Em4885 irrigation management practices to protect ground water

SECTION 1 - OVERALL GOOD PRACTICES
IP 3.01.01 - Assess the Risk of Contamination of Ground and
Surface Water Due to Fertilizer/Chemical Leaching or Runoff
Objective
Use fertilizer formulations that are minimally susceptible to leaching considering
the soil on which they are used.
Description
Some combinations of soil and fertilizer are more susceptible to leaching than others.
The Washington State Water Quality Guide contains a section that explains the
development of a Leaching Index. The Leaching Index “allows for evaluation of
the potential for contaminating the ground water with soluble nutrients.” The major
concern is nitrate-nitrogen leaching through lighter soils.
65
CHAPTER 4
The Soil Conservation Service also has developed a method for estimating the potential
for phosphorus losses. This includes losses of soluble phosphorus as well as material
that is adsorbed to soil particles. Note, the current edition of the Washington State Water
Quality Guide states, “. . . The key to preventing agriculturally applied phosphorus from
becoming an environmental problem is to prevent erosion.”
The Washington State Water Quality Guide also lists important questions to be answered
when considering the fertilizer/pesticide program. These include:
1. What is the sensitivity of the surface/ground water resource? Is it a sole source
aquifer for drinking water? What about contact recreation?
2. Will surface water or wind drift losses affect adjacent field or aquatic vegetation?
3. Where is the nearest water well in relation to application points?
4. If ground water is a concern, how deep is the aquifer and what type of geologic
structures are present from the surface to the aquifer? That is, are there confining
(fine clays or bedrock) layers that could provide additional protection from
leached chemicals?