Atrazine distribution measured in soil and leachate following infiltration conditions

Atrazine transport through packed 10 cm soil columns representative of the 0-10 cm soil horizon was observed by measuring the atrazine recovery in the total leachate volume, and upper and lower soil layers following infiltration of 7.5 cm water using a mechanical vacuum extractor (MVE). Measured recoveries were analyzed to understand the influence of infiltration rate and delay time on atrazine transport and distribution in the column. Four time periods (0.28, 0.8, 1.8, and 5.5 h) representing very high to moderate infiltration rates (26.8, 9.4, 4.2, and 1.4 cm/h) were used. Replicate soil columns were tested immediately and following a 2-d delay after atrazine application. Results indicate atrazine recovery in leachate was independent of infiltration rate, but significantly lower for infiltration following a 2-d delay. Atrazine distribution in the 0-1 and 9-10 cm soil layers was affected by both infiltration rate and delay. These results are in contrast with previous field and laboratory studies that suggest that atrazine recovery in the leachate increases with increasing infiltration rate. It appears that the difference in atrazine recovery measured using the MVE and other leaching experiments using intact soil cores from this field site and the rain simulation equipment probably illustrates the effect of infiltrating water interacting with the atrazine present on the soil surface. This work suggests that atrazine mobilization from the soil surface is also dependent on interactions of the infiltrating water with the soil surface, in addition to the rate of infiltration through the surface soil.     

Leaching of atrazine, metolachlor and diuron in the field in relation to their injection depth into a silt loam soil

A field experiment was conducted on a Calcaric Cambisol soil to study the consequences of the penetration depth and properties of pesticides on the risk of subsequent leaching. Three pesticides with different mobility characteristics and bromide were injected at 30 cm (where soil organic matter (OM) was 2%) and 80 cm (soil OM 0.5%) on irrigated plots without a crop. The migration of injected solutes was assessed for two years by sampling the soil solution using six porous cups installed at 50 and 150 cm depth and by relating solute contents to drainage water flux estimated by the STICS model (Simulateur mulTIdisciplinaire pour les Cultures Standard). Pesticides injected at 30 cm were strongly retained so that no metolachlor or diuron was detected at 50 and 150 cm. The ratio of atrazine peak concentration in the soil solution to concentration in the injected solution (C/C₀) was 1 × 10⁻³ and 0.2 × 10⁻³, respectively, at 50 and 150 cm. When injected at 80 cm, (C/C₀) of atrazine, metolachlor and diuron were 10 × 10⁻³, 1 × 10⁻³ and 0.3 × 10⁻³ at 150 cm, respectively; 1/(C/C₀) was correlated with K_oc values reported from databases. The ratio of drainage volume to the amount of water at field capacity in the soil layer between the injection point at 30 cm and the water sampling level (V/V₀) at 50 and 150 cm was 0.6 and 0.9, respectively, for bromide and 1.6 and 1.0 for atrazine. V/V₀ of the injected solutes at 80 cm was for bromide, atrazine, metolachlor and diuron 0.6, 0.9, 1.2 and 1.7, respectively; pesticide V/V₀ was correlated with K_oc. The retardation factor was a good indicator of migration risk, but tended to overestimate retardation of molecules with high K_oc. Atrazine desorption represented an additional leaching risk as a source of prolonged low contamination. The large variability in soil solution of bromide and pesticide concentrations in the horizontal plane was attributed to flow paths and clods in the tilled soil layer. This heterogeneity was assumed to channel water fluxes into restricted areas and thereby increase the risk of groundwater contamination. The methodology used in the field proves to provide consistent results.     

Interactions of tillage and rainfall on atrazine leaching under field and laboratory conditions

Laboratory studies were conducted to evaluate effects of tillage reversal and rainfall on ¹⁴C-atrazine (2-chloro4-ethylamino-6-isopropylamino- -triazine) leaching patterns. Twelve intact soil cores (16 cm dia x 20 cm deep) were collected from 8-yr no-till (NT) fields. Half the cores were tilled (5 cm deep) prior to ¹⁴C-atrazine treatment (2.7 mg core⁻¹) to all cores. All cores received two rains (27 mm rain in 1.5 h, one day after application followed, two days later, by a 17 mm rain in 2.5 h) and leachate was collected and analyzed for atrazine. These rains simulated the timing, amount and duration of natural rainfall events from a tillage reversal field study. During the first high inte ity rainfall event, a pulse (2.1 μg L-1) of atrazine leached through tilled cores while leaching rate was linear and decreased (1.25 to 0.9 μg L-1) through un-tilled cores. Leaching rate was linear for both the tilled and un-tilled cores during the second rain. Less atrazine was left in the surface 5 cm of tilled soil than un-tilled after the two rains. Results confirmed field observations and suggested that when tillage is reversed on well structured soils, pesticide leaching may increase relative to un-tilled soil but these effects are probably confined to the first rain events after application only.     

Leaching and persistence of ametryn and atrazine in red–yellow latosol

This study evaluated the mobility and persistence of atrazine and ametryn in red–yellow latosols using polyvinyl chloride columns with a diameter of 100 mm and a height of 15 cm. The assays simulated 60-mm rainfall events at 10-day intervals for 70 days. The persistence and leaching were evaluated for these two herbicides. The analytes obtained from the samples were quantified by gas chromatography using flame ionization detection. Compared with ametryn, atrazine showed a greater potential to reach depths below 15 cm over 30 days of simulated rain. Ametryn, however, showed greater persistence in soil at 70 days after application. The persistence of atrazine and ametryn in soil under sunlight was 10 and 144 days respectively. Atrazine was more susceptible to sunlight than ametryn because sunlight favored atrazine degradation in hydroxyatrazine. The results indicate that in red–yellow latosol, atrazine has a high leaching potential in short term, but that ametryn is more persistent and has a high leaching potential in long term.     

Atrazine sorption and fate in a Ultisol from humid tropical Brazil

This study combined laboratory based microcosm systems as well as field experiments to evaluate the mobility of atrazine on a Ultisol under humid tropical conditions in Brazil. Results from sorption experiments fit to the Freundlich isotherm model [K(f) 0.99 mg kg(-1)/(mg l(-1))(1/n)], and indicate a low sorption capacity for atrazine in this soil and consequently large potential for movement by leaching and runoff. Microcosm systems using (14)C-atrazine to trace the fate of the applied herbicide, showed that 0.33% of the atrazine was volatilized, 0.25% mineralized and 6.89% was recorded in the leachate. After 60 d in the microcosms, 75% of the (14)C remained in the upper 5 cm soil layer indicating atrazine or its metabolites remained close to the soil surface. In field experiments, after 60 d, only 5% of the atrazine applied was recovered in the upper soil layers. In the field experiments atrazine was detected at a depth of 50 cm indicating leaching. Simulating tropical rain in field experiments resulted in 2.1% loss of atrazine in runoff of which 0.5% was adsorbed onto transported soil particles and 1.6% was in solution. Atrazine runoff was greatest two days after herbicide application and decreased 10 fold after 15 d. The use of atrazine on Ultisols, in the humid tropics, constitutes a threat to water quality, causing surface water and ground water pollution.     

Leaching of glyphosate and AMPA under two soil management practices in Burgundy vineyards (Vosne-Romanée, 21-France)

Some drinking water reservoirs under the vineyards of Burgundy are contaminated with herbicides. Thus the effectiveness of alternative soil management practices, such as grass cover, for reducing the leaching of glyphosate and its metabolite, AMPA, through soils was studied. The leaching of both molecules was studied in structured soil columns under outdoor conditions for 1 year. The soil was managed under two vineyard soil practices: a chemically treated bare calcosol, and a vegetated calcosol. After 680 mm of rainfall, the vegetated calcosol leachates contained lower amounts of glyphosate and AMPA (0.02% and 0.03%, respectively) than the bare calcosol leachates (0.06% and 0.15%, respectively). No glyphosate and only low amounts of AMPA (<0.01%) were extracted from the soil. Glyphosate, and to a greater extent, AMPA, leach through the soils; thus, both molecules may be potential contaminants of groundwater. However, the alternative soil management practice of grass cover could reduce groundwater contamination by the pesticide.     

Mobility of atrazine from alginate-bentonite controlled release formulations in layered soil

The mobility of atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine] from alginate-bentonite-based controlled release (CR) formulations was investigated by using soil columns. Two CR formulations based on sodium alginate (14.0 g kg(-1), atrazine (6.0 g kg(-1), natural or acid-treated bentonite (50 g kg(-1), and water (924 g kg(-1) were compared to technical grade product and commercial liquid (CL) formulation (Gesaprim 500FW). All herbicide treatments were applied to duplicate layered bed systems simulating the typical arrangement under a plastic greenhouse, which is composed of sand (10 cm), peat (2 cm), amended soil (20 cm) and native soil (20 cm). The columns were leached with 39 cm (1500 ml) and 156 cm (6000 ml) of 0.02 M CaCl2 solution to evaluate the effect of water volume applied on herbicide movement. When 39 cm of 0.02 M CaCl2 solution was applied, there was no presence of herbicide in the leachate for the alginate-bentonite CR treatments. However, 0.11% and 0.14% of atrazine appeared in the leachate when the treatment was carried out with technical grade and CL formulations, respectively. When 156 cm of 0.02 M CaCl2 solution was applied, the use of the alginate-acid treated bentonite CR formulation retards and reduces the presence of atrazine in the leachate as compared to technical product. Analysis of the soil columns showed the highest atrazine concentration in the peat layer. Alginate-bentonite CR formulations might be an efficient system for reducing atrazine leaching in layered soil and thus, it could reduce the risks of pollution of groundwater.     

Leaching and degradation of corn and soybean pesticides in an Oxisol of the Brazilian Cerrados

Pesticide pollution of ground and surface water is of growing concern in tropical countries. The objective of this pilot study was to evaluate the leaching potential of eight pesticides in a Brazilian Oxisol. In a field experiment near Cuiabá, Mato Grosso, atrazine, chlorpyrifos, lambda-cyhalothrin, endosulfane alpha, metolachlor, monocrotofos, simazine, and trifluraline were applied onto a Typic Haplustox. Dissipation in the topsoil, mobility within the soil profile and leaching of pesticides were studied for a period of 28 days after application. The dissipation half-life of pesticides in the topsoil ranged from 0.9 to 14 d for trifluraline and metolachlor, respectively. Dissipation curves were described by exponential functions for polar pesticides (atrazine, metolachlor, monocrotofos, simazine) and bi-exponential ones for apolar substances (chlorpyrifos, lambda-cyhalothrin, endosulfane alpha, trifluraline). Atrazine, simazine and metolachlor were moderately leached beyond 15 cm soil depth, whereas all other compounds remained within the top 15 cm of the soil. In lysimeter percolates (at 35 cm soil depth), 0.8-2.0% of the applied amounts of atrazine, simazine, and metolachlor were measured within 28 days after application. Of the other compounds less than 0.03% of the applied amounts was detected in the soil water percolates. The relative contamination potentials of pesticides, according to the lysimeter study, were ranked as follows: metolachlor > atrazine = simazine > monocrotofos > endsulfane alpha > chlorpyrifos > trifluraline > lambda-cyhalothrin. This order of the pesticides was also achieved by ranking them according to their effective sorption coefficient Ke, which is the ratio of Koc to field-dissipation half-life.     

Long-term dynamics of the atrazine mineralization potential in surface and subsurface soil in an agricultural field as a response to atrazine applications

The dynamics of the atrazine mineralization potential in agricultural soil was studied in two soil layers (topsoil and at 35-45 cm depth) in a 3 years field trial to examine the long term response of atrazine mineralizing soil populations to atrazine application and intermittent periods without atrazine and the effect of manure treatment on those processes. In topsoil samples, ¹⁴C-atrazine mineralization lag times decreased after atrazine application and increased with increasing time after atrazine application, suggesting that atrazine application resulted into the proliferation of atrazine mineralizing microbial populations which decayed when atrazine application stopped. Decay rates appeared however much slower than growth rates. Atrazine application also resulted into the increase of the atrazine mineralization potential in deeper layers which was explained by the growth on leached atrazine as measured in soil leachates recovered from that depth. However, no decay was observed during intermittent periods without atrazine application in the deeper soil layer. atzA and trzN gene quantification confirmed partly the growth and decay of the atrazine degrading populations in the soil and suggested that especially trzN bearing populations are the dominant atrazine degrading populations in both topsoil and deeper soil. Manure treatment only improved the atrazine mineralization rate in deeper soil layers. Our results point to the importance of the atrazine application history on a field and suggests that the long term survival of atrazine degrading populations after atrazine application enables them to rapidly proliferate once atrazine is again applied.     

Laboratory study on leachability of five herbicides in South Australian soils

Abstract

Norflurazon, oxadiazon, oxyfluorfen, trifluralin and simazine are herbicides widely used in the vineyards of the Barossa Valley, South Australia. The leaching behaviour of norflurazon, oxadiazon, oxyfluorfen and trifluralin was investigated on four key soils in the Barossa Valley. Leaching potential on packed soil columns and actual mobility using intact soil columns were investigated. On the packed soil columns, norflurazon was the most leachable herbicide. More of the herbicides were detected in the leachates from the sandy soils (Mountadam and Nuriootpa) than from the clayey soils (Lyndoch and Tanunda). Organic matter is generally low in soils in the Barossa region. Porosity and saturated conductivity significantly affect herbicide movement and in the sandy Mountadam and Nuriootpa soils, the water flux is greater than for the higher clay content Lyndoch and Tanunda soils. Increasing the time interval between herbicide application and the incidence of “rainfall”; reduced the amounts of herbicides found in the leachates. The use of intact soil columns and including simazine for comparison showed that both norflurazon and simazine were present in the leachates. Simazine was the first herbicide to appear in leachates. Sectioning of the intact soil columns after leaching clearly demonstrated that norflurazon and simazine reached the bottom of the soil columns for all soils studied. Greater amounts of norflurazon were retained in the soil columns compared with simazine. The other herbicides were mostly retained in the initial sections of the soil columns.     

Laboratory study on leachability of five herbicides in South Australian soils

Norflurazon, oxadiazon, oxyfluorfen, trifluralin and simazine are herbicides widely used in the vineyards of the Barossa Valley, South Australia. The leaching behaviour of norflurazon, oxadiazon, oxyfluorfen and trifluralin was investigated on four key soils in the Barossa Valley. Leaching potential on packed soil columns and actual mobility using intact soil columns were investigated. On the packed soil columns, norflurazon was the most leachable herbicide. More of the herbicides were detected in the leachates from the sandy soils (Mountadam and Nuriootpa) than from the clayey soils (Lyndoch and Tanunda). Organic matter is generally low in soils in the Barossa region. Porosity and saturated conductivity significantly affect herbicide movement and in the sandy Mountadam and Nuriootpa soils, the water flux is greater than for the higher clay content Lyndoch and Tanunda soils. Increasing the time interval between herbicide application and the incidence of "rainfall" reduced the amounts of herbicides found in the leachates. The use of intact soil columns and including simazine for comparison showed that both norflurazon and simazine were present in the leachates. Simazine was the first herbicide to appear in leachates. Sectioning of the intact soil columns after leaching clearly demonstrated that norflurazon and simazine reached the bottom of the soil columns for all soils studied. Greater amounts of norflurazon were retained in the soil columns compared with simazine. The other herbicides were mostly retained in the initial sections of the soil columns.     

Fate of atrazine in a soil under different agronomic management practices

Agricultural management affects the movement of atrazine in soil and leaching to groundwater. The objective of this study was to determine atrazine adsorption in a soil after 20 years of atrazine application under agronomic management practices differing in tillage practice (conventional and zero tillage), residue management (with and without residue retention) and crop rotation (wheat-maize rotation and maize monoculture). Atrazine sorption was determined using batch and column experiments. In the batch experiment, the highest distribution coefficient K_d (1.1 L kg^?1) at 0–10 cm soil depth was observed under zero tillage, crop rotation and residue retention (conservation agriculture). The key factor in adsorption was soil organic matter content and type. This was confirmed in the column experiment, in which the highest K_d values were observed in treatments with residue retention, under either zero or conventional tillage (0.81 and 0.68 L kg^?1, respectively). Under zero tillage, the fact that there was no soil movement helped to increase the K_d. The increased soil organic matter content with conservation agriculture may be more important than preferential flow due to higher pore connectivity in the same system. The soil's capacity to adsorb 2-hydroxyatrazine (HA), an important atrazine metabolite, was more important than its capacity to adsorb atrazine, and was similar under all four management practices (K_d ranged from 30 to 40 L kg^?1). The HA adsorption was attributed to the type and amount of clay in the soil, which is unaffected by agronomic management. Soils under conservation agriculture had higher atrazine retention potential than soils under conventional tillage, the system that predominates in the study area.     

Erratum

The influence of soil macro-porosity and manure on atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine) transport was investigated under laboratory conditions using disturbed and undisturbed soil columns. The macro-porosity in the soil column was obtained with CT scanning technique. Liquid manure was applied at the surface of soil column, 19 cm long and 8 cm in diameter, at a rate of 60 m³/ha. Experimental results revealed that atrazine moves faster through the soils in the presence of manure compared to soil without application of manure. The average time for elusion and the relative peak concentration in the disturbed soil column without manure was 14.5 h and 3.1%, respectively compared to 11.0 h and 6.9% in the presence of manure, respectively. Similar behavior was observed in the case of disturbed soil columns. Soil macro-porosity has shown large impact on atrazine transport, especially in the presence of manure.     

Macroporosity and manure influence on atrazine transport through soil

The influence of soil macro-porosity and manure on atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) transport was investigated under laboratory conditions using disturbed and undisturbed soil columns. The macro-porosity in the soil column was obtained with CT scanning technique. Liquid manure was applied at the surface of soil column, 19 cm long and 8 cm in diameter, at a rate of 60 m3/ha. Experimental results revealed that atrazine moves faster through the soils in the presence of manure compared to soil without application of manure. The average time for elusion and the relative peak concentration in the disturbed soil column without manure was 14.5 h and 3.1%, respectively compared to 11.0 h and 6.9% in the presence of manure, respectively. Similar behavior was observed in the case of disturbed soil columns. Soil macro-porosity has shown large impact on atrazine transport, especially in the presence of manure.     

Particle-facilitated lead and arsenic transport in abandoned mine sites soil influenced by simulated acid rain

The role of acid rain in affecting Pb and As transport from mine tailings was investigated by pumping simulated acid rain at a infiltration rate of 10.2 cm/h through soil columns. Simulated acid rain with pH of 3.0, 4.5 and 5.6 were used as leaching solutions. Results showed that 86.9–95.9% of Pb and 90–91.8% of As eluted from the columns were adsorbed by particles in the leachates. Scanning electron microscopy (SEM) analysis showed that particles released from the columns were mainly composed of flocculated aggregates and plate or rod shaped discrete grains. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray analysis (EDX) showed that these particles were predominantly silicate minerals. Results from our experiments demonstrated that when rapid infiltration conditions or a rainstorm exist, particle-facilitated transport of contaminants is likely to the dominant metal transport pathway influenced by acid rain.     

Behavior of atrazine in soils of tropical zone

Abstract

Movement and degradation of ¹⁴C‐atrazine (2‐chloro 4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine, was studied in undisturbed soil columns (0.50m length × 0.10m diameter) of Gley Humic and Deep Red Latosol from a maize crop region of Sao Paulo state, Brazil. Atrazine residues were largely confined to the 0–20cm layers over a 12 month period Atrazine degraded to the dealkylated metabolites deisopropylatrazine and deethylatrazine, but the major metabolite was hydroxyatrazine, mainly in the Gley Humic soil. Activity detected in the leachate was equivalent to an atrazine concentration of 0.08 to 0.11μg/1.

The persistence of ¹⁴C‐atrazine in a maize‐bean crop rotation was evaluated in lysimeters, using Gley Humic and Deep Red Latosol soils. Uptake of the radiocarbon by maize plants after 14‐days growth was equivalent to a herbicide concentration of 3.9μg/g fresh tissue and was similar in both soils. High atrazine degradation to hydroxyatrazine was detected by tic of maize extracts. After maize harvest, when beans were sown the Gley Humic soil contained an atrazine concentration of 0.29 μg/g soil and the Deep Red Latosol, 0.13 μg/g soil in the 0–30 cm layer. Activity detected in bean plants corresponded to a herbicide concentration of 0.26 (Gley Humic soil) and 0.32μg/g fresh tissue (Deep Red Latossol) after 14 days growth and 0.43 (Gley Humic soil) and 0.50 μg/g fresh tissue (Deep Red Latossol) after 97 days growth. Traces of activity equivalent to 0.06 and 0.02μg/g fresh tissue were detected in bean seeds at harvest. Non‐extractable (bound) residues in the soils at 235 days accounted for 66.6 to 75% (Gley Humic soil and Deep Red Latossol) of the total residual activity.     

Influence of soil texture and tillage on herbicide transport

Two long-term no-till corn production studies, representing different soil texture, consistently showed higher leaching of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] to groundwater in a silt loam soil than in a sandy loam soil. A laboratory leaching study was initiated using intact soil cores from the two sites to determine whether the soil texture could account for the observed differences. Six intact soil cores (16 cm dia by 20 cm high) were collected from a four-year old no-till corn plots at each of the two locations (ca. 25 km apart). All cores were mounted in funnels and the saturated hydraulic conductivity (Ksat) was measured. Three cores (from each soil texture) with the lowest Ksat were mixed and repacked. All cores were surface treated with 1.7 kg ai ha(-1) [ring-14C] atrazine, subjected to simulated rainfall at a constant 12 mm h(-1) intensity until nearly 3 pore volume of leachate was collected and analyzed for a total of 14C. On an average, nearly 40% more of atrazine was leached through the intact silt loam than the sandy loam soil cores. For both the intact and repacked cores, the initial atrazine leaching rates were higher in the silt loam than the sandy loam soils, indicating that macropore flow was a more prominent mechanism for atrazine leaching in the silt loam soil. A predominance of macropore flow in the silt loam soil, possibly due to greater aggregate stability, may account for the observed leaching patterns for both field and laboratory studies.     

Pesticide storage and release in unsaturated soil in Illinois, USA.

The chemical fate and movement of pesticides may be subject to transient storage in unsaturated soils during periods of light rainfall, and subsequent release into shallow groundwater by increased rainfall. The objective of this study was to conduct field-scale experiments to determine the relative importance of transient storage and subsequent release of agrichemicals from the vadose zone into potential aquifers. Two field-scale experiments were conducted under a rain exclusion shelter. In the 1x experiment, atrazine and chlorpyrifos were applied at application-rate equivalents (1.6 kg ha(-1) and 1.3 kg ha(-1), respectively). In the 4x experiment, atrazine was applied in an amount that was four times greater than that usually applied to fields (6.7 kg ha(-1)). Water was either applied to simulate rain or withheld to simulate dry periods. In the 1x experiment, atrazine was detected in the water samples whereas chlorpyrifos was not detected in the majority of the samples. The dry period imposed on the treatment plot did not appear to result in storage of the chemicals, whereas the wet period resulted in greater leaching of atrazine, although the concentrations remained less than the Maximum Contaminant Level of 3 microg L(-1). Both chemicals were detected in soil samples collected from a 20- to 30-cm depth, but it appeared that both chemicals dissipated before the field experiment was concluded. It appeared that the one-time application of atrazine and chlorpyrifos at the label rates did not result in a sufficient mass to be stored and flushed in significant concentrations to the saturated zone. When atrazine was applied at 4x and a longer drought period was imposed on the treatment plot, the resulting concentrations of dissolved atrazine were still less than 3 microg L(-1) . Atrazine was detected in only the near-surface (0 to 15 cm) soil samples and the herbicide dissipated before the onset of the dry period in the treatment plot. The results of this field study demonstrated that atrazine and chlorpyrifos were not sufficiently persistent to be stored and then released in significantly large concentrations to the saturated zone. The dissipation half-life of atrazine in the 4x application was about 44 days. This study, in addition to others, suggested that atrazine may be less persistent in surface soil than has been generally reported.     

Atrazine and metolachlor residues in brookston Cl following conventional and conservation tillage culture

Atrazine and metolachlor are extensively used in Ontario, Canada for control of broadleaf weeds and annual grasses in corn. Conservation tillage may alter the physical and biological environment of soil affecting herbicide dissipation. The rate of dissipation of these two herbicides in soil from conventional, ridge and no-tillage culture was followed. Herbicide dissipation was best described by first order reaction kinetics. Half life, the time for herbicide residues to dissipate to half their initial concentration, was unaffected by tillage. Half life for atrazine and metolachlor was similar and ranged from 31 to 66 d. The rate of dissipation decreased in dry years when soil moisture content was low. In a dry year, herbicide residues during the growing season were significantly greater on ridge tops than in the other tillage treatments. However, after harvest no differences in herbicide residues were detected among tillage treatments. Residues of atrazine (6 to 9% of applied) and metolachlor (4 to 6%) were detected in soil before planting a year after application. De-ethyl atrazine, the primary degradation product of atrazine, increased in concentration during the growing season with the greatest concentrations measured at harvest and in years when atrazine dissipated fastest. De-ethyl atrazine one year after application accounted for about 12% of the remaining triazine residue. These herbicide residues would not be phytotoxic to subsequent crops but are a potential source for leaching to ground and surface waters.     

Modelling of atrazine transport in the presence of surfactants

Laboratory experiments were conducted to examine the effect of detergents on transport of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] through loam and sandy loam soils under saturation conditions. The Convection Dispersion Equation (CDE) was used to model and quantify the effects of detergents on atrazine model parameters: the retardation factor (R), pore velocity (v) and dispersion coefficient (D). The transport parameters were estimated using moment technique and partition coefficient obtained from batch experiments and compared with best-fitted parameters, R and D, keeping pore velocity constant. Results indicated the CDE model was not successful in predicting atrazine transport in the presence of surfactants at high concentrations. In the case of anionic surfactant with Elora loam, the average predicted R and D from moment technique of 3.4 and 11.1 cm2/h, respectively were significantly different than fitted parameters (R = 39 and D = 227 cm2/h). The poor performance of CDE in the presence of surfactants results from physiochemical changes in herbicide solubility and retention to the soil matrix rather than changes in soil hydraulic properties since the predicted pore water velocities from moment technique were similar to those measured during leaching experiments. Nevertheless, BTC analysis with CDE showed that land application of anionic surfactant (sulphonic) significantly increased R and D and decrease v for both soils. Addition of sulphonic increased R of atrazine by 12 and 26 folds for loam and sandy loam soils, respectively. On the other hand non-ionic surfactants seemed to decrease R, especially in sandy loam soil, thus facilitating atrazine leaching through soil. Non-equilibrium conditions seemed to govern atrazine transport in the presence of surfactants; double peaks in breakthrough curves were observed, indicating a need for mathematical models to account for such phenomena. Atrazine dispersion and tailing seemed to be higher through Elora loam compared to Caledon sandy loam due to higher aggregation of the Elora soil.