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DIVISION S-2—SOIL CHEMISTRY

Leaching of Imidacloprid and Procymidone in a Greenhouse of Southeast of Spain

^a Dep. of Inorganic Chemistry, Univ. of Almeria, La Cañada San Urbano, s/n, 04120-Almeria, Spain
^b Dep. of Agricultural and Environmental Science, Univ. of Newcastle, NE1 7RU - Newcastle upon Tyne, UK

* Corresponding author (egonzale{at}ual.es)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The leaching processes of the insecticide imidacloprid [1-(6-chloro-3-pyridinylmethyl)-N-nitro-2-imidazolidinylideamine] and the fungicide procymidone [N-(3,5-dichlorophenyl)-1,2-dimethyl-1,2-cyclopropanedicarboximide] in a greenhouse soil from the southeastern of Spain were investigated. Four separate pesticide applications were made at dose rates considerably higher than the recommended in normal agronomic practice, representing a worst case scenario. Soils samples were taken to a depth of 40 cm at time intervals after each application and analyzed by high performance liquid chromatography (HPLC). The partition coefficients (K_d) of the samples for imidacloprid and procymidone were calculated by carrying out batch experiments and fitting the experimental data point to the linear isotherm equation. Soil tension, water content, and temperature measurements were also determined during all the experiments. Although the results show a high degree of variability, rapid transport of pesticides through the soil occurred which increases the possibility of groundwater pollution. The leaching of these pesticides, particularly procymidone, generally thought of as immobile, might be possible through formation of stable soluble organic fraction–pesticide interactions in solution, allowing an increased groundwater contamination potential.

Abbreviations: DOM, dissolved organic matter • EU, European Union • HPLC, high performance liquid chromatography • K_d, partition coefficient

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE FATE AND TRANSPORT of organic pollutants and pesticides in the environment has become a matter of great concern, because of the increasing use of soils for the disposal of chemical wastes, and the generalized use of manufactured chemicals for several purposes in agricultural practices. The environmental impact of pesticides will be affected by properties such as their solubility, ionizability, and reactivity with mineral and organic surface groups or persistence in soil media (i.e., volatility and resistance to biodegradation). Likewise, soil properties such as organic matter and clay content, pH, and ion-exchange capacity affect the behavior of the chemical.

Owing to the development of intensive horticulture in the Almería province (Tout, 1990), pesticide use (mainly foliar and soil insecticide and fungicide application) has increased considerably and this southeastern region of Andalucía is now a focus for research of the occurrence of pesticides in groundwater (Chiron et al., 1993, 1995; Fernández-Alba et al., 1998). This intensive horticultural production is based on a raised layered bed system and conducted in plastic greenhouses. The Campo de Dalías is the most economically important agricultural area in the whole of the province of Almería. The climatic conditions (semi-arid) permit the cultivation of numerous horticultural crops at times when the market has not been well supplied, which contributes to the economic development of Almería (Tout, 1990).

These intensive greenhouse activities involve a demand for irrigation supplied from wells drilled into the subterranean aquifers running through the local rocks. There is concern that these aquifers are becoming depleted and also are contaminated with pesticides (Bosch, et al., 1991; Parrilla, et al., 1994; Chiron et al., 1995).

Although the groundwater level in the Campo de Dalías is very deep in most areas (>100 m), the vadose zone is largely made up of limestone-dolomite fractured rock corresponding to the Medium Pliocene, which will allow very rapid penetration of water after passing through the soil layer. Clearly, if groundwater pollution is due to pesticide leaching from normal agricultural usage, then vadose transport of water and pesticides must be studied. In particular, it is important to have information about the movement of water and pesticides through the soil layer, which acts as a restrictive boundary between the surface where the pesticides are applied, and the regolith below. Such study will help to provide information on the conditions under which pesticide transport is most likely, and will assist in the development of mitigation strategies to reduce groundwater pollution.

Organic matter amendment to soil is a very common practice in areas of intensive horticultural production (Pérez de los Cobos, 1960). There is evidence that increased soil organic matter can lead to increased concentrations of dissolved organic matter which interacts with organic contaminants, so affecting the fate of these contaminants in the soil or aquatic systems (Chiou et al., 1986; Nelson et al., 1998; Johnson et al., 1997; Guo et al., 1993).

Imidacloprid is a systemic insecticide used as a seed-dressing soil and foliar treatments in a range of crops (Tomlin, 1999; Kagabu et al., 1992; Moriya et al., 1992). Procymidone is a systemic fungicide with protective and curative properties (Tomlin, 1999). Both pesticides are widely applied in Almería greenhouses.

The main objective of this paper is to study the leaching process of the pesticides imidacloprid and procymidone, in the artificial soil of a typical Almería greenhouse to generate a set of data that could validate laboratory studies and existing models for the movement of applied pesticides through the surface layers of the soil, so obtaining further information about the fate of pesticides in the soil environment.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Layout and Preparation
A series of four studies were made to investigate pesticide leaching in a greenhouse, which was selected at the CIDH experimental center (La Mojonera, Almería, Spain). The greenhouse was typical for the area, being covered by plastic sheeting and containing a layered soil (sand overlaying clay, overlaying the native soil). Two plots within the greenhouse were selected. One was used for the first and third studies, and the other was used for the second and fourth studies.

A total of 11 tensiometers were installed to allow monitoring of soil water potential at three depths: 20, 40, and 70 cm. Soil thermometers were installed at depths of 20, 40, and 70 cm, and connected to a datalogger. A thermometer was installed near the middle of the greenhouse to measure the air temperature at a height of 1.6 m during periods in which a crop was in the greenhouse.

Prior to application of pesticides, soil samples were taken to check for the presence of the test pesticides. No residues of the pesticides studied were found.

Agronomic Practices
The crop planted in each of the four experiments was climbing beans (Phaseolus vulgaris). The sowing, emergence, and removal dates are shown in Table 1. Maintenance of the crop during the course of the experiment followed typical practice for the greenhouse.

Soil
The soils in greenhouses in Almería generally consist of a layered substrate laid upon the original soil surface, as the original soil is of poor quality. In this greenhouse, the anthropogenic layers consisted of a layer of clay-rich soil (for good water retention, an 8- to 10-cm depth), and a layer of sand (10 cm, for a good growing medium for plant roots) (Cermeño, 1990). Peat that was laid down between the clay and the sand when the greenhouse was installed had dissipated and a distinct layer could not be identified. The anthropogenic soil layers are laid down to an uniform thickness over the ground area. Two distinct horizons of the native soil were distinguishable with the upper horizon having a depth of around 40 cm. The native soil was a calcareous soil (Camborthid) and the clay-rich layer may be classified as Xerosol-Luvic, both from the Almería region.

The individual layers of the soil from the greenhouse were characterized in terms of their physical properties. Air-dried samples, <2-mm particle size, were analyzed by standard methods. Soil pH was determined in a 1:2.5 soil/water suspension using a glass electrode (Jackson, 1982); organic C content was determined by dry combustion (Nieuwenhuize et al., 1994); soil texture was determined by the hydrometer method (Black et al., 1982); water retention characteristics were determined following the guidelines by Hall et al., (1977) and corrected to take into account the stoniness of the soil; bulk density was determined by using intact soil cores (Uhland, 1949). The properties of the various layers are shown in Table 2.

View larger version (13K): [in this window] [in a new window]   Fig. 1. Molecular formula and selected properties of imidacloprid and procymidone.

The spray operations were carried out using a hydraulic sprayer at a tank pressure of 2 MPa and a single lance with a flow rate of approximately 0.65 L min^-1. The chemigation applications of imidacloprid were made by adding Confidor to a dispensing tank in the irrigation system.

Sampling
Soil samples were taken to a depth of 40 cm using an Ackermann Core Sampler (Van Walt Ltd., Surrey, UK). Deeper sampling was not possible because of the stoniness of the soil below the clay-rich layer. Samples were taken at six to eight time points after each application in each portion of the greenhouse. At each time point, three soil cores of a 7-cm diam. were taken midway between two plants from randomly selected plots from each portion of the greenhouse. Occasionally, extra samples were taken instead of the usual three, as part of a wider study into the effects of drip irrigation on pesticide movement (data not shown). The cores were split into 10-cm long sections, mixed in plastic bags and stored at -20°C until analysis. The holes were filled with fresh soil to avoid preferential flow pathways for water and pesticide movement.

Sorption Experiments
As sorption of pesticides by soil is one of the most important factors affecting their fate in the soil environment (Troester et al., 1984; Calvet, 1980), we have studied the adsorption processes of imidacloprid and procymidone on the greenhouse soil. Solutions of the pesticides were prepared in CaCl₂ (0.01 M) and 2 mL of these solutions were added to 1 g of air-dried soil in 25-mL glass vials with caps. Triplicate samples of each soil were used with four initial concentrations ranging from 0.5 to 2 mg L^-1. After shaking for the equilibrium time at 25 ± 1°C, the samples were centrifuged at 10000 rpm for 10 min. Preliminary experiments were conducted for various time intervals to determine when sorption equilibrium was reached. The time required for equilibrium to be reached between the pesticides sorbed and the pesticides in solution were 2 h for procymidone and 24 h for imidacloprid.

The concentrations of procymidone and imidacloprid in the supernatant solutions (C_e) were determined by HPLC as described in the section of analysis. The amount of pesticide sorbed (X) was calculated from the difference between the initial and equilibrium solution concentrations.

Extraction and Analysis
Two subsamples of approximately 20 g of the individual layers of the soil were placed in glass stoppered conical flasks and shaken with 50 mL of acetone. After shaking for 24 h at 25 ± 1°C, the samples were decanted and centrifuged at 12 000 x g (10000 rpm) for 10 min. The supernatants were evaporated in a rotary evaporator at 60°C and taken up to a final volume of 2 mL with an (1:1) acetonitrile/water mix.

The solutions were filtered through 0.45-µm nylon filters and analyzed for pesticides by HPLC using a Beckman liquid chromatographic system (Beckman-Coulter Inc., Fullerton, CA) equipped with diode-array detector and data station. Both pesticides were determined by isocratic elution using a mobile phase of acetonitrile/water (35:65) for imidacloprid and acetonitrile: 25 mM H₃PO₄ water solution (70:30) mix for procymidone measurements. The flow rate of the mobile phase was 1 mL min^-1, and the injection volume, 20 µL. Imidacloprid was analyzed at 269 nm and procymidone at 212 nm, their wavelengths of maximum absorption. The extraction efficiencies obtained were 79% for imidacloprid and 86% for procymidone. Blanks containing no pesticide were used for each series of experiments. Additionally, the moisture content was determined gravimetrically for all soil sections by heating soil samples to 110°C.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sorption Studies
The sorption capacity was calculated from the K_d parameter of the linear isotherm equation (Voice et al., 1983) (Eq. [1]),

[1]

Where X is the amount of pesticide adsorbed per kilogram of soil (mg kg^-1), C is the equilibrium solution concentration (mg L^-1), and K_d is the adsorption coefficient (L kg^-1). The adsorption of both pesticides was compared using the K_d parameter of the linear isotherm and values are shown in Table 4 together with the correlation coefficients.

Soil Tension, Water Content, and Temperature
Soil tension over time is shown in Fig. 2 . The tension was generally greater for deep soil than for the soil at the 20-cm depth. Between November 1997 and March 1998, the soil tension at each depth remained fairly constant (5 KPa at 20 cm, 15 KPa at 40 cm, 20 KPa at 70 cm). The direction of water flow was downwards under these circumstances. Between April and May 1998, the tension at all depths increased. At the same time, the variability of the tension also increased. The variability was greatest at the 20-cm depth, which indicates that there was significant soil drying between irrigation events. On days when the tension at the 20-cm depth was greater than the tension at the 70-cm depth, the direction of water flow was upwards (capillary rise).

View larger version (30K): [in this window] [in a new window]   Fig. 2. Soil tension measurements at three depths taken between 0900 and 1000 h.

Figure 3 shows the average nighttime soil temperature at three depths. There is a very little difference between the temperature values at the three depths, which indicates that the thermal conductivity of the soil is high. As can be seen, the minimum values are reached in January (around 15°C) and the maximum values in summer (at least 25°C). According to this, the degradation rate in summer is expected to be much higher than that in winter, potentially by a factor of 2.2 (FOCUS, 1996; Walker and Allen, 1984).

View larger version (19K): [in this window] [in a new window]   Fig. 3. Average night-time (1800–0600 h) soil temperature measurements at three depths

View larger version (50K): [in this window] [in a new window]   Fig. 4. Imidacloprid residues from the (a) first, (b) second, (c) third, and (d) fourth experiments.

View larger version (54K): [in this window] [in a new window]   Fig. 5. Procymidone residues from the (a) first, (b) second, (c) third, and (d) fourth experiments.

In the first experiment (Fig. 4a), imidacloprid concentration in the sand layer dropped by a factor of ten over the course of the experiment. In the 10- to 20- and 20- to 30-cm layers the concentrations of the pesticide were generally in the range from 0.01 to 0.1 mg kg^-1 and the concentrations in the 30- to 40-cm layer were in the range 0.001 to 0.01 mg kg^-1 in the period from 28 d to 155 d. In the second experiment (Fig. 4b), imidacloprid dissipated from the sand layer more quickly than in the first experiment, probably because of higher temperatures. The concentrations below the 10-cm depth were lower than in the first experiment, probably because of the lower application rate of the pesticide. The variability was greater than in the first experiment, and in some samples, concentrations in excess of 0.01 mg kg^-1 were found in the 30- to 40-cm layer. The third and fourth experiments (Fig. 4c and 4d, respectively) have a similar pattern to the first experiment, but the concentrations were higher. The higher concentrations may be because of carry-over from the application in the first experiment. Additionally, they may be because of the greater amount of irrigation in Exp. 3 and 4 compared with Exp. 1 and 2, and the fact that chemigation was used as the application method in Exp. 3 and 4 (rather than spray).

To understand the significance of the imidacloprid soil concentrations above obtained, it is useful to predict the fate of the pesticide, to consider how the imidacloprid soil concentration relates to the imidacloprid soil water concentration. The expected soil water concentration in the upper native soil can be calculated as follows: assuming that T is the bulk concentration of imidacloprid in the soil, that is, imidacloprid sorbed + imidacloprid in solution (mg kg^-1),

[2]

where C is the concentration of imidacloprid in the soil water (mg L^-1), is the soil water content (L L^-1); D_b is the bulk density of the soil (kg L^-1); X is the amount of pesticide adsorbed per kilogram of soil (mg kg^-1).

Substituting in Eq. [2] the value of X by the linear isotherm expression, X = K_d xC, then,

[3]

The value for was assumed to be 0.13 L L^-1, according to the soil water content at 33 KPa (see Table 2). For the bulk density parameter we assumed a value of 1.6 kg L^-1 (Table 2). Therefore, for bulk concentrations (T) of 0.001, 0.01, and 0.1 mg kg^-1 (the concentration range obtained in the experiment planned), the corresponding concentrations of imidacloprid in the soil water (C) are 1.95, 19.6, and 195.6 µg L^-1, respectively. It is interesting to note that all these values exceed the maximum permitted levels in the groundwater as defined by the European Union (EU) directive 91/414 (0.1 µg L^-1). The imidacloprid soil water concentrations calculated here do not directly transform into groundwater concentrations, but they are indicative of the potential risk of contamination of the groundwater especially when the pesticide is continuously used in a great amount. Therefore, considering these data taken in conjunction with the slow degradation rates found by other authors for this same pesticide (López-Capel et al., 2000), there is clearly a potential risk that should be investigated more extensively.

Procymidone
The data about the residues of procymidone fall into two groups: those from the first two experiments (Year 1) and those from the final two experiments (Year 2). In the first two experiments (Fig. 5a,b) very little movement was observed; procymidone reached the clay-rich soil layer only rarely, and was hardly ever observed below this level. In the final two experiments (Fig. 5c,d), considerable amounts of procymidone were found in lower layers, frequently at levels in excess of 0.1 mg kg^-1. It is possible that residues from the previous year contributed to the residues in the lower layers, or that procymidone was transported faster through the soil in the second year. At the same time, the occurrence of procymidone at levels below the 20-cm depth is interesting because it would not be expected from its physicochemical properties. Procymidone is generally thought of as immobile (Gustafson, 1989), and short-lived in alkaline conditions because of hydrolysis.

Fast transport may have been possible by preferential flow through macropores as described by Larsson and Jarvis (1999) or by binding of procymidone to soluble organic matter, decreasing its effective sorption as has been described for other pesticides by other authors (Williams et al., 2000). Previous studies have indicated that low concentrations of dissolved or suspended particulate-bound natural organic matter in water can significantly enhance the solubility and stability of many hydrophobic organic compounds (Chiou et al., 1986; Carter et al., 1983; Landrum et al., 1984). The data here reported suggest that procymidone leaches and is protected from degradation from alkaline hydrolysis so it might be inferred that the movement of procymidone in the greenhouse soil is because of transport facilitated by the binding of this pesticide to the dissolved organic matter. This fact is in accordance with previous experimental results obtained by these same authors by studying the mobility of diuron [N'-(3,4-dichorophenyl)N, N-dimethylurea] and atrazine (6-chloro-N²-ethyl-N⁴-isopropyl-1,3,5-triazine-2,4-diamine) in soil columns filled with peat-amended soils (González-Pradas et al., 1998; Socias-Viciana et al., 1999). Besides, the same soil was used for studying the mobility of imidacloprid in presence of dissolved organic matter (DOM) and it was experimentally checked that the higher content of DOM, the higher mobility of the pesticide (Flores-Céspedes et al., 2002).

As discussed in the case of imidacloprid, the expected procymidone soil water concentrations for the native soil were also calculated from Eq. [2]. Assuming a sorption coefficient as found in Table 4, for bulk soil concentrations of 0.001, 0.01, and 0.1 mg kg^-1, the equivalent procymidone soil water concentrations are 0.6, 5.6, and 55.8 µg L^-1, all of them exceeding the level of 0.1 µg L^-1 set out in the EU drinking water directive. Therefore, and although the hydrophobic character of procymidone, the values obtained indicate a potential risk for this pesticide to leach through the soil.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This study has provided a large body of data on the physical soil environment as well as the residue of pesticides in a greenhouse soil in Almería. The pesticide residue data show a high degree of variability, which is thought to be because of the heterogeneous nature of this environment. Despite the variability in the data, both pesticides are shown to have been transported throughout the top 40 cm of the soil profile within 2 yr of their first application. It is postulated that either preferential flow pathways are responsible, or that the dissolved organic C has reduced pesticide sorption through stable soluble organic fraction–pesticide interactions in solution so allowing enhanced leaching process and an increased groundwater potential contaminant for these pesticides. The calculated concentrations in the soil water are sufficiently high to prompt further study of pesticide movement in this environment and there is some evidence of carry-over of pesticide from one year to the next. Clearly, this could lead to build-up of pesticide concentrations if repeated applications are made, which is an undesirable situation. It must be remembered that the dose rates in the experiment were higher than what would be used in normal agronomic practice, and that certain conditions of application represent a worst case scenario, the data point to a need for further understanding of the processes governing pesticide transport in this environment.

Obvious steps to take with respect to improving our understanding of pesticide fate in this environment are the application of a relevant model, and further study of spatial heterogeneity, particularly with respect to the effect of the drip irrigation method employed.

	ACKNOWLEDGMENTS

 
This research was supported by the European project with contract no. SMT4-CT96-2048 (DGXII-RSTM).

Received for publication October 15, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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