Herbicides in ground water beneath Nebraska's Management Systems Evaluation Area

Profiles of ground water pesticide concentrations beneath the Nebraska Management Systems Evaluation Area (MSEA) describe the effect of 20 yr of pesticide usage on ground water in the central Platte Valley of Nebraska. During the 6-yr (1991-1996) study, 14 pesticides and their transformation products were detected in 7848 ground water samples from the unconfined water table aquifer. Triazine and acetamide herbicides applied on the site and their transformation products had the highest frequencies of detection. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4,-diamine] concentrations decreased with depth and ground water age determined with 3H/3He dating techniques. Assuming equivalent atrazine input during the past 20 yr, the measured average changes in concentration with depth (age) suggest an estimated half-life of >10 yr. Hydrolysis of atrazine and deethylatrazine (DEA; 2-chloro-4-amino-6-isopropylamino-s-triazine) to hydroxyatrazine [6-hydroxy-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] appeared to be the major degradation route. Aqueous hydroxyatrazine concentrations are governed by sorption on the saturated sediments. Atrazine was detected in the confined Ogallala aquifer in ultra-trace concentrations (0.003 microg L(-1)); however, the possibility of introduction during reverse circulation drilling of these deep wells cannot be eliminated. In fall 1997 sampling, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was detected in 57% of the 230 samples. Metolachlor oxanilic acid [(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl) amino]oxo-acetic acid] was detected in most samples. In ground water profiles, concentrations of metolachlor ethane sulfonic acid [2-[(ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxo-ethanesulfonic acid] exceeded those of deethylatrazine. Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] was detected in <1% of the samples; however, alachlor ethane sulfonic acid [2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2-oxoethanesulfonic acid] was present in most samples (63%) and was an indicator of past alachlor use.     

Stream transport of herbicides and metabolites in a tile-drained, agricultural watershed

The occurrence of metabolites of many commonly used herbicides in streams has not been studied extensively in tile-drained watersheds. We collected water samples throughout the Upper Embarras River watershed [92% corn, Zea mays L., and soybean, Glycine max (L.) Merr.] in east-central Illinois from March 1999 through September 2000 to study the occurrence of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), metolachlor 12-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) acetamide], and their metabolites. River water samples were collected from three subwatersheds of varying tile density (2.8-5.3 km tile km(-2)) and from the outlet (United States Geological Survey [USGS] gage site). Near-record-low totals for stream flow occurred during the study, and nearly all flow was from tiles. Concentrations of atrazine at the USGS gage site peaked at 15 and 17 microg L(-1) in 1999 and 2000, respectively, and metolachlor at 2.7 and 3.2 microg L(-1); this was during the first significant flow event following herbicide applications. Metabolites of the chloroacetanilide herbicides were detected more often than the parent compounds (evaluated during May to July each year, when tiles were flowing), with metolachlor ethanesulfonic acid [2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid] detected most often (> 90% from all sites), and metolachlor oxanilic acid [2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoacetic acid] second (40-100% of samples at the four sites). When summed, the median concentration of the three chloroacetanilide parent compounds (acetochlor, alachlor, and metolachlor) at the USGS gage site was 3.4 microg L(-1), whereas it was 4.3 microg L(-1) for the six metabolites. These data confirm the importance of studying chloroacetanilide metabolites, along with parent compounds, in tile-drained watersheds.     

The acetochlor registration partnership state ground water monitoring program

The Acetochlor Registration Partnership (ARP) conducted a 7-yr ground water monitoring program at a total of 175 sites in seven states: Illinois, Indiana, Iowa, Kansas, Minnesota, Nebraska, and Wisconsin. While acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)-acetamide] was the primary focus, the analytical methods also quantified alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-acetamide], atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-acetamide], and two classes of soil degradates for acetochlor, alachlor, and metolachlor. Ground water samples were collected monthly for five years and quarterly for two additional years. All samples were analyzed for the presence of parent herbicides, and degradates were monitored during the last three years. Parent acetochlor was detected above 0.1 microg L(-1) in three or more samples at just seven sites. Alachlor and metolachlor were also rarely detected, but atrazine was detected in 36% of all samples analyzed. Even more widespread were the tertiary amide sulfonic acid (ethanesulfonic acid, ESA) degradates of acetochlor, alachlor, and metolachlor, which were detected at 81, 76, and 106 sites, respectively. The other class of monitored soil degradates (oxanilic acid, OXA) was detected less frequently, at 26, 16, and 63 sites for acetochlor OXA, alachlor OXA, and metolachlor OXA, respectively. The geographic distribution of detections did not follow the pattern originally expected when the study began. Rather than being a function primarily of soil texture, the detection of these herbicides in shallow ground water was related to site-specific factors associated with local topography, the occurrence of surface water drainage features, irrigation practices, and the vertical positioning of the well screen.     

The acetochlor registration partnership surface water monitoring program for four corn herbicides

A surface drinking water monitoring program for four corn (Zea mays L.) herbicides was conducted during 1995-2001. Stratified random sampling was used to select 175 community water systems (CWSs) within a 12-state area, with an emphasis on the most vulnerable sites, based on corn intensity and watershed size. Finished drinking water was monitored at all sites, and raw water was monitored at many sites using activated carbon, which was shown capable of removing herbicides and their degradates from drinking water. Samples were collected biweekly from mid-March through the end of August, and twice during the off-season. The analytical method had a detection limit of 0.05 microg L(-1) for alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-acetamide] and 0.03 microg L(-1) for acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)-acetamide], atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-acetamide]. Of the 16528 drinking water samples analyzed, acetochlor, alachlor, atrazine, and metolachlor were detected in 19, 7, 87, and 53% of the samples, respectively. During 1999-2001, samples were also analyzed for the presence of six major degradates of the chloroacetanilide herbicides, which were detected more frequently than their parent compounds, despite having higher detection limits of 0.1 to 0.2 microg L(-1). Overall detection frequencies were correlated with product use and environmental fate characteristics. Reservoirs were particularly vulnerable to atrazine, which exceeded its 3 microg L(-1) maximum contaminant level at 25 such sites during 1995-1999. Acetochlor annualized mean concentrations (AMCs) did not exceed its mitigation trigger (2 microg L(-1)) at any site, and comparisons of observed levels with standard measures of human and ecological hazards indicate that it poses no significant risk to human health or the environment.     

Fate of pesticides in tropical soils of Brazil under field conditions

The potential of pesticides for nonpoint ground water pollution depends on their dissipation and leaching behavior in soils. We investigated the fate of 10 pesticides in two tropical soils of contrasting texture in the Brazilian Cerrado region near Cuiabá during an 80-d period, employing topsoil dissipation studies, soil core analyses, and lysimeter experiments. Dissipation of pesticides was rapid, with field half-lives ranging from 0.8 to 20 d in Ustox and 0.6 to 11.8 d in Psamments soils. Soil core analyses showed progressive leaching of polar pesticides in Psamments, whereas in Ustox pesticides were rapidly transported to 40 cm soil depth regardless of their sorption properties, suggesting that leaching was caused by preferential flow. In lysimeter experiments (35 cm soil depth), cumulative leaching was generally low, with < or = 0.02% and < or = 0.19% of the applied amounts leached in Ustox and Psamments, respectively. In both soils, all pesticides but the pyrethroids were detected in percolate at 35 cm soil depth within the first 6 d after application. Cumulative efflux and mean concentrations of pesticides in percolate were dosely correlated with their Groundwater Ubiquity Score (GUS). The presence of alachlor (2-chloro-2', 6'-diethyl-N-methoxymethylacetanilide), atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], simazine [2-chloro-4,6-bis(ethylamino)-1,3,5-triazine], and trifluralin (2,6-dinitro-N,N-dipropyl-4-trifluoromethylaniline) throughout the soil profile and in percolate of wick lysimeters at 95 cm soil depth indicated that a nonpoint pollution of ground water resources in tropical Brazil cannot be ruled out for these substances.     

Occurrence and fate of pesticides in four contrasting agricultural settings in the United States

Occurrence and fate of 45 pesticides and 40 pesticide degradates were investigated in four contrasting agricultural settings--in Maryland, Nebraska, California, and Washington. Primary crops included corn at all sites, soybeans in Maryland, orchards in California and Washington, and vineyards in Washington. Pesticides and pesticide degradates detected in water samples from all four areas were predominantly from two classes of herbicides--triazines and chloroacetanilides; insecticides and fungicides were not present in the shallow ground water. In most samples, pesticide degradates greatly exceeded the concentrations of parent pesticide. In samples from Nebraska, the parent pesticide atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] was about the same concentration as the degradate, but in samples from Maryland and California atrazine concentrations were substantially smaller than its degradate. Simazine [6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine], the second most detected triazine, was detected in ground water from Maryland, California, and Washington. Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] rarely was detected without its degradates, and when they were detected in the same sample metolachlor always had smaller concentrations. The Root-Zone Water-Quality Model was used to examine the occurrence and fate of metolachlor at the Maryland site. Simulations accurately predicted which metolachlor degradate would be predominant in the unsaturated zone. In analyses of relations among redox indicators and pesticide variance, apparent age, concentrations of dissolved oxygen, and excess nitrogen gas (from denitrification) were important indicators of the presence and concentration of pesticides in these ground water systems.     

ATRAZINE AND METOLACHLOR OCCURRENCE IN SHALLOW GROUND WATER OF THE UNITED STATES, 1993 TO 1995: RELATIONS TO EXPLANATORY FACTORS1

ABSTRACT: Since 1991, the U.S. Geological Survey has been conducting the National Water Quality Assessment (NAWQA) Program to determine the quality of the Nation's water resources. In an effort to obtain a better understanding of why pesticides are found in shallow ground water on a national scale, a set of factors likely to affect the fate and transport of two herbicides in the subsurface were examined. Atrazine and metolachlor were selected for this discussion because they were among the most frequently detected pesticides in ground water during the first phase of the NAWQA Program (1993 to 1995), and each was the most frequently detected compound in its chemical class (triazines and acetanilides, respectively). The factors that most strongly correlated with the frequencies of atrazine detection in shallow ground‐water networks were those that provided either: (1) an indication of the potential susceptibility of ground water to atrazine contamination, or (2) an indication of relative ground‐water age. The factors most closely related to the frequencies of metolachlor detection in ground water, however, were those that estimated or indicated the intensity of the agricultural use of metolachlor. This difference is probably the result of detailed use estimates for these compounds being available only for agricultural settings. While atrazine use is relatively extensive in nonagricultural settings, in addition to its widespread agricultural use, metolachlor is used almost exclusively for agricultural purposes. As a result, estimates of agricultural applications provide a less reliable indication of total chemical use for atrazine than for metolachlor. A multivariate analysis demonstrated that the factors of interest explained about 50 percent of the variance in atrazine and metolachlor detection frequencies among the NAWQA land‐use studies examined. The inclusion of other factors related to pesticide fate and transport in ground water, or improvements in the quality and accuracy of the data employed for the factors examined, may help explain more of the remaining variance in the frequencies of atrazine and metolachlor detection.     

Runoff and drainage losses of atrazine, metribuzin, and metolachlor in three water management systems

Rainfall can transport herbicides from agricultural land to surface waters, where they become an environmental concern. Tile drainage can benefit crop production by removing excess soil water but tile drainage may also aggravate herbicide and nutrient movement into surface waters. Water management of tile drains after planting may reduce tile drainage and thereby reduce herbicide losses to surface water. To test this hypothesis we calculated the loss of three herbicides from a field with three water management systems: free drainage (D), controlled drainage (CD), and controlled drainage with subsurface irrigation (CDS). The effect of water management systems on the dissipation of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] in soil was also monitored. Less herbicide was lost by surface runoff from the D and CD treatments than from CDS. The CDS treatment increased surface runoff, which transported more herbicide than that from D or CD treatments. In one year, the time for metribuzin residue to dissipate to half its initial value was shorter for CDS (33 d) than for D (43 d) and CD (46 d). The half-life of atrazine and metolachlor were not affected by water management. Controlled drainage with subsurface irrigation may increase herbicide loss through increased surface runoff when excessive rain is received soon after herbicide application. However, increasing soil water content in CDS may decrease herbicide persistence, resulting in less residual herbicide available for aqueous transport.     

Anaerobic degradation of atrazine and metolachlor and metabolite formation in wetland soil and water microcosms

The half-lives, degradation rates, and metabolite formation patterns of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] were determined in an anaerobic wetland soil incubated at 24 degrees C for 112 d. At 0, 7, 14, 28, 42, 56, and 112 d, the soil and water were analyzed for atrazine and metolachlor, and their major metabolites. The soil oxidation-reduction potential reached -200 mV after 14 d. Degradation reaction rates were first-order for atrazine in anaerobic soil and for metolachlor in the aqueous phase. Zero-order reaction rates were best fit for atrazine in the aqueous phase and metolachlor in anaerobic soil. In anaerobic soil, the half-life was 38 d for atrazine and 62 d for metolachlor. In the aqueous phase above the soil, the half-life was 86 d for atrazine and 40 d for metolachlor. Metabolites detected in the anaerobic soil were hydroxyatrazine and deethylatrazine for atrazine, and relatively small amounts of ethanesulfonic acid and oxanilic acid for metolachlor. Metabolites detected in the aqueous phase above the soil were hydroxyatrazine, deethylatrazine, and deisopropylatrazine for atrazine, and ethanesulfonic acid and oxanilic acid for metolachlor. Concentrations of metabolites in the aqueous phase generally peaked within the first 25 d and then declined. Results indicate that atrazine and metolachlor can degrade under strongly reducing conditions found in wetland soils. Metolachlor metabolites, ethanesulfonic acid, and oxanilic acid are not significantly formed under anaerobic conditions.     

Pesticide removal from container nursery runoff in constructed wetland cells

The increased use of pesticides by container nurseries demands that practices for removal of these potential contaminants from runoff water be examined. Constructed wetlands may be designed to clean runoff water from agricultural production sites, including container nurseries. This study evaluated 14 constructed wetlands cells (1.2 by 4.9 m or 2.4 by 4.9 m, and 30 or 45 cm deep) that collected pesticide runoff from a 465-m2 gravel bed containerized nursery in Baxter, TN. One-half of the cells were vegetated with bulrush, Scirpus validus. The cells were loaded at three rates or flows of 0.240, 0.120, and 0.060 m3 d(-1). Herbicides-simazine (Princep) [2-chloro-4,6-bis(ethylamino)-s-triazine] and metolachlor (Pennant) [2-chloro-N-(2-ethyl-6-methylphenyl)-N-2-methoxy-1-methylethyl-acetamide] -were applied to the gravel portion of the container nursery at rates of 4.78 and 239 kg ha(-1), respectively, 9 July 1998, and at rates of 2.39 and 1.19 kg ha(-1), respectively, 17 May 1999. Pesticides entering the wetland and wetland cell water samples were analyzed daily to determine pesticide removal. At the slower flow rate, which corresponds to lower mass loading and greater hydraulic retention times (HRTs), a greater percentage of pesticides was removed. During the 2-yr period, cells with plants removed 82.4% metolachlor and 77.1% simazine compared with cells without plants, which removed 63.2% metolachlor and 64.3% simazine. At the lowest flow rate and mass loading, wetland cells removed 90.2% metolachlor and 83% simazine. Gravel subsurface flow constructed wetlands removed most of the pesticides in runoff water with the greatest removal occurring at lower flow rates in vegetated cells.     

Tillage, intercrop, and controlled drainage-subirrigation influence atrazine, metribuzin, and metolachlor loss

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] have been found with increasing occurrence in rivers and streams. Their continued use will require changes in agricultural practices. We compared water quality from four crop-tillage treatments: (i) conventional moldboard plow (MB), (ii) MB with ryegrass (Lolium multiflorum Lam.) intercrop (IC), (iii) soil saver (SS), and (iv) SS + IC; and two drainage control treatments, drained (D) and controlled drainage-subirrigation (CDS). Atrazine (1.1 kg a.i. ha-1), metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one] (0.5 kg a.i. ha-1), and metolachlor (1.68 kg a.i. ha-1) were applied preemergence in a band over seeded corn (Zea mays L.) rows. Herbicide concentration and losses were monitored from 1992 to spring 1995. Annual herbicide losses ranged from < 0.3 to 2.7% of application. Crop-tillage treatment influenced herbicide loss in 1992 but not in 1993 or 1994, whereas CDS affected partitioning of losses in most years. In 1992, SS + IC reduced herbicide loss in tile drains and surface runoff by 46 to 49% compared with MB. The intercrop reduced surface runoff, which reduced herbicide transport. Controlled drainage-subirrigation increased herbicide loss in surface runoff but decreased loss through tile drainage so that total herbicide loss did not differ between drainage treatments. Desethyl atrazine [6-chloro-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine] comprised 7 to 39% of the total triazine loss.     

Preferential bromide and pesticide movement to tile drains under different cropping practices

Subsurface drainage systems are useful tools to study chemical leaching in soils. Our objective was to compare the breakthrough behavior of bromide, atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamid] to tile drains under two fall tillage practices (conventional tillage [CT] with a moldboard plow, and reduced tillage [RT] with a chisel plow) in field plots cultivated with corn (Zea mays L.). Leachate volume were greater in RT than in CT, with no statistical differences. Soil analysis showed that bromide migrated deeper in the soil profile than both herbicides, with little tillage effect. All chemicals were detected in drainage water at the same time and followed an event-driven behavior. Tillage had no effect on atrazine and metolachlor found in drainage water, while bromide concentration peaks were higher in RT than in CT in 1999. Concentration peaks were recorded earlier for atrazine and metolachlor than for bromide. Plots of cumulative relative chemical mass (cumulative mass divided by total mass measured in drainage) as a function of cumulative drainage were mostly linear for bromide, while they were S-shaped for both herbicides. Drainage that corresponded to 50% of relative cumulative mass ranged from 40 to 55% for bromide and from 5 to 28% for both herbicides. Rapid chemical movement to tile drains suggested that preferential flow was important in both CT and RT, and that these tillage practices had little influence on this phenomena.     

Herbicide loading to shallow ground water beneath Nebraska's Management Systems Evaluation Area

Better management practices can counter deterioration of ground water quality. From 1991 through 1996 the influence of improved irrigation practices on ground water pesticide contamination was assessed at the Nebraska Management Systems Evaluation Area. Three 13.4-ha corn (Zea mays L.) fields were studied: a conventional furrow-irrigated field, a surge-irrigated field and a center pivot-irrigated field, and a center pivot-irrigated alfalfa (Medicago sativa L.) field. The corn fields received one identical banded application of Bicep (atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4,-diamine] + metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamidel) annually; the alfalfa field was untreated. Ground water samples were collected three times annually from 16 depths of 31 multilevel samplers. Six years of sample data indicated that a greater than 50% reduction in irrigation water on the corn management fields lowered average atrazine concentrations in the upper 1.5 m of the aquifer downgradient of the corn fields from approximately 5.5 to <0.5 microg L(-1). Increases in deethylatrazine (DEA; 2-chloro-4-amino-6-isopropylamino-s-triazine) to atrazine molar ratios indicated that reducing water applications enhanced microbial degradation of atrazine in soil zones. The occurrence of peak herbicide loading in ground water was unpredictable but usually was associated with heavy precipitation within days of herbicide application. Focused recharge of storm runoff that ponded in the surge-irrigated field drainage ditch, in the upgradient road ditch, and at the downgradient end of the conventionally irrigated field was a major mechanism for vertical transport. Sprinkler irrigation technology limited areas for focused recharge and promoted significantly more soil microbial degradation of atrazine than furrow irrigation techniques and, thereby, improved ground water quality.     

Adsorption and desorption of metolachlor and metolachlor metabolites in vegetated filter strip and cultivated soil

Previous studies have indicated that dissolved-phase metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide] transported in surface runoff is retained by vegetative filter strips to a greater degree than either metolachlor oxanilic acid 12-[(2-ethyl-6-methylphenyl) (2-methoxy-1-methylethyl)amino]-2-oxo-acetic acid] (OA) or metolachlor ethanesulfonic acid [2-[(2-ethyl-6-methylphenyl) (2-methoxy-1-methylethyl-1)amino]-2-oxoethanesul-fonic acid] (ESA), two primary metabolites of metolachlor. Adsorption-desorption of ESA and OA in vegetated filter strip soil (VFSS) has not been evaluated, yet these data are required to assess the mobility of these compounds in VFSS. The objective of this experiment was to compare metolachlor, ESA, and OA adsorption and desorption parameters between VFSS and cultivated soil (CS). Adsorption and desorption isotherms were determined using the batch equilibrium procedure. With the exception of a 1.7-fold increase in organic carbon content in the VFSS, the evaluated chemical and physical properties of the soils were similar. Sorption coefficients for metolachlor were 88% higher in VFSS than in CS. In contrast, sorption coefficients for ESA and OA were not different between soils. Relative to metolachlor, sorption coefficients for ESA and OA were at least 79% lower in both soils. Metolachlor desorption coefficients were 59% higher in the VFSS than in the CS. Desorption coefficients for ESA and OA were not different between soils. Relative to metolachlor, desorption coefficients for ESA and OA were at least 66% lower in both soils. These data indicate that the mobility of ESA and OA will be greater than metolachlor in both soils. However, higher organic carbon content in VFSS relative to CS may limit the subsequent transport of metolachlor from the vegetated filter strip.     

Differentiating nonpoint sources of deisopropylatrazine in surface water using discrimination diagrams

Pesticide degradates account for a significant portion of the pesticide load in surface water. Because pesticides with similar structures may degrade to the same degradate, it is important to distinguish between different sources of parent compounds that have different regulatory and environmental implications. A discrimination diagram, which is a sample plot of chemical data that differentiates between different parent compounds, was used for the first time to distinguish whether sources other than atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) contributed the chlorinated degradate, deisopropylatrazine (DIA; 6-chloro-N-ethyl-1,3,5-triazine-2,4-diamine) to the Iroquois and Delaware Rivers. The concentration ratio of deisopropylatrazine to deethylatrazine [6-chloro-N-(1-methylethyl)1,3,5-triazine-2,4-diamine], called the D2R, was used to discriminate atrazine as a source of DIA from other parent sources, such as cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile) and simazine (6-chloro-N,N'-diethyl-1,3,5-triazine-2,4diamine). The ratio of atrazine to cyanazine (ACR) used in conjunction with the D2R showed that after atrazine, cyanazine was the main contributor of DIA in surface water. The D2R also showed that cyanazine, and to a much lesser extent simazine, contributed a considerable amount (approximately 40%) of the DIA that was transported during the flood of the Mississippi River in 1993. The D2R may continue to be a useful discriminator in determining changes in the nonpoint sources of DIA in surface water as cyanazine is currently being removed from the market.     

Reduced surface runoff losses of metolachlor in narrow-row compared to wide-row soybean

Cultural management practices that reduce the off-site transport of herbicides applied to row crops are needed to protect surface water quality. A soybean [Glycine max (L.) Merr.] field study was conducted near Stoneville, MS on Sharkey clay to evaluate row spacing (50 cm vs. 100 cm) effects on metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide] transport. One day after the foliar application of metolachlor to 2.03 m wide by 2.43 m long plots, 60 mm h(-1) of simulated rainfall was applied until 25 min of runoff was generated per plot. The calculated mass of metolachlor intercepted by the soybean foliage was greater in narrow-row than wide-row soybean, 0.39 kg ha(-1) vs. 0.23 kg ha(-1), respectively. Field and laboratory studies indicated that less than 2% of the metolachlor intercepted by the soybean foliage was available for foliar wash-off 1 d after application. Antecedent soil water content at the start of the simulations was lower in narrow-row soybean. In turn, there was a 1.7-fold greater time to runoff on narrow-row plots. The greater time to runoff likely contributed to lower metolachlor concentration in runoff from narrow-row plots. Cumulative metolachlor losses were significantly greater in wide-row than narrow-row soybean, 3.7% vs. 2.2%, respectively. Findings indicate that narrow-row planting systems may reduce metolachlor runoff following a post-emergence application.     

Field-scale remediation of a metolachlor-contaminated spill site using zerovalent iron

Pesticide spills are common occurrences at agricultural cooperatives and farmsteads. When inadvertent spills occur, chemicals normally beneficial can become point sources of ground and surface water contamination. We report results from a field trial where approximately 765 m3 of soil from a metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] spill site was treated with zerovalent iron (Fe0). Preliminary laboratory experiments confirmed metolachlor dechlorination by Fe0 in aqueous solution and that this process could be accelerated by adding appropriate proportions of Al2(SO4)3 or acetic acid (CH3COOH). The field project was initiated by moving the stockpiled, contaminated soil into windrows using common earth-moving equipment. The soil was then mixed with water (0.35-0.40 kg H2O kg(-1)) and various combinations of 5% Fe0 (w/w),2% Al2(SO4)3 (w/w), and 0.5% acetic acid (v/w). Windrows were covered with clear plastic and incubated without additional mixing for 90 d. Approximately every 14 d, the plastic sheeting was removed for soil sampling and the surface of the windrows rewetted. Metolachlor concentrations were significantly reduced and varied among treatments. The addition of Fe0 alone decreased metolachlor concentration from 1789 to 504 mg kg(-1) within 90 d, whereas adding Fe0 with Al2(SO4)3 and CH3COOH decreased the concentration from 1402 to 13 mg kg(-1). These results provide evidence that zerovalent iron can be used for on-site, field-scale treatment of pesticide-contaminated soil.     

Pesticide and Transformation Product Detections and Age‐Dating Relations from Till and Sand Deposits1

Abstract:  Pesticide and transformation product concentrations and frequencies in ground water from areas of similar crop and pesticide applications may vary substantially with differing lithologies. Pesticide analysis data for atrazine, metolachlor, alachlor, acetochlor, and cyanazine and their pesticide transformation products were collected at 69 monitoring wells in Illinois and northern Indiana to document occurrence of pesticides and their transformation products in two agricultural areas of differing lithologies, till, and sand. The till is primarily tile drained and has preferential fractured flow, whereas the sand primarily has surface water drainage and primary porosity flow. Transformation products represent most of the agricultural pesticides in ground water regardless of aquifer material – till or sand. Transformation products were detected more frequently than parent pesticides in both the till and sand, with metolachlor ethane sulfonic acid being most frequently detected. Estimated ground‐water recharge dates for the sand were based on chlorofluorocarbon analyses. These age‐dating data indicate that ground water recharged prior to 1990 is more likely to have a detection of a pesticide or pesticide transformation product. Detections were twice as frequent in ground water recharged prior to 1990 (82%) than in ground water recharged on or after 1990 (33%). The highest concentrations of atrazine, alachlor, metolachlor, and their transformation products, also were detected in samples from ground water recharged prior to 1990. These age/pesticide detection relations are opposite of what would normally be expected, and may be the result of preferential flow and/or ground‐water mixing between aquifers and aquitards as evident by the detection of acetochlor transformation products in samples with estimated ground‐water ages predating initial pesticide application.     

Enhancing metolachlor destruction rates with aluminum and iron salts during zerovalent iron treatment

Pesticide-contaminated soil may require remediation to mitigate ground and surface water contamination. We determined the effectiveness of zerovalent iron (Fe(0)) to dechlorinate metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methyl ethyl) acetamide] in the presence of aluminum and iron salts. By treating aqueous solutions of metolachlor with Fe(0), we found destruction kinetics were greatly enhanced when Al, Fe(II), or Fe(II) salts were added, with the following order of destruction kinetics observed: Al2(SO4)3 > AlCl3 > Fe2(SO4)3 > FeCl3. A common observation was the formation of green rusts, mixed Fe(II)-Fe(III) hydroxides with interlayer anions that impart a greenish-blue color. Central to the mechanism responsible for enhanced metolachlor loss may be the role these salts play in facilitating Fe(II) release. By tracking Al and Fe(II) in a Fe(0) + Al2(SO4)3 treatment of metolachlor, we observed that Al was readily sorbed by the corroding iron with a corresponding release of Fe(II). The manufacturing process used to produce the Fe(0) also profoundly affected destruction rates. Metolachlor destruction rates with salt-amended Fe(0) were greater with annealed iron (indirectly heated under a reducing atmosphere) than unannealed iron. Moreover, the optimum pH for metolachlor dechlorination in water and soil differed between iron sources (pH 3 for unannealed, pH 5 for annealed). Our results indicate that metolachlor destruction by Fe(0) treatment may be enhanced by adding Fe or Al salts and creating pH and redox conditions favoring the formation of green rusts.     

Residual and contact herbicide transport through field lysimeters via preferential flow

Usage of glyphosate [N-(phosphonomethyl)-glycine] and glufosinate [2-amino-4-(hydroxy-methylphosphinyl)butanoic acid] may reduce the environmental impact of agriculture because they are more strongly sorbed to soil and may be less toxic than many of the residual herbicides they replace. Preferential flow complicates the picture, because due to this process, even strongly sorbed chemicals can move quickly to ground water. Therefore, four monolith lysimeters (8.1 m(2) by 2.4 m deep) were used to investigate leaching of contact and residual herbicides under a worst-case scenario. Glufosinate, atrazine (6-chloro-N(2)-ethyl-N(4)-isopropyl-1,3,5-triazine-2,4-diamine), alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], and linuron (3-3,4-dichlorophenyl-1-methoxy-1-methylurea) were applied in 1999 before corn (Zea mays L.) planting and glyphosate, alachlor, and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were applied in 2000 before soybean [Glycine max (L.) Merr.] planting. A high-intensity rainfall was applied shortly after herbicide application both years. Most alachlor, metribuzin, atrazine, and linuron losses occurred within 1.1 d of rainfall initiation and the peak concentration of the herbicides coincided (within 0.1 d of rainfall initiation in 2000). More of the applied metribuzin leached compared with alachlor during the first 1.1 d after rainfall initiation (2.2% vs. 0.035%, P < 0.05). In 1999, 10 of 24 discrete samples contained atrazine above the maximum contaminant level (atrazine maximum contaminant level [MCL] = 3 mug L(-1)) while only one discrete sample contained glufosinate (19 mug L(-1), estimated MCL = 150 mug L(-1)). The results indicate that because of preferential flow, the breakthrough time of herbicides was independent of their sorptive properties but the transport amount was dependent on the herbicide properties. Even with preferential flow, glyphosate and glufosinate were not transported to 2.4 m at concentrations approaching environmental concern.