Effect of a full-grown vegetative filter strip on herbicide runoff: maintaining of filter capacity over time

Narrow vegetative filter strips (VFS) proved to effectively reduce herbicide runoff from cultivated fields mainly due to the ability of vegetation to delay surface runoff, promote infiltration and adsorb herbicides. Since VFS are dynamic systems, their performance would not remain constant over the years indicating the need to define suitable buffer management. In order to evaluate the performance of different five and six year-old VFS, the runoff of the herbicides metolachlor and terbuthylazine was monitored in 2002 and 2003 in an experimental site in northern Italy. The structure of the herbaceous cover in the buffers changes over time. When rows of trees are present, the grass cover is decreased by the shading action of the trees, but the leaf litter gains importance. In VFS with grass cover only, the cover composition changes because of the substitution of grass by broadleaf species. Six metres wide VFS are very effective in reducing runoff volume and concentration during both wet and dry years. Classification analysis showed that runoff concentration and volume are linked to the characteristics of the rainfall event, buffer, source of herbicides and time after application. Regression analysis showed that the significant predictors for runoff volume are rainfall amount and intensity, total vegetal cover in the VFS, crop leaf area index and time after treatment; for concentration they are rainfall intensity, crop leaf area index and total vegetal cover in the VFS. The role of VFS is complex, so appropriate management is required to maintain its increasing filtering capacity over time.     

Comparison between the polar organic chemical integrative sampler and the solid-phase extraction for estimating herbicide time-weighted average concentrations during a microcosm experiment

Polar organic chemical integrative samplers (POCIS) were exposed for 9 days in two different microcosms that contained river waters spiked with deethylterbuthylazine, terbuthylazine and isoproturon. The experiment was performed with natural light and strong turbulence (flow velocities of about 15-50cms(-1)) for reproducing natural conditions. The concentrations were kept relatively constant in the first microcosm (2.6-3.6mugl(-1)) and were variable in the second microcosm (peak concentrations ranged from 15 to 24mugl(-1) during the 3 day pulse phase). The time-weighted average (TWA) concentrations were determined with both POCIS and repetitive grab sampling followed by solid-phase extraction. The results showed a systematic and significant overestimation of the TWA concentrations with the POCIS most probably due to the use of sampling rates derived under low flow scenario. The results showed also that peak concentrations of pollutants are fully integrated by this passive sampler. Even if the POCIS should not provide very accurate concentration estimates without the application of adequate sampling rate values or the use of performance reference compounds, it can be a really useful tool for detecting episodic or short-term pollution events (e.g. increased herbicide concentrations during a flood), which may be missed with classical and low frequency grab sampling.     

Sorption behavior of metolachlor, isoproturon, and terbuthylazine in soils

The sorption-desorption of metolachlor [2-chloro-N-(ethyl-6-methyl phenyl)-N-(2-methoxy-1-methyl ethyl) acetamide], isoproturon [3-(4-isopropyl phenyl)-1,1-dimethyl urea] and terbuthylazine [N6-tert butyl-6-chloro-N4-ethyl-1,3,5-triazine-2,4-diamine] herbicides was studied in two German soils at 1:10 soil to water ratio by batch method. Equilibrium of herbicides between soil and water (0.01 M CaCl2) was attained in 2 h. Sorption data fitted very well to Freundlich equation, represented by very high correlation coefficient (r2 > 0.934). Comparison of Freundlich K values indicated that sorption of all the three herbicides was most pronounced in soil having higher organic carbon content. Koc values were as expected nearly identical for each herbicide in the two soils. The Freundlich constant (1/n) was about 1 for metolachlor and less than 1 for terbuthylazine and isoproturon indicating a L-type of sorption isotherms. Desorption of all the three herbicides showed hysteresis. Nearly equal amounts of metolachlor, isoproturon and terbuthylazine were desorbed from both soils. There was a good correlation between Koc and solubility.     

Estimating the spatial scale of herbicide and soil interactions by nested sampling, hierarchical analysis of variance and residual maximum likelihood

An unbalanced nested sampling design was used to investigate the spatial scale of soil and herbicide interactions at the field scale. A hierarchical analysis of variance based on residual maximum likelihood (REML) was used to analyse the data and provide a first estimate of the variogram. Soil samples were taken at 108 locations at a range of separating distances in a 9 ha field to explore small and medium scale spatial variation. Soil organic matter content, pH, particle size distribution, microbial biomass and the degradation and sorption of the herbicide, isoproturon, were determined for each soil sample. A large proportion of the spatial variation in isoproturon degradation and sorption occurred at sampling intervals less than 60 m, however, the sampling design did not resolve the variation present at scales greater than this. A sampling interval of 20-25 m should ensure that the main spatial structures are identified for isoproturon degradation rate and sorption without too great a loss of information in this field.     

Terbuthylazine persistence in an organic amended soil

The aim of this work was to study the effect of the application of a solid waste from olive oil production (alperujo) on the movement and persistence of the herbicide terbuthylazine (N2-tert-butyl-6-chloro-N4-ethyl-1,3,5-triazine-2,4-diamine). An experimental olive grove was divided in two plots: (i) Plot without organic amendment (blank) and (ii) Plot treated with alperujo during 3 years at a rate of 17920 kg of alperujo ha(- 1). Terbuthylazine was applied to both plots at a rate of 2 kg ha(- 1) a.i. Triplicates from each plot were sampled at 3 depths (0-10, 10-20 and 20-30 cm), air-dried, remains of olive leaves, grass roots, and stones removed and sieved through a 5 mm mesh sieve. Terbuthylazine was extracted with methanol 1:2 weight:volume ratio, the extracts were evaporated to dryness, resuspended in 2 mL of methanol, filtered and anylized by high performance liquid chromatography (HPLC). Higher amounts of terbuthylazine were detected at each sampling depth in plots treated with alperujo. The increase in soil organic matter content upon amendment with alperujo slightly increased sorption, suggesting that other factors beside sorption affect terbuthylazine degradation rate in organic amended soils.     

Effect of vegetative filter strips on herbicide runoff under various types of rainfall

Narrow vegetative filter strips proved to effectively reduce herbicide runoff from cultivated fields mainly due to the ability of vegetation to delay surface runoff, promote infiltration and adsorb herbicides. A field trial was conducted from 2007 to 2009 in north-east Italy in order to evaluate the effectiveness of various types of vegetative filter strips to reduce spring-summer runoff of the herbicides mesotrione, metolachlor and terbuthylazine, widely used in maize, and to evaluate the effect of the rainfall characteristics on the runoff volume and concentration. Results show that without vegetative filter strip the herbicide load that reaches the surface water is about 5-6 g ha⁻¹ year⁻¹ for metolachlor and terbuthylazine (i.e. 0.5-0.9% of the applied rate), confirming that runoff from flat fields as in the Po Valley can have a minor effect on the water quality, and that most of the risk is posed by a few, or even just one extreme rainfall event with a return period of about 25-27 years, causing runoff with a maximum concentration of 64-77 μg L⁻¹. Mesotrione instead showed rapid soil disappearance and was observed at a concentration of 1.0-3.8 μg L⁻¹ only after one extreme (artificial) rainfall. Vegetative filter strips of any type are generally effective and can reduce herbicide runoff by 80-88%. Their effectiveness is steady even under severe rainfall conditions, and this supports their implementation in an environmental regulatory scheme at a catchment or regional scale.     

The fate of isoproturon in a freshwater microcosm with Lemna minor as a model organism

Degradation, bioaccumulation and volatile loss of the 14C-labeled phenylurea herbicide isoproturon (IPU) was examined in a freshwater microcosm with the free floating macrophyte species Lemna minor during a 21-day exposure time. Isoproturon volatilisation was very low with 0.13+/-0.01% of the initially applied herbicide. Only a minor amount of the herbicide was completely metabolised, presumably by rhizosphere microorganisms and released as 14CO2. In total, about 9% isoproturon was removed from the aquatic medium during 21 days. The major portion of the pesticide was removed by bioaccumulation of Lemna minor (5.0+/-0.8%) and the bioconcentration factor (BCF) based on freshweight was 15.8+/-0.2. However, this study indicated a high persistence of IPU in freshwater ecosystems and a potential hazard due to bioaccumulation in non-target species. The novel experimental system of this study, developed for easy use and multiple sampling abilities, enabled quantitatively studying the fate of isoproturon and showed high reproducibility with a mean average (14)C-recovery rate of 97.1+/-0.7%.     

Biodegradation in laboratory activated sludge plants and aquatic toxicity of herbicides

The biodegradation and the aquatic toxicity of four herbicides (isoproturon, terbuthylazine, mecoprop, metamitron) were investigated. Laboratory activated sludge plants were used for biodegradation experiments. The biodegradation of mecoprop reached nearly 100%, the other herbicides were not eliminated by biodegradation. The acute Daphnia magna 24-h assay, the algal 72-h inhibition test, and the recently developed lemna growth inhibition 7-d test were applied to evaluate the biological effects of herbicides as original substances. EC 50 and EC 10 values were determined. Algal and lemna test show that isoproturon and terbuthylazine are both much more toxic than mecoprop and metamitron. Daphnids are generally less sensitive against herbicides than plants. Biodegradation and toxicity test were coupled for mecoprop to assess biological long-term effects of possible biodegradation products of this herbicide. The effluents of the laboratory activated sludge units were used in toxicity tests (Daphnia magna 21-d reproduction test, lemna growth inhibition 7-d test). No inhibiting effect on the tested organisms was observed.     

Tillage management to mitigate herbicide loss in runoff under simulated rainfall conditions

Conservation tillage mitigates soil loss in cropland because plant residues help protect the soil, but effects on pesticide movement in surface runoff are not as straightforward. Effects of soil disturbance on surface runoff loss of chlorimuron and alachlor were evaluated utilizing runoff trays. Soil in the trays was either disturbed (tilled) and kept bare or was not tilled, and existing decomposed plant residue was left on the surface. Rainfall (25mm, 20min) was simulated 1d after alachlor (2.8kg ha(-1)) or chlorimuron (54g ha(-1)) application, and runoff was collected. Runoff fractions were analyzed for herbicide and sediment. Total alachlor loss from bare plots was greater than that in no-tillage plots (4.5% vs. 2.3%, respectively). More than one-third of total alachlor lost from bare plots occurred in the first l of runoff, while no-tillage plots had less runoff volume with a more even distribution of alachlor concentration in the runoff during the rainfall simulation and subsequent runoff period. In contrast, more chlorimuron was lost from no-tillage plots than bare plots (12% vs. 1.5%) even though total runoff volume was lower in the no-tillage plots (10.6mm vs. 13.6mm). This was attributed to dense coverage with partially decomposed plant residue in no-tillage plots (1652kg ha(-1)) that intercepted chlorimuron. It was likely that chlorimuron, a polar compound, was more easily washed off surface plant residues and transported in runoff.     

Determination of herbicides in stemflow and throughfall of beeches (Fagus sylvatica L.) and in rainfall

The pesticide contamination of water samples collected in and nearby a beech forest in northern Germany was evaluated. For this purpose, a method for the collection of water samples from stemflow and throughfall of beeches (Fagus sylvatica L.) and rainfall was developed in response to the demands for the analysis of organic contaminants in water samples. Furthermore a sensitive and selective multiresidue method was developed to determine 18 pesticides, frequently used in Germany, in aqueous samples. The samples collected were taken from the stemflow, the crown throughfall and the rainfall between May and November 2001. Analysis based on reversed-phase liquid chromatography with a pneumatically assisted electrospray mass spectrometer followed a solid phase extraction using C-18 extraction cartridges. Isoproturon, metolachlor, prosulfocarb and terbuthylazine were found during and shortly after their application period. In rainfall metolachlor, terbuthylazine and prosulfocarb were detectable in concentrations between 5 and 65 ng l(-1) and isoproturon in concentrations between 20 and 360 ng l(-1) respectively. In most of the samples, concentrations of those four pesticides were higher and detectable for a longer time in stemflow than in rainfall. Crown throughfall samples were collected from the end of August to November. Absolute deposition of isoproturon to forest soil were up to 70 times higher in comparison to rainwater samples.     

Runoff and leaching of atrazine and alachlor on a sandy soil as affected by application in sprinkler irrigation

Rainfall simulation was used with small packed boxes of soil to compare runoff of herbicides applied by conventional spray and injection into sprinkler-irrigation (chemigation), under severe rainfall conditions. It was hypothesized that the larger water volumes used in chemigation would leach some of the chemicals out of the soil surface rainfall interaction zone, and thus reduce the amounts of herbicides available for runoff. A 47-mm rain falling in a 2-hour event 24 hours after application of alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-acetamide) and atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2, 4-diamine) was simulated. The design of the boxes allowed a measurement of pesticide concentrations in splash water throughout the rainfall event. Initial atrazine concentrations exceeding its' solubility were observed. When the herbicides were applied in 64,000 L/ha of water (simulating chemigation in 6.4 mm irrigation water) to the surface of a Tifton loamy sand, subsequent herbicide losses in runoff water were decreased by 90% for atrazine and 91% for alachlor, as compared to losses from applications in typical carrier water volumes of 187 L/ha. However, this difference was not due to an herbicide leaching effect but to a 96% decrease in the amount of runoff from the chemigated plots. Only 0.3 mm of runoff occurred from the chemigated boxes while 7.4 mm runoff occurred from the conventionally-treated boxes, even though antecedent moisture was higher in the former. Two possible explanations for this unexpected result are (a) increased aggregate stability in the more moist condition, leading to less surface sealing during subsequent rainfall, or (b) a hydrophobic effect in the drier boxes. In the majority of these pans herbicide loss was much less in runoff than in leachate water. Thus, in this soil, application of these herbicides by chemigation would decrease their potential for pollution only in situations where runoff is a greater potential threat than leaching.     

Winter runoff losses of phosphorus from paddy soils in the Taihu Lake Region of South China

A winter wheat field plot experiment was conducted on two types of paddy soils, from November, 2000 to June, 2001 to assess P losses to its surrounding watercourses by runoff in the Taihu Lake Region. Commercial NPK compound fertilizer and single superphosphate fertilizer were applied to furnish 0, 20, 80, and 160 kg P ha(-1). The experiments consisted of six replicates of each treatment in Changshu site and four replicates in Anzhen site, with a plot size of 5x6 m2 in a randomized block design. Results revealed that the average concentration of dissolved P (DP), particulate P (PP), and total P (TP) in runoff water during the winter season was 0.13, 0.90 and 1.04 mg P l(-1) respectively, from P20 plots in Anzhen site. While it was 0.67, 1.08 and 1.75 mg P l(-1) respectively, from P20 plots in Changshu site. The seasonal TP load (mass loss) from P20 plot ranged from a low of 290.88 g P ha(-1)season(-1) to a high of 483.54 g P ha(-1)season(-1), with a mean of 382.29 g P ha(-1)season(-1) in Anzhen, but from 444.92 to 752.21 g P ha(-1)season(-1), with a mean of 539.13 g P ha(-1)season(-1) in Changshu. Both in Anzhen and Changshu PP represented a major portion of the TP lost in runoff, the average PP/TP was about more than 80% in P0 and P20 plot, but it was decreased with the increase of P rate. The average seasonal P loads (DP, PP, and TP) in Changshu were greater than in Anzhen although runoff volume in Anzhen (45 mm season(-1)) was more than in Changshu (36 mm season(-1)). This was probably associated with the differences of soil physical and chemical properties between the two sites. Phosphate fertilizer rate significantly affected P concentrations and P loads by runoff. Both the mean concentrations and the average seasonal P loads from the P80 plots were lower than from the P160 plots, but obviously higher than from the P20 and P0 plots. There was no significant difference found between the P20 plots and the P0 plots both in Anzhen and Changshu sites. It indicated that P loads by runoff would be greatly increased in 5-10 years due to the accumulation of soil P if 20 kg P ha(-1) applied each wheat season in this area.     

Effect of temperature on the disappearance of four triazine herbicides in environmental waters

The influence of temperature on the disappearance of four s-triazine herbicides, terbuthylazine, simazine, atrazine and prometryn was studied in sea, river and groundwaters spiked with approx. 5 mg l(-1) of each during long-term laboratory incubation. Residues were analyzed by GC-NPD and confirmed by GC-MSD. No clean-up was necessary and a micro on-line method for the determination of herbicide residues was used. The results showed that temperature had little effect on the behaviour of the four herbicides in river and seawaters but strongly affected their behaviour in groundwater. Simazine was the most readily affected compound in sea, river and groundwaters, while terbuthylazine and atrazine were the most persistent in all cases, especially in riverwater. Half-lives ranged from 41 days (constant rate = 0.017 days(-1)) to 196 days (constant rate = 0.003 days(-1)) for simazine (40 degrees C) and terbuthylazine (20 degrees C), respectively, in riverwater. Only for terbuthylazine in riverwater was the remaining percentage at the end of the experiment higher than 50% (58%, 3.21 mg l(-1)). In the other cases, the remaining percentage varied from 4% (0.20 mg l(-1), 40 degrees C) to 43% (2.25 mg l(-1), 20 degrees C) for simazine and terbuthylazine, respectively, in groundwater.     

Uptake of terbuthylazine and its medium polar metabolites into maize plants

The uptake of terbuthylazine and its medium polar metabolites into maize plants under outdoor conditions is investigated. For this purpose, a dynamical fate model consisting of soil, plant and air is developed. The model calculations are compared with experimental results of outdoor lysimeter tests, carried out with¹⁴C-labelled herbicide applied to sandy agricultural soil at a single application rate of 890 g/ha. Approximately 0.3 % of the applied activity remains in all the plants after the vegetation period. The model predicts that about three times that amount is volatilized from the plants into the air. Activity uptaken from soil and volatilized from plant surface into air is predominately associated with metabolites. During the whole vegetation period the fraction of unchanged terbuthylazine in the plants is very small (less than 1 % of the extractable activity).     

Leaching of pesticides through normal-tillage and low-tillage soil--a lysimeter study. I. Isoproturon

Isoproturon is a herbicide, which was used in Denmark against grass weeds and broad-leaved weeds until 1998. Isoproturon has frequently been detected in ground water monitoring studies. Leaching of isoproturon (N,N-dimethyl-N'-(4-(1-methylethyl)-phenyl)urea) and its metabolites, N'-(4-isopropylphenyl)-N-methylurea and N'-(4-isopropylphenyl)urea was studied in four lysimetres, two of them being replicates from a low-tillage field (lysimeter 3 and 4), the other two being replicates from a normal tillage field (lysimeter 5 and 6). In both cases the soil was a sandy loam soil with 13-14% clay. The lysimetres had a surface area of 0.5 m2 and a depth of 110 cm. Lysimeter 3 and 4 were sprayed with unlabelled isoproturon while lysimeter 5 and 6 was sprayed with a mixture of 14C-labelled and unlabelled isoproturon. The total amount of isoproturon sprayed onto each lysimeter was 63 mg, corresponding to 1.25 kg active ingredient per ha. The lysimeters were sprayed with isoproturon on October 26, 1997. The lysimetres were installed in an outdoor system in Research Centre Flakkebjerg and were thus exposed to normal climatic conditions of the area. A mean of 360 l drainage water were collected from lysimeter 3 and 4 and a mean of 375 litres from lysimeter 5 and 6. Only negligible amounts of isoproturon and its primary metabolites were found in the drainage water samples, and thus no significant difference between the two lysimeter sets was shown. In a total of 82 drainage water samples, evenly distributed between the four lysimetres isoproturon was found in detectable amounts in two samples and N'-(4-isopropylphenyl)urea was found in detectable amounts in two other samples. The detection limit for all the compounds was 0.02 microg/l. 48% and 54% of the added radioactivity were recovered from the upper 10 cm soil layer in lysimeter 5 and 6, respectively, and 17 and 14% from 10-20 cm's depth. By extraction first with an aquatic CaCl2 solution 0.49% of the added radioactivity was extracted from the upper 10 cm layer in lysimeter 5. In the subsequent extraction with acetonitril, 1.19% of the added radioactivity was extracted. In lysimeter 6, upper 10 cm, 0.2% were extracted with water and 0.56% were extracted with acetonitril. Below 10 cm's depth no measurable amounts could be extracted.     

Mapping field spatial distribution patterns of isoproturon-mineralizing activity over a three-year winter wheat/rape seed/barley rotation

The temporal and spatial variability of the activity of soil microorganisms able to mineralize the herbicide isoproturon (IPU) pesticide was investigated over a three-year long crop rotation between 2008 and 2010. Isoproturon mineralization was higher in 2008, when winter wheat was treated with this herbicide, than in 2009 and 2010, when rape seed and barley were treated with different herbicides. Under laboratory conditions, we showed that isoproturon mineralization was not promoted by sulfonylurea herbicide applied on barley crop in 2010. IPU mineralization was shown to be highly variable at the field scale in years 2009 and 2010. Principal component analyses and analyses of similarities revealed that soil pH and equivalent humidity, and to a lesser extent soil organic matter content and cation exchange capacity (CEC) were the main drivers of isoproturon-mineralizing activity variance. Using a rather simple model that yields the rate of isoproturon mineralization as a function of soil pH and equivalent humidity, we explained up to 85% of the variance observed. Mapping field-scale distribution of isoproturon mineralization over the three-year survey indicated higher variability in 2009 and in 2010 as compared to 2008, suggesting that isoproturon treatment applied to winter wheat promoted isoproturon mineralization activity and reduced its spatial variability. Field-scale distribution of isoproturon mineralization showed important similarity to the distribution of soil pH, equivalent humidity and to a lesser extent to soil organic matter and cation exchange capacity (CEC) thereby confirming our model.     

Microbiological characterization of the biological treatment of aircraft paint stripping wastewater

Leaching rates of the herbicide dichlorprop [(+/--2-(2,4-dichlorophenoxy)propanoic acid] and nitrate were measured together in field lysimeters containing undisturbed clay and peat soils. The purpose of the study was to investigate the leaching pattern of the two solutes in structured soils under different precipitation regimes. Spring barley (Hordeum distichum L.) was sown on each monolith and fertilized with 100 kg N ha(-1). Dichlorprop was applied at a rate of 1.6 kg active ingredient (a.i.) ha(-1). Each soil type received supplemental irrigation at two levels ('average' and 'worst-case'), giving total water inputs (irrigation and precipitation) of 664 and 749 mm year(-1), respectively. The larger water input approximately doubled the nitrate loads, from, on average, 11.6 to 21.8 kg N ha(-1) year(-1) in the clay soil and from 37.6 to 65.4 kg N ha(-1) year(-1) in the peat soil. In contrast, dichlorprop leaching was reduced by more than one order of magnitude when the water input was increased, from average amounts of 3.22 to 0.26 g a.i. ha(-1) during an S-month period in the clay and from 28.9 to 2.67 g a.i. ha(-1) in the peat. This leaching pattern of dichlorprop was explained in terms of preferential flow. The dried-out topsoil of 'average' watered monoliths may have allowed water flow in cracks, thus moving some of the herbicide rapidly through the topsoil to the subsoil. Once the compound reached the subsoil, degradation rates would be reduced and the herbicide residues would be stored for later leaching. Nitrate was presumably more evenly distributed in the soil matrix; therefore, water rapidly moving through macropores would not carry significant amounts of nitrate. In contrast, leaching would occur more evenly through the soil matrix, causing larger nitrate loads in the 'worst-case' watered monoliths. These results show that wet years may constitute a worst case scenario in terms of nitrate leaching, but not pesticide leaching, if macropore flow exerts a significant influence on leaching.     

Influence of humic fractions on retention of isoproturon residues in two Moroccan soils

The influence of different fractions of soil organic matter on the retention of the herbicide isoproturon (IPU) has been evaluated. Water and methanol extractable residues of (14)C labeled isoproturon have been determined in two Moroccan soils by beta -counting-liquid chromatography. The quantification of bound residues in soil and in different fractions of soil humic substances has been performed using pyrolysis/scintillation-detected gas-chromatography. Microbial mineralization of the herbicide and soil organic matter has been also monitored. Retention of isoproturon residues after 30-days incubation ranged from 22% to 32% (non-extractable fraction). The radioactivity extracted in an aqueous environment was from 20% to 33% of the amount used for the treatment; meanwhile, methanol was able to extract another 48%. Both soils showed quantities of bound residues into the humin fraction higher than humic and fulvic acids. The total amount of residues retained into the organic matter of the soils was about 65 % of non-extractable fraction, and this percentage did not change with incubation time; on the contrary, the sorption rate of the retention reaction is mostly influenced by the clay fraction and organic content of the soil. Only a little part of the herbicide was mineralized during the experimental time.     

Runoff of trifluralin,metolachlor, and metribuzin from a clay loam soil of Louisiana

Trifluralin[2,6-dinitro-N,N-dipropyl-4-(trifluormethyl)benzenamine], metolachlor[2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] and metribuzin[4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)one] were applied as pre-emergent herbicides to soybean plots in Louisiana (LA) at the rate of 1683 g/ha, 2759 g/ha and 609 g/ha, respectively. The concentrations of trifluralin in the runoff water ranged between 0.09 ng/mL and 0.02 ng/mL, which is lower than the 2 ng/mL US Environmental Protection Agency (EPA) advisory level for trifuralin in drinking water. Metolachlor concentrations in the runoff water ranged from 9.0 ng/mL to 221.5 ng/mL, which is both lower and higher than the 175 ng/mL EPA advisory level for metolachlor. Similarly, metribuzin concentrations in the runoff water ranged between 1.5 ng/mL and 56.2 ng/mL, which is also lower and higher than the 10 ng/mL EPA advisory level for metribuzin. Accordingly, from the field plots located on a Commerce clay loam soil in LA, although the concentration of trifluralin in runoff water were substantially lower than the EPA advisory level, metolachlor and metribuzin concentrations are likely to exceed the EPA advisory levels early on in the application season with a subsequent rapid decrease to safe levels. The total loss of trifluralin in runoff water was 0.005% of the applied amount over an 89 day period after application. The total loss of metolachlor and metribuzin in the runoff water was 4.67% and 5.36% of the applied amount, respectively, over a 22 day period after application. As such, there was almost no movement of trifluralin in the runoff water, whereas metolachlor and metribuzin were much more easily moved.     

Penetration of herbicides to groundwater in an unconfined chalk aquifer following normal soil applications

The persistence and penetration of the herbicides isoproturon and chlorotoluron in an unconfined chalk aquifer has been monitored over a 4-year period through soil sampling, shallow coring and groundwater monitoring. Chlorotoluron was applied on plots as a marker compound, having never been used previously on that, or surrounding fields. The fieldsite had a 5 degree slope with soil depths of 0.5 to 1.5 m and a water table between 20 and 5 m from the soil surface. Where the water table was deepest (9-20 m below surface (mbs)) little or no positive herbicide detections were made. However, where the water table was at only 4-5 mbs, a regular pesticide signal of around 0.1 microg/l for isoproturon and chlorotoluron could be distinguished. Over the winter recharge period automatic borehole samplers revealed a series of short-lived peaks of isoproturon and chlorotoluron reaching up to 0.8 microg/l. This is consistent with a preferential flow mechanism operating at this particular part of the field. Such peaks were occurring over 2 years after the last application of these compounds. Shallow coring failed to uncover any significant pesticide pulse moving through the deep unsaturated zone matrix at the fieldsite.