility - a measure of how fast a chemical evaporates. Highly volatile chemicals
should be injected into the soil or worked in quickly to prevent losses.
Site Conditions Affecting the Pollution Process
Site conditions include the soil conditions in the immediate rootzone, the depth to ground water,
climate, and geologic conditions. The shallower the aquifer, the less time it will take leached
nutrients and chemicals to reach it. This means less time for chemicals to break down and less
opportunity for adsorption by soil particles. Large and variable rainfall make it difficult to plan
chemical and nutrient applications to prevent leaching. Colder climates decrease the rate of chemical
reactions that might degrade harmful substances.
The main soil factors that affect the pollution process are soil texture, organic material content,
amount of soil water, soil structure, and soil temperature.
1. Soil texture - refers to the relative amounts of sand, silt, and clay present. It is important
because the soil texture governs the amount of surface area available for adsorptivity,
that is, how much total soil surface there is available for chemical attachment. Sand
particles are relatively large. Clay particles are very small. Thus, there are fewer soil
particles in the same volume of sandy soil than clay soil. Consequently there is
much less surface area in sandy soil than clay soil. Thus, less chemical will attach to
sand particles and there will be more chemical available for leaching than in a clay soil.
Coarse-textured soil holds less water and has a higher permeability than fine-textured soil
making it more susceptible to leaching from rainfall or over-irrigation. Very fine-textured
soil may be susceptible to excessive surface runoff from either rainfall or over-irrigation.
Leaching can transport contaminants to ground water and surface runoff can transport
sediments, along with any adsorbed chemicals, to surface water.
2. Organic matter - important because organic matter greatly increases the area available
for adsorption.
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