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Journal of Environmental Quality 32:171-179 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Landscape and Watershed Processes

Diuron Occurrence and Distribution in Soil and Surface and Ground Water Associated with Grass Seed Production

Jennifer A. Field*,a, Ralph L. Reeda, Thomas E. Sawyera, Steven M. Griffithb and P. J. Wigington, Jr.c

a Department of Environmental and Molecular Toxicology, Oregon State University, Corvallis, OR 97331
b USDA Agricultural Research Service, Corvallis, OR 97331
c USEPA, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Western Ecology Division, Corvallis, OR 97333

* Corresponding author (Jennifer.Field{at}orst.edu)

Received for publication September 27, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Little is known about the occurrence and distribution of the herbicide diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl urea] in soil, ground water, and surface water in areas affected by grass-seed production. A field study was designed to investigate the occurrence and distribution of diuron and its transformation products at a poorly drained field site located along an intermittent tributary of Lake Creek in the southern Willamette Valley of Oregon. The experimental sites consisted of a field under commercial grass seed production with a cultivated riparian zone and a second site that was part of the same grass seed field but with a noncultivated riparian zone. Diuron and its transformation product DCPMU [3-(3,4-dichlorophenyl)-1-methylurea] were the only significant residues detected in this study. Concentrations of diuron in surface water declined from a maximum of 28 µg/L immediately following application to low levels that persisted as long as flow was present. Diuron and DCPMU concentrations in shallow ground water (15–36 cm below ground surface) were highest (2–13 µg/L) in the zone immediately adjacent (0.5 m) to Lake Creek and indicated the influence of stream water on shallow ground water near the stream. Diuron and DCPMU detected in soil prior to the second season's application indicated the persistence of diuron and DCPMU from the previous year's application. Surface runoff during the rainy season removes only a very small percentage (<1%) of the applied herbicide. In addition, no evidence was obtained for the downward transport of diuron or its transformation products to deep ground water.

Abbreviations: CR, cultivated riparian zone • DCA, 3,4-dichloroaniline • DCPMU, 3-(3,4-dichlorophenyl)-1-methylurea • DCPU, 1-(3,4-dichlorophenylurea) • FLD, perennial ryegrass seed production zone • LCE, Lake Creek East • LCM, Lake Creek Middle • LCW, Lake Creek West • NCR, noncultivated riparian zone


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE OCCURRENCE OF PESTICIDES in surface waters and their movement to ground water is an important issue in intensively managed agricultural regions. Diuron is a principal herbicide used in grass seed production with 250 610 kg/yr applied in 1996 in the Willamette Valley of Oregon (Anderson et al., 1997). In a survey of ground water in the United States, diuron was among the top 10 pesticides most frequently detected in shallow and deep ground water (Barbash et al., 1997). Diuron is characterized as a potential threat to ground water resources due to its moderate water solubility (42 mg/L at 25°C; Montgomery, 1993), moderately low tendency to sorb to soils (log Koc = 2.21–2.87; Lennartz et al., 1997; Montgomery, 1993), and relatively long half-life (>30 d) in soils (Alva and Singh, 1990; Hill et al., 1955; Montgomery, 1991; Newman, 1995; Wauchope et al., 1992) (Table 1). Because diuron is moderately water soluble and has a relatively low vapor pressure (0.41 mPa), potential losses due to volatilization are considered insignificant (Hill et al., 1955). Diuron sorbs to soils by a combination of hydrophobic interactions, hydrogen bonding, and the formation of charge-transfer complexes (Senesi and Testini, 1983; Spurlock, 1995).


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  		Table 1. Physical properties of diuron.

 
Biological degradation of diuron is the principal means of diuron transformation under field conditions whereas nonbiological degradation of diuron is insignificant (Hill et al., 1955). The aerobic biodegradation pathway for diuron is well-established and proceeds by successive demethylation steps to form DCPMU, DCPU [1-(3,4-dichlorophenyl)urea], and DCA (3,4-dichloroaniline) (Dalton et al., 1966; Geissbuhler, 1969). Reductive dechlorination was observed in anaerobic pond sediments and leads to the formation of the dechlorinated product, 3-(3-chlorophenyl)-1,1-dimethylurea (Attaway et al., 1982; Stepp et al., 1985).

Relatively few field studies have been conducted to determine the fate and transport of these compounds in soil, surface waters, and ground water. Most data on pesticide loss by overland flow come from studies conducted in temperate or humid climates while almost no data are available for climates that are characterized by cool, rainy winters and warm, dry summers, such as those that occur in western Oregon (Lennartz et al., 1997). Dry summer conditions potentially inhibit the microbial degradation of any diuron remaining in soils. The combination of sorption to surface soils and the inhibition of microbial degradation during dry summer months potentially results in long soil half-lives (Table 1). It is not well understood if diuron that persists through dry summers will be remobilized during the period of fall and winter rains.

For this study, we have investigated the occurrence and distribution of diuron and its transformation products in a poorly drained agricultural landscape in western Oregon. The hypothesis of this study was that noncultivated riparian areas would mitigate the movement of diuron from grass seed fields to surface water compared with cultivated riparian areas. Noncultivated riparian zones have more diverse vegetation and biogeochemistry that may act as a buffer, thereby reducing the flow of diuron to receiving waters. To this end, diuron and its transformation products were measured in samples of surface runoff, ground water, and soil collected from a field site located adjacent to an intermittent stream, Lake Creek, which is located in the Willamette Valley of Oregon. The field site included zones under grass seed production and the adjacent cultivated and noncultivated riparian zones along Lake Creek. The goal of this project was to obtain data that can be used to support the assessment of grass seed production practices in the region and their effect on water quality.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site
The two adjacent field study sites located in the Willamette Valley are described in detail by Wigington et al. (2003). Briefly, the study sites are located on an intermittent tributary of Lake Creek in the southern Willamette Valley (Fig. 1) . The research sites consisted of Lake Creek West (LCW), which is under commercial perennial ryegrass (Lolium perenne L.) seed production (FLD) and is adjacent to a 30- to 48-m-wide noncultivated riparian (NCR) zone. The Lake Creek East (LCE) site is part of the same grass seed field but is located upstream on Lake Creek. This site consists of a FLD zone under perennial ryegrass production but with a cultivated (perennial ryegrass seed) riparian (CR) zone bordering Lake Creek. A swale with an intermittent stream is located between LCW and LCE and is termed Lake Creek Middle (LCM).



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  		Fig. 1. Lake Creek riparian study site. LCW, Lake Creek West; LCE, Lake Creek East; NCR, noncultivated riparian zone; CR, cultivated riparian zone; FLD, perennial ryegrass seed field.

 
To examine the changes in water quality of ground water as it moved from the grass seed fields through the riparian zones at LCW and LCE, a series of nested piezometers were installed along three transects at each site essentially perpendicular to Lake Creek (Fig. 1). All piezometers were constructed with 51-mm-i.d. TIMCO (Prairie du Sac, WI) PVC slotted, high-flow piezometers with a slot width of 0.25 mm and vented caps (Wigington et al., 2003). Both shallow and deep piezometers had watertight screw-joints that allowed for each piezometer to be disassembled to allow for harvesting and tillage operations. Each piezometer nest consisted of two piezometers with one piezometer in the A/E horizon and one in the C horizon at each location. Piezometer nest positions along the transects were determined by the relative position of riparian vegetation, grass seed fields, and soil types (Wigington et al., 2003).

The shallow piezometers located in the A/E horizon were screened from 15 to 36 cm below land surface. The deep piezometers located in the C horizon were screened from 18 to 53 cm below land surface. At these sites, the soils are poorly drained and are well suited to crops such as perennial ryegrass, which can tolerate waterlogged conditions. A band of Dayton (fine, smectitic, mesic Vertic Albaqualf) soil encompasses Lake Creek to a width of about 25 m at LCE and 18 m at LCE while Holcomb (fine, smectitic, mesic Typic Argialboll) soils occur in the rest of the study site. The Dayton series is characterized by a very slowly permeable clay 2Bt horizon. The Holcomb soil that comprises the remainder of the study site has a clayey texture 2Bt horizon (Wigington et al., 2003).

At LCW, piezometer nests 17.5 m apart were located along each of three transects that were sited along the predominant ground water flow path. Piezometers in the NCR zone were located at 0.5 m (Row 1), 8 to 9 m (Row 2), 15 to 17 m (Row 3), and 24 to 32 m (Row 4) from Lake Creek. The FLD-zone piezometers were located 36 to 44 m (Row 5) and 51 to 58 m (Row 6) from Lake Creek (Fig. 1). The NCR had Dayton soil near the stream (0.5–17 m), which corresponded to Rows 1 through 3, and Holcomb soil farther from the stream (24–32 m), which corresponded to Row 4 in the FLD. All piezometers in FLD Rows 5 and 6 were located in Holcomb soil.

At LCE, four piezometer nests were located along three transects that were 15 m apart. Piezometers in the CR zone were located 5 m (Row 1) and 16 m (Row 2) from Lake Creek while piezometers in the FLD zone were located 22 m (Row 3) and 40 to 47 m (Row 4) from Lake Creek. The CR piezometers were located in Dayton soil (Rows 1 and 2) and the FLD piezometers (Rows 3 and 4) were located in Holcomb soil.

Diuron Application
A formulation (90% active ingredient) containing diuron was applied by broadcast application at a rate of 2.0 kg/ha on 25 Oct. 1995 over the entire area depicted in Fig. 1 except in the NCR region, which is shaded gray. A second application of the diuron formulation (90% active ingredient) was applied on 24 Oct. 1996 at a rate of 1.5 kg/ha.

Surface Water Samples
Surface water samples were collected as individual single grab samples from the Lake Creek West sampling site (Fig. 1) from November 1995 until the end of May 1996. From 31 Oct. 1996 until 10 Dec. 1996, surface water samples were collected from the Lake Creek East and Lake Creek West sampling sites in 500-mL polyethylene bottles with autosamplers (Model 2900; ISCO, Lincoln, NE). After December, single samples were collected every other day through 14 May 1997. The surface water sampled for this study ranged from 1 to 8 mg/L dissolved organic carbon and from pH 7.4 to 7.8. All samples were stored at 4°C for no more than two weeks before analysis.

Ground Water Samples
Before collecting samples from the piezometers, one well casing volume was removed with a peristaltic pump. All ground water samples were collected in 250-mL baked brown glass bottles with Teflon-lined lids. Ground water ranged from less than 1 mg/L (detection limit) dissolved organic carbon to 17 mg/L and from pH 6.8 to 7.3. Single ground water piezometer samples were obtained bimonthly from November 1995 until June 1996 and from October to December 1996 while monthly samples were collected from January to June 1997. All samples were stored at 4°C for no more than two weeks before analysis.

Soil Samples
On 10 Sept. 1996, which was before the diuron application on 26 Oct. 1996, single, composited soil samples from five depth intervals corresponding to 0 to 40, 40 to 50, 50 to 100, 100 to 150, and 150 to 250 cm below land surface were obtained from soil pits. The soil pits (dug by backhoe) located in the LCW FLD zone were characterized by Holcomb series soils, while the pit located in the NCR zone of LCW was characterized by Dayton series soils (Fig. 1).

Diuron and Metabolite Analysis
Diuron and its three major transformation products (DCPMU, DCPU, and DCA) were concentrated from surface and ground water samples by solid phase extraction and quantified by high performance liquid chromatography with ultraviolet detection (Beckman [Fullerton, CA] System Gold HPLC with a Beckman 167 Scanning Detection Module) as described in Field et al. (1997). Briefly, 250-mL water samples were first filtered through a 0.45-µm polypropylene filter (J.T. Baker, Union City, CA) and then extracted onto 25-mm C18 Empore disks. The analytes were eluted from the disk with methanol and acetonitrile (50:50) and analyzed by HPLC with ultraviolet detection at 252 nm (Hewlett-Packard [Wilmington, DE] Model 1050 HPLC with diode array detector). The detection limit of this method was 0.5 µg/L for diuron and each of its transformation products. The accuracy of this method, as indicated by the recovery of diuron and its transformation products spiked into six replicate, blank surface water samples was 98% for diuron, DCPMU, and DCPU while the recovery of DCA was 78% (Field et al., 1997). The precision of the method, as indicated by the relative standard deviation for the six replicate analyses, was ±5% (Field et al., 1997).

Soil samples (15 g) were analyzed for diuron and its DCPMU metabolite by Sohxlet extraction with 175 mL of methanol for 12 h. Diuron and DCPMU in the extracts were quantified by high performance liquid chromatography with ultraviolet detection at 250 nm. The method detection limit as defined as the concentration that gave a signal-to-noise ratio of <=3 was 0.02 mg/kg for diuron and DCPMU. The average recovery of diuron and DCPMU spiked onto replicate soil samples was 104 and 74%, respectively. Neither DCPU nor DCA were analyzed for in soils by this method for soil analysis. The extraction of DCA from soils requires more vigorous (e.g., basic) extraction conditions (You and Bartha, 1982). As a result, no data are reported for DCPU nor DCA for soil samples.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Surface Water
Lake Creek is an intermittent stream that carries water typically from November until early June. The intermittent nature of Lake Creek is illustrated by the break in sampling at LCW from June until November when no flow occurred. From November until early June the creek flowed continuously in both the study years (Fig. 2) . Flow (data not shown) typically occurred at the LCM sampling site from November until April, after which it became intermittent.



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  		Fig. 2. Temporal trend in (a) diuron concentrations and (b) stage height of Lake Creek measured at Lake Creek West (LCW) from November 1995 to June 1997 at the Lake Creek West sampling site. The diuron application dates were 25 Oct. 1995 and 24 Oct. 1996.

 
In the first year of the study (November 1995 to June 1996), diuron concentrations determined from grab samples collected at LCW indicated that high concentrations (27.7 µg/L) occurred on the first sampling date after application and in two subsequent time periods in January (12.7 to 17.6 µg/L) and in late April (25.8 µg/L). The increase in diuron concentration in January 1996 did not coincide with a particular hydrologic event (Fig. 2) nor is it probably due to a diuron application at this study site or upstream. A late-season peak in diuron concentrations also occurred on 15 April, which is a time of the year in which no grass seed–related applications typically occur. The increase in diuron concentrations was not associated with meteorological events. For this reason, the increase was hypothesized to be the result of upstream diuron applications unrelated to grass seed production, such as roadside weed control.

In second year of the study (November 1996 to June 1997), diuron concentrations in Lake Creek at the LCW and LCE sampling sites were very similar and were greatest in the fall after the October diuron application and decreased toward spring with some late-season increases associated with rain events (Fig. 2 and 3a) . The reported runoff of diuron from vineyards in a similar climate that coincided with major rain events and declined with subsequent events (Lennartz et al., 1997) was interpreted as decreasing availability with time.



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  		Fig. 3. Temporal trend in surface water concentrations at the Lake Creek West (LCW), Lake Creek East (LCE), and Lake Creek Middle (LCM) sampling sites for (a) diuron and (b) DCPMU [3-(3,4-dichlorophenyl)-1-methylurea]. The date of diuron application was 24 Oct. 1996.

 
The transformation product, DCPMU, which results from the biological demethylation of diuron, was detected in all samples that contained >3 µg L diuron (Fig. 3). When the diuron concentrations fell below 3 µg/L, DCPMU generally was not detected. A second metabolite that forms during a second demethylation step, DCPU, and its corresponding hydrolysis product, DCA, were not detected in any surface water samples collected from this study site. The absence of DCA in surface water can be potentially attributed to its strong binding through the formation of covalent bonds with soil organic matter (You and Bartha, 1982). In addition, the dechlorination product 3-(3-chlorophenyl)-1,1-dimethylurea (Attaway et al., 1982; Stepp et al., 1985), which forms under reducing conditions, was not detected.

The seasonal variation in the contribution of LCM to measured diuron and DCPMU concentrations at LCW may be seen by examining the ratio of diuron to DCPMU. In general, the diuron to DCPMU ratios decreased during the year from 10.7 to approximately 2.5 in May (Fig. 4) and indicate a relative decrease in diuron relative to DCPMU over time. These values are in contrast to the diuron to DCPMU ratio of 120 that was measured in puddle water collected at the LCW site just after application in October (data not shown). The lower diuron to DCPMU ratios in LCW compared with LCE are consistent with the input of water from LCM, which has significantly higher DCPMU concentrations relative to diuron (Fig. 4). It is not clear why LCM has higher concentrations of DCPMU relative to diuron from November to January. These differences in stream water composition may reflect differences in the diuron formulation and/or application times within the area drained by LCM compared with the area that drains into LCE. No information was available on the application rate, timing, or formulations for the grass seed fields upstream of LCE.



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  		Fig. 4. Temporal trend in the diuron to DCPMU [3-(3,4-dichlorophenyl)-1-methylurea] ratio for Lake Creek West (LCW), Lake Creek East (LCE), and Lake Creek Middle (LCM) sampling sites. The date of diuron application was 24 Oct. 1996.

 
Ground Water
Samples were obtained from shallow ground water piezometers in Rows 1 through 4 at LCE. The highest diuron concentrations were obtained for samples collected from Row 1 piezometers even though diuron had been applied to the entire LCE site (Fig. 5) . The variation in diuron concentrations was similar to the variation in diuron concentrations observed for Lake Creek (Fig. 2). Lower concentrations of diuron were detected in Rows 2 and 3. Some samples from Row 4 had higher concentrations of diuron, yet there was no trend among the data. DCPMU was detected in samples collected from Rows 1 through 3 and was limited to samples collected from November 1996 to January 1997 (Fig. 5). At no time over the course of the season were DCPU, DCA, or any dechlorination products detected in ground water samples collected from either deep or shallow piezometers at either sampling site.



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  		Fig. 5. Temporal trends in diuron concentrations in rows of shallow ground water piezometers at Lake Creek West (LCW) and Lake Creek East (LCE).

 
The highest concentrations of diuron at LCW were detected in Row 1 even though no diuron was applied to the soils in the NCR zone during this study (Fig. 2). Although there were fewer samples with detectable concentrations of diuron collected from LCW, the trend in diuron concentration with time is similar to that of diuron concentrations in Row 1 at LCE (Fig. 5) and in Lake Creek surface water (Fig. 2). No diuron was measured above the detection limit (0.5 µg/L) for Rows 2 through 4 in the NCR zone. However, diuron was detected in Rows 5 and 6, which were located in the FLD zone that received an application of diuron in both years of the study. Low concentrations of DCPMU were detected in Row 1 and in Rows 5 and 6 during the 1996–1997 study year.

The number of shallow ground water piezometers for which the concentrations of diuron and DCPMU were both above detection were few in 1996 and were limited to the months of November and December (Table 2). However, in December the higher diuron to DCPMU ratios detected in NCR zone Row 1 of LCW are more similar to those of Lake Creek (Table 2) than to Rows 5 through 6, which are in the FLD zone at LCW. For LCE, the diuron to DCPMU ratio for CR zone Row 1 also was more similar to Lake Creek than that of Rows 2 through 4 (Table 2). These findings suggest that shallow ground water in Row 1 is influenced by Lake Creek rather than by subsurface ground water flow from soils in Rows 2 through 6 in LCE and LCW. The lower diuron to DCPMU ratios detected in Rows 5 and 6 at LCW and 2 through 4 at LCE are more similar to that of LCM surface water (ratio of approximately 2). This finding indicates that LCM surface water has a composition similar to soil porewater within the study site, which is different from the surface water flowing into LCE from grass seed fields upstream. Griffith et al. (Griffith et al., 1997) also observed higher concentrations of nitrate in NCR zone Row 1 of LCW compared with Rows 2 through 6 and attributed this finding to the influence of Lake Creek on shallow ground water in Row 1 piezometers. Because infiltration of Lake Creek affected the composition of shallow ground water in piezometer Row 1 in the NCR and CR zones, it was not possible to determine the effect that NCR and CR zones had on water quality.


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  		Table 2. Diuron to DCPMU [3-(3,4-dichlorophenyl)-1-methylurea] ratios for shallow ground water collected from Lake Creek West (LCW) and Lake Creek East (LCE) on three sampling dates.

 
Wigington et al. (2003) found that very little of the water in Lake Creek was derived from lateral flow though A/E horizon soils. Rather, water reaching Lake Creek primarily originates from ground water rising to the surface and into surface impressions and rainfall directly onto surfaces with rapid movement to channels, swales, and other types of depressions. During periods of over-bank flooding, water moves from the Lake Creek stream channel into riparian zone soils, which results in higher concentrations of diuron and nitrate in shallow piezometers in Row 1, which are the closest to Lake Creek. Consequently, most of the water in Lake Creek does not interact significantly with riparian zone soils along LCE and LCW such that the riparian zone appears to have little influence on stream water quality in Lake Creek.

Deep ground water samples were obtained on 21 October from deep piezometers located at LCE in CR zone Rows 1 through 3 and from only one well of the NCR zone Row 1 at LCW. Diuron was detected in these deep piezometers at concentrations ranging from below detection (0.5 µg/L) to 8.3 µg/L even though the diuron application for that year had not yet occurred (24 October). A week later in these same wells, diuron concentrations decreased from below detection (0.5 µg/L) to a maximum of 0.9 µg/L and no DCPMU, DCPU, and DCA were detected. From November through December, diuron and its transformation product concentrations decreased to below the detection limit (0.5 µg/L). The timing of the decline in diuron concentrations in the deep ground water piezometers coincided with an increase in the water table and the appearance of water in shallow ground water piezometers. We hypothesize that the diuron detected in water from the deep ground water piezometers resulted from preferential flow down through cracks in the soil that had developed during the dry season. During the dry season the Dayton and Holcomb soils produce cracks and macropores due to their shrink-well clay content (Langridge, 1987). During the wet season, the number of cracks in these soils is greatly reduced or nonexistent; the impermeable nature of the 2Bt clay present at this site prevents downward transport of diuron during the wet season.

Soils
Soil samples were obtained from the FLD zone of the LCW site on 10 Sept. 1996, which was prior to the diuron application for this study. However, this zone had received a direct application of diuron the previous year. Soils collected from 0 to 40 cm had diuron and DCPMU concentrations of 0.38 mg/kg (1.6 mol/kg) and 0.53 mg/kg (2.4 mol/kg), respectively. Assuming that all the measured DCPMU was derived from diuron, the combined diuron and DCPMU soil concentration expressed on a molar basis was equal to approximately 46% of the diuron that was applied at a rate of 2.0 kg/ha (8.6 mol/kg) on 25 Oct. 1995.

Although diuron was not applied to the soil in NCR zone during this study, diuron and DCPMU were detected at concentrations of 0.9 mg/kg (3.9 mol/kg) and 1.2 mg/kg (5.5 mol/kg), respectively. When combined on a molar basis, the diuron and DCPMU soil concentrations were greater than that that could result from the diuron applied (2.0 kg/ha or 8.6 mol/kg) to the adjacent grass seed field. The residues of diuron and DCPMU detected within the NCR zone may be residues remaining from the previous year's application; it is not known whether diuron was applied to the NCR zone in years prior to this study. Diuron and DCPMU concentrations at or above the application rate at the end of the growing season are consistent with reports of long soil half-lives for diuron of 100 to 4000 d (Table 1) (Alva and Singh, 1990; Madhun and Freed, 1987).

The lower concentration of diuron and DCPMU in the FLD zone may be due, in part, to the lower soil organic carbon content of the A/E horizon, which has a fraction of organic carbon (foc) of 0.011 to 0.026 compared with that of the A/E horizon of NCR zone, which had a foc of 0.045 to 0.085. The sorption of diuron and DCPMU are unaffected by soil pH; however, diuron adsorption is related to the organic carbon content of soils (Liu et al., 1970). Neither diuron nor DCPMU were detected at any of the depths below 40 cm at either of the soil sampling sites. The depth concentration profiles for diuron and DCPMU indicate that at the end of the dry season, diuron and DCPMU had not been transported downward to a significant extent during the year. The lack of diuron transport downward and soil concentration equal to the application rate suggest that the amount of diuron removed from the field site was small (e.g., <1%). Others have reported only a small percentage (0.7%) removal of the total applied mass of diuron from a field site by runoff under similar climatic conditions (Lennartz et al., 1997).

The diuron to DCPMU ratio was 0.72 in the FLD zone and 0.75 in the NCR zone at LCW, which is in contrast to the diuron to DCPMU ratio of 120 measured in puddles on the day of diuron application and the surface water diuron to DCPMU ratios (approximately 10) early in the crop season. The low diuron to DCPMU ratios in soil indicate that DCPMU is preferentially enriched in soils over time relative to diuron. At the time the soils were sampled, which was just prior to diuron application, the concentration of DCPMU was similar to that of diuron, which suggests that DCPMU is more stable in the soil environment compared with diuron. Laboratory soil dissipation studies have indicated that DCPMU loss from soil is slower than that of diuron (Walker, 1978; Walker and Roberts, 1978). Others have shown that DCPMU is strongly sorbed to soil and the amount that can be recovered by chemical means (e.g., solvent extraction) decreases with time (Lennartz et al., 1997).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Diuron and its transformation product, DCPMU, were detected over the two-year study in surface water, ground water, and soil associated with grass seed production fields. DCPMU was the major transformation product observed and was present when diuron concentrations were >3 µg/L. While the risk that diuron poses to aquatic species was not investigated as part of this study, the diuron concentrations measured during this study were lower than those that affect the survival, growth, and malformation of frogs that live in the riparian zones of the Willamette Valley (>7.6 mg/L) (Schuytema and Nebeker, 1998). The ratio of diuron to DCPMU in surface water and soil differed significantly and may reflect different application formulations or practices. The high concentrations of diuron and DCPMU in the surface soils obtained from the soil pit at LCW and lack of downward leaching into ground water indicate that the mass of diuron and its transformation products exported from the site is relatively small (<1%). The effect of cultivated versus noncultivated riparian zones on diuron transport and transformation could not be assessed because Lake Creek surface water influenced the ground water composition in shallow wells near the creek in both the cultivated and noncultivated riparian zones.


   ACKNOWLEDGMENTS
 
We thank Fred Bramble of Du Pont Agricultural for providing technical assistance, Ron Burr and Western Farm Service for application information, Steve Thun of Pacific Agricultural Laboratory for soil analyses, and Margaret Sparrow for assistance with field sampling. The information in this paper has been funded by the USEPA under Interagency Agreement DW12936582, the USDA Agricultural Research Service, and the Oregon Department of Agriculture, Division of Natural Resources. This paper has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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G. W. Gee, G. W. Gee, Z. F. Zhang, and A. L. Ward
A Modified Vadose Zone Fluxmeter with Solution Collection Capability
Vadose Zone J., November 1, 2003; 2(4): 627 - 632.
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P. J. Wigington Jr., S. M. Griffith, J. A. Field, J. E. Baham, W. R. Horwath, J. Owen, J. H. Davis, S. C. Rain, and J. J. Steiner
Nitrate Removal Effectiveness of a Riparian Buffer along a Small Agricultural Stream in Western Oregon
J. Environ. Qual., January 1, 2003; 32(1): 162 - 170.
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