B-6050

Pesticide Properties ThatA�ect Water Quality

Douglass E. Stevenson, Extension Associate,

Paul Baumann, Associate Professor and Extension Weed Specialist,

and

John A. Jackman, Professor and Extension Entomologist,

The T exas A&M University System

Research and manuscript preparation were performed under contract by Jerry L. Cook, Post Doctoral Research Associate, Department of Entomology, T exas A&M University

over the land before running intorivers, aquifers and lakes. It alsoseeps into underground aquifers.Irrigation and drinking watercome from both surface andground water. Eventually, all ofthe chemicals we use can polluteour water supplies (see Fig. 2).

There are many materials thatendanger our water quality. Mostcome from urban and industrialactivity. Some, however, comefrom agriculture. Whether inagricultural operations or inurban environments, the improperapplication, handling or disposalof pesticides can lead to waterpollution. There is reason foroptimism, however. Withoutbeing oppressive, the regulationof pesticides is reducing pesticidepollution of surface and groundwater.

UnderstandingPesticides

Pesticides are poisons designedto destroy unwanted life forms.Used properly, modern pesticidescan perform their functionswithout causing signi�canthazards to humans or the environ-ment. Federal and state lawsrequire the registration of anychemical that claims to controlpests, and these laws specify howand where such pesticides can beused.

Pesticides have many uses inhomes, gardens, farms, forestsand public health. It is di�cultto imagine what life would be likewithout modern pesticides. Yet, ithas been less than half a centurysince they became widely used.Before modern pesticides, human-ity was at nature’s mercy.

The U.S. farmer, through use ofthe latest management technol-

Three factors are neces-sary to all life on earth.These are an oxidizing

agent (usually oxygen), nutrientsand water. Water may be theuniversal chemical compoundrequired by all living organisms.The chemical content of thewater in a speci�c ecosystemdetermines what life forms canexist. Humans require water withlow levels of minerals and organicmaterial. We also require waterwith low concentrations ofchemical toxins. We considerwater with these properties to behigh quality water.

Most people in the UnitedStates expect high quality wateras one of the privileges of mod-ern society. Technology makes itpossible to turn on the faucet andhave clean, clear water readilyavailable. However, the technol-ogy that makes this possible alsocreates pressure on the verywater resources that are nowtaken for granted.

Why is Water QualityImportant?

Water is a part of everyday life,yet it is not an unlimited resource.Fresh water accounts for less than2.5 percent of all the earth’swater. Of all the fresh water onearth, nearly 80 percent is ice inthe polar ice caps and glaciers ofthe world. This leaves only about0.2 percent of earth’s fresh wateravailable for our use (Environmen-tal Protection Agency, 1990).

Since water is the currency oflife, we can look at it in terms ofmoney. If $1,000 represented allthe water on earth, only about $2would be available as fresh water.Most of this would be locked upin ice and other unavailablesources. Only a few pennies would

be available to spend. So, wecan’t a�ord to lose it or waste it.

We depend on water to sustainus, our domestic and wild ani-mals, and the growing plants inforests, �elds, yards and gardens.If water becomes contaminatedby toxins, it can harm all lifeforms. Pollution a�ects all of us—o�ce workers and housewives,the farmer and the �eld mouse.

Most of the available freshwater is ground water. A muchsmaller percentage is in rivers,lakes, soil moisture, and theatmosphere. This might appearinadequate. However, if it is ofhigh quality, the amount we haveis enough. At present, onlyabout2 percent of ground waterin the United States shows pollu-tion. However, an increasingamount of surface water is be-coming at least somewhat con-taminated (Environmental Protec-tion Agency, 1990).

More than 600 million poundsof pesticides enter the environ-ment each year in the UnitedStates. Pesticides control thou-sands of di�erent weeds, insectsand other pests; they protectcrops, human health, propertyand domestic animals almosteverywhere; and, they evenprotect our drinking water fromcontamination by algae and otherdangerous organisms. However,information about the health andenvironmental e�ects of pesti-cides has increased public con-cerns and led to more regulationof these chemicals.

We must understand howpesticides can pollute waterthroughout the hydrologic sys-tem (Fig. 1).

The contamination of water isdirectly related to the degree ofpollution of our environment.Rainwater �ushes airborne pollu-tion from the skies. It then washes

Classes of Pesticides

Pesticides have several classi�-cations. First, they fall into neatgroups on the basis of their targetpests—herbicides, insecticides,fungicides and several others. Thethree most widely used groups ofpesticides are the herbicides,insecticides and fungicides. Herbi-cides eliminate unwanted anddangerous vegetation. Insecticidesprevent injury and damage fromharmful insects, mites and ticks.Fungicides protect our food supplyfrom dangerous disease organisms.

The Environmental ProtectionAgency (EPA) classi�es pesticidesinto two types. These are general-use and restricted-use pesticides.If the EPA believes a pesticide ishazardous to humans or theenvironment, it is placed in therestricted-use category. To usethese chemicals, applicators musthave training and acquire a speciallicense. These regulations helpprevent pollution.

Before a pesticide is registeredfor use, the EPA estimates itspotential to pollute water. Pesti-cide manufacturers and the EPAuse this information to developspeci�c precautions to preventpesticides from entering water.These precautions are printed onthe product’s label. The EPAfrequently cancels or restrictspesticides that have a record ofcontaminating water even whenused according to the label.

Modern pesticides ordinarily donot get into water when usedaccording to label directions.However, there is always a poten-tial for water pollution if pesti-cide applicators do not followlabel precautions. Table 1 shows afew common pesticides and theirpotential as water pollutants. TheEPA develops this type of informa-tion for all pesticides that itregisters.

It is not always possible to usepesticides that pose a low poten-tial risk to water. There are fewchemicals to choose from forcontrolling some pests. When youhave to use a chemical that caneasily contaminate water, alwaysfollow label precautions. Payspecial attention to informationabout the water pollution poten-tial of the chemical you are using.You can then plan your applicationto reduce the pollution risks.Follow label directions and guide-lines at the end of this manual toavoid problems with pollution.

ogy, equipment, chemicals andimproved hybrid varieties, pro-duces food for this country andthe world. In 1994, the averageAmerican farmer fed his familyand 129 other people, including97 people in the United Statesand 32 in other countries. Becausemost of us don’t know muchabout how our food is producedon farms and ranches, pesticidesoften are a source of public fearand misunderstanding. Explainingwhat these chemicals are andwhat they do is not easy, becausemost consumers aren’t interestedin the details. However, it isimportant to understand boththeir bene�ts and hazards.

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Figure 1. The hydrologic cycle. Water also will �ow to the lowest point allowed by the geologic and soil structures present in the environment. (Moon et al. 1957.)

streams. Using excessive amountsof chemicals on open or poroussoils where there are shallowwater tables can allow pesticidesto leach or percolate into theground water.

Improperly cleaning or dispos-ing of containers, as well asmixing and loading pesticides inareas where residues or run-o�are likely to threaten surface orground water, are other potentialsources of contamination. Somepesticide labels and some statestatutes specify safe distancesfrom well heads for pesticidemixing and loading.

Agricultural chemicals also canpollute surface water throughirrigation return �ow and rainfallruno�. Carefully following labeldirections about proper dosageand application methods cangreatly reduce the possibility ofwater contamination.

Pesticide Properties

Properties that a�ect apesticide’s potential to pollutewater include formulation, toxic-ity, persistence, volatility, solubil-ity in water, and soil adsorption.Of course, pollution risk also isa�ected by soil characteristics,application methods, weather andother factors.

Formulation

Pesticides come in severalphysical forms or formulationsthat make them easy to store,transport and apply, and that helpin controlling target pests. Com-mon formulations include waterdispersable granules, wettablepowders, dusts, aerosols, solid orliquid baits, granules, emulsi�ableand �owable concentrates andsolutions. There are other lesscommon formulations designed

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Chemical Water Solubility Half-life (days)

Ways AgriculturalPesticides CanContaminate Water

The over-application or misuseof pesticides and other agricul-

tural chemicals (such as fertiliz-ers) can allow these chemicals toenter surface and ground water.Drift, evaporation and winderosion can carry pesticideresidues into the atmosphere.From there they can fall in rain orsnow to contaminate lakes and

Methyl Parathion insecticide

Carbaryl insecticide

PCNB fungicide

Disulfoton insecticide

Malathion insecticide

Chlorthalonil fungicide

Phorate insecticide

Diazinon insecticide

Methamidophos insecticide

low

low

very low

low

low

very low

low

low

high

5

10

21

30

1

30

60

40

6

Figure 2. Pathways of pesticide movement in the hydrologic cycle. (After USGS, 1996)

Table 1. Properties of some of the most commonly used pesticides in Texas.

The e�ective dose is theamount of a substance needed tokill or otherwise a�ect a targetpest. Amounts less than thee�ective dose will likely not killthe target pest. Amounts greaterthan the e�ective dose will notnecessarily be more e�ective inkilling the target pest. Instead,this larger dose may kill more non-target organisms, cost more, andpollute the environment.

Common measures of achemical’s toxicity are the LD50

and LC 50 . These measures refer todoses that kill 50 percent of theanimals in a test group. Thesetoxicity terms can apply to targetpests or non-target organisms,including humans. The toxicity ofa substance determines its properdosage.

The LD50 is the dose of a par-ticular material, taken through themouth, skin, or inhaled, that islethal to 50 percent of a group oftest animals. The higher an LD50 is,

to give special properties to thepesticide mixture or to takeadvantage of properties of activeingredients or protect the envi-ronment. These includemicrocapsules, plastic beads,plastic membranes, plastic ropes,controlled release dispensers andothers.

While most environmentalhazards come from the activeingredient in a pesticide, the wayits formulation interacts with theenvironment determines theoverall hazard of a pesticide.Spray formulations can drift withthe wind or vaporize into the air.Other formulations can leach intoground water or be carried intosurface water by rainfall or irriga-tion runo�. Even pesticides informulations that bind them tosoil particles can �nd their wayinto surface waters if soil iseroded by wind or water.

Toxicity

The active ingredient is thechemical compound in a pesticidethat kills or otherwise a�ects thetarget pest. Other substances in apesticide formulation are inertingredients that act as carriersand preservatives for activeingredients, and also make mixingand application easier.

When determining whether andhow to register a pesticide, theEPA considers the toxicity of theactive ingredient. Toxicity isdetermined by the amount re-quired to produce biologicale�ects.

Dose and E�ective Dose

A dose is the amount of asubstance used at one time. Mostsubstances are toxic at largeenough doses, but harmless oreven bene�cial at lower doses.

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LD50 (Rat) Other product withPesticide in mg/kg about equal toxicity

TCDD (Dioxin® )tcartxe naeb rotsac( niciR2000.0)

Saran (GB nerve gas) 0.2 Black widow spider venom

Flocoumafen (rodenticide) 0.25 Strychnine

Aldicarb (insecticide) 0.9 Nicotine alkaloid (free base)

Phorate (insecticide) 1.0 Heroin

Parathion (insecticide) 2.0 Morphine

Carbofuran (insecticide) 8 Codeine

Nicotine sulfate (insecticide) 50 Ca�eine

Paraquat (herbicide) 150 Benadryl (antihistamine)

Carbaryl (insecticide) 250 Vitamin A

Acephate (insecticide) 833 Salt substitute (KCl)

Allethrin (insecticide) 1,160 Gasoline

Diazinon (insecticide) 1,250 Tobacco

Malathion (insecticide) 5,500 Castor oil

Ferbam (fungicide) 16,900 Mineral oil

Drinking water is an example.People need to drink some waterevery day. However, drinking theequivalent of 15 percent of one’sbody weight can be fatal. Simi-larly, table salt is absolutelynecessary for proper health, butas little as 1 ounce (2 Table-spoons) of table salt woulddeliver a lethal dose to a 1-year-old child. There is a lethal dose ofca�eine in 100 cups of co�ee.There is a lethal dose of alcohol ina quart of whiskey. There is alethal dose of oxalic acid in 20pounds of spinach. There is alethal dose of aspirin in 100tablets. We can compare aspirinwith two chemical pesticides.Malathion is about half as toxic asaspirin. Parathion is 70 times moretoxic than aspirin. The hazards ofpesticide residues are negligiblecompared to the dangers fromcommon household chemicals andmedicines. Table 2 comparestoxicities of common productswith pesticides.

Methoprene (hormone) Sugar34,600

Table 2. Comparative toxicity of pesticides and natural products (1995 Farm Chemicals Handbook; Gosselin et al. 1984; SIPRI 1973).

the lower the toxicity of thesubstance. Items with low LD50sare extremely toxic. Basic measur-ing units used are milligrams oftoxin per kilograms of bodyweight, or “mg/kg.” Table 2 showsthe LD50 values in rats for variouspesticides and other familiarchemicals. Aspirin, table salt andother common natural productsprovide comparisons.

EPA uses LD50 s to determine thesafe level of pesticide residues inwater. The rat is a common testanimal for LD 50 s, but certainenvironmental studies requireLD 50s for animals such as rabbitsand mice, birds such as bobwhitequail and mallard ducks, �sh suchas trout and bluegill, andarthropods such as house�ies,honeybees and daphnia (a smallfresh-water crustacean).

LC50 is another measure oftoxicity. LC50 stands for theconcentration of a material in airor water that will kill 50 percentof the animals tested.

The toxicity of a pesticide isdi�erent from the hazard itrepresents. Hazard refers to thelikelihood that a substance willcause harm under certain condi-tions. For example, the pesticideparaquat is highly toxic. Just a fewdrops can kill an adult human.There is no antidote for paraquatpoisoning. Used properly andstored in a tight container,paraquat has high toxicity and alow hazard. If the contents of thecontainer spill, however, thetoxicity remains the same but thehazard increases enormously.

Regulating Toxinsin Water

The EPA uses the properties ofchemicals to establish standardsfor toxins in water. The standardfor water is the MCL or Maximum

Contaminant Level. When drinkingwater exceeds the MCL set for aspeci�c chemical, EPA must takeaction to increase regulation ofthe o�ending product.

EPA sets MCLs at a very low,very safe level. They are less than1/1,000th of the dose required tohave a measurable e�ect.

Scientists measure pesticideresidues in water in parts permillion (ppm), parts per billion(ppb), parts per trillion (ppt) andparts per quadrillion (ppq). Onepart per million is equivalent toone drop of pesticide in 21.7gallons of water. This is enough to�ll a small garbage can. One partper billion is equal to one drop ina 21,700-gallon swimming pool.One part per trillion is one drop in1,000 swimming pools. One partper quadrillion (ppq) is equal toone drop in a million swimmingpools. This is enough water to �lla volume 1 mile long, 1 mile wideand 1 mile deep.

Table 3 shows MCLs for severalpesticides found in water. Watercontaining these amounts of thevarious pesticides shown is com-pletely safe to drink. Furthermore,a 150-pound man would have to

drink at least 75 gallons of waterdaily to consume even theseamounts of pesticides.

Persistence

Persistence describes how longa pesticide remains active. Half-life is one measure of persistence.The half-life of a substance is thetime required for that substanceto degrade to one-half its originalconcentration. In other words, if apesticide has a half-life of 10 days,half of the pesticide normallybreaks down by 10 days afterapplication. After this time, thepesticide continues to breakdown at the same rate. The half-life of a pesticide is not an abso-lute factor. Soil moisture, tem-perature, organic matter, availableoxygen, microbial activity, soil pH,photodegradation and otherfactors may cause the half-life ofa substance to vary. In general,the longer a pesticide persists inthe environment, the more likelyit is to move from one place toanother and be a potential sourceof pollution.

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Table 3. MCLs for pesticides found in drinking water.

Contaminant Product type MCL (ppm)

500.0tnagimuFenaporporolhciD 2,1

70.0edicibreHD-4,2

200.0edicibreHrolhcalA

300.0edicitcesnIbracidlA

300.0edicibreHenizartA

Dibromochloropropane (DBCP) Fumigant 0.0002

Ethylene dibromide (EDB) Fumigant 0.00005

7.0edicibreHetasohpylG

2.0edicitcesnIlymaxO

5.0edicibreHmarolciP

Volatility

Many pesticides, includingseveral types of herbicides and soilfumigants can escape from soils asgases (see Fig. 2). Some can distilfrom soils and enter the atmo-sphere with evaporating water.Pesticide particles in the atmo-sphere can come back to earth inrain or snow, and then either leachinto ground water or be carried byruno� into surface waters.

Water Solubility

The water solubility of a pesti-cide determines how easily it goesinto solution with water. Whenthese compounds go into solutionwith water they can travel with itas it runs o� the land or leachesthrough the soil. The solubilities ofmaterials such as pesticides areusually given in parts per million(ppm), or in some cases as milli-grams per liter (mg/l). The solubil-ity of a substance is the maximumnumber of milligrams that willdissolve in 1 liter of water.

Simply being water soluble doesnot mean that a pesticide willleach into ground water or run o�into surface water. However,solubility does mean that if asoluble pesticide somehow getsinto water, it will probably staythere and go where the watergoes. Some pesticides must besomewhat soluble in water to workproperly. Others cannot be watersoluble to work properly. Manufac-turers and the EPA consider solubil-ity carefully when registering apesticide product. It is importantnot only to apply pesticidescorrectly, but also to mix, load,handle and dispose of pesticidesand their containers according tolabel directions. Care with clean-up and disposal is critical whenhandling pesticides that aresoluble in water.

Soil Adsorption

Soil adsorption is the tendencyof materials to attach to thesurfaces of soil particles. If asubstance is adsorbed by the soil,it stays on or in the soil and is lesslikely to move into the watersystem unless soil erosion occurs.A soil’s texture, structure and

organic matter content a�ect itsability to adsorb chemicals. If youdon’t know what type of soil youhave, send a sample to a labora-tory for analysis. Once you knowyour soil type you can �nd out itspotential risk for pollution byreferring to a U.S.D.A. publicationcalled “Soil Ratings for Determin-ing Water Pollution Risks forPesticides.”

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Figure 3. Pesticides can pollute water through either surface runo� or leaching.

How PesticidesEnter Surface andGround Water

Pesticides can enter waterthrough surface runo�, leachingor erosion. Water that �owsacross the surface of the land,whether from rainfall, irrigation,snow melt or other sources,always �ows downhill until itmeets a barrier, joins a body ofwater, or begins to percolate intothe soil. Some pesticides andfertilizers can be carried alongwith runo�.

Wind and water can erode soilthat contains pesticide residues andcarry them into nearby bodies ofwater. Even comparatively insolublepesticides and pesticides with highsoil adsorption properties can movewith eroding soil.

With increasing frequency, soil-applied pesticides also are beingfound in ground water across theU.S., and regulating agencies aretaking action to prevent this fromoccurring. Pesticides have to haveseveral characteristics before theypose a risk to ground water. Theyhave to be water soluble enough tomove in the soil. They have topersist long enough to be carriedbeyond the region of bacterialactivity in the soil. They have to beapplied at rates high enough toallow them to persist. They have tobe applied to soils that will not bindthem tightly or deactivate them.They must be applied in regionswhere climatic factors, includingprecipitation, will allow them tomove through the soil. And, theyhave to be applied in regions whereground water exists and where it isshallow enough for substancesleaching from the surface to reachit.

Pesticides that enter watersupplies can come either frompoint sources or from non-pointsources (Fig. 4). Point sources aresmall, easily identi�ed objects orareas of high pesticide concentra-

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Figure 4. Point and non-point source pollution.

Figure 6. Percolation can transport water soluble pollutants from one body of waterto another.

Figure 5. Water soluble pesticides leach more readily into ground water.

tion such as tanks,containers or spills.Non-point sources arebroad, unde�nedareas in which pesti-cide residues arepresent.

Insecticides

By far, insecticidesare the largest groupof pesticides. Insecti-cides are chemicalsused to kill, repel,alter the growthpatterns, or manipu-

late the behavior of insects, otherarthropods and nematodes.Insecticides include a wide vari-ety of chemical compoundsranging from highly toxic nervepoisons to practically non-toxicpheromones.

Table 4 shows the four mostused insecticides in the UnitedStates. Hundreds of others alsohave very wide use. Other pesti-cides that kill animals are therodenticides for rodents, mollus-cicides for slugs and snails,piscicides for �sh, avicides forbirds, and predacides for preda-tors. These are not as widely usedas insecticides, but some of themhave similar properties.

Insecticides have varying toxic-ity for aquatic organisms. Somecan kill �sh; some disrupt the foodchain by killing aquatic insects andother organisms upon which �shdepend for food. Table 5 shows thecharacteristics of several insecti-cides used in homes, gardens andagriculture. Some are general-useand others are restricted-usepesticides. Two restricted-useinsecticides, aldicarb and oxamyl,have been reported in surface andground water in several states.

Insecticide Leachingand Solubility

Several systemic insecticidesthat are applied to the soil arewater soluble to allow them to betaken up through plant roots.Many of these are highly toxic tomammals. Table 6 shows some ofthe soil-applied systemic insecti-cides, their LD50s and watersolubility. Soluble systemic insec-

ticides such as aldicarb and oxamylare used at heavy rates (more than10 pounds per acre) for nematodecontrol. They persist for weeks,sometimes months, in the soil.Erosion can carry them into sur-face waters where they dissolvereadily. Leaching can drive theminto ground water. The EPA and theU.S. Geological Survey have re-ported their presence in groundwater in several eastern statessince the 1970s. Aldicarb was the�rst pesticide to be regulated bythe EPA in an attempt to protectground water.

Herbicides

Herbicides are among the mostwidely used chemicals in the U.S.They account for more than 70percent of the total volume ofpesticides applied in agriculture.Herbicides generally work byaltering one or more of thefollowing processes: seedling

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Table 5. Common insecticides with their chemical properties and toxicity to �sh (Environmental Protection Agency, 1992).

Solubility in Mobility in Half-life in Relative toxicityInsecticide runo� soil water days to �sh 1

Hydramethylnone high small 10 high

Diazinon medium high 30 high

Aldicarb medium high >30 very high

Oxamyl high high 10 very high

Chlorpyrifos high small 30 very high

Malathion small small 1 very high

Acephate small small 3 very low

Carbaryl medium small 10 medium

Dimethoate small medium 7 medium

Trichlorfon small high 27 high

Dicofol high small 60 high

Propargite high small 56 high1 Fish toxicity based on cat�sh and bluegill. LC50 categories are rated as follows: very low = morethan 100 mg/l, low = 10 to 100 mg/l, medium = 1 to 10 mg/l, high = 0.1 to 1 mg/l, very high = lessthan 0.1 mg/l.

Most insecticides applied toagricultural crops and in urbanareas break down after a giventime. However, some are verypersistent and may remain in theenvironment for a long time.Persistence is a good quality forsome insecticides, because itmakes them e�ective in killingpests for a long time. However,persistent insecticides are moreapt to �nd their way into watersupplies at a level of toxicity thatcan cause problems. These sub-stances can build up in inverte-brates and �sh. They can passthrough the food chain to �sh,birds, mammals, and even humans.

Usage inmillion pounds

active ingredientInsecticide (avg. 1991-1992)

Chlorpyrifos 15.0

Carbaryl 12.5

Malathion 12.5

Terbofos 10.0

Table 4. Approximate volumes of the most widely used insecticides in the United States (U.S. Environmental Protection Agency, 1992.)

growth, transport of water andnutrients, production of plantfoods (photosynthesis), plant celldevelopment, and plant proteinor lipid synthesis. Most herbicidesare not very toxic to mammals.

The range of plants a�ected bya particular herbicide may bebroad or very narrow. Someherbicides are toxic to almost allplants. These chemicals areappropriately named non-selec-tive herbicides. Non-selectiveherbicides are useful for control-ling vegetation along roadsidesand railroad rights-of-way, onparking lots, or around petroleumstorage facilities and electricpower stations. Non-selectiveherbicides also can be used tocontrol weeds when the physicalcharacteristics of the targetweeds are di�erent from those ofdesirable plants nearby.

Many herbicides are designedto kill only certain plants. Theseare called selective herbicides.Most of the herbicides presentlyregistered are selective, and theyare used most widely in agricul-ture.

Selective herbicides may a�ectonly a few weeds or a widevariety of plants. Most selective

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ingredient. Table 7 gives proper-ties of some common herbicides.

Herbicides in Surfaceand Ground Water

Herbicides vary widely. Someare water soluble enough to enterlakes or streams with rainfall orruno� irrigation water, but thehazard they represent depends ontheir persistence and interactionwith the soil. They can also leachinto ground water or move witheroding soil.

Many herbicides designed tobe applied to emerged plants areinactivated once they reach thesoil surface. Soil-applied herbi-cides, however, must be soluble insoil water in order to move intothe root zones of target weeds.Some move deeply into theground to kill deep-rooted peren-nials. Others don’t move as deeplyin order to kill shallow-rootedweeds and spare a deeper rootedcrop.

Among the soil-applied herbi-cides that are taken up by plantroots are the triazines. Several ofthese have been detected in

Table 6. Some systemic insecticides leach into ground water because of their solubility, persistence, soil adsorption, rates of application, orwidespread use. Weather, climate, precipitation and soil characteristics also can in�uence leaching.

Systemic insecticide Toxicity (LD50) Persistence incommon name Solubility (ppm) (rat) in mg/kg the soil Soil adsorption

wolmuidem9.0000,6bracidla

muidemmuidem1005etarohp

muidemmuidem252notoflusid

wolmuidem5.451sofubret

wolmuidem652sohpimanef

wolmuidem482lymaxo

muidemwol000,5elbulosdirpolcadimi

muidemmuidem4153narufobrac

muidemwol744,1000,056etahpeca

herbicides are very broad-spec-trum plant killers. Some killgrasses and broadleaf plants anda few desirable plants. Others killonly broadleaf plants or onlygrasses. Some of the most highlyselective herbicides kill only asingle weed species, and only atone particular point in the plant’sgrowth cycle. The usefulness of aselective herbicide lies not only inwhat it will kill, but also in what itwill leave alive. One very broad-spectrum herbicide, clopyralid, isalmost universally toxic to broad-leaf plants, but does not a�ectseedling sugar beets.

The persistence of some herbi-cides can be looked upon aseither a detriment or advantage.Obviously, the longer thesematerials remain active in the soil,the less appealing they are envi-ronmentally. However, to thefarmer, weed control throughoutthe crop growing season (gener-ally 3 to 6 months) is essential toensure a good quality, pro�tablecrop.

Sometimes the herbicide’sactive ingredient is not as toxic asits inert ingredients. Therefore,the formulation may have moreimpact on the toxicity of theproduct than the active herbicide

Fungicides

Fungicides are used to controlmicroorganisms. We could notfeed this country without modernfungicides to control plant dis-eases. Moreover, toxic plantdisease organisms would makefood far more dangerous thanfungicide residues at the maxi-mum levels prescribed by the EPA.If you want to save your lawn,crops, garden or ornamental treesand shrubs, you must use fungi-cides.

Fungicides are of small concern inprotecting water quality. They areused less frequently than otherpesticides, and most are not persis-tent. However, they can be a pos-sible source of pollution if applied,stored or disposed of improperly.Even when applied correctly, thesesubstances can drift away from theapplication area, leach into groundwater and be carried away byruno�. Table 9 lists some fungicidescommonly used by homeowners andfarmers, and in industry. Fungicidesare seldom found in water, with theexception of some of the heavymetal fungicides that containedmercury. The EPA has cancelled most

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surface and ground waters acrossthe United States at levels nearthe MCL. Triazine herbicides areof particular concern to the EPA.Some triazines are very stable inthe environment and may persistfor long periods in the soil. Thediscovery of two widely -usedtriazines in surface and groundwaters prompted the EPA to starta special review of all triazines in1994. Table 8 shows some triazineherbicides, both those reportedand not reported in U.S. waters.

Many triazines that have foundtheir way into water supplies areimportant herbicides in corn.They are applied at the rate ofseveral pounds per acre on mil-lions of acres of corn. Studiesshow that the half-life of atrazinecan exceed 170 days. Althoughsimazine is not as soluble, it alsohas found its way into the nation’swater supplies. Other, moresoluble, triazines have not beenfound in ground water for reasonsthat include soil adsorption, shortpersistence, use patterns, and thedepth of the ground water wherethey are used (Table 8).

Several other herbicides such asalachlor, diquat, glyphosate,picloram, and 2,4-D also havebeen detected in surface andground water, and the EPA hasassigned MCLs to all of them(Table 3).

Table 7. Characteristics of some commonly used herbicides, with relative toxicity to �sh(U.S. Environmental Protection Agency, 1992).

Solubility in Mobility in Half-life Relative toxicityHerbicide runo� soil water in days to �sh1

wol yrev001hgihhgihAMSM

hgih yrev03wolhgihnifeneB

Dicamba Salt low high 14 low

2,4-D Amine Salt low medium 10 very low

MCPP Amine Salt low high 21 low

Pendimethalin high low 90 high

Glyphosate Amine Salt high low 47 very low

Metribuzin medium large 40 medium1 Fish toxicity based on cat�sh and bluegill. LC50 categories are rated as follows: very low = morethan 100 mg/l, low = 10 to 100 mg/l, medium = 1 to 10 mg/l, high = 0.1 to 1 mg/l, very high = lessthan 0.1 mg/l.

Table 8. The solubility, persistence and soil adsorption characteristics of triazines.

Triazines - Solubility Persistence Soilcommon name (ppm) (half-life) adsorption

metribuzin 1,200 medium medium

promoton 620 high low

hexazinone 330 medium medium

ametryn 185 low medium

atrazine 33 high low

prometryn 33 medium medium

cyanazine 16 low medium

wolhgih5.3enizamis

gants can produce serious chronice�ects at low concentrations,and the EPA has assigned themvery low MCLs (Table 3).

Soil fumigants are usuallyapplied at much higher rates thanother pesticides. Rates of severalhundred pounds per acre arecommon. Most of this use occursin California, Florida and Texas.Residues of dibromochloropro-pane in California’s ground waterin�uenced EPA to cancel thatproduct. Dichloropropene alsohas been detected in groundwater in the San Joaquin Valley ofCalifornia. In October, 1996, theEPA levied heavy �nes against anIdaho company for misapplicationof the fumigant metam-sodiumwhich caused contamination ofthe Snake river.

Water QualityProtection

Most water pollution does notcome from the normal, correctusage of pesticides. Problemsarise from misuse or carelesshandling. Here is a checklist to usewhen applying any pesticide.These guidelines can help safe-guard the future of our waterquality.

11

uses of these fungicides. Whenusing a fungicide, always followinstructions on the label to mini-mize the risk of water pollution.

Soil Fumigants andGround Water

Soil fumigants are gaseouschemicals applied to the soil tocontrol various pests such asplant disease organisms, insectsand weed seeds. They are non-selective, and many are toxic toall life forms. They have variouschemical properties. Fumigantsare very nonpersistent. They lastfrom a few days to a few weeksafter application. With the excep-tion of metam-sodium, most areonly slightly soluble in water.Fumigants can move rapidlythrough the soil-gas interface andcan dissolve in various amounts insoil water. The same factors thata�ect insecticides and herbicidesalso govern the movement of soilfumigants into ground and sur-face waters.

Soil fumigants such as ethylenedibromide, dibromochloropro-pane, metam-sodium anddichloropropane have beendetected in ground and surfacewaters. The chlorinated fumi-

Table 9. Risk factors of some commonly used fungicides.

Fungicide Hazards

)aeruoihtenelyhtE( recnaCbezocnaM

stcefed htrib ,nosiop evreNmarihT

stcefed htriBlymoneB

Thiophanate Mutations, birth defects

Pentachloronitrobenzine Accumulates in food chains, hormone e�ects

Phenyl mercuric acetate Heavy metal poisoning

Fixed Copper Toxic to plants and phytoplankton

nosiop evreNP-nizatiK

Streptomycin Allergic reaction

• Read all product labels andfollow label directions.

• When possible, use pesticidesand fertilizers with less poten-tial for surface runo� or leach-ing.

• Use integrated pest manage-ment (IPM) tactics to controlpests, using pesticides onlywhen necessary.

• Don’t apply pesticides whenconditions are most likely topromote runo� or excessiveleaching.

• Have soil tested to determinethe fertilizer needs of a givencrop.

• Store potential water pollut-ants away from water sourcessuch as wells, ponds andstreams.

• Don’t spray pesticides on awindy day (wind more than 4mph).

• Calibrate all pesticide applica-tion devices to ensure that thecorrect dosage is being ap-plied.

• Prevent pesticide spills andleaks from application equip-ment.

• Make sure product containersdo not leak.

• Do not dispose of leftovermaterials by dumping them indrains or on the ground. Dis-pose of pesticides according tolabel directions.

• Use low-toxicity productswhen a choice is possible.

• Use narrow spectrum productswhen a choice is possible.

• Prevent back flow duringmixing operations by maintain-ing an air gap between thewater �ll hose and the waterlevel in the spray tank

• Always mix, handle and storepesticides down slope fromand at least 150 feet fromwater wells.

• Consider the vulnerability ofthe site; be sure that weatherand irrigation won’t increasethe risk of water pollution.

• Evaluate the location of watersources.

• Leave buffer zones aroundsensitive areas such as wells,irrigation ditches, ponds,

streams, drainage ditches,septic tanks, and other areasthat lead to ground or surfacewater. Don’t apply pesticides inthese locations.

• If you use a spray systemhooked to your hose, use abackup nozzle on your houseconnection to prevent pesti-cides from flowing back intoyour home water system.

• Use up pesticides on your shelfbefore buying more.

• Use up older pesticides beforethey exceed their shelf life.

• Do not water pesticide-treatedareas immediately after appli-cation unless indicated on labelinstruction. Runoff could carrypesticides into storm drainsthat empty into lakes, rivers orstreams.

• Do not use banned or canceledpesticides. Such materialsshould be stored safely until ahazardous waste disposal eventis organized in your community.

Glossary

Adsorption - The adhesion ofmaterials to the surface of a solid.

Bioaccumulation - The storage oraccumulation of materials in thetissues of living organisms.

Broad spectrum - A pesticide thatwill control a wide variety of organ-isms.

Carcinogenic - A property thatmakes a material more likely to causecancer in humans or animals that areexposed to it.

E�cacy range - How many or howfew organisms a pesticide willcontrol.

Ground water - A region withinthe earth that is wholly saturatedwith water.

Inert - A substance that is notreactive in the environment and doesnot contribute to the action of the

active ingredient. Inert materialsoften function as carriers and dilutorsof active ingredients.

Leaching - Dissolving and trans-porting of materials by the action ofpercolating water.

Narrow spectrum - A pesticidethat will control only a few organ-isms.

Non-selective - A pesticide thatwill kill or control both target pestsand desirable organisms.

Non-target - An organism towardswhich an application is not directed.

Persistence - The ability of asubstance to remain in its originalform without breaking down.

Pesticide - A material used to killan unwanted pest.

Selectivity - The ability of apesticide to control target pests but

not desirable or bene�cial crops andorganisms.

Solubility - The ability to be putinto solution.

Target pest - An unwanted speciestoward which a pesticide applicationis directed.

Target weed - An unwanted weedspecies toward which an herbicideapplication is directed.

Toxic - Poisonous to an organismwith which it comes in contact.

Toxin - A substance that is poison-ous to a given organism.

Water pollution - A detrimentalchange in the chemical or physicalproperties of water.

Water table - the upper limit ofthe saturated level of the soil.

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