Movement of Pesticides
and Best Management Practices
by Heather Burner, Scott Crosswell,
Kara Kaufman, Scott O'Rourke, and Lisa Shelton
Spring 1997
- Problem Statement and Introduction
- Movement of Pesticides - Diagram
- Movement Processes
- Retention
- Transformation
- Transport
- Plant Uptake
- Best Management Practices
- Conclusion
- Related Links
- References
- Author Contacts
For centuries farmers have used compounds
containing elements such as arsenic, lead, and mercury to control
insects and other pests. These chemicals achieved only limited
success for pest control compared to, the 1939 discovery, DDT.
Since this time, other pesticides such as 2,4-D
and MCP have been introduced to control pests and help increase
crop yields. It has always been known that pesticides are toxic,
however the adverse side effects towards humans and the
environment have become known only within the past few years.
Pesticides were first identified in groundwater
less than 10 years ago. A 1990 study by the United States Environmental Protection
Agency (USEPA) highlighted the presence of 74 pesticides in
the groundwater of 38 states. This high level of pesticides in
the groundwater threatens human health since over 50% of the US
population relies on groundwater as their source of drinking
water.
Figure 1 - The Water Cycle
Source: United
States Geological Survey
The purpose of this webpage is to provide information about
the pathways for pesticide movement in the soil environment.
We will discuss the following five processes:
- Transport
- Retention or Sorption
- Transformation/Degradation
- Volatilization
- Plant uptake
In addition this page provides information about the Best
Management Practices (BMPs) that can be initiated to control
pesticide movement in the environment.
Back to the top
Figure 2 - Movement of
Pesticides in the Environment
Click on the picture to see a full size version
Back to the top
Retention, commonly known as adsorption is the
soil's ability to hold, or retain, the pesticide on its surface.
Several soil characteristics affect the adsorption rate of a
pesticide, these being pH, moisture content, clay content, oxide
content, cation exchange capacity, specific surface area, and
organic matter content.
Retention affects the characteristics of the
pesticides for several reasons. First, adsorption provides the
pesticide with a place that it can collect and force chemical
transformations. These chemical changes can create very complex
problems that may make remediation very difficult. Adsorption can
also affect the pesticide's ability to be transported. Very
simply, the more pesticide adsorped by the soil, the less that
will be transported.
Back to the top
One of the main concerns when dealing with
pesticides is the process of transformation.
When pesticides enter the soil and groundwater they can be
degraded, transformed, or stored by micro-organisms, plants, and
animals. According to Sanjay Shukla, "the term
transformation includes all the changes in the chemical structure
or composition of the pesticide."1.
There are three main transformation processes.
- Photochemical processes
- Chemical processes
- Microbial processes
Microbial processes than chemical processes because of the
increased microbial activities in the root zone.1
"Microbial degradation of pesticides, ... is a
process whereby micro-organisms adapt to the pesticide and
produce enzymes suitable for degrading the pesticide during a
phase called 'lag phase' followed by the phase of
'enrichment' as the adapted micro-organsisms multiply while
utilizing the substrate as the preferred energy source."2
When pesticides are absorbed into the soil they affect the
metabolic processes in soil micro-organsisms. Microbes are a
major controlling factor in the subsurface movement of pesticides
because they break down the pesticide into H2O and CO2
molecules.
The five basic processes involved in the microbial
transformation of pesticides are: biodegradation, cometabolism,
polymerization or conjugation, accumulation, and secondary
effects of microbial activity.2
Major factors influencing microbial population are:
- pH
- Organic matter content
- Soil moisture status
- Temperature
- Aeration
- Cation exchange capacity3
It is very difficult to distinguish between microbial
transformations and chemical transformations.
Chemical transformation takes place on pesticide molecules as
soon as the pesticide enters the water. There are several
physical and chemical properties that affect chemical reactions
in water. These include pH, buffering, general acid and base
catalysis, temperature, dissolved organics and suspended solid,
metal ions, and the redox state of the water column.4
The type of soil to which the pesticide is applied also affects
the chemical transformation. Hydrolysis and redox reactions in
sediments are generally considered to be the most important modes
of transformations.
Photochemical transformation is "an important degradation
pathway for many pesticides, especially those that are surface
applied."
Back to the top
A pesticide can be transported in several
different ways. The most common being 'mass flow' and
'diffusion'.
Mass flow is when a pesticide is transported
by a flow of water in which the pesticide has been retained and
can transport the pesticide molecules very quickly over large
distances. Mass flow in a soil is a function of the rate of water
movement and the soil's characteristics.
Diffusion occurs in a much more random manner than mass flow. It
occurs when a pesticide travels from an area of high
concentration to an area of low concentration by way of random
molecular movement. Diffusion is a much slower process than mass
flow because of its randomness. It is dependent upon the soil's
characteristics and the molecular structure of the pesticide.
Back to the top
Plant uptake is the process whereby
pesticides are transported into and within the plant's structure.
This process can be seperated into two distinct methodic
pathways, sorption by the roots of the plant, and adsorption with
subsequent movement to the plant's supersurface structure.
The most important factor governing sorption
and movement within the plant is the solubility of the pesticide
in water. The content of the surrounding soil is also important
to the plant uptake. For non-polar pesticides the volume of
organic matter is particularly important. Other factors such as
pH and clay and microbial activity are more important as the
polarity of the pesticide increases.
The accumulation of pesticide through plant
uptake can have severe consequences through the food chain if the
pesticide is translocated into the section of the plant that will
be subsequently harvested.
Back to the top
There are four general types of Best Management
Practices (BMPs): structural, biological, biological plus
structural, and cultural.
Structural management includes
the construction of reservoirs which can remove pesticides. In
addition, reservoirs can store peak storm water flows, minimizing
biological stress on receiving waters. Research has shown that
over 70% of nutrients (pesticide and fertilizer) from
agricultural runoff can be trapped.5
Biological practices include riparian zones
(vegetation on river banks) and natural flood plains. In each
case sediment is trapped and chemicals which are bound to these
particulates are assimilated.
Biological and structural management can be
in the form of wetlands which provide both biological and
physical remediation. These systems are commonly referred to as
'Nutrient/Sediment Control Systems' and are composed of a
combination of sedimentation basins, grass filter strips, and
constructed wetlands.5
- Sedimentation basins are designed to
reduce runoff rates and to remove pollutants. The
remediation obtained through the usage of sedimentation
basins primarily depends upon the retention time of the
pond. For example ponds that are always filled with water
have the most effective removal of pollutants.
- Filter strips remove sediment and other
pollutants from surface runoff. These strips alter the
hyraulics of the flow, enhancing infiltration,
deposition, filtration, adsorption, and absorption of the
sediment bound pollutants.
- Wetlands act similarly to detention
basins/ponds, however they have the additional capability
to provide biological assimilation.
Cultural practices are considered to have the
greatest potential for lessening the effects of agricultural
runoff. Examples of such practices are conservation tillage and
pesticide application optimization.
- Conservation tillage is the process in
which 30% of the soil surface remains covered with crop
residue after planting. This residue helps decrease soil
erosion and surface runoff and increases infiltration.
- Pesticide application has been receiving
much attention because of its potential for reducing
pesticide runoff and leaching.
Many times crops are watered and nutrients and pesticides are
applied uniformly over an area, based on average conditions for
that area. However, variablility in soil type (sandy, clayey,
rocky etc.) and soil water content will cause some areas to be
overwatered resulting in pesticide runoff. Systems are being
developed which can optimize application rates for small areas
(approximately 30 feet by 30 feet) based upon stored data or soil
measurements.6
Back to the top
From the information that we have provided on
this webpage we can conclude that the movement of pesticides
through the soil structure and into the groundwater proceeds by
an array of pathways. Consequently the inhibition and control of
pesticide movement can be difficult to implement. Through this
difficulty it has been experienced that the most effective route
to pesticide control is through Best Management Practices.
Even though we have arrived at this situation
we are currently limited in the number of BMPs that are
technologically, socially and economically viable and even though
they all produce desirable effects, unwanted detrimental
consequences may still occur. A prime example are structural
practices that may improve surface surface water quality yet
increase groundwater contamination. Each pollution site must also
be treated as an individual, there is no generic plan for the
treatment of pollution due to the very site specific nature of
the BMPs and a BMP that is very effective at one site has no
gurantee of effectiveness at another.
From what we have seen we must draw the
conclusion that a more appropriate way forward to address the
issue of ground and surface water contamination would be to focus
upon its prevention rather than the subsequent removal. It would
be a far easier route to facilitate but until such time as the
issue is viewed from this angle we must rely upon the
effectiveness of BMPs as our answer.
Back to the top
The numbered parts of the text of this page
have been based upon extractions from the following texts.
Shukla, Sanjay. "Assessment of
Groundwater Vulnerability to Pesticide Contamination in Albemarle
and Louisa Counties, Virginia (1995)"
- Page 14
- Page 16
- Page 17
- Page 15
Schreiber, J. D. "The Occurrence,
Distribution, and Remediation of Transient Pollution Events in
Agricultural and Silvicultural Environments."
- Page 23
Camp, C. R. "Site Specific Water
Nutrient and Pesticide Management."
- Page 14
Back to the top
The following links are related to the subject
of this page.
Virginia Tech Pesticide
Programs
Virginia
Integrated Pest Management Site
United
States Department of Agriculture - Integrated Pest Management
Initiative
United States Environmental
Protection Agency
The Groundwater
Foundation
Back to the top
- Shukla, Sanjay. "Assessment of
Groundwater Vulnerability to Pesticide Contamination in
Albemarle and Louisa Counties, Virginia (1995)".:
1-34
- Schreiber, J. D., Smith Jr., S., and
Cooper, C. M. "The Occurrence, Distribution,
and Remediation of Transient Pollution Events in
Agricultural and Silvicultural Environments." Water,
Science and Technology 33 (1996): 17-26.
- Camp, C. R. and Sadler, E. J. "Site
Specific Water, Nutrient and Pestecide Management."
Irrigation Journal 47.3 (1997): 14-16
- "Chemigation, A Viable,
Efficient Alternative." Irrigation Journal 47.4
(1997): 16-18
- Evans, Robert O., Skaggs, R. Wayne, and
Gilliam, J. Wendell. "Conventional Drainage
Effects on Water Quality." Journal of Irrigation and
Drainage Engineering (1995): 271-275
Back to the top
Send comments or suggestions to:
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
