Terrestrial Field Dissipation Studies - American Chemical Society

Chapter 18

Determining the Dissipation and Potential Offsite Movement of Pendimethalin through a Monitoring Survey and Environmental Modeling Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: December 15, 2002 | doi: 10.1021/bk-2002-0842.ch018

Patricia J. Rice, Gary D. Mangels, and Maximilian M. Safarpour Department of Environmental Chemistry and Metabolism, BASF Agro Research Division, Princeton, NJ 08543-0400

Dissipation, accumulation, and the potential offsite movement of pendimethalin, a soil-applied herbicide, were determined by conducting a retrospective monitoring survey and environmental modeling. The test area for the monitoring survey had a history of annual pendimethalin applications. Soil, water, and sediment samples were collected and analyzed. Information from laboratory and field dissipation studies was utilized in the environmental modeling to predict potential long-range transport and deposition to soil, water, and sediment. The monitoring survey has shown very low concentrations of pendimethalin in the treated soil. Residues were not detected in the non-agricultural soil, surface water or sediment, which indicates no accumulation of pendimethalin, at levels greater than the limit of detection, as a result of degradation, offsite movement through runoff and/or atmospheric transport and re-deposition. Model simulations estimate that the concentration of pendimethalin in soil, water, and sediment resulted in concentrations below the level of ecological concern.

© 2003 American Chemical Society

In Terrestrial Field Dissipation Studies; Arthur, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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274 Pendimethalin, a pre-emergence and early post-emergence herbicide used to control broad leaf weeds, has been applied to soil throughout the world for over 25 years. Currently this compound is undergoing reregistration in Europe. In the evaluation process concerns have been raised that long-term use, persistence, and slow volatilization of pendimethalin may result in soil accumulation, off-site movement, and atmospheric transport and deposition. Laboratory studies indicate pendimethalin persists in soil and slow volatilization (3-7% applied dose within 4 months) occurs over time (1-4). European terrestrial field dissipation studies have shown moderate persistence of this compound in soil with half-lives ranging from 27 to 155 days (84 day average) and dissipation resulting from degradation and volatilization rather than leaching ( i ) . As a result of these concerns, a retrospective monitoring survey of a site, in Spain, with a history of annual application of pendimethalin was performed to evaluate the environmental fate of this herbicide. The objectives of the monitoring survey were to collect and analyze soil, water, and sediment samples from the lower Tietar Valley to determine: 1) soil concentrations and potential accumulation of pendimethalin in treated agricultural soils, and 2) the potential transport, deposition, and accumulation of pendimethalin in non-agricultural areas of the watershed as a result of offsite transport. For the environmental modeling a Level III fiigacity model was utilized to predict the potential long-range atmospheric transport of pendimethalin to remote areas due to volatilization and deposition. Fugacity models have been shown to accurately predict the atmospheric transport of several organic chemicals (5). This chapter summarizes the monitoring survey and atmospheric transport modeling that was conducted for Europe.

Experimental Design Monitoring Survey The retrospective monitoring survey was conducted in December 1999 in the Tietar Valley in Talayuela, Spain. The test area had annual applications of pendimethalin to agricultural fields. Soil samples were collected from the treated fields and remote non-agricultural land (several kilometers down wind from treated field) within the lower Tietar Valley. Water and sediment was sampled at various sites along the Tietar River and Torrejon Tietar reservoir, which were located adjacent to, and down stream from, the pendimethalin treated fields. Selection of the test site, site characteristics, agricultural practices, location of sampling sites, and the sample collection and analysis


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procedures were recorded in Rice et. al. (2000) (6) and are briefly discussed in the following text. Test Site. The survey was conducted in the Tietar Valley, in Talayuela, Spain. The Tietar Valley contains large areas of intensive cultivation along with nonagricultural land used for pastures and the growing of oak trees. Seasonal brooks run through the treated agricultural land and empty into the Tietar River, which is adjacent to many agricultural fields. A n average of 13 tons of pendimethalin are applied in the Talayuela area each year, which accounts for 20% of the annual pendimethalin use in Spain. This area was selected based on several factors which include: 1) highest average annual pendimethalin application in the last five years within a province, 2) largest concentration of pendimethalin used within the sub-regions of the province with the largest load (tons/year), 3) proximity of the water courses through the treated site, 4) significant erosion factors: field slopes and rainfall intensity, 5) high cultivation intensity and occurrence of repeated treatments in the same fields, and 6) potential areas of sedimentation including dams or other water retention points within the water system.

Sample Collection SoilfromΡ endimethalin-treated Agricultural Fields Sample Sites. Soil samples were collected from agricultural fields located within the test area. Fifteen representative agricultural fields with a history of pendimethalin use were selected for the survey. These fields were in crop production and had received annual applications of pendimethalin over the last five years, with the exception of one field that was not treated in 1998. The treated fields were characterized as sand, loamy sand, or loam soil and were planted with tobacco for five consecutive years, with the exception of one field that was rotated with peppers. Three types of applications were distinguished: pre-transplanting, between rows, and de-suckering (Table I). Annual applications of pendimethalin ranged from 0.83 to 2.31 kg/ha (6). Sample Procedure. Soil samples were collected in December 1999, approximately 4 to 8 months after the last pendimethalin application. Ten soil cores were taken at random within each field from undisturbed soil using 30 cm χ 23 mm diameter P E T G zero contamination tubes (model J M C Backshaver Handle). The upper and lower ends of the cores were clearly marked. Each soil core was labeled, and placed in freezer within hours of sampling. Samples remained frozen during shipping and storage, prior to analysis.


276 Table I. The application history of pendimethalin to fifteen agricultural fields (1995-1999) Type of Application Pre-transplanting Between Rows De-suckering

Rate (kglhaf 0.83 -1.32 03 0.5-0.99

Tuneof Application April/May June August

Application Technique Broadcast Spray/Incorporation' Directed Spray Broadcast Spray 1


"Range of annual pendimethalin application rates within the fifteen fields ^Pendimethalin was sprayed onto the soil surface and incorporated to a depth of 3-7 cm

SoilfromUntreated Test Area Sampling Sites. Soil samples were collected from remote, nonagricultural areas located within the test site. Five representative areas with no history of pendimethalin use were selected for the survey. The nonagricultural areas were pastures with grasses, leguminous plants, rockroses, thyme and a few hoalm oak trees. Soil textures ranged from loamy sand to clay (6). Sample Procedures. Soil within the untreated test area was compact since this non-agricultural area was not regularly tilled. Therefore, bulb planters were used to collect the soil rather than the soil core samplers mentioned above. Ten 5-cm deep soil samples were randomly collected from each untreated test area using bulb planters. Different bulb planters were used at every site to prevent cross-contamination of the sample media. Ten soil samples from each site were combined to make one representative sample. Soil samples were stored frozen until analysis.

Water and Sediment Samples Sampling Sites. Water and sediment samples were collected at 14 locations from the Tietar River and reservoir system within the test site. Control samples were collected from one location in the mountain area, Sierra de la Serrana, which is a natural reservation without agricultural use. The accumulation of sediment particles, that may be a result of agricultural runoff, were found at regions of the river where water flow was slow. These regions were located in areas where the river branched, along the outer bank at the bend of the river, and near rock formations or small islands. Sampling Procedures. Water and sediment samples were taken from each sampling site. The sampling technique depended on the depth of the river and


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the nature of the riverbed. Water samples were collected prior to sediment in order to minimize the quantity of sediment particles in the water layer. Immediately after sampling, several physical parameters (pH, temperature, Redox, conductivity, dissolved oxygen) were measured to characterize the nature of the sample material. Detailed descriptions of the sample locations and observations have been documented (6). Collection of Water Samples. Depending on the depth of the water, the water samples were collected either by means of a sampling device (System Dr. Blasy) or by filling the glass bottles directly. Each water sample consisted of approximately 1200 ml of water. At least two replicate water samples were collected at every sampling site. The Dr. Blasy water-sampling device was used from a boat or a bridge. Before taking the water sample the depth of the river was measured using a plumb line. The Dr. Blasy device consisted of a sampling tube, an upper seal, lower seal with a drain, clip, and a messenger that were suspended from a cable. The decontaminated sampling device was preset with the upper and lower seals pulled away from the sampling tube, allowing the water to pass through the tube while it was lowered to the predetermined depth (mid-depth of water body). After the sampler had reached the required depth, the messenger was sent down to release the tube and to enclose the water sample between the seals. Water samples were retrieved and stored in labeled glass bottles. The procedure was repeated until sufficient sample material was collected. The direct sampling technique was utilized in shallow areas of the river (< 3 meters deep). Sample bottles were submerged in the water with the opening held down. Once the bottle was mid-depth in the water, it was turned 180 degrees so that the opening faced the surface of the water and water could flow into the bottle. Collection of Sediment Samples. The sampling technique for the sediment samples depended on the depth of the water, and the nature of the sediment and the riverbed at the corresponding position. The Ekmann-Birger dredge consisting of a sample container to entrap sediment in spring-loaded jaws was arranged so that the jaws were in open position. The sampler was lowered to a point just above the sediment surface by means of a cable and dropped sharply onto the sediment. The release mechanism was triggered by lowering the messenger down the line. The sampler dredge was raised and the sediment was transferred into a stainless steel bucket where it sat undisturbed until the sediment particulates had settled out of the water. The supernatant water was carefully decanted and the required amount of sediment was transferred into two labeled polyethylene containers. These containers were considered the two replicate sediment samples from each sampling site.


278 A stainless steel bucket was used to collect sediment from beneath a shallow water layer. The decontaminated bucket was used to scoop the top layer of sediment from a designated area. The sediment sample was collected in one continuous motion starting down stream and moving up stream to prevent the loss of any fine sediment particulates. The sediment was handled and placed into the appropriate containers as stated above.


Sample Preparation and Analysis Soil, water, and sediment samples were stored frozen until analysis (< 1 month after collection). The sample preparation and analysis are summarized below. Samples were extracted and the extracts were analyzed on a gas chromatograph (GC) equipped with an electron capture detector (ECD). The presence of pendimethalin residues was confirmed by G C / M S / M S . B A S F (formerly American Cyanamid Company) supplied the pendimethalin analytical standard. Soil. The top 5-cm of the ten cores from each treated field were removed, combined, homogenized, and placed in labeled containers. These soil samples were combined and sieved through a 3.5 mm mesh sieve to remove stones and debris and were thoroughly mixed to further homogenize the soil. Aliquots of the soil samples were analyzed for pendimethalin according to validated B A S F (formerly American Cyanamid) Method M 2514 (limit of quantification (LOQ) - 10 ppb, limit of detection (LOD) = 2 ppb) (7). Soil aliquots were extracted with acidic methanol (2%) and filtered. Hydrolchoric acid (0.1 N) was added to aliquots of the filtrate in a 1:1 ratio. The acidified filtrate was then passed through a conditioned C I 8 solid phase extraction cartridge (SPE). SPE cartridges were washed with deionized water and the pendimethalin was eluted with 1% methanol in hexane. Eluates were evaporated to dryness, redissolved in hexane, and analyzed by G C . Additional soil aliquots were removed and used to determine the moisture content. The same procedure was followed for the 5-20 cm soil core segments. The 20-30 cm portion of the soil cores remained intact and stored in a freezer. Identical protocols were followed to analyze the untreated soil samples, except these samples only consisted of the top 0-5 cm of soil collected with the bulb planters (6). Water. Replicate water samples from each sampling site were combined and homogenized. Water samples were analyzed according to the validated B A S F (formerly American Cyanamid) Method M 1715 (LOQ 0.1 ppb) (6,8). Aliquots of the water samples were passes through conditioned C I 8 SPE cartridges. These SPE cartridges were washed with deionized water and eluted with hexane. Eluates were concentrated and analyzed for pendimethalin by G C .


279 Sediment. Sediment samples were allowed to thaw and settle. The standing water at the top of each sample was carefully removed and the sediment was passed through a 3.5 mm mesh sieve to remove stones and debris. The sieved sediment was thoroughly mixed. Subsamples of each sediment were removed and analyzed for pendimethalin according to B A S F (formerly American Cyanamid) Method M 2514 (6,7) as previously described for the soil samples.


Modeling Long-Range Atmospheric Transport and Deposition A Level III fugacity model was used to evaluate the potential long-range transport of pendimethalin to remote areas due to volatilization and deposition (/). We selected this model since it had been shown to accurately predict the atmospheric transport of several organic chemicals (5). Mackay et al. {5,9-11) described the Level III fugacity model and its usefulness to evaluate the environmental fate of both new and existing chemicals. Each model simulation represented a 100,000-km region (approximate area of Greece), which was composed of four major compartments: soil, air, water, and sediment. The soil consisted of the top 10 cm of soil with a 2% organic carbon content. The air was considered the 1000-m of atmosphere directly above the region. Water accounted for 10% of the surface area of the region and had a depth of 20 m. The sediment had a depth of 1 cm and a 4% organic carbon content (5,9) The average annual application of pendimethalin in Europe is 127,000 kg/100,000 km of area (7). Because the fiigacity model is a steady state model, the mass of pendimethalin input into the system needs to be a constant value over time. Since the average field dissipation half-life in European soil is approximately 84 days and pendimethalin can be applied in both the spring and fall, it is reasonable for an initial assessment to simulate the applications of pendimethalin to the soil as occurring uniformly over time. Therefore pendimethalin was simulated as being introduced into the soil at a rate of 40 kg/hour, that corresponds to a total loading of 350,400 kg/100,000 k m and represents the highest loading in any major region in Europe. This total loading represents a realistic "worst-case" scenario for the use of pendimethalin. Residues of pendimethalin remain in the top centimeters of the soil profile, since pendimethalin is strongly adsorbed by soil. Most applications in Europe are made to the surface of the soil and are not mechanically incorporated. Therefore, the environment that was modeled was changed so the depth of the soil was 2.5 cm rather than the 30-cm depth routinely used in the model. This change was performed in order to ensure that potential movement of pendimethalin from the soil into the atmosphere was not underestimated by simulating incorporation into the soil profile. Results from this revised modeling scenario can therefore be expected to reflect or overestimate a realworld scenario. 2

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280 A series of modeling simulations were run with a combination of pendimethalin half-lives in various phases and the properties that are listed below. Atmospheric half-lives of 4 and 28 hours, which correspond to the rate of reaction with O H radicals as calculated by the method of Atkinson and the rate of direct photolysis corrected for sunlight intensity in Europe, were evaluated (72,75). Simulations included water half-lives of 7 and 28 days, which bracket the rates of degradation due to photolysis and biotic degradation in water (7). The DT50 range of 84 (2016 hours) to 365 days (8760 hours) was selected to represent the average half-life determined from the European field dissipation studies and a conservative estimate of the longest DT50 (331 days) calculated in a laboratory aerobic soil metabolism study (2). A sediment halflife of 14 days (336 hours), determined by a water/sediment study, was used for all simulations (14). This accelerated degradation rate observed in the sediment is a result of the rapid degradation of pendimethalin under anaerobic conditions (14,15). The properties of pendimethalin used in the evaluation were as follows: molecular weight of 281.3 g/mol, melting point of 47 °C, Log K of 5.18, water solubility of 0.275 ppm (mg/1), and a vapor pressure of 0.00125 Pa at 25 °C (7,75,76). o w

Results and Discussion Monitoring Survey Pendimethalin-treated Agricultural Fields. Pendimethalin residues were detected in the soil samples from fields containing a history of annual pendimethalin applications. Soil residues in the 0-5 cm and 5-20 cm depths are listed in Table Π. Concentration of pendimethalin within the top 5 cm ranged from < 10 ppb (LOQ) to 194 ppb. The 5-20 cm soil contained pendimethalin residues between < 10 ppb (LOQ) to 73 ppb with 40% of the analyzed samples below the limit of quantification. If no degradation/dissipation of pendimethalin occurred in the soil following an annual application and the total quantity applied reached the soil surface, the estimated concentrations of 1107 and 3080 ppbfag/kg)would be expected within the top 5 cm of soil. Pendimethalin has a low water solubility (0.275 ppm) and is strongly sorbed to soil (Koc > 13,000 ml/g), therefore its tendency to leach is minimal (15). Cultivation practices within these fields (disc harrow, 15-20 cm deep, autumn and early spring) can result in an incorporation of pendimethalin down to a depth of 15 to 20 cm (6). The pendimethalin detected in the 5-20 cm segments are believed to be a result of the agricultural practices rather than downward movement with water.


281 Table Π. Pendimethalin Residue Detected in the Treated Field Soil (0-5 cm and 5-20 cm depths) Treated Residues at Residues at Field No. 0-5 cm depth {\iglkgf 5-20 cm depth (iiglkg)" 1 194" 25" 3 33 67 4 15 30 5 26 175" 8 16 55°

Terrestrial Field Dissipation Studies - American Chemical Society

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