er-18.nd
March 1994
NDSU EXTENSION SERVICE
North Dakota State University,
Fargo, ND 58105
An Assessment System for Potential Groundwater
Contamination from Agricultural Pesticide Use
in North Dakota -Technical Guideline
Bruce Seelig
Water Quality Specialist
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.
er-18.fig1.nd.gif
Figure 1. Stepwise algorithm for determination of ground water
sensitivity to pesticides (first order of priority).
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 1 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 11
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 range land 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: 1) depth to the saturated aquifer
combined with predominant waterflow direction; 2) soil and geologic
strata permeability; 3) soil organic matter content; 4) 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 so
mapping unit description.
blkb
--------------------------------------------
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
--------------------------------------------
blke
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 K00 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)
blkb
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-CPAsodiumsalt 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
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
DNQC 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
Enthalfluralin 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
-----------------------------------------------------------------------
Imazamethab nz-
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
MOPA 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 salt1,000 1,000,000 >2,000 Low
Metalaxyl 70 50 7 High
Metaldehyde 10 240 240 Intermediate
-----------------------------------------------------------------------
Metham (metam)
sodiumsalt 7 10 14 High
Methamidophos 6 5 12 High
Methanearsonic acid
sodiumsalt 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 5,000 553 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 1 High
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