An Assessment System for Potential Groundwater Contamination from Agricultural
Pesticide Use in North Dakota — Technical Guideline
Extension Report No. 18, March 1994
Bruce Seelig, Water Quality Specialist
Introduction
STEP 1. Aquifers versus groundwater
STEP 2. Pesticide use
STEP 3. Filtration potential
An EXAMPLE of GROUNDWATER ASSESSMENT for Pesticide
Contamination
A NOTE OF CAUTION
A groundwater assessment system is proposed to help develop
and implement best management practices (BMP) to protect groundwater from pesticide
contamination. This system will help producers organize natural resource information into
groundwater sensitivity categories. BMP recommendations will be adapted for each
groundwater sensitivity category. To determine the groundwater sensitivity of a given
area, a guided path, or stepwise algorithm, (Figure 1) is used.
Figure 1. Stepwise algorithm for determination of
ground water sensitivity to pesticides (first order of priority). (12KB
b&w image)
A variety of systems have been developed to determine groundwater vulnerability and/or
sensitivity. Vulnerability assessment requires physical information about materials
that overlie and protect aquifers from contamination (Pettyjohn et. al., 1991). Sensitivity
assessment includes a measure of human activity above an aquifer in addition to the
hydrogeologic factors.
Unfortunately all assessment systems have weaknesses and none of them adequately
address the complexity of the natural system. Perhaps the greatest weakness is that
assessment schemes are often based on computer simulations and have not been verified for
actual field conditions (Wagenet and Rao, 1990). If the user is aware of the basic
assumptions and consequent weaknesses in each computer simulation, they can be useful for
groundwater assessment. However, extrapolation beyond the boundaries of those assumptions
can lead to nonsensical conclusions.
Aspects of several different assessment systems have been utilized to create a system
that best fits conditions in North Dakota. It should be noted that the assessment system
for North Dakota does not rely on artificial values and weights. Considering our limited
understanding of the complexity of natural systems, rating systems often build the
illusion of relative importance when in fact there is none. However, some general trends
common to most groundwater assessment systems have been incorporated into the North Dakota
system. Key factors that determine vulnerability or sensitivity will be assessed to assign
each site to a specific category. The categories will emphasize similarities in factors
and will represent a rating system only in the broadest sense. Hopefully this will avoid
the usual criticisms leveled at the inconsistencies of a rating system, and the
credibility problems that follow. Instead, the focus will be on placement in categories
that allow logical development of an effective system of management practices that protect
groundwater.
STEP 1. Aquifers versus groundwater
The first step requires the user to determine if an aquifer with a useable supply of
water exists. We recognize that because there is linkage between all forms of groundwater,
protection of all groundwater is desirable. However, in the real world of limited funds,
priorities must be set. As identified in the position paper developed by the North Dakota
Technical Advisory Committee for the Pesticide/Groundwater Protection Management Plan,
water resources that serve human needs are of the highest priority. Emphasis must be
placed on protection of readily accessible groundwater or shallow aquifers with useable
water.
In North Dakota, aquifers located in glacially derived materials are of greatest value
due to their generally good water quality, high yields, and shallow depths. The water must
be of such quality that it is useable for human needs. Useable water quality is considered
to be Class I groundwater or water having less than 10,000 ppm total dissolved solids,
according to the North Dakota State Department of Health. The term shallow has been used
to describe vulnerable aquifers with less than 50 feet of material overlaying them,
similar to a combination of Pettyjohn et al.'s (1991) Class I and Class II aquifers. Many
glacial and alluvial aquifers in North Dakota meet this definition; however, many are
deeper than 50 feet. All glacial and alluvial aquifers will be considered as worthy of
protection, particularly those shallower than 50 feet.
Those aquifers that supply useable water to a significant number of people must also
take on a higher level of importance than those that don't. Glacial or alluvial aquifers
with useable water that are extensive enough to be used by significant numbers of people
are identified in the groundwater studies report (North Dakota Geological Survey and State
Water Commission), for each county in North Dakota.
In general, aquifers located in bedrock in North Dakota have poor quality water, are
deep, and have variable yields. Some of these aquifers even exceed the standard of 10,000
ppm. As a whole, these aquifers are not worthy of the same level of protection as glacial
aquifers or alluvial aquifers. However, in some parts of the state, particularly the
unglaciated southwest, bedrock aquifers are the only source of groundwater. Even though
bedrock aquifers underlay extensive areas, water quality and yield is unpredictable. Areas
of useable water within each bedrock aquifer are not as readily identifiable compared to
glacial and alluvial aquifers. Within the county groundwater studies report, information
about bedrock aquifers is quite general and difficult to apply to a specific area. This
makes the sensitivity assessment more difficult, because important information about the
aquifer is not as easily extracted from the report.
The first step in the assessment process requires the user to locate the extent of the
appropriate aquifers. The first order of priority is the determination of glacial or
alluvial aquifers under the area of interest. Only in areas of southwestern North Dakota,
where bedrock aquifers are the sole source of groundwater, should the assessment be
extended to include these aquifers as second order of priority.
STEP 2. Pesticide use
Distribution of land use has been recognized as an important factor in protecting
groundwater from agricultural chemicals (Thomas, 1992a). Different types of land use will
require different levels of agricultural inputs. Land use is a general indicator of the
amount and type of pesticide applied above an aquifer. Pesticide use will be combined with
land use in the following land use – pesticide categories: 1) cropland with
pesticides; 2) hayland, pastureland, forestland, and rangeland with pesticides; and 3) no
pesticides.
For regional assessments, the land use – pesticide category can be determined from
a combination of ASCS records and maps, Pesticide Use on Major Crops in North Dakota,
North Dakota Agricultural Statistics, and the USDA Agricultural Census. For farm
assessments, land use – pesticide categories may be determined from personal
knowledge.
STEP 3. Filtration potential
After the location of vulnerable aquifers and pesticide usage over them is assessed,
the site properties that affect pesticide movement must be determined. In simple terms,
the soil and geologic materials act as a filter to protect aquifers from contamination.
That filtering process is often referred to as pesticide "attenuation" in
scientific parlance. Attenuation can be defined as lessening the amount, force, or value
of something. In this case, the amount of pesticide is lessened as it is filtered-out on
soil and geologic materials. An estimate of the potential for materials to attenuate or
filter-out pesticides will be presented as the "filtration potential" for
this sensitivity assessment system.
In reality, pesticide attenuation is a complex process that depends not only on the
physical and chemical characteristics of the overlaying materials, but also on the
physical and chemical characteristics of the pesticide. Analysis of contamination
potential of groundwater requires solutions to complex formulas for water and solute
transport. It also requires large amounts of many different types of data. Manipulation of
large amounts of data within complex formulas has only become possible in recent years due
to computers. A growing number of computer programs are now available to help assess
contaminant movement within a set of assumed conditions. These programs have been utilized
to help predict contamination under various conditions, thereby identifying groundwater
sensitivity. Unfortunately, these programs are generally accurate within narrow
conditions, and the user must be aware of the basic assumptions used to develop the
program before realistic interpretations can be made from the results.
Because monitoring and interpreting data from actual field sites is expensive and time
consuming, computer simulations will continue to be used as a tool for assessing
vulnerability and sensitivity. Computer simulations must be used with caution,
particularly where little field validation has been demonstrated (Thomas, 1992b). Computer
simulation studies have identified several factors that are commonly recognized to affect
groundwater contamination. These factors will be used in this assessment system to
identify categories of groundwater sensitivity; however, computer simulations will not be
used in the categorization process.
Depth to the aquifer and vadose zone texture have been recognized as important factors
in several groundwater assessment systems (Cates and Madison, 1991; Pettyjohn et al.,
1991; Trojan and Perry, 1988; Aller et al. 1985). Goss (1992) determined soil organic
matter to be the most important soil characteristic influencing pesticide movement through
soils. Brown et al. (1991) recognized permeability and the presence or absence of organic
layers as the most important soil factors affecting pesticide leaching in Florida soils.
Groundwater vulnerability maps in North Dakota use soil permeability, soil organic matter
content, and depth to water table as the most important factors in groundwater
vulnerability determination. Cates and Madison (1991) incorporated soil texture and
organic matter content into their system for site evaluations for potential groundwater
contamination in Wisconsin.
Pesticide properties must also be accounted for when determining groundwater
sensitivity. Pesticide half-life (T1/2) and organic carbon adsorption
coefficient (Koc) have been used to rate pesticide potential to leach (Goss,
1992; Hornsby, 1992;).
The assessment of filtration potential of materials overlaying an aquifer will include
the following:
- depth to the saturated aquifer combined with predominant waterflow direction;
- soil and geologic strata permeability;
- soil organic matter content;
- pesticide Koc and T1/2.
Aquifer depth - water flow direction. Depth to the saturated aquifer can
be determined from the county groundwater studies report. Depths less than 50 feet are
considered to be shallow. Soils are an excellent indicator of long term water flow
direction (Bigler and Richardson, 1984; Arndt and Richardson, 1989; Knuteson et al., 1989;
Seelig and Richardson, 1993). Water flow through a soil to the groundwater can be
categorized as recharge (downward through the soil to groundwater) and discharge
(upward through the soil from the groundwater). Flowthrough is the term used to
described lateral movement of groundwater through the soil.
The presence and depth of calcium carbonate (lime) and a water table will be used to
assess the long-term hydrologic environment. As the depth of calcium carbonate increases,
so does the groundwater recharge potential. For this assessment system, soils of recharge
areas lack calcium carbonate in the upper 30 inches of the soil profile. Soils of discharge
and flowthrough areas have calcium carbonate in the surface horizon (usually
throughout the soil profile) and will have a water table within 6 feet of the surface.
Soils of an intermediate hydrologic environment that may be inactive or have a
relatively even balance between recharge and discharge will be characterized by a
combination of calcium carbonate and water table depths that do not fall in either of the
two categories described above. Depth to calcium carbonate and water table can be
determined from a county soil survey report (USDA, Soil Conservation Service). Presence of
calcium carbonate in each soil horizon is indicated by effervescence when dilute
hydrochloric acid is applied to the soil. This information is available in the soil series
descriptions.
Irrigation increases the potential for groundwater recharge. Many factors such as
timing of water application, tile drainage, soil texture, and pumping of wells influence
groundwater recharge under irrigated fields. Despite these extenuating factors, the
hydrologic environment for irrigated soils will be considered recharge.
A groundwater recharge area overlaying a shallow aquifer constitutes low potential
for filtration of contaminants from percolating water. All other combinations of
groundwater flow and aquifer depth have high filtration potential.
Soil and geologic material permeability. Soil permeability is closely
related to soil texture. Soils in the sandy and sandy skeletal textural families that
overlie sand and gravel geologic materials have low potential for filtration. Soils
in the fine textural family that overlie geologic material finer than sand and gravel have
high potential for filtration. All other textures or combination of textures will
have intermediate potential for filtration. Family textural classification of soils
can be determined from a county soil survey. Texture of geologic material overlaying the
aquifer can be determined from a county groundwater studies report or sometimes from the
county soil survey report.
Organic matter content. Soil organic matter (o.m.) content has the largest
influence on pesticide attenuation compared to the other soil factors. Organic matter
content of < 2% in the A horizon (very low to moderately low) will have low
potential to filter pesticides from percolating water. As o.m. content increases,
filtration potential also increases. Soils with > 2% o.m. (moderate to very high) in
the A horizon have a high potential to filter pesticides from percolating water.
Soil organic matter classes are given in the map unit descriptions in most county soil
survey reports (Table 1). If this information is not in the county soil survey report, the
local SCS office should be contacted.
Table 1. Soil organic matter content (percent) conversion
from soil mapping unit description.
organic matter organic matter content
descriptor by weight
---------------------------------------
(%)
Very Low < 0.5
Low 0.5 - 1.0
Moderately Low 1.0 - 2.0
Moderate 2.0 - 4.0
High 4.0 - 8.0
Very High > 8.0
Pesticide chemistry. The tendency for a pesticide
to move with water through soils is also influenced by its chemistry. This is referred to
as leaching potential. It is just the opposite of filtration potential or pesticide
tendency to be removed from the water and trapped or filtered by the soil. Hornsby's index
for pesticide leaching potential (Table 2) will be utilized because it is a combination of
the Koc and T1/2. The ratio of Koc and T1/2 is
multiplied by 10 to give a leaching index for each pesticide. The smaller the index, the
more likely the pesticide will not be filtered but will leach to the groundwater. A
pesticide with an index of 10 or less or Koc of 100 or less (Hornsby, 1992) would have a low
filtration potential and high leaching potential. If the index is 2000 or greater
(Hornsby, 1992) the pesticide would have a high filtration potential and low
leaching potential. Pesticides that do not meet these criteria are considered to have both
intermediate filtration potential and leaching potential. Their Hornsby index is a
relative indication of how close they may be to pesticides that are considered leachable.
Table 2. Pesticide properties and leaching potential (After Wauchope, et. al., 1992)
Soil
Half-life Sorption Hornsby Leaching
Pesticide (T1/2)days (Koc) Index Potential
----------------------------------------------------------------------
1,3-Dichloropropene 10 32 32 High
1-Naphthaleneacetamide 10 100 100 High
2,4,5-T amine salts 24 80 33 High
2,4-D acid 10 20 20 High
2,4-D dimethylamine
salt 10 20 20 High
----------------------------------------------------------------------
2,4-D esters or oil-sol.
amines 10 100 100 High
2,4-DB butoxyethyl ester 7 500 714 Intermediate
2,4-DB dimethylamine
salt 10 20 20 High
3-CPA sodium salt 10 20 20 High
Acephate 3 2 7 High
----------------------------------------------------------------------
Acifluorfen sodium salt 14 113 81 Intermediate
Alachlor 15 170 113 Intermediate
Aldicarb 30 30 10 High
Aldoxycarb
(aldicarb sulfone) 20 10 5 High
Ametryn 60 300 50 Intermediate
----------------------------------------------------------------------
Amitraz 2 1,000 >2,000 Low
Amitrole (aminotriazole) 14 100 71 High
Ancymidol 120 120 10 High
Anilazine 1 1,000 >2,000 Low
Arsenic Acid 10,000 100,000 100 Intermediate
----------------------------------------------------------------------
Asulam sodium salt 7 40 57 High
Atrazine 60 100 17 High
Azinphos-methyl 10 1,000 1,000 Intermediate
Bendicoarb 5 570 1,140 Intermediate
Benefin (benfluralin) 40 9,000 >2,000 Low
----------------------------------------------------------------------
Benomyl 67 1,900 283 Intermediate
Bensulfuron methyl 5 370 740 Intermediate
Bensulide 120 1,000 83 Intermediate
Bentazon sodium salt 20 34 17 High
Bifenox 7 10,000 >2,000 Low
----------------------------------------------------------------------
Bienthrin 26 240,000 >2,000 Low
Bromacil acid 60 32 5 High
Bromacil lithium salt 60 32 5 High
Bromoxynil butyrate
ester 7 1,079 1,541 Intermediate
Bromoxynil octanoate
ester 7 10,000 >2,000 Low
----------------------------------------------------------------------
Butylate 13 400 308 Intermediate
Captan 2.5 200 800 Intermediate
Carbaryl 10 300 300 Intermediate
Carbofuran 50 22 4 High
Carboxin 3 260 867 Intermediate
----------------------------------------------------------------------
Chloramben salts 14 15 11 High
Chlordimeform
hydrochloride 60 100,000 >2,000 Low
Chlorimuron ethyl 40 110 28 Intermediate
Chlorobenzilate 20 2,000 1,000 Intermediate
Chlorneb 130 1,650 127 Intermediate
Chloropicrin 1 62 620 Intermediate
Chlorothalonil 30 1,380 460 Intermediate
Chloroxuron 60 3,000 500 Intermediate
Chlorpropham (CIPC) 30 400 133 Intermediate
Chlorpyrifos 30 6,070 202 Intermediate
----------------------------------------------------------------------
Chlorsulfuron 40 40 10 High
Clomazone
(dimethazone) 24 300 125 Intermediate
Clopyralid amine salt 40 6 2 High
Cyanazine 14 190 136 Intermediate
Cycloate 30 430 143 Intermediate
----------------------------------------------------------------------
Cyfluthrin 30 100,000 >2,000 Low
Cypermethrin 30 100,000 >2,000 Low
Cyromazine 150 200 13 Intermediate
Dalapon sodium salt 30 1 <1 High
DBCP 180 70 4 High
----------------------------------------------------------------------
DCNA (dicloran) 60 1,000 167 Intermediate
DPCA
(chlorthal-dimethyl) 100 5,000 500 Intermediate
Desmedipham 30 1,500 500 Intermediate
Diazinon 40 1,000 250 Intermediate
Dicamba salt 14 2 1 High
----------------------------------------------------------------------
Dichlobenil 60 400 67 Intermediate
Dichlorprop (2,4-DP)
ester 10 1,000 1,000 Intermediate
Diclofop-methyl 30 16,000 >2,000 Low
Dicofol 45 5,000 1,110 Intermediate
Dicofol 45 5,000 1,110 Intermediate
Dicrotofos 20 75 38 High
----------------------------------------------------------------------
Diethatyl-ethyl 30 1,400 467 Intermediate
Difenzoquat
methylsulfate salt 100 54,500 >2,000 Low
Diflubenzuron 10 10,000 >2,000 Low
Dimethipin 120 10 1 High
Dimethoate 7 20 29 High
----------------------------------------------------------------------
Dinocap 5 550 1,100 Intermediate
Dinoseb phenol 20 500 250 Intermediate
Dinoseb salts 20 63 32 High
Diphenamid 30 210 70 Intermediate
Dipropetryn 100 900 90 Intermediate
----------------------------------------------------------------------
Diquat dibromide salt 1,000 1,000,000 >2,000 Low
Disulfoton 30 600 200 Intermediate
Diuron 90 480 53 Intermediate
DNOC sodium salt 20 20 10 High
Dodine acetate 20 100,000 >2,000 Low
----------------------------------------------------------------------
Endosulfan 50 12,400 >2,000 Low
Endothall (endothal)
salt 7 20 29 High
EPTC 6 200 333 Intermediate
Esfenvalerate 35 5,300 1,510 Intermediate
Ethalfluralin 60 4,000 667 Intermediate
----------------------------------------------------------------------
Ethephon 10 100,000 >2,000 Low
Ethion 150 10,000 667 Intermediate
Ethofumesate 30 340 113 Intermediate
Ethoprop (ethoprophos) 25 70 28 High
Etridiazole 103 1,000 97 Intermediate
----------------------------------------------------------------------
Fenac (chlorfenac) salt 180 20 90 High
Fenamiphos 50 100 20 High
Fenarimol 360 600 17 Intermediate
Fenbutatin oxide 90 2,300 256 Intermediate
Fenoxaprop-ethyl 9 9,490 >2,000 Low
----------------------------------------------------------------------
Fenoxycarb 1 1,000 >2,000 Low
Fenthion 34 1,500 441 Intermediate
Fenvalerate 35 5,300 1,510 Intermediate
Ferbam 17 300 176 Intermediate
Fluazifop-p-butyl 15 5,700 >2,000 Low
----------------------------------------------------------------------
Flucythrinate 21 100,000 >2,000 Low
Flumetralin 20 10,000 >2,000 Low
Fluometuron 85 100 12 High
Fluridone 21 1,000 476 Intermediate
Fluvalinate 7 1,000,000 >2,000 Low
----------------------------------------------------------------------
Fomesafen sodium salt 100 60 6 High
Fonofos 40 870 218 Intermediate
Formetanate
hydochloride salt 100 1,000,000 >2,000 Low
Fosamine ammonium
salt 8 150 188 Intermediate
Fosetyl-aluminum 0.1 20 2,000 Low
----------------------------------------------------------------------
Glufosinate ammonium
salt 7 100 143 High
Glyphosate
isopropylamine salt 47 24,000 >2,000 Low
Hexazinone 90 54 6 High
Hexythiazox 30 6,200 >2,000 Low
Hydramethylnon
(amdro) 10 730,000 >2,000 Low
----------------------------------------------------------------------
Imazamethabenz-
methyl (m-isomer) 45 66 15 High
Imazamethabenz-
methyl (p-isomer) 45 35 8 High
Imazapyr acid 90 100 11 High
Imazapyr
isopropylamine salt 90 100 11 High
Imazaquin ammonium
salt 60 20 33 High
----------------------------------------------------------------------
Imazethapyr 90 10 1 High
Iprodione 14 700 50 Intermediate
Isazofos 34 100 29 High
Isofenphos 150 600 40 Intermediate
Isopropalin 100 10,000 1,000 Intermediate
----------------------------------------------------------------------
Lactofen 3 10,000 >2,000 Low
Lambda-cyhalothrin 30 180,000 >2,000 Low
Lindane 400 1,100 28 Intermediate
Linuron 60 400 67 Intermediate
Malathion 1 1,800 >2,000 Low
----------------------------------------------------------------------
Maleic hydrazide
potassium salt 30 20 15 High
Mancozeb 70 >2,000 >286 Intermediate
Maneb 70 >2,000 >286 Intermediate
MCPA dimethylamine
salt 25 20 8 High
MCPA ester 25 1,000 400 Intermediate
----------------------------------------------------------------------
MCPB sodium salt 14 20 7 High
Mecoprop (MCPP)
dimethylamine salt 21 20 9 High
Mepiquat chloride salt 1,000 1,000,000 >2,000 Low
Metalaxyl 70 50 7 High
Metaldehyde 10 240 240 Intermediate
----------------------------------------------------------------------
Metham (metam)
sodium salt 7 10 14 High
Methamidophos 6 5 12 High
Methanearsonic acid
sodium salt 1,000 100,000 1,000 Intermediate
Methazole 14 3,000 >2,000 Low
Methidathion 7 400 570 Intermediate
----------------------------------------------------------------------
Methicarb
(mercaptodimethur) 30 300 100 Intermediate
Methomyl 30 72 24 High
Methoxychlor 120 80,000 >2,000 Low
Methyl bromide 55 22 4 High
Methyl isothiocyanate 7 6 9 High
----------------------------------------------------------------------
Methyl parathion 5 5,100 >2,000 Low
Metiram 20 500,000 >2,000 Low
Metolachlor 90 200 22 Intermediate
Metribuzin 40 60 15 High
Metsulfuron-methyl 30 35 12 High
----------------------------------------------------------------------
Mevinphos 3 44 15 High
Molinate 21 190 90 Intermediate
Monocrotophos 30 1 <1 High
NAA ethyl ester 10 300 300 Intermediate
NAA sodium salt 10 20 20 High
----------------------------------------------------------------------
Naled 1 180 1,800 Intermediate
Napropamide 70 700 100 Intermediate
Naptalam sodium salt 14 20 14 High
Nitrapyrin 10 570 570 Intermediate
Norflurazon 30 700 233 Intermediate
----------------------------------------------------------------------
Oryzalin 20 600 300 Intermediate
Oxadiazon 60 3,200 1,600 Intermediate
Oxamyl 4 25 62 High
Oxycarboxin 20 95 48 High
Oxydemeton-methyl 10 10 10 High
----------------------------------------------------------------------
Oxyfluorfen 35 100,000 >2,000 Low
Oxythioquinox
(quinomethionate) 30 2,300 767 Intermediate
Paraquat dichloride
salt 1,000 1,000,000 >2,000 Low
Parathion
(ethyl parathion) 14 5,000 >2,000 Low
PCNB 21 5,000 >2,000 Low
----------------------------------------------------------------------
Pebulate 14 430 307 Intermediate
Pendimethalin 90 5,000 556 Intermediate
Permethrin 30 100,000 >2,000 Low
Petroleum oil 10 1,000 1,000 Intermediate
Phenmedipham 30 2,400 800 Intermediate
----------------------------------------------------------------------
Phorate 60 1,000 167 Intermediate
Phosalone 21 1,800 857 Intermediate
Phosmet 19 820 432 Intermediate
Phosphamidon 17 7 4 High
Picloram salt 90 16 2 High
----------------------------------------------------------------------
Piperalin 30 5,000 167 Intermediate
Pirimiphos-methyl 10 1,000 1,000 Intermediate
Prochloraz 120 500 42 Intermediate
Profenofos 8 2,000 >2,000 Low
Prometon 500 150 3 High
----------------------------------------------------------------------
Prometryn 60 400 67 Intermediate
Pronamide
(propyzamide) 60 800 133 Intermediate
Propachlor 6.3 80 127 Intermediate
Propamocarb 30 1,000,000 >2,000 Low
Propanil 1 149 1,490 Intermediate
----------------------------------------------------------------------
Propargite 56 4,000 714 Intermediate
Propazine 135 154 11 Intermediate
Propham (IPC) 10 200 200 Intermediate
Propiconazole 110 650 59 Intermediate
Propoxur 30 30 10 High
----------------------------------------------------------------------
Pyrazon (chloridazon) 21 120 57 Intermediate
Quizalofop-ethyl 60 510 85 Intermediate
Sethoxydim 5 100 200 Intermediate
Siduron 90 420 47 Intermediate
Simazine 60 130 22 Intermediate
----------------------------------------------------------------------
Sulfometuron-methyl 20 78 39 High
Sulprofos 140 12,000 857 Intermediate
Tebuthiuron 360 80 2 High
Temephos 30 100,000 >2,000 Low
Terbacil 120 55 5 High
----------------------------------------------------------------------
Terbufos 5 500 1,000 Intermediate
Terbutryn 42 2,000 476 Intermediate
Thiabendazole 403 2,500 62 Intermediate
Thidiazuron 10 110 110 Intermediate
Thifensulfuron-methyl 12 45 38 High
----------------------------------------------------------------------
Thiobencarb 21 900 429 Intermediate
Thiodicarb 7 350 500 Intermediate
Thiophanate-methyl 10 1,830 1,830 Intermediate
Thiram 15 670 447 Intermediate
Toxaphene 9 100,000 >2,000 Low
----------------------------------------------------------------------
Tralomethrin 27 100,000 >2,000 Low
Triadimefon 26 300 115 Intermediate
Triallate 82 2,400 293 Intermediate
Tribufos 10 5,000 >2,000 Low
Trichlorfon 10 10 10 High
----------------------------------------------------------------------
Triclopyr amine salt 46 20 4 High
Triclopyr ester 46 780 170 Intermediate
Tridiphane 28 5,600 2,000 Low
Trifluralin 60 8,000 1,330 Intermediate
Triforine 21 200 95 Intermediate
----------------------------------------------------------------------
Trimethacarb 20 400 200 Intermediate
Triphenyltin hydroxide 75 23,000 >2,000 Low
Vernolate 12 260 217 Intermediate
----------------------------------------------------------------------
* These values are based on results of field and laboratory
measurements found in the literature. References for T1/2 and Koc
values and footnotes regarding factors that influence
interpretation of these values is presented in Wauchope et.al.
(1992). The complex interaction of site factors such as soil water
content, temperature, pH, and application procedures make precise
extrapolation of results beyond each study site impossible.
Nevertheless, T1/2 and Koc have been demonstrated to directly
affect the environmental fate of most pesticides. The values
expressed in this table should be interpreted only in a broad and
relative sense.
[ MORE . . . ]
[ An EXAMPLE of GROUNDWATER ASSESSMENT
for Pesticide Contamination ]
[ A NOTE OF CAUTION ]
Extension Report No. 18, March 1994
|
