AGRICULTURAL NONPOINT SOURCE POLLUTION: Watershed Management and Hydrology - Chapter 5 pot

The major losses of pesticides to the environmentare through volatilization into the atmosphere and aerial drift, runoff to surface waterbodies in dissolved and particulate forms, and le

Pesticides and Water

nico-In the mid-1960s, the use of these classes of pesticides declined; they were replaced

by amide and triazine herbicides and carbonate and organophosphate insecticides.Some pesticides have been banned from use mainly because of toxicities In the past

10 years, the use of triazine herbicides and organophosphate and carbamate cide has declined These groups of pesticides have been replaced by other classes ofpesticides that have shorter half-lives and are applied in smaller amounts Some of theolder pesticides such as cyanazine have been banned and the use of others has been

insecti-5

restricted Today there are more than 30 classes of chemicals with pesticidal ties that are registered for weed, insect, and fungal control.1These classes are sum-marized in Table 5.1.

proper-On-farm pesticide use increased from about 182 million kg in the mid-1960s tonearly 386 million kg by 1980 Since the mid-1980s, total pesticide consumption hasincreased only modestly to 411 million kg in 1996.1Atrazine and alachlor are the twomost widely used pesticides.2

Pesticide formulations include emulsifiable concentrates, wettable powders,granules, and flowables Emulsifiable concentrates are the bulwark product for pes-ticide sprays

5.2 FATE AND TRANSPORT PROCESSES

The environmental fates of pesticides applied to cropland are summarized in Figure5.1 Pesticides applied to cropland can be degraded by microbial action and chemi-cal reactions in the soil Pesticides are also immobilized through sorption onto soilorganic matter and clay minerals Pesticides that are taken up by pests or plants eithercan be transformed to degradation products or, in some cases, can accumulate in plant

or animal tissue A certain amount of pesticides applied are also removed when thecrop is harvested Pesticides not degraded, immobilized, or taken up by the crop orinsects are lost to the environment The major losses of pesticides to the environmentare through volatilization into the atmosphere and aerial drift, runoff to surface waterbodies in dissolved and particulate forms, and leaching to groundwater

TABLE 5.1

Classes of pesticides

FIGURE 5.1 Pesticide transport and transformation in the soil-plant environment and the vadose zone. (Reprinted with permission of American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America.)

© 2001 by CRC Press LLC

5.2.1 P ESTICIDE P ROPERTIES

Chemical characteristics of pesticides that influence transport include strength(cationic, anionic basic or acidic), water solubility, vapor pressure, hydrophobic/hydrophilic characters, partition coefficient, and chemical photochemical and bio-logical reactivity Pesticides that dissolve readily in water are considered highly soluble These chemicals have a tendency to be leached through the soil to ground-water and to be lost as surface water runoff from rainfall events or irrigation practices.Pesticide vapor pressures are extremely low in comparison with other organicchemicals such as alcohols or ethers Taylor and Spencer4 cited values ranging overabout six orders of magnitude from 2800 m Pr for EPTC to 0.00074 m Pr for piclo-ram Pesticides with high vapor pressures are easily lost to the atmosphere by voli-talization Some highly volatile pesticides, however, may also move downward intothe groundwater

Pesticides may be sorbed to soil particles, particularly the clays and soil organicmatter The linear and Freundlich isotherm equations have been most often used todescribe pesticide adsorption on soils These equations are given by

and

C5  kf CLN (N  1) (5.2)where kdand kfare the sorption coefficients, C is the sorbed-phase concentration(g/g), CL is the total solute concentration (mg/L), and N is an empirical constant.Green and Karickoff and Koskinen and Harper discuss the pesticide sorption process

in detail Sorption coefficient data has been published for many pesticides.7,8 Thevalue of kd or kf is a measure of the extent of pesticide sorption by the soil The soilorganic C (OC) content is the single best predictor of the sorption coefficient formonionic hydrophobic pesticides When the pesticide sorption coefficient is normal-ized with respect to soil OC, it is essentially independent of soil type This has led tothe OC-normalized sorption coefficient, Koc as

koc  (k%d o

O

rC

kp)

  100 (5.3)Pesticides may be degraded by chemical and biological processes Chemical degra-dation processes include photolysis (photochemical degradation), hydrolysis, oxida-tion, and reduction The degradation of pesticides through microbial metabolicprocesses is considered to be the primary mechanism of biological degradation.9Rao and Hornsby8 have summarized pesticide sorption coefficients and half-lives (Table 5.2) They classify pesticides as nonpersistent if they have half-lives of

30 days or less, moderately persistent if they have half-lives longer than 30 days but less than 100 days, and persistent if their half-lives are more than 100 days.Published half-lives are generally based upon laboratory data; it is difficult to predictthe half-life of a chemical in the field because of dependent variables such as soil

TABLE 5.2

Sorption Coefficients and Half-Lives of Pesticides Used In Florida

(common name) (ml/g of organic chemical)

temperature, moisture, microbial populations, and soil type Pesticides most likely to

contaminate groundwater are those with low sorption coefficients, long half-lives,

and a high water solubility.10

5.2.2 S OIL P ROPERTIES

Soil properties have significant influences on the fate and transport on pesticides Soil

organic matter is the most important soil property in the sorption process

of most pesticides Fine-textured soils have a higher sorptive capacity than

coarse-TABLE 5.2 (continued)

(common name) (ml/g of organic chemical)

textured soils because of the high clay content Soil water has an important role in theretention of pesticides by soil in that it is both a solvent for the pesticide and a solutethat can compete for adsorption sites It also plays a direct role in many of the adsorp-tion mechanisms such as water bridging and liquid exchange.

Infiltration rate and hydraulic conductivity influence pesticide transport Soilswith higher infiltration rates will generally have lower surface runoff rates, so a pes-ticide that readily infiltrates into the soil is more likely to be leached to ground-water than lost in surface runoff Soil water will also move through soils morerapidly with greater hydraulic conductivity rates, so pesticides will be leached to thegroundwater more rapidly and have less time to degrade In general, coarse-texturedsoils have greater infiltration rates and hydraulic conductivity rates than fine-textured soils

Soil pH is an important property for those pesticides degrading by hydrolysis.The hydrolysis or dehalogenation of DBCP occurs in the soil at a faster rate underalkaline conditions

Soil structure, which reflects the manner in which soil particles are aggregatedand cemented, influences erosion and infiltration rates A soil with a weak structurewill likely be eroded and have lower infiltration rates, which will result in sorbed pes-ticides being lost in runoff Macropores and cracks can have a major effect on pesti-cide transport Under particular water application rate conditions, pesticides willmove through the macropores and cracks and reach the water table in a shorter period

of time

5.2.3 S ITE C ONDITIONS

A shallow depth of groundwater offers less opportunity for pesticide sorption anddegradation If the groundwater is shallow, the soil is permeable and rainfall exceedsthe water-holding capacity of the soil; the travel time of the pesticide to reach thewater table may be from a few days to a week

Hydrogeologic conditions may dictate both the direction and rate of chemicalmovement The presence of impermeable lenses in the soil profile may limit the ver-tical movement of pesticides but could contribute to the lateral flow of groundwaterand the eventual discharge of groundwaters and pesticides into surface waters Thepresence of karsts and fractured geologic materials generally allow for rapid trans-port of water and chemicals to the groundwater

Climatic and weather conditions other than rainfall also affect the fate of cides Higher temperatures tend to accelerate degradation High winds and high evaporation rates may accelerate volatilization and other processes that contribute togaseous losses of pesticides

pesti-The slope will influence runoff and erosion rates Increasing slope may increaserunoff rate, soil detachment, and transport and increase effective depth for chemicalextraction

Soil crusting and compaction decrease infiltration rates and reduces time to off, resulting in increasing the initial concentration of soluble pesticides in runoff

run-5.3 GROUNDWATER IMPACTS

5.3.1 M ONITORING S TUDIES

Numerous state, local, and multistate investigations have been carried out Parsonsand Witt11summarized data on the occurrence of pesticides in groundwater in 35states A more comprehensive database on pesticides in groundwater is the Pesticides

in Groundwater Database (PGDB) compiled by the U S Environmental ProtectionAgency (EPA), which contains data from 45 states and 68,824 wells from 1971 to

1991.12The only study that has measured pesticides in groundwater in all 50 states isthe EPA National Pesticide Survey (NPS).13Other multistate studies include the MidContinent Pesticide Study (MCPS)14 by the U.S Geological Survey (USGS),Cooperative Private Well Testing Program15 (PGWDB), National Alachlor WellWater Survey,16 Metolachlor Monitoring Study,17 and the USGS National WaterQuality Assessment Program (NWQAP).18

Statewide monitoring surveys that have been conducted include Kansas, Iowa,Ohio, New York, Wisconsin, Massachusetts, Minnesota, Nebraska, Illinois,Louisiana, Indiana, Oregon, Arizona, and Connecticut.19All statewide and multistatesurveys sampled existing community or domestic wells The most extensive moni-toring of groundwater has been carried out in California, Florida, New York, most ofthe states in New England, the central Atlantic Coastal Plain, and the central andnorthern midcontinent The types of pesticides analyzed have been largely deter-mined by the extent of use or concern at the time of sampling Most site-specific stud-ies that involve the application of one or more pesticides under controlled conditionsare usually analyzed only for the pesticides applied and perhaps some of their trans-formation products The principal objective of most monitoring studies, on the otherhand, is to determine which pesticides are present in groundwater in the areas ofinterest, thereby requiring a broad spectrum of pesticides to be analyzed With theincrease in the use of triazine and acetanilide herbicides over the past three decades,more recent studies have increased the attention devoted to them Ongoing concernover pesticides whose use had been discontinued, but that still persist in groundwaterwhere former use was heavy, is reflected in the considerable number of recent stud-ies of the long-term subsurface fate of the fumigants DBCP and 1,2-dibromoethane(EDB)

The MCPS study conducted in 12 states involved preplanting sampling in 1991and postplanting sampling in July and August in 1991 and 1992 In total, 55% ofcompounds and eight degradation products were analyzed in 1992 Sixty-two percent

of the wells sampled had detectable amounts of parent compound pesticides or theirbreakdown products in 1992 In 1991, only 11 pesticides were analyzed and 27.8%

of the wells had detectable amounts of pesticides In 1991, none of the pesticide centrations were above the maximum contaminant level (MCL), whereas in 1992,0.1% of the samples had concentrations above the MCL Atrazine dominated theMCPS herbicide detections with 43% of the samples having atrazine concentrationsabove the detection limit of 0.005 µg/L in 1992 Simazine and metolachlor were alsodetected in more than 10% of the samples in 1992 along with the alachlor transfor-mation products ethanesulfonic acid and 2-6-diethylaniline Atrazine detections were

con-generally more frequent in areas with heavier atrazine use, except in much of Ohioand Indiana, where atrazine was detected infrequently.

In the NPS program, atrazine and cyanazine were the most frequently detectedpesticides.13Atrazine was also detected in 11.7% of the samples of the NationalAlachlor Well Water Survey; alachlor was detected in only 0.78%.16

The USGS NAWQA study was derived from 2227 wells and springs in 20 majorhydrologic basins across the U.S from 1993 to 1995 In total, 55 pesticides were ana-lyzed, but the major emphasis was on the herbicides atrazine, cyanazine, simazine,alachlor, metolachlor, prometon, and acetochlor All of these herbicides except ace-tochlor were detected in shallow groundwater (groundwater recharged within the past

10 years) in a variety of agricultural and nonagricultural areas, as well as in severalaquifers that are sources of drinking water supply.18

Acetochlor was detected at two of 953 sites in the NAWQA study and in shallowgroundwater in a statewide USGS study in Iowa in 1995 and 1996 Because ace-tochlor was first registered for use in 1994, the results are in agreement with thosefrom previous field studies in that some pesticides may be detected in the shallowgroundwater within 1 year following their application More than 98% of pesticidedetections in the NAWQA study were at concentrations of less than 1.0 µg/L.Frequencies of detection at or above 0.01 µg/L in shallow groundwater beneath agri-cultural areas were significantly correlated at the 0.05 level with agricultural use foratrazine, cyanazine, alachlor, and metolachlor, but not simazine

Barbash and Resik19found no significant correlation between total pesticide useper unit area and the overall pesticide detection frequencies in states with data from

100 or more wells in the PGWDB Of the herbicide classes examined in the PGWDB,the numbers of triazines and acetamilides detected in individual states appear to showthe closest relations with use In contrast, less of a geographic correspondencebetween occurrence and use is apparent for the chlorophenoxy acid, urea, and mis-cellaneous herbicides The most frequently detected herbicides were atrazine,cyanazine, simazine, propazine, metribuzen, alachlor, metolachlor, propachlor, triflu-ralin, dicamba, DCPA, and 2-4-D The most frequently detected insecticides werealdicarb and its degradates and carbofuran, whereas the most widely detected fumi-gants were 1,2-dibromo-3-chloropropance (DBCP), 1,2-dibromoethane (EDB) and1,2-dichloropropane Because of the health risks associated with the presence of thesethree fumigants in groundwater, their agricultural use has been cancelled in the U.S

In a number of state studies, direct relations between the frequency of pesticidedetection and pesticide use have been reported Kross et al.20reported lower frequen-cies of atrazine detection in wells located on Iowa farms where herbicides had notbeen applied during the recent growing season, compared with farms where they hadbeen applied LeMasters and Doyle21 also reported a direct relationship betweenatrazine use and occurrence in groundwater beneath various areas on Wisconsingrade A dairy farms across the state Koterba et al.,22in a study of the groundwaterbeneath the Delmarva Peninsula, found that the pesticides detected in wells locatednear areas planted in corn, soybeans, or small grains were (with one exception) com-pounds that were commonly applied to those crops in that region The single excep-tion was hexazinone, an herbicide used to control brush and weeds in noncrop areas

Wade et al sampled 97 wells in the surficial aquifer in areas that were morevulnerable to contamination in North Carolina Twenty-three pesticides or pesticidedegradates were detected in 26 of the 97 wells Nine of the pesticides or degradateswere no longer registered for use; dibromochloropropane and methylene chloride hadconcentrations above the state groundwater quality standards They also found thatareas with a high soil leaching potential index based on the pesticide DRASTICmodel were no more likely to have pesticides detected in groundwater than areas withlow soil-leaching potential index value.

5.3.2 W ATERSHED AND F IELD -S CALE S TUDIES

Atrazine and some of the other triazine herbicides have also been detected frequently

in groundwater in many plot and watershed studies Hallberg24reported that in theBig Springs watershed, the flow-weighted mean atrazine concentrations for ground-water discharge increased steadily from 1981 to 1985 Maximum concentrations ofatrazine in the groundwater from 1981 to 1985 ranged from 2.5 to 10.0 µg/L.Atrazine has also been found in the groundwater in Delaware.25Atrazine wasdetected in the groundwater in the Appoquinimink watershed in New Castle County

in 11 of 23 monitoring wells in a Matapeake silt loam soil at depths of 6–9 m.Concentrations ranged from 1 to 45 µg/L

Hallberg24 also found cyanazine and alachlor in the groundwater in the BigSprings watershed Maximum concentrations from 1981 to 1985 ranged from 0.5 to4.6 µg/L,24

and alachlor concentrations as high as 16.6 µg/L were measured

Pionke et al.26detected atrazine, simazine, and cyanazine in groundwater in anagricultural watershed in Pennsylvania; the soils on the watershed ranged fromcoarse to fine textured Atrazine was detected in 14 of 20 wells ranging in concentra-tion from 0.013 to 1.1 µg/L Simazine was detected in 35% of the wells at concen-trations ranging from 01 to 1.7 µg/L and cyanazine was detected only in one well(0.09 µg/L)

Brinsfield et al.27studied pesticide leaching on no-till and conventional tillagewatersheds on a silt loam Coastal Plain soil in Maryland Over a 3-year period,atrazine was detected in the groundwater more frequently than simazine, cyanazine,

or metolachlor Pesticides were detected more frequently in the groundwater on theno-till watershed than on the conventional tillage watershed

Dillaha et al.28found atrazine had the highest mean concentration of 20 cides detected in the groundwater on an agriculture watershed with a Rumford loamysand soil in Virginia The average concentration of 129 samples was 0.46 µg/L withconcentrations ranging from 0 to 25.6 µg/L

pesti-Isensee et al.29found atrazine in nearly all of their monitoring wells for a 3-yearperiod in both conventional tillage and no-till plots The wells were from 1.5 to 3.0

m deep Atrazine concentrations ranged from 0.005 to 2.0 µg/L Alachlor wasdetected in fewer than 5% of the wells

In 1990, the Management Systems Evaluation Areas (MSEA) Program was initiated in eight states in the Midwest by USDA30to study the impact of prevailing

and modified farming systems on groundwater and surface water quality Manyreports have been published on the results In the Walnut Creek watershed in Iowa,annual atrazine losses in tile drainage water ranged from 0.02 to 2.16 g/ha in a cornand soybean rotation during the 4-year study.31Fewer than 3% of the groundwatersamples contained atrazine concentration exceeding 3 µg/L Metribrizan, which wasapplied to soybeans, was also found in groundwater, but only half as frequently asatrazine.

A number of researchers have found pesticides can move rapidly to the water by macropore flow Steenhuis et al.32 found atrazine in the groundwater 1month after it was applied in conservation tillage but did not detect any atrazine inthe groundwater in conventional tillage until late fall They concluded atrazinemoved to the groundwater under conservation tillage by macropores that were con-nected to the surface, but under conventional tillage most of the atrazine wasadsorbed in the root zone

ground-Ritter et al.33studied the movement of alachlor, atrazine, simazine, cyanazine,and metolachlor on an Evesboro loamy sand soil that had a water table near the sur-face Over a period of 9 years in four different experiments, they found these pesti-cides may move to shallow groundwater by macropore flow if more than 30 mm ofrainfall occurs shortly after they are applied They found no large difference in pes-ticide transport between conventional tillage and no-tillage

Gish et al.34found that average field-scale solute phase atrazine concentrations

at 1 m resulting from 48 mm of rainfall 12 h after application on a loam soil were 243µg/L for no-tillage and 59 µg/L for conventional tillage Cyanazine concentrationswere 184µg/L for no-tillage and 69 µg/L on conventional tillage They concludedthese high concentrations were a result of preferential flow

5.3.3 M ANAGEMENT E FFECTS

Management practices such as tillage and method of application can influence theamount of pesticide leached to groundwater The attempts by researchers to discernthe influence of tillage practices on pesticide movement to groundwater are beset by

a number of complicating factors First, the effects of tillage on infiltration capacityare seasonal Conventional tillage leads to transient increases in soil permeabilityrelative to an untilled soil Over the course of an entire growing season, however,long-term infiltration rates tend to be higher under reduced tillage than under con-ventional tillage.35Second, both the placement of pesticides during application andthe magnitude of individual recharge events may influence the effect of tillage onpesticide transport

The results of the effect of tillage practices on pesticide concentrations in thesubsurface have not always been consistent among different investigations In gen-eral, reduced tillage gives rise to pesticide distributions in the subsurface that aremarkedly different from those observed under conventional tillage Although pesti-cide concentrations are typically higher in surficial soils under conventional tillagethan under reduced tillage, the reverse is often observed at greater depths in the soil