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a Dipartimento di Agronomia, Selvicoltura e Gestione del Territorio, Università di Torino, Via Leonardo da Vinci, 44, 10095 Grugliasco (TO), Italy
b Istituto di Chimica Agraria, Università di Catania, via S. Sofia, 98, 95100 Catania
c Dipartimento di Valorizzazione e Protezione delle Risorse Agroforestali, Sezione Chimica Agraria, Università di Torino, Via Leonardo da Vinci, 44, 10095 Grugliasco (TO), Italy
Corresponding author (ferrero{at}agraria.unito.it)
Received for publication December 24, 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DAT, days after treatment • HPLC, high performance liquid chromatography
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The main aim of the study, carried out in 1997 and 1998, was to determine the behavior of cinosulfuron in the surface waters of rice paddies and the ground water below the paddies. Another objective was to investigate cinosulfuron behavior in water at various pH levels so as to gather information on its degradation dynamics for use in the interpretation of field data. The study was conducted in a traditional rice-growing area with three relevant characteristics: high representativeness of the paddy environment, good hydrological characterization, and no other cinosulfuron-treated paddies uphill or upgradient. These three conditions are not usually found together. It is, in fact, very hard to find rice-growing areas that are both sufficiently isolated from others (in hydrological terms) and representative of the paddy environment.
Paddies have two peculiarities. First, a paddy's surface water covers its entire area almost continuously. Water is supplied serially from the topmost to the bottommost paddy and is regulated by floodgates placed in the embankments that separate paddies. This means that the effects of the distribution of a herbicide in a paddy are not only felt in the area of direct application. The results provided by sampling superficial water and ground water at individual points can therefore be influenced by actions that do not pertain to the survey site. Second, flooding of the ground for most of the growing period has a marked influence on the flow of solutes from the surface to the subsoil waters and results in abundant exchanges between these two bodies of water. Changes in the quality of the surface water are thus also likely to appear relatively quickly in the ground water.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Sampling Site Characteristics
Within the study area, a specific paddy field (test paddy) was considered. The test paddy is located within and downhill from the study area (Fig. 2). It measures about 90 x 240 m and extends from west to east. The test paddy is the most downhill field of a series of connected fields. As for the other fields of the series (except the first), the test paddy receives the water from the uphill paddy field. As is usual in this area, the water inlet and outlet floodgates of the test paddy are set opposite each other (Fig. 3)
in prefabricated reinforced concrete housings. The soil characteristics of the test paddy were determined by considering three core samples taken to a depth of 60 cm from the surface. Each core was divided into six 10-cm portions, representing the corresponding soil layers. The soil characteristics of the test paddy are reported in Table 1. The texture, pH, and organic matter content values varied by the depth. A comparison between the first and the last 10 cm shows that the rock fragments ranged from 54.5 to 533.4 g kg-1 and the sand (coarse + fine) from 54 to 71.8% of the fine earth. The pH rose from 5.3 to 6.6 and the organic carbon content decreased from 1.3 to 1.1%.
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The pond was artificially formed with the extraction of gravel. It lies some 500 m to the southeast of the test paddy, and 250 m south to the nearest cinosulfuron-treated paddy. Its area is 10500 m2 and its average depth is 9.5 m below the site plane. Its water depth ranged from 1.5 to 4 m in 1997 and 1998 and the volume of water varied from 15750 to 42000 m3. The lowest level was reached in 1997, when it was frequently exploited to make up for surface water shortages. About 90% of its water comes from the first ground water body, which is exposed, owing to the shape of the basin. The balance is supplied by a drainage pipe, at a minimum depth of about 0.4 m, which intercepts water percolating from a series of rice fields placed beside the study area and not treated with cinosulfuron. The pipe depth does not disturb any mechanical operation as the maximum tillage depth in Italian rice fields is about 20 cm.
The two wells, one uphill and one downhill to the study area, were included in 1998. The first well is in the courtyard of the Bischiavino farm buildings. It is 80 m from the nearest rice field (to the west) and about 250 m from the test paddy. The second well is in the courtyard of the Pompogno farm buildings, about 700 m to the south of the test paddy. As already mentioned, the rice fields between this well and the test paddy have never been treated with cinosulfuron. The uphill well was sunk in 1994 and the downhill one in 1989 and these are used for irrigation purposes when there is a shortage of surface waters. According to the available drawings in the farms, both wells are characterized by the same stratigraphy (as mentioned before) and are surrounded by a packing of fine gravel along their full depth. Their inside diameter is 114 mm and they have openings at a depth of about 60 to 68 m.
Two piezometer pipes (shown as A, nearer the resurgence, and B in Fig. 3) were installed in 1998 ten meters from each end of the southern embankment of the test paddy. They consist of lengths of 63-mm-diameter steel tubing driven into the ground to a depth of about 7 m. There are 240 holes (2 mm diameter) in approximately the lowest 2 m of each pipe.
The following parameters were investigated from the moment the herbicide was applied until the end of the sampling period:
Herbicide Application and Water Management
Cinosulfuron was applied in test paddy as well as in the entire study area (35 ha) at 70 g a.i. ha-1, as the commercial herbicide Setoff (Novartis Crop Protection AG, Basel, Switzerland), with a rear-mounted boom sprayer. This implement had a 600-L-capacity tank and a 12-m-long boom, equipped with flat-fan nozzles. The forward speed during the treatment was 1.7 m s-1, the pressure was 240 kPa, and the application volume was 400 L ha-1. The herbicide was applied to fields flooded with 16 cm of water at the second leaf stage of the crop, on 24 Apr. 1997 and 19 Apr. 1998. After the treatment, water circulation in the paddy was stopped for 14 d in 1997 and 22 d in 1998. Further interruptions of water circulation occurred from 22 to 30 and from 52 to 57 DAT in 1997 and from 30 to 37 and 52 to 65 DAT in 1998. When the floodgates were open, the water depth in the test paddy ranged between 9 and 11 cm. In order to control those weeds that are not controlled by cinosulfuron, such as barnyard grass [Echinochloa crus-galli (L.) P. Beauv.] and mudplantain (Heteranthera reniformis Ruiz et Pavon), the flooded paddy was treated 2 to 3 d before rice planting with a mixture of oxadiazon (5-tert-butyl-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4-oxadiazol-2(3H)-one) (Ronstar; Rhone-Poulenc AG Company, North Carolina; 0.25 kg a.i. ha-1) and thiobencarb (S-4-chlorobenzyl diethylthiocarbamate) (Saturn 90 EC; Dow AgroSciences, Indianapolis, IN; 1.84 kg a.i. ha-1).
Samplings
Samples were collected from the sampling stations (test paddy, spring, pond, wells and piezometers). Six 1-L subsamples were taken from each location in 1-L LD-PE flasks (Bracco, Milan, Italy) on each occasion. Within 3 h after the sampling, the six subsamples were pooled, and then mixed in closed glass flasks for 10 min using a laboratory stirrer. Six samples (three for analysis and three for reserve) of 500 mL each were withdrawn from this pool, and immediately stored at -18 to -20°Cuntil they were analyzed. The sampling procedures and scheduling for each station were as follows:
Test paddy: samples of paddy water were taken in the top 5 cm of the water layer, by direct filling the flasks, in the east, center, and west parts of the paddy before and immediately after the treatment, after 2 d, and then every 7 d for 60 d. Two samples were taken from each floodgate before treatment, and after the first opening of the floodgates every 7 d for 60 d. Sediment samples were collected using a stainless steel shovel shaped to take the top 1 cm of soil in the east, center, and west parts of the paddy before and immediately after treatment, and after 21, 42, and 64 d. In 1998, a further sample was taken 15 DAT.
Spring: samples were taken before and 42 and 293 DAT (there was no water in the intervening period) in 1997 and before and 29, 57, 90, 131, and 159 DAT in 1998.
Pond: samples were taken before and 8, 42, 71, 92, 123, and 293 DAT in 1997 and before and 29, 57, 90, 131, and 159 DAT in 1998.
Wells: samples were taken before and 29, 57, 90, and 131 DAT.
Piezometers: samples were taken before and 8, 22, 29, 36, 43, 50, 57, 64, 90, and 131 DAT. Samples were taken with a piston pump. Prior to sampling, the piezometers were pumped for about 10 min to allow for complete removal of stagnant water and to provide a true representation of the current state of the ground water.
Analysis of the Samples
Extraction of Cinosulfuron from the Water Samples
Liquid–liquid separation was performed with dichloromethane starting from 100 mL water acidified to pH 2 with 1 M HCl. This was done three times: once with 100 mL and twice with 50 mL dichloromethane. The organic phases were combined and dried with a rotary evaporator at 30°C. The residue was taken up with 2 mL water and acetonitrile (50:50 v/v) and analyzed by high performance liquid chromatography (HPLC).
Extraction of Cinosulfuron from the Sediment Samples
The analysis was performed on water saturated samples. The sediment was allowed to settle for about 2 h. The excess water was drawn off with a pipette and then with blotting paper. The samples now had an average of 50% (w/w) water. Extraction was carried out on 15 g of sediment by adding 15 g kieselguhr (Extrelut 1.13076, Merck) and 1 mL 0.1 M HCl, shaking, and adding 100 mL of 5% methanol in dichloromethane. The suspension was transferred to 250-mL screw-top glass bottles and shaken on a reciprocating mechanical shaker at 150 rpm for 30 min. Following decantation of the suspended material phase, the liquid phase was transferred to a vacuum flask. The solid phase underwent another two extraction operations each with 75 mL of the extracting solvent and 15 min shaking. The combined liquid extracts were dried using a rotary evaporator at 30°C. The residue was taken up with 2 mL water and acetonitrile (50:50 v/v).
Analytical Determination
This was done with a PerkinElmer (Wellesley, MA) 250 HPLC equipped with a 50-µL loop and a PerkinElmer 235 diode array detector set at 220 nm. A Spherisorb-octyl (5 µm, 250 mm x 4.6 mm) analytical column (Supelco, Milan, Italy) was used. The mobile phase was water acidified to pH 3 with orthophosphoric acid (A) and Far-UV HPLC grade acetonitrile (B) (Aldrich, Milan, Italy). This phase was 50% A and 50% B for the water analysis. In this analytical condition, retention time was 5.35 min. The following gradient was used for sediments: 58% A for 8 min, from 58 to 30% A in 5 min, 30% A for 7 min, from 30 to 58% A in 5 min. The flow rate was always 1 mL min-1. In this analytical condition, retention time was 7.13 min.
Concentrations of cinosulfuron were determined in duplicate by reference to peak areas of standards solutions. The recovery of cinosulfuron was 95 ± 3% from water and 81 ± 5% from sediments. The detection limits were 0.10 µg L-1 for water and 0.50 µg kg-1 for sediments. The recovery tests from water and sediments were made during the methodological set up, for fortifying concentrations ranging from 0.5 to 50 µg L-1.
Soil Analysis
The soils were analyzed in accordance with the official methods of the Italian Soil Science Society (Società Italiana della Scienza del Suolo, 1985). The texture was determined by the hydrometer method, the soil pH was determined on a 1:10 sample to water suspension, and the organic carbon content was assessed by K2Cr2O7 oxidation.
Degradation of Cinosulfuron in Solutions Buffered to Different pH Values
A series of screw-top Erlenmeyer flasks containing 15 mL of buffer solution at pH 4, 5, 6, 7, and 9 were sterilized in an autoclave, allowed to cool, and supplemented with 3 mL of an aqueous solution of cinosulfuron (50 µg L-1) sterilized by filtration. The flasks were then kept in the dark at a constant 25°C temperature. Three flasks per pH value were taken for HPLC analysis immediately after the addition of cinosulfuron and then after 17, 41, 66, 95, 111, and 152 d, as described for the water samples.
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RESULTS AND DISCUSSION |
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The rainfall during the study period in 1998 amounted to 355 mm. This figure is near the historical average for the area (Biancotti et al., 1998).
The inlet and outlet water volumes corresponded to 1400 and 1142 mm, respectively. These figures correspond to the mean quantities stated for the requirements of Piedmontese rice-growing areas (Luppi and Finassi, 1981).
The reference crop evapotranspiration (ET0) was calculated with the Priestley–Taylor formula, which uses the daily maximum and minimum temperature and relative humidity data and the incident radiation. The weather station that was set up on the site provided the required maxima and minima. The temperatures for 1998 corresponded to the average for the area. The radiation was estimated with the model proposed by Donatelli and Campbell (1998) using the parameter values published by Ducco et al. (1998) following their calibration and validation of the model for the entire Po Valley. The ET0 for 1998 was 425 mm with a maxima of 5.7 mm d-1 in July. These values agree with those of previous studies of the Piedmontese rice-growing area. In the conditions of the paddy, the crop evapotranspiration (ETc) can be regarded as equal to the ET0. The mean daily percolation was 2.3 mm in 1998. This value is within the normal range given in literature (0.2–15.6 mm d-1) for similar rice fields (International Rice Research Institute, 1978; Luppi and Finassi, 1981; Yoshida, 1981; Mikkelsen and DeDatta, 1991).
The ground water in the tract between the two piezometers showed a remarkable slope from B (west position) to A (east position). The ground water level, with reference to the site plane, was between -5.82 and -4.49 m (mean ± standard deviation: -5.60 ± 0.09 m) in Piezometer A and between -5.31 and -4.32 m (-4.67 ± 0.31 m) in Piezometer B. The ground water level in Piezometer A was always deeper. This data, together with the indications given in the Fig. 1 regarding the ispophreatic contours of the study area, suggest that the water table flows in the northwest to southeast direction.
Cinosulfuron in the Test Paddy: Flood Water, Inlet and Outlet Floodgate Water, Sediment
No cinosulfuron was present in the paddy water before the 1997 or 1998 treatments were carried out (Table 3). Samples taken immediately after the treatments gave a mean content of 39.52 and 41.45 µg L-1 for the two years, respectively. These figures agree with the amounts used and the current volume of water in the paddy. In both years the concentration fell by about 60% during the period when the inlet and outlet floodgates were kept closed (14 and 22 d in 1997 and 1998, respectively). This reduction can only in part be attributed to degradation phenomena due to hydrolysis and microbial degradation. On the basis of the degradation tests (see below), it is possible to estimate that only about 5% of the product is subject to hydrolysis at pH values of the paddy water (pH 6 to 7) during the period the floodgates are closed. As far as the microbiological degradation is concerned, it was not possible to identify a microbiological population that was able to degrade the molecule in other tests that the authors carried out (Gennari et al., unpublished data, 1999). The slow degradation of the molecule would lead one to believe that the majority of the product no longer found in the water moved into the soil profile or underwent photolysis. After the floodgate is opened a great decrease of the cinosulfuron concentration can be observed in the paddy water, mainly due to the water flow. This behavior is evident from the higher concentrations of product in the outgoing water than in the incoming water that flows from uphill of the treated paddy. When the floodgates were reopened, the cinosulfuron content continued to fall and was no longer detectable by the 64th day in 1997 and the 78th day in 1998. In the 1998 experiment the herbicide concentration showed an increase on the 57th and 64th days. This behavior was most likely ascribable to the closure of the inlet and outlet floodgates between the 52nd and 65th days, which prevented cinosulfuron dilution by means of water circulation and resulted in a marked fall of water volume by evaporation loss or deep percolation. Moreover, desorption of cinosulfuron from sediment could occur. The highest concentrations were always observed in the west end of the paddy where there was less circulation, since the inlet and outlet floodgates were located at the east end.
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Cinosulfuron in the Pond and Spring Water
In both years, no cinosulfuron was present in the pond and spring water before the treatment began (Table 7). Cinosulfuron detected in the pond water in the 1997 experiment ranged from 0.29 to 0.18 µg L-1 and was only recorded between the 71st and the 133rd DAT. In 1998 lower concentrations (max 0.17 µg L-1) were found over a shorter though earlier period than the previous year. This cinosulfuron presence can probably be related to the origin of the water that fed the pond even though it was quite far (about 250 m) from the treated area. The pond water derives from the upper watertable and superficial draining pipe, placed under paddies that were not treated with cinosulfuron (Fig. 2).
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Degradation of Cinosulfuron in Various pH Conditions
Cinosulfuron degraded rapidly in the presence of high acidity (Table 8). At pH 4 it was, in fact, no longer detectable 66 d after the contamination. In a neutral or alkaline environment, on the other hand, its degradation was considerably slower. About 86% of the initial amount was still present after 111 d. These results are in agreement with those reported for other sulfonylurea herbicides (Harvey et al., 1985; Bray et al., 1997).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The hydrogeology of the study area and its water-bearing strata indicate that it can be regarded as a specimen unit of the rice-growing system. It can therefore be supposed that the fate of cinosulfuron in the water bodies is very similar, in quantitative terms, to that which would be detectable in a more extensive rice-growing zone.
The concentration of cinosulfuron in the water of the test paddy immediately after the treatment in both years was similar to values expected for the average doses stated on the label of the commercial formulation. Its progressive decrease in the period prior to the opening of the inlet and outlet floodgates can be in part attributed to adsorption by the sediment and leaching of the product. This is clear from the gradual accumulation of cinosulfuron in the sediment in the first 21 to 22 DAT. Hydrolytic degradation of the molecule, while very slow at the pH values of the paddy water during this initial period, may have only partly contributed to the decrease in its concentration. No data are available on possible photolytic degradation of the product. In view of the low Henry constant and vapor pressure values, its substantial volatilization can be regarded as unlikely.
In the days following the opening of the floodgates, the amount of cinosulfuron in the water of the test paddy gradually fell to levels below the detection limit of the analytical method about 2 mo after the treatment. This behavior follows the results of a previous study conducted by the Experimental Plant Nutrition Institute, Rome (Sequi et al., unpublished data, 1996), in other rice environments. It should also be pointed out that the reduction of the presence of the herbicide was quick, despite the continuous new input from the uphill rice fields.
The cinosulfuron levels also fell in the sediment after the floodgates were opened. This shows that the molecule was adsorbed by means of reversible mechanisms. This behavior agrees with the observation by Pantani et al. (1994) of a weak, reversible interaction between cinosulfuron and Al-, Na- and Ca-saturated montmorillonite.
Cinosulfuron in the subsoil ground water were found in the first 90 DAT in both piezometers and was higher in the downhill piezometer (A), which was under the influence of a large part of the treated area. Cinosulfuron was never detected in the deep waters (wells) during the study. The presence of cinosulfuron in the spring and pond was due to infiltration of the rice field waters into the initial horizons of the soil profile. In the case of the pond this infiltration was particularly attributable to the interception of percolating water by means of the drainage pipe.
The results of this study indicate that cinosulfuron is not subject to accumulation in the rice-growing conditions of study. The stability tests in water at different pH values also show that cinosulfuron is not stable, even in neutral and alkaline conditions, and that its degradation rate increases greatly below pH 6. This pH is near that of the soil in the top layer and water when the herbicide was applied.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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