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Fate of the herbicide linuron in outdoor experimental ditches
jPergamon
clumosphcre, Vol. 36, No. 10,pp. 2175-2190,1998 Q 1998Elswier Scia~ccLtd Allrights-cd.RintediaGratBritaii 0045-6535/98 $19.00+0.00
PII: soo45-6535010190-4
FATE OF THE HERBICIDE LINURON IN OUTDOOR EXPERIMENTAL DITCHES
S.J.H. Crum’, G.H. Aalderink, and T.C.M. Brock
DLO Winand Staring Centre for Integrated Land, Soil and Water Research, PO Box 125, 6700 AC Wageningen, The Netherlands (kceived in Germany11september1997;accepted 3 November1997)
Abstract
The fate of the urea-herbicide linuron was studied in experimental ditches. Eight ditches were sprayed three times at monthly intervals, while two served as controls. As a simulation of spray drift, four doses (nominal: 0.5; 5; 15 and 50 pg.dni3) were applied in duplicate. After each herbicide application, the ditches were kept stagnant for a week, followed by a flow period of three weeks with fresh water. No clear stratifican n of linuron could be found in the water compartment. During the stagnant period, half-lives ‘a for the disappearance of linuron in the water compartment ranged from 7.2 to 11.8 days. The rate of disappearance was slower in the ditches treated with the highest dose and in colder treatment periods. A maximum of 6% of the linuron dose was found in the sediment and 1% in the macrophyte compartment. Approximately 20% of the fraction in the sediment compartment was present in the pore water. 01998 Else&r Science Ltd. All rights reserved
Introduction
Herbicides are widely used for weed control in agriculture. During and after the application of herbicides on agricultural fields, these chemicals may enter adjacent aquatic ecosystems by spray drift, runoff and leaching. It has frequently been reported that herbicides may have adverse effects on the structure and functioning of freshwater ecosystems [ 1; 21. To prevent adverse side-effects of agrochemicals in freshwater ecosystems, authorities have set criteria that have to be met before herbicides are allowed on the market. Recently, the member states of the European Union adopted the Uniform Principles (Council Directive 91/414/EEC, concerning the placing of plant protection products on the market; EU, 1994). This directive states that environmental concentrations of a pesticide in surface water should be calculated with
2175
2176 a validated simulation model, accepted by the member states. The outcome of such a model, the Predicted Environmental
Concentration (PEC), is compared with toxicity data for standard test species (algae;
Daphnia; fish). Of these species, algae are usually the most susceptible to herbicides. The PEC should not exceed 0.1 times the NOEC (no observed effect concentration) of the test algae.. However, if additional (semi-)field studies clearly indicate that the environmental concentration of ,the herbicide does not pose a significant risk to the viability of aquatic organisms, the above criteria may be overruled. In 1994, a research project was initiated on the fate and effects of herbicides in freshwater model ecosystems (microcosms; mesocosms). The urea-herbicide linuron (applied as Afalon Flow) was adopted as a benchmark compound, while mesocosms were used which intended to simulate drainage ditches. The experiments were performed to: a) validate the criteria adopted in the Uniform Principles to protect aquatic organisms; b) investigate the ability of the freshwater community to recover from herbicide stress; and c) validate the TOXSWA fate model for a herbicide applied to ditches with flowing water. The TOXSWA fate model has been developed to calculate the long-term exposure of aquatic organisms to pesticides [3; 41, and it is intended to be used in the Dutch registration procedure.
Water flow in Dutch drainage ditches varies from stagnant to slowly flowing. In our experiment, the water in the ditches was kept stagnant for one week after the application of linuron, followed by a threeweek period of fresh water flow. This yielded an exposure regime which would be realistic for Dutch drainage ditches, and allowed us to study the recovery potential of the freshwater community. To allow comparison of exposure regimes and responses of effect endpoints, linuron was measured at several locations in the water compartment of all ditches. Validation of the TOXSWA model, however, required more detailed information. Therefore, the fate of linuron was studied in more detail in the ditches treated with the highest dose (50 pg.dm-‘). This paper presents methods and results on the fate of linuron in the experimental ditches. Results of our mesocosm study are compared with literature data on the fate of linuron and other pesticides in freshwater ecosystems. Finally, this paper is intended as a reference article for studies dealing with model validation of TOXSWA, and with effects of linuron on ecosystem structure and functioning in the same experiment.
MaterhIs and methods
Experimental design. Twenty experimental ditches were constructed in 1988 on the Sinderhoeve experimental station, Renkum, the Netherlands [5]. Twelve of these are in use for ecotoxicological research. The experimental ditches have a length of 40 m and a width at the sediment surface of approximately 1.6 m. At a water depth of 0.50 m, the width at the water surface is 3.4 m and the ditches then contain
2177 approximately 50 m3 water. The materials for the sediment layer was taken from a mesotrophic lake. Aquatic organisms and macrophytes were introduced in the experimental ditches together with the sediment. The water in the ditches originates from a supply reservoir (2500 m’) at the experimental station. This reservoir contains a sediment layer of the same origin as that of the ditches, and is regularly replenished with unpolluted well water, pumped up from a depth of 90 m. The well water is diluted with the water in the reservoir to maintain its surface water characteristics. Each individual ditch can be connected with the reservoir in order to maintain the desired hydrological regime. Effluent water from the ditches is discharged to the discharge reservoir. The amount of water entering and leaving each experimental ditch is recorded. The sediment layer of the ditches can be characterised as a sandy loam in which a well-developed macrophyte vegetation was rooting. Some characteristics of different sediment layers are presented in Table 1. Abundant macrophytes in the ditches were Myriophyllum spicatzun,Elodea nuttallii and Sag&aria sagittifolia. The ditches in which the fate of linuron was studied in detail were dominated by Myriophyllum spicatun. In the ditches with the highest treatment level, the total dry weight biomass of macrophytes at
the start of the experiment was 0.02 kg.m-‘, increasing to 0.11 kg.m-* in October. The ditch water was relatively poor in nutrients, while the average dissolved and total organic carbon contents were found to be 4 and 5 mg.dm-‘, respectively.
Table 1:
Some characteristics of different sediment layers in the experimental ditches.
Sediment layer
Organic matter
Dry bulk density
Volume fraction of
(cm)
(%)
(kg.dm-3)
liquid phase
o-1
26
0.1
0.9
l-2
19
0.2
0.8
2-4
6
0.7
0.7
2
1.6
0.4
4-
10
Herbicide application and hydrological regime. Linuron was applied as technical grade Afalon Plow.
This is an emulsifiable formulation with linuron (450 g.dme3) as the active ingredient. Linuron (3-(3,4dichlorophenyl)-1-methoxy-1-methylurea)
is a urea herbicide which inhibits the photosynthetic electron
transport. Some physico-chemical properties have been described by Tomlin [6]. Spray equipment had been designed and constructed for the application of pesticides to the experimental ditches [7]. Eight experimental ditches were treated with four different doses in duplicate, resulting in nominal concentration levels in the water of the ditches of 50; 15; 5 and 0.5 pg.dme3, respectively. The concentration of 0.5 ng.dmm3represents the norm concentration based on the Uniform Principles. The other concentrations are chosen to obtain a dose-response relationship for sensitive ecological endpoints. Two experimental
2178 ditches served as control units. The herbicide was applied on the 91hof May, on the 6’i’of June and on the 4’ of July 1995. After each application, the ditches were kept stagnant for seven days. Thereafter, flow of fresh water from the supply reservoir was introduced, at a velocity of approximately supply and discharge
were recorded during the flow periods. Differences
a
5 m.d”. Water
in the volume of water stored
in the ditches were calculated by regular measuring the water level with a fixed water-level gauge. During the experiment
the temperature
and pH were measured continuously
the ditches. These measurements
were carried out as described by Drent and Kersting [5].
Linuron in water. To estimate the exposure concentrations, with the help of perspex application
in the water column in the centre of
depth-integrated
tubes (length 42 cm, internal diameter
(for time intervals see Fig. 5). Depth-integrated
water samples were taken
3.9 cm) at several moments
water samples were collected
after
by vertically
inserting the tube into the water layer and closing the tube with rubber stoppers. Finally the water samples were transferred
to 500 cm’ flasks and transported to the laboratory for extraction
depth-integrated
water samples were taken at three locations (5; 22 and 33 m from water inlet). In the
ditches treated with the highest dose, depth-integrated
and analysis. These
samples were collected at five locations (5; 14; 22;
27 and 33 m from inlet). Stratification
of linuron in the water column was studied in the ditches treated with the highest dose,
by placing a stainless steel gradient sampling apparatus at five locations (3; 8; 12; 25 and 35 m from inlet). This gradient sampling apparatus consisted of three perforated tubes, fixed in a framework and placed at three different levels (0.05 m; 0.25 m and 0.45 m) under the water surface. The tubes were connected
to
sampling flasks by stainless steel tubing. After the perforated tubes had been flushed, water samples were collected
simultaneously
flasks individually, concentration.
by lowering the pressure in the flasks with a vacuum pump. After shaking the
subsamples
were transferred into 2 cm’ HPLC-vials
for direct analysis of the linuron
A more detailed description of the gradient sampling apparatus has been given in a previous
paper L31. The transverse distribution of linuron in the water compartment by means of the purpose-built allowed
15 different
apparatus framework.
consisted
CATS apparatus (Crum and Aalderink Transversal
water samples within the ditch profile to be collected of fifteen
stainless
steel tubes (internal diameter
Sampler, Fig. 1), which
simultaneously.
The CATS
2 mm), fixed in an aluminium
Each of the stainless steel tubes was individually connected to 250 cm3 flasks by a short piece
of polyethylene direction.
was investigated during the flow period
tubing. The lower end (e 0.5 cm) of each individual
tube was bent in a horizontal
During sampling, the framework with the tubes was carefully inserted into the water column.
After the tubes had been flushed, water samples were collected simultaneously in the flasks with a vacuum pump. These measurements
were performed
by lowering the pressure
at seven different locations (for
details see Fig. 3) in a ditch treated with the highest dose. After shaking the flasks, subsamples transferred
into 2 cm’ HPLC-vials
for direct analysis of the linuron concentration.
were
2179
Figure 1. The CATS sampling apparatus used for water sampling at fifteen locations in the cross-section
of a ditch.
Samples with linuron concentrations below 10 pg.dm-3were concentrated in octadecyl (C-18) solid-phase extraction columns, which were prepared by filling a 5 cm3 cartridge with 0.5 g octadecyl material (Bakerbond C-18 40 pm lotno. 702501, J.T. Baker Inc., Phillipsburg, New Jersey, USA), fixed between two polypropylene filters. The extraction columns were conditioned with 5 cm3 methanol and 5 cm3 distilled water, respectively. After extraction of a known volume of water, linuron was eluted from the column with 1.5 cm3 acetonitrile. Finally, the samples were diluted with water to a volume of 5 cm3 and subsamples were transferred to HPLC-vials for analysis. Recovery rates of linuron from spiked distilled water and from natural water samples were 101.2% (s.d.= 0.7%; n=3) and 100.3% (s.d.= 0.9%; n=3), respectively. Calculations of half-lives for linuron in the water compartment of the ditches were based on nominal concentrations for t=O and on measured concentrations for subsequent samplings. The measured linuron concentrations were corrected for the varying water levels due to evaporation and/or rainfall.
Linuron
in macrophytes.
At several moments after application, macrophyte samples were collected
at five locations in the ditches treated with the highest dose, by taking at least two shoots of the dominant macrophyte Myriophyllum spicutum at each location. The locations and time intervals were equal to those of the depth-integrated water sampling. The macrophyte samples were packed in aluminium foil and stored in a freezer (-20 “C) until analysis. The macrophyte samples were frozen by adding liquid nitrogen and then crushed in a mortar. A subsample was weighed into a 250 cm3 flask. After 50 cm3 distilled ethyl acetate and 50 cm3 distilled water had been added, linuron was extracted by shaking intensively for two hours. After phase separation
2180 a fraction of the ethyl acetate layer was removed and subjected to clean-up. The remaining content of the flask was transferred
into an aluminium tray and dried in an oven at 105 “C for 24 hours, to measure the
dry mass of the macrophyte
sample.
The ethyl acetate extract was evaporated
to dryness and the residue was redissolved
in 0.5 cm3 ethyl
acetate and 2 cm3 n-hexane. The clean-up column consisted of a 5 cm3 cartridge filled with 0.5 g florisil and 2 g sodium sulfate on top of it. It was rinsed with 5 cm3 ethyl acetate followed by 5 cm3 n-hexane, respectively.
The sample was transferred quantitatively
20 cm3 n-hexane evaporated
containing
to the clean-up column and linuron was eluted with
10% ethyl acetate. The eluate was collected
to dryness in a water bath (40 “C) under a gentle airstream. The residue was redissolved
0.5 cm3 acetonitrile
by ultrasonification
for analysis. Recovery
Millipore) and transferred to a 2 cm3 HPLC-vial
of linuron from spiked macrophyte
in sediment.
in
and then diluted with 2 cm3 distilled water. This solution was
filtered through a 0.45 pm disposable filter (Millex-HV,,,
Linuron
in a 25 cm3 glass tube and
samples was 96.9% (s.d.= 6.0%; n=lO).
Sediment cores were taken to investigate
the concentration
of linuron in the
sediment layer and its distribution between the liquid and solid phases. The cores were collected by means of the perspex tubes described
above (internal diameter 3.9 cm). A tube was inserted carefully into the
sediment by hand, to a depth of approximately
0.1 m, and then turned once along its vertical axis. Its top
opening was then closed with a rubber stopper below the water surface, and the tube with sediment was pulled up carefully. The lower end of the tube was then also closed with a rubber stopper, below the water surface, and the water layer was decanted immediately.
The cores were taken at several moments
after
application in the ditches treated with the highest dose, each time at five locations per ditch. The locations and time intervals were equal to those of the depth-integrated
water sampling. The tubes with the sediment
were stored vertically in a freezer (-20 “C) until extraction and analysis. The frozen sediment cores were divided into four layers (O-l cm, l-2 cm, 2-4 cm, > 4 cm), in order to study the stratification tubes and weighed. extraction intensively
efficiency.
of linuron in the sediment. The layers were transferred
Occasionally,
to 100 cm3 centrifuge
distilled water was added to relatively dry samples to obtain a better
The sediment was extracted with 40 cm3 ethyl acetate, by shaking the suspension
for two hours. After phase separation,
the same clean-up procedure remaining after extraction
a fraction of the ethyl acetate layer was removed and
as that for the macrophyte
was transferred
samples was followed. The content of the tube
into an aluminium tray. The dry mass of the sediment sample
was measured by drying in the oven at 105 “C for 24 hours. The recovery rate of linuron from spiked sediment samples was 92.4% (s.d.= 2.7%, n=18).
Analysis of linuron. Linuron was analyzed using a high performance Subsamples
liquid chromatography
of 100 mm3 were injected with a Perkin Elmer ISS-100 autosampler.
was a mixture of acetonitrile, methanol and water (UV,,,-purified)
technique.
The mobile phase used
in a ratio of 25:25:50. The mobile phase
2181 was pumped at a flow rate of 0.8 cm3.min” with a Waters M590 pump through a Chrompack Lichrospher
100 BP-18 (length: 100 mm; internal diameter: 3 mm; particle size: 5 pm) analytical column, provided with a guard column of the same origin. The column was mounted in a Waters TCM column oven, which was set at a temperature of 40 “C. The herbicide was detected using a Perkin Elmer LC-90 UV-detector set at a wavelength of 254 nm. The retention time for linuron was 12.6 min, with a detection limit of 2 pg.dm-‘. The sediment extracts were separated on a Waters Novapak C-18 (length: 150 mm; internal diameter: 4,6 mm; particle size: 4 urn) analytical column, provided with a guard column of the same origin. The mobile phase consisted of a mixture of water and acetonitrile in a ratio of 55:45 (v/v), adjusted to a flow rate of 0.7 cm3.min~‘. In this set-up the retention time of linuron was 10.7 min.
Results and discussion
Linurun in water. In neither of the periods was a clear stratification of linuron found in the water column (Fig. 2). Within a few hours after application, the linuron concentration at all sampled depths was higher than 90% of the nominal concentration. In addition, concentration differences between the three sampled depths were less than 10%. This indicates a rather high mixing rate of linuron in the water column. The literature contained no data on linuron stratification in the overlying water of aquatic ecosystems, though studies with more lipophilic compounds [9; 10 and 1 l] found pesticide stratification for at least
b depth 0.05 m -*. depth 0.25 m -I- depth 0.45 m 1 5
10
15
Time after application (h)
20
25
*depth 0.05 m -.- depth 0.25 m *-depth 0.45 m 5
20 10 15 Time after application (h)
25
Figure 2. Mean linuron concentrations (n=lO) at three depths in the ditches after application of a nominal dose of 50 ug.dmS3 in May (A) and July (B).
2182 24 hours after application. Compared with a chlorpyrifos study conducted in the same ditches [ 101, the high mixing rate of linuron in the present study can be partly explained by the lower macrophyte biomass. Moreover, linuron has a much higher water solubility compared to the pesticides used in the abovementioned studies, allowing a higher mixing rate. A comparable high mixing rate was found [ 121 for the poorly soluble diflubenzuron, applied as a liquid with Dimilin wettable powder in relatively shallow ponds (0.27 m deep). This relatively high mixing rate can be explained by the absence of macrophytes and the smaller depth of the ponds. In addition, the effect of the additives in the formulation might have played a role, since application of the technical product in the study of Schaefer and Dupras [12] showed stratification of diflubenzuron for at least 48 hours. During the flow period, the linuron concentrations in the water compartment showed slight longitudinal variations (Fig. 3). The average linuron concentration near the water supply was found to be approximately 20 to 30% lower than that near the water discharge pipe. No clear differences in linuron concentration were found in the transverse cross-section of the ditch, due to the differentiated transverse flow profile. At a distance of 3 m from the inlet introducing uncontaminated water into the ditches, the variation in the concentration of linuron at the fifteen different sampling points was somewhat higher than at other locations. The maximum standard deviation within a transversal cross-section was found to be 15%.
*day
1
-.-day 4 A- day 7
I
I
10 Distance
I
20 from
30 inlet
I
40
(m)
Figure 3. Average and range in linuron concentrations (n=15) at seven different locations in a ditch (8) treated with the highest dose of linuron. Results are shown for 1, 4 and 7 days after flow started in the ditches in May.
Differences in the rate of disappearance of linuron from the water compartment seem to be relatively small between the May, June and July applications and between the treatment levels (Fig. 4). During the stagnant period, the half-life (ti/;) for the disappearance of linuron from the water compartment varied from 7.2 to 11.8 days (Table 2). Treatment level and test period, however, significantly affected the half-life.
2183
00
14
7
21
o1
,
‘-,‘_N
0
7
14
28
_ _ ~ 21
28
1s I;
3
cn r,
30 0
D
10-
7
14
Time
after application
2 28
21
0
0
7 Time
(d)
14
21
after application
Cd)
28
Figure 4. Mean linuron concentrations (n=3; 50 pg.dm? n=5) by depth-integrated sampling of the water compartment after application to the ditches in May, June and July. Nominal concentrations of 0.5 (A), 5 (B), 15 (C) and 50 (D) pg.dme3 were used. Table 2. Half-lives of linuron in water (for the stagnant period of one week after each application) for the four treatment levels and the three applications. The values found for each individual ditch are given in parentheses.
Half-life (d) after application in
Initial concentration (pg.dm-3) 0.5
8.2
5
10.5 (9.4 - 11.5)
15 50
June
May
9.6
(7.6 - 8.5)
(9.4 -9.7)
11.8 (10.1 - 13.5)
July
8.0
(7.8 - 8.2)
7.2
(7.0 - 7.4)
7.5
(6.7 - 8.2)
7.2
(6.6 - 7.9)
8.6
(7.9 - 9.3)
7.3
(7.0 - 7.5 )
11.4 (11.1 - 11.7)
9.1 (8.2 - 10.0)
Analyzing our results with ANalysis Of VAriance (ANOVA), we found significantly (p < 0.01) higher half-lives for the highest linuron concentration used in our study. Comparable significant differences (ANOVA, p < 0.01) in half-lives were found between the application periods. These differences can be attributed to differences in average temperature between treatment periods, and to differences in pH between treatment levels. In July, the average water temperature during the first two weeks after application was almost 10 “C higher than that in the other periods (Table 3), while differences in water temperature between May and June were relatively small. It must be mentioned that the summer of 1995 was relatively hot by Dutch standards. In the ditches treated with the highest dose, the pH of the water was found to be distinctly lower than in those treated with lower doses (Table 4).
2184
Table 3. Average water temperatures in the ditches for the four weeks after linuron application in May, June and July 1995. Flow period
Stagnant period Period
week 1
week 2
week 3
week 4
May
13 “C
14 “C
20 “C
19 “C
June
15 “C
16 “C
20 “C
24 “C
July
23 “C
24 “C
23 “C
23 “C
After the introduction of flow into the ditches, the rate of disappearance of linuron from the water was more rapid than in the stagnant period. For the flow period, an average half-life could be calculated of 3.8 days. After three weeks of flowing the ditches, less than 2% of the nominal concentration was left in the water compartment. Calculated half-lives of linuron in water in our experiment ( 7.2 - 11.8 days) are somewhat shorter than those (16 - 40 days) found for linuron in pond enclosures [ 131.The half-lives calculated for our experiment, however, are based on a period of six days after application, whereas the results obtained by Stephenson and Kane [ 131 were based on a period of 23 to 42 days after application. The difference in nominal linuron concentrations may constitute an additional explanation. The nominal linuron concentration at the start of the pond enclosure experiment was approximately 1000 pg.dme3,which is 20-fold higher than the highest nominal concentration used in our experiment. The concentration dependence of the disappearance rate of linuron in the water compartment is in accordance with results obtained in indoor microcosm
Table 4. Average pH values of ditch water for treatment levels of 0, 0.5, 5, 15 and 50 pg.dme3 linuron during the stagnant period (7 days) after each application. Maximum and minimum pH values are given in parentheses. pH at treatment level Period May
June
July
0
0.5
5
15
50
9.1
8.6
7.9
7.7
7.2
(7.9 - 9.8)
(8.0 -9.0)
(7.5-8.5)
(7.5 -7.8)
(7.0 - 7.4)
8.3
9.2
8.0
8.1
7.2
(7.6 - 8.8)
(9.1 - 9.4)
(7.5 - 8.7)
(7.5 - 8.6)
(7.0 - 7.4)
8.5
9.1
8.5
8.5
7.3
(8.0 - 9.2)
(8.9 - 9.3)
(7.9 - 8.9)
(8.2 - 8.7)
(7.2 - 7.3)
2185 experiments in which chronic levels of linuron (0.5; 5; 15; 50 and 150 pg.dn?) were applied [14]. In these indoor microcosms half-lives for the disappearance of linuron from the water column were calculated to range from 11 to 49 days, and were found to be strongly dependent on the level of treatment and the pH of the overlying water (which was lower at higher treatment levels). The finding of a faster linuron disappearance from the water in the warmer treatment period is in agreement with results reported by Cserhati et al.[ 151, who studied the decomposition of linuron in aqueous solutions in the laboratory and found it to be faster at higher temperatures.
Linuron
in macrophytes.
Linuron showed a relatively high sorption rate onto macrophytes (Fig. 5).
Three days after application of the highest dose, an average content of 7.3 ug.g-’linuron based on dry mass was found for the three application periods. In laboratory microcosms, linuron residues in stems and roots of sorghum plants (Sorghum sp.) have been found to range from 12 to 44 mg.kg-’ after application of linuron to the sediment [16]. The dose of linuron used in the latter experiment [ 161, was about fifteen times as high as that used in our study. In contrast with our results, linuron could not be detected in aquatic macrophytes (Potamogeton
pedoliatus
L. and Myriophyllum
spicatum L.) at the end of a microcosm
experiment containing estuarine sediment and water [17]. In this context it is striking that, in spite of the absence of linuron from the macrophytes, effects were found on the photosynthesis of the macrophytes in these estuarine microcosms.
5
+ Water
pgldrrr3 -t- Sediment
I 7
@di3*
I 14
Time after application
I 21
28
(d)
Figure 5. Linuron concentrations in the water and sediment compartments per dm3, and in the macrophytes per g dry weight, averaged for the ditches treated with the highest dose and for the three application periods.
2186 During the flow period, linuron concentrations The half-life
for the disappearance
in the macrophyte
compartment
of linuron from the macrophyte
decreased
compartment
in this period was
estimated to be 6.5 d. This half-life is somewhat longer than that in the water compartment the flow period, indicating
a relatively slow desorption
gradually.
(3.8 d.) during
of linuron from macrophytes.
Linuron in the sediment. Linuron showed a relatively high sorption rate to the sediment (Fig. 5). The maximum amount of linuron sorbed to the sediment was found on day 6 after application.~During period the linuron concentration disappearance
in the sediment compartment
the flow
decreased gradually, with a half-life for the
of linuron from the sediment calculated at 4.6 d. This half-life is slightly longer than that
calculated for the water compartment
(3.8 d.) during the flow period, indicating that desorption
of linuron
from sediment seems to be a relatively fast process. The concentrations
of linuron measured in the sediment decreased
with depth (Fig. 6). Less than 1%
of the linuron sorbed to the sediment was found in the sediment at depths exceeding 4 cm. Approximately 80% of the linuron in the sediment compartment
was sorbed to the sediment, while the remaining fraction
was present in the pore water. From day 1 onwards, the concentration
of linuron in the pore water of the
upper layer (O-l cm) was approximately
in the overlying water. Only seven
hours after application overlying
water. Apparently,
concentration 50
the same as the concentration
(day 0.3) the linuron concentration the equilibrium
in the pore water was incomplete ml
150
50
100
between
in the pore water was lower than that in the the linuron
sorbed
to the sediment
and the
at that time. 150
50
Concentration (mgm”) 100 150 50
150
I
1
3 5
I 3 5
Depth (cm)
Sorbed linuron
0
Dissolved
linuron
Figure 6. Linuron concentrations in the sediment compartment as a function of the depth, averaged for the three application periods. The fractions of linuron sorbed to the sediment and dissolved in the pore water are indicated separately.
Low residues (1.4 ug.kg-‘) of linuron have been found at the end of an experiment estuarine microcosms
[ 173. At that time, the average concentration
these results were based on averaged concentrations
using sediment of
in water was 100 pg.dme3. However,
for a sediment column with a length of 8 cm. When
2187 we calculated
the average concentration
based on the dry mass of a sediment column with a length of
8 cm, we found values of approximately
10 ug.kg-‘, with a corresponding
40 ug.dmm3. This relatively high concentration
concentration
in the sediment compartment
in the water of
of our study is probably due
to the relatively high organic matter content of the upper layer of the sediment (20 - 25%).
Material
balance.
Seven hours after the application
was found in the water compartment quantitatively
compartment
for linuron. After the stagnant periods, about
was found in the water compartment
During the flow periods, a large proportion discharge
about 95% of the linuron dose
(Fig. 7). During all application periods, the overlying water remained,
speaking, the main environmental
60% of the nominal concentration
of the herbicide,
of linuron could be estimated
of all ditches.
of the linuron dose was discharged
by multiplying
from the ditches. The
the linuron concentrations
measured
before the
water discharge pipe and the registered volumes of water discharged from the ditches. The total discharge of water in the application period of July was slightly lower (125 m3) than that in May and June (140 m3). Approximately
60% of the linuron dose was discharged
In comparison,
only 4 1% of the linuron dose was discharged
in discharge
can be explained
during the May and June flow period (Fig. 7). during the July flow period. The difference
from the lower water discharge
in July in comparison
application periods. In addition, the half-life for the disappearance during the stagnant period in July was significantly
lower (ANOVA, p < 0.01) than the half-lives
other application periods (Table 2). A higher rate of disappearance less linuron is available to be discharged.
with the other
of linuron from the water compartment in the
of linuron from the water implies that
At the end of the flow periods, approximately
2% of the linuron
dose was left in the ditches treated with the highest dose. In the ditches treated with the highest dose, the macrophyte a sink for linuron (Fig. 7). Three days after the applications of the linuron macrophytes.
dose of 0.6%, 0.7% and l.l%,
compartment
in May, June and July, maximum proportions
respectively,
were found to be associated
The literature provided no other data on the role of macrophytes
In a chlorpyrifos by macrophytes of macrophytes
study [IO] approximately
only played a minor role as
40% of the chlorpyrifos
in the partitioning of linuron.
dose applied was found to be sorbed
(mainly Elodea nutfullii and Chara globularis). In this chlorpyrifos was considerably
is more lipophilic.
1 010 and 50 000, respectively
study, the biomass
higher (0.35 kg.m-‘) than that in our linuron study (0.02 - 0.11 kg.m-‘).
In addition, the sorption of linuron to macrophytes because chlorpyrifos
with the
can be expected to be lower than that of chlorpyrifos,
The partition coefficients
(K,,,) of linuron and chlorpyrifos
are
[6].
The sediment was found to contain maximum proportions of 7.3%, 6.7% and 6.0% of the linuron dose, respectively, involving
six days after the applications
the application
chlorpyrifos had expected
of chlorpyrifos
dose in the sediment. a greater difference
in May, June and July (Fig. 7). In a previous experiment in the same ditches,
Based on the difference
we found approximately
10% of the
in solubility of linuron and chlorpyrifos,
in the amounts of the two pesticides
[lo]
sorbed to the sediment.
we
That this
2188
I Start of flow 100 al -3 %
75
& 2
5o
c k
25
0
25
0
7
14
21
28
0 0
7
14
Time after application
I
0
7
Time
14
after application
21
21
28
(d)
I
28
(d)
Figure 7. Linuron as a percentage of the dose in the water, macrophyte and sediment compartments of
the ditches treated with 50 pg.dmm3,as well as the percentage of the dose discharged from the ditches during the flow period. Results are shown for the application periods of May (A), June (B) and July (C).
was not the case can be partly explained from the lower biomass (0.02 - 0.11 kg.m-*) of macrophytes during the linuron experiment compared to the chlorpyrifos experiment (0.35 kg.m-*). It has been shown that in microcosms devoid of macrophytes a relatively large proportion of chlorpyrifos is sorbed by the sediment, compared to the situation in macrophyte-dominated microcosms [lo]. Finally, the average organic matter content for the two upper sediment layers( O-1 cm and 1-2 cm) was approximately 20 to 25% during the linuron experiment (Table l), while the organic matter content of the sediment in the ditches during the chlorpyrifos experiment was only approximately 5% [lo]. A relatively high degree of sorption of linuron to sediment richer in organic matter is very likely.
Acknowledgements
This study was supported by the Dutch Ministry of Agriculture, Nature Management and Fisheries. We are indebted to Edwin Koopman for his assistance in the sampling and analyzing of the macrophytes. We thank Carolien van Rhenen, Jan Drent and Kees Kersting for providing us with the data on the environmental conditions. Last but not least, we are indebted to Paulien Adriaanse, Bram ten Cate, Jan Klerkx, Minze Leistra, Rene van Wijngaarden and the reviewers for their helpful comments on this paper.
2189
References
1. Brock, T.C.M., and B.J. Budde (1994). On the choice of structural parameters and endpoints to indicate responses of freshwater ecosystems to pesticide stress. In Freshwater Field Testsfor Hazard Assessment of Chemicals (Edited by LA. Hill, F. Heimbach, P. Leeuwangh and P. Mathiessen) Lewis, Boca Raton,
FL, USA, pp. 19-56.
2. Kersting, K. (1994). Functional endpoints in field testing. In Freshwater
FieM Tests for Hazard
Assessment of Chemicals (Edited by LA. Hill, F. Heimbach, P. Leeuwangh and P. Mathiessen), Lewis,
Boca Raton, FL, USA, pp. 57-81.
3. Adriaanse, P.I., (1996). Fate of pesticides in field ditches: the TOXSWA simulation model. Report 90, DLO Winand Staring Centre, Wageningen, The Netherlands.
4. Adriaanse, P.I., (1996). Exposure assessment of pesticides in field ditches: the TOXSWA model. Pesticide Science, 49, 2 1O-212.
5. Drent, J. and K. Kersting (1993). Experimental ditches for research under natural conditions. Water Research, 27, 1497- 1500.
6. Tomlin, C. (1994). The pesticide manual. 10” edition. BCPC Publication Sales, Bear Farm, Binfield, Bracknell, Berks RG12 5QE, United Kingdom.
7. Ronday, R., G.H. Aalderink and S.J.H. Crum (1997). Spray drift simulation of pesticides to an aquatic mesocosm. Water Research (accepted for publication)
8. Brock, T.C.M., S.J.H. Crum, R. van Wijngaarden, B.J. Budde, J. Tijink, A. Zuppelli and P. Leeuwangh (1992). Fate and effects of the insecticide Dursban 4E in indoor Elodea-dominated and macrophyte-free freshwater model ecosystems. I. Fate and primary effects of the active ingredient chlorpyrifos. Archives of Environmental Contamination and Toxicology, 23, 69-84.
9. Caquet, T., E. Thybaud, S. Le Bras, 0. Jonot and F. Ramade (1992). Fate and biological effects of lindane and deltamethrin in freshwater mesocosm. Aquatic Toxicology, 23, 261-278.
2190
10.
Crum, S.J.H. and T.C.M. Brock (1994). Fate of chlorpyrifos in indoor microcosms and outdoor experimental ditches. In Freshwater Field Testsfor Hazard Assessment of Chemicals (Edited by IA. Hill, F. Heimbach, P. Leeuwangh and P. Mathiessen), Lewis, Boca Raton, FL, USA, pp. 315322.
11.
Heinis, L.J. and M.L. Knuth (1992). The mixing, distribution and persistence of esfenvalerate within littoral enclosures. Environmental Toxicology and Chemistry, 11,1 l-25.
12.
Schaefer, C.H. and E.F. Dupras Jr. (1976). Factors affecting the stability of Dimilin in water and the persistence of Dimilin in field waters. Journal Agricultural and Food Chemistry, 24, 733-739.
13.
Stephenson, R.R. and D.F. Kane (1984). Persistence and effects of chemicals in small enclosures in ponds. Archives of Environmental Contamination and Toxicology, 13, 3 13-326.
14.
Brink, P.van den, E.M. Hartgers, U. Fettweis, S.J.H. Crum, E. van Donk, and T.C.M. Brock (1997). Sensitivity of macrophyte-dominated freshwater microcosms to chronic levels of the herbicide linuron I. Primary products. Ecotoxicology and Environmental Safety (accepted for publication).
15.
Cserhati, T., A. VCgh and A. Dobrovolszky
(1976). Study on the factors influencing
the
decomposition of linuron. Acta Phytopathologica Academiae Scientiarum Hungaricae, 11,325330.
16.
Francis, B.M., R.L. Lampman and R.L. Metcalf (1985). Model ecosystem studies of the environmental
fate of five herbicides used in conservation tillage. Archives of Environmental
Contamination and Toxicology, 14, 693-704.
17.
Kemp, W.M., W.R. Boynton, J.J. Cunningham, J.C. Stevenson, T.W. Jones and J.C. Means (1985). Effects of atrazine and linuron on photosynthesis and growth of the macrophytes, Potamogeton per$oliatus L. and Myriophyllum spicatum L. in an estuarine environment. Marine Enviranmenta~ Research, 16, 255-280.
clumosphcre, Vol. 36, No. 10,pp. 2175-2190,1998 Q 1998Elswier Scia~ccLtd Allrights-cd.RintediaGratBritaii 0045-6535/98 $19.00+0.00
PII: soo45-6535010190-4
FATE OF THE HERBICIDE LINURON IN OUTDOOR EXPERIMENTAL DITCHES
S.J.H. Crum’, G.H. Aalderink, and T.C.M. Brock
DLO Winand Staring Centre for Integrated Land, Soil and Water Research, PO Box 125, 6700 AC Wageningen, The Netherlands (kceived in Germany11september1997;accepted 3 November1997)
Abstract
The fate of the urea-herbicide linuron was studied in experimental ditches. Eight ditches were sprayed three times at monthly intervals, while two served as controls. As a simulation of spray drift, four doses (nominal: 0.5; 5; 15 and 50 pg.dni3) were applied in duplicate. After each herbicide application, the ditches were kept stagnant for a week, followed by a flow period of three weeks with fresh water. No clear stratifican n of linuron could be found in the water compartment. During the stagnant period, half-lives ‘a for the disappearance of linuron in the water compartment ranged from 7.2 to 11.8 days. The rate of disappearance was slower in the ditches treated with the highest dose and in colder treatment periods. A maximum of 6% of the linuron dose was found in the sediment and 1% in the macrophyte compartment. Approximately 20% of the fraction in the sediment compartment was present in the pore water. 01998 Else&r Science Ltd. All rights reserved
Introduction
Herbicides are widely used for weed control in agriculture. During and after the application of herbicides on agricultural fields, these chemicals may enter adjacent aquatic ecosystems by spray drift, runoff and leaching. It has frequently been reported that herbicides may have adverse effects on the structure and functioning of freshwater ecosystems [ 1; 21. To prevent adverse side-effects of agrochemicals in freshwater ecosystems, authorities have set criteria that have to be met before herbicides are allowed on the market. Recently, the member states of the European Union adopted the Uniform Principles (Council Directive 91/414/EEC, concerning the placing of plant protection products on the market; EU, 1994). This directive states that environmental concentrations of a pesticide in surface water should be calculated with
2175
2176 a validated simulation model, accepted by the member states. The outcome of such a model, the Predicted Environmental
Concentration (PEC), is compared with toxicity data for standard test species (algae;
Daphnia; fish). Of these species, algae are usually the most susceptible to herbicides. The PEC should not exceed 0.1 times the NOEC (no observed effect concentration) of the test algae.. However, if additional (semi-)field studies clearly indicate that the environmental concentration of ,the herbicide does not pose a significant risk to the viability of aquatic organisms, the above criteria may be overruled. In 1994, a research project was initiated on the fate and effects of herbicides in freshwater model ecosystems (microcosms; mesocosms). The urea-herbicide linuron (applied as Afalon Flow) was adopted as a benchmark compound, while mesocosms were used which intended to simulate drainage ditches. The experiments were performed to: a) validate the criteria adopted in the Uniform Principles to protect aquatic organisms; b) investigate the ability of the freshwater community to recover from herbicide stress; and c) validate the TOXSWA fate model for a herbicide applied to ditches with flowing water. The TOXSWA fate model has been developed to calculate the long-term exposure of aquatic organisms to pesticides [3; 41, and it is intended to be used in the Dutch registration procedure.
Water flow in Dutch drainage ditches varies from stagnant to slowly flowing. In our experiment, the water in the ditches was kept stagnant for one week after the application of linuron, followed by a threeweek period of fresh water flow. This yielded an exposure regime which would be realistic for Dutch drainage ditches, and allowed us to study the recovery potential of the freshwater community. To allow comparison of exposure regimes and responses of effect endpoints, linuron was measured at several locations in the water compartment of all ditches. Validation of the TOXSWA model, however, required more detailed information. Therefore, the fate of linuron was studied in more detail in the ditches treated with the highest dose (50 pg.dm-‘). This paper presents methods and results on the fate of linuron in the experimental ditches. Results of our mesocosm study are compared with literature data on the fate of linuron and other pesticides in freshwater ecosystems. Finally, this paper is intended as a reference article for studies dealing with model validation of TOXSWA, and with effects of linuron on ecosystem structure and functioning in the same experiment.
MaterhIs and methods
Experimental design. Twenty experimental ditches were constructed in 1988 on the Sinderhoeve experimental station, Renkum, the Netherlands [5]. Twelve of these are in use for ecotoxicological research. The experimental ditches have a length of 40 m and a width at the sediment surface of approximately 1.6 m. At a water depth of 0.50 m, the width at the water surface is 3.4 m and the ditches then contain
2177 approximately 50 m3 water. The materials for the sediment layer was taken from a mesotrophic lake. Aquatic organisms and macrophytes were introduced in the experimental ditches together with the sediment. The water in the ditches originates from a supply reservoir (2500 m’) at the experimental station. This reservoir contains a sediment layer of the same origin as that of the ditches, and is regularly replenished with unpolluted well water, pumped up from a depth of 90 m. The well water is diluted with the water in the reservoir to maintain its surface water characteristics. Each individual ditch can be connected with the reservoir in order to maintain the desired hydrological regime. Effluent water from the ditches is discharged to the discharge reservoir. The amount of water entering and leaving each experimental ditch is recorded. The sediment layer of the ditches can be characterised as a sandy loam in which a well-developed macrophyte vegetation was rooting. Some characteristics of different sediment layers are presented in Table 1. Abundant macrophytes in the ditches were Myriophyllum spicatzun,Elodea nuttallii and Sag&aria sagittifolia. The ditches in which the fate of linuron was studied in detail were dominated by Myriophyllum spicatun. In the ditches with the highest treatment level, the total dry weight biomass of macrophytes at
the start of the experiment was 0.02 kg.m-‘, increasing to 0.11 kg.m-* in October. The ditch water was relatively poor in nutrients, while the average dissolved and total organic carbon contents were found to be 4 and 5 mg.dm-‘, respectively.
Table 1:
Some characteristics of different sediment layers in the experimental ditches.
Sediment layer
Organic matter
Dry bulk density
Volume fraction of
(cm)
(%)
(kg.dm-3)
liquid phase
o-1
26
0.1
0.9
l-2
19
0.2
0.8
2-4
6
0.7
0.7
2
1.6
0.4
4-
10
Herbicide application and hydrological regime. Linuron was applied as technical grade Afalon Plow.
This is an emulsifiable formulation with linuron (450 g.dme3) as the active ingredient. Linuron (3-(3,4dichlorophenyl)-1-methoxy-1-methylurea)
is a urea herbicide which inhibits the photosynthetic electron
transport. Some physico-chemical properties have been described by Tomlin [6]. Spray equipment had been designed and constructed for the application of pesticides to the experimental ditches [7]. Eight experimental ditches were treated with four different doses in duplicate, resulting in nominal concentration levels in the water of the ditches of 50; 15; 5 and 0.5 pg.dme3, respectively. The concentration of 0.5 ng.dmm3represents the norm concentration based on the Uniform Principles. The other concentrations are chosen to obtain a dose-response relationship for sensitive ecological endpoints. Two experimental
2178 ditches served as control units. The herbicide was applied on the 91hof May, on the 6’i’of June and on the 4’ of July 1995. After each application, the ditches were kept stagnant for seven days. Thereafter, flow of fresh water from the supply reservoir was introduced, at a velocity of approximately supply and discharge
were recorded during the flow periods. Differences
a
5 m.d”. Water
in the volume of water stored
in the ditches were calculated by regular measuring the water level with a fixed water-level gauge. During the experiment
the temperature
and pH were measured continuously
the ditches. These measurements
were carried out as described by Drent and Kersting [5].
Linuron in water. To estimate the exposure concentrations, with the help of perspex application
in the water column in the centre of
depth-integrated
tubes (length 42 cm, internal diameter
(for time intervals see Fig. 5). Depth-integrated
water samples were taken
3.9 cm) at several moments
water samples were collected
after
by vertically
inserting the tube into the water layer and closing the tube with rubber stoppers. Finally the water samples were transferred
to 500 cm’ flasks and transported to the laboratory for extraction
depth-integrated
water samples were taken at three locations (5; 22 and 33 m from water inlet). In the
ditches treated with the highest dose, depth-integrated
and analysis. These
samples were collected at five locations (5; 14; 22;
27 and 33 m from inlet). Stratification
of linuron in the water column was studied in the ditches treated with the highest dose,
by placing a stainless steel gradient sampling apparatus at five locations (3; 8; 12; 25 and 35 m from inlet). This gradient sampling apparatus consisted of three perforated tubes, fixed in a framework and placed at three different levels (0.05 m; 0.25 m and 0.45 m) under the water surface. The tubes were connected
to
sampling flasks by stainless steel tubing. After the perforated tubes had been flushed, water samples were collected
simultaneously
flasks individually, concentration.
by lowering the pressure in the flasks with a vacuum pump. After shaking the
subsamples
were transferred into 2 cm’ HPLC-vials
for direct analysis of the linuron
A more detailed description of the gradient sampling apparatus has been given in a previous
paper L31. The transverse distribution of linuron in the water compartment by means of the purpose-built allowed
15 different
apparatus framework.
consisted
CATS apparatus (Crum and Aalderink Transversal
water samples within the ditch profile to be collected of fifteen
stainless
steel tubes (internal diameter
Sampler, Fig. 1), which
simultaneously.
The CATS
2 mm), fixed in an aluminium
Each of the stainless steel tubes was individually connected to 250 cm3 flasks by a short piece
of polyethylene direction.
was investigated during the flow period
tubing. The lower end (e 0.5 cm) of each individual
tube was bent in a horizontal
During sampling, the framework with the tubes was carefully inserted into the water column.
After the tubes had been flushed, water samples were collected simultaneously in the flasks with a vacuum pump. These measurements
were performed
by lowering the pressure
at seven different locations (for
details see Fig. 3) in a ditch treated with the highest dose. After shaking the flasks, subsamples transferred
into 2 cm’ HPLC-vials
for direct analysis of the linuron concentration.
were
2179
Figure 1. The CATS sampling apparatus used for water sampling at fifteen locations in the cross-section
of a ditch.
Samples with linuron concentrations below 10 pg.dm-3were concentrated in octadecyl (C-18) solid-phase extraction columns, which were prepared by filling a 5 cm3 cartridge with 0.5 g octadecyl material (Bakerbond C-18 40 pm lotno. 702501, J.T. Baker Inc., Phillipsburg, New Jersey, USA), fixed between two polypropylene filters. The extraction columns were conditioned with 5 cm3 methanol and 5 cm3 distilled water, respectively. After extraction of a known volume of water, linuron was eluted from the column with 1.5 cm3 acetonitrile. Finally, the samples were diluted with water to a volume of 5 cm3 and subsamples were transferred to HPLC-vials for analysis. Recovery rates of linuron from spiked distilled water and from natural water samples were 101.2% (s.d.= 0.7%; n=3) and 100.3% (s.d.= 0.9%; n=3), respectively. Calculations of half-lives for linuron in the water compartment of the ditches were based on nominal concentrations for t=O and on measured concentrations for subsequent samplings. The measured linuron concentrations were corrected for the varying water levels due to evaporation and/or rainfall.
Linuron
in macrophytes.
At several moments after application, macrophyte samples were collected
at five locations in the ditches treated with the highest dose, by taking at least two shoots of the dominant macrophyte Myriophyllum spicutum at each location. The locations and time intervals were equal to those of the depth-integrated water sampling. The macrophyte samples were packed in aluminium foil and stored in a freezer (-20 “C) until analysis. The macrophyte samples were frozen by adding liquid nitrogen and then crushed in a mortar. A subsample was weighed into a 250 cm3 flask. After 50 cm3 distilled ethyl acetate and 50 cm3 distilled water had been added, linuron was extracted by shaking intensively for two hours. After phase separation
2180 a fraction of the ethyl acetate layer was removed and subjected to clean-up. The remaining content of the flask was transferred
into an aluminium tray and dried in an oven at 105 “C for 24 hours, to measure the
dry mass of the macrophyte
sample.
The ethyl acetate extract was evaporated
to dryness and the residue was redissolved
in 0.5 cm3 ethyl
acetate and 2 cm3 n-hexane. The clean-up column consisted of a 5 cm3 cartridge filled with 0.5 g florisil and 2 g sodium sulfate on top of it. It was rinsed with 5 cm3 ethyl acetate followed by 5 cm3 n-hexane, respectively.
The sample was transferred quantitatively
20 cm3 n-hexane evaporated
containing
to the clean-up column and linuron was eluted with
10% ethyl acetate. The eluate was collected
to dryness in a water bath (40 “C) under a gentle airstream. The residue was redissolved
0.5 cm3 acetonitrile
by ultrasonification
for analysis. Recovery
Millipore) and transferred to a 2 cm3 HPLC-vial
of linuron from spiked macrophyte
in sediment.
in
and then diluted with 2 cm3 distilled water. This solution was
filtered through a 0.45 pm disposable filter (Millex-HV,,,
Linuron
in a 25 cm3 glass tube and
samples was 96.9% (s.d.= 6.0%; n=lO).
Sediment cores were taken to investigate
the concentration
of linuron in the
sediment layer and its distribution between the liquid and solid phases. The cores were collected by means of the perspex tubes described
above (internal diameter 3.9 cm). A tube was inserted carefully into the
sediment by hand, to a depth of approximately
0.1 m, and then turned once along its vertical axis. Its top
opening was then closed with a rubber stopper below the water surface, and the tube with sediment was pulled up carefully. The lower end of the tube was then also closed with a rubber stopper, below the water surface, and the water layer was decanted immediately.
The cores were taken at several moments
after
application in the ditches treated with the highest dose, each time at five locations per ditch. The locations and time intervals were equal to those of the depth-integrated
water sampling. The tubes with the sediment
were stored vertically in a freezer (-20 “C) until extraction and analysis. The frozen sediment cores were divided into four layers (O-l cm, l-2 cm, 2-4 cm, > 4 cm), in order to study the stratification tubes and weighed. extraction intensively
efficiency.
of linuron in the sediment. The layers were transferred
Occasionally,
to 100 cm3 centrifuge
distilled water was added to relatively dry samples to obtain a better
The sediment was extracted with 40 cm3 ethyl acetate, by shaking the suspension
for two hours. After phase separation,
the same clean-up procedure remaining after extraction
a fraction of the ethyl acetate layer was removed and
as that for the macrophyte
was transferred
samples was followed. The content of the tube
into an aluminium tray. The dry mass of the sediment sample
was measured by drying in the oven at 105 “C for 24 hours. The recovery rate of linuron from spiked sediment samples was 92.4% (s.d.= 2.7%, n=18).
Analysis of linuron. Linuron was analyzed using a high performance Subsamples
liquid chromatography
of 100 mm3 were injected with a Perkin Elmer ISS-100 autosampler.
was a mixture of acetonitrile, methanol and water (UV,,,-purified)
technique.
The mobile phase used
in a ratio of 25:25:50. The mobile phase
2181 was pumped at a flow rate of 0.8 cm3.min” with a Waters M590 pump through a Chrompack Lichrospher
100 BP-18 (length: 100 mm; internal diameter: 3 mm; particle size: 5 pm) analytical column, provided with a guard column of the same origin. The column was mounted in a Waters TCM column oven, which was set at a temperature of 40 “C. The herbicide was detected using a Perkin Elmer LC-90 UV-detector set at a wavelength of 254 nm. The retention time for linuron was 12.6 min, with a detection limit of 2 pg.dm-‘. The sediment extracts were separated on a Waters Novapak C-18 (length: 150 mm; internal diameter: 4,6 mm; particle size: 4 urn) analytical column, provided with a guard column of the same origin. The mobile phase consisted of a mixture of water and acetonitrile in a ratio of 55:45 (v/v), adjusted to a flow rate of 0.7 cm3.min~‘. In this set-up the retention time of linuron was 10.7 min.
Results and discussion
Linurun in water. In neither of the periods was a clear stratification of linuron found in the water column (Fig. 2). Within a few hours after application, the linuron concentration at all sampled depths was higher than 90% of the nominal concentration. In addition, concentration differences between the three sampled depths were less than 10%. This indicates a rather high mixing rate of linuron in the water column. The literature contained no data on linuron stratification in the overlying water of aquatic ecosystems, though studies with more lipophilic compounds [9; 10 and 1 l] found pesticide stratification for at least
b depth 0.05 m -*. depth 0.25 m -I- depth 0.45 m 1 5
10
15
Time after application (h)
20
25
*depth 0.05 m -.- depth 0.25 m *-depth 0.45 m 5
20 10 15 Time after application (h)
25
Figure 2. Mean linuron concentrations (n=lO) at three depths in the ditches after application of a nominal dose of 50 ug.dmS3 in May (A) and July (B).
2182 24 hours after application. Compared with a chlorpyrifos study conducted in the same ditches [ 101, the high mixing rate of linuron in the present study can be partly explained by the lower macrophyte biomass. Moreover, linuron has a much higher water solubility compared to the pesticides used in the abovementioned studies, allowing a higher mixing rate. A comparable high mixing rate was found [ 121 for the poorly soluble diflubenzuron, applied as a liquid with Dimilin wettable powder in relatively shallow ponds (0.27 m deep). This relatively high mixing rate can be explained by the absence of macrophytes and the smaller depth of the ponds. In addition, the effect of the additives in the formulation might have played a role, since application of the technical product in the study of Schaefer and Dupras [12] showed stratification of diflubenzuron for at least 48 hours. During the flow period, the linuron concentrations in the water compartment showed slight longitudinal variations (Fig. 3). The average linuron concentration near the water supply was found to be approximately 20 to 30% lower than that near the water discharge pipe. No clear differences in linuron concentration were found in the transverse cross-section of the ditch, due to the differentiated transverse flow profile. At a distance of 3 m from the inlet introducing uncontaminated water into the ditches, the variation in the concentration of linuron at the fifteen different sampling points was somewhat higher than at other locations. The maximum standard deviation within a transversal cross-section was found to be 15%.
*day
1
-.-day 4 A- day 7
I
I
10 Distance
I
20 from
30 inlet
I
40
(m)
Figure 3. Average and range in linuron concentrations (n=15) at seven different locations in a ditch (8) treated with the highest dose of linuron. Results are shown for 1, 4 and 7 days after flow started in the ditches in May.
Differences in the rate of disappearance of linuron from the water compartment seem to be relatively small between the May, June and July applications and between the treatment levels (Fig. 4). During the stagnant period, the half-life (ti/;) for the disappearance of linuron from the water compartment varied from 7.2 to 11.8 days (Table 2). Treatment level and test period, however, significantly affected the half-life.
2183
00
14
7
21
o1
,
‘-,‘_N
0
7
14
28
_ _ ~ 21
28
1s I;
3
cn r,
30 0
D
10-
7
14
Time
after application
2 28
21
0
0
7 Time
(d)
14
21
after application
Cd)
28
Figure 4. Mean linuron concentrations (n=3; 50 pg.dm? n=5) by depth-integrated sampling of the water compartment after application to the ditches in May, June and July. Nominal concentrations of 0.5 (A), 5 (B), 15 (C) and 50 (D) pg.dme3 were used. Table 2. Half-lives of linuron in water (for the stagnant period of one week after each application) for the four treatment levels and the three applications. The values found for each individual ditch are given in parentheses.
Half-life (d) after application in
Initial concentration (pg.dm-3) 0.5
8.2
5
10.5 (9.4 - 11.5)
15 50
June
May
9.6
(7.6 - 8.5)
(9.4 -9.7)
11.8 (10.1 - 13.5)
July
8.0
(7.8 - 8.2)
7.2
(7.0 - 7.4)
7.5
(6.7 - 8.2)
7.2
(6.6 - 7.9)
8.6
(7.9 - 9.3)
7.3
(7.0 - 7.5 )
11.4 (11.1 - 11.7)
9.1 (8.2 - 10.0)
Analyzing our results with ANalysis Of VAriance (ANOVA), we found significantly (p < 0.01) higher half-lives for the highest linuron concentration used in our study. Comparable significant differences (ANOVA, p < 0.01) in half-lives were found between the application periods. These differences can be attributed to differences in average temperature between treatment periods, and to differences in pH between treatment levels. In July, the average water temperature during the first two weeks after application was almost 10 “C higher than that in the other periods (Table 3), while differences in water temperature between May and June were relatively small. It must be mentioned that the summer of 1995 was relatively hot by Dutch standards. In the ditches treated with the highest dose, the pH of the water was found to be distinctly lower than in those treated with lower doses (Table 4).
2184
Table 3. Average water temperatures in the ditches for the four weeks after linuron application in May, June and July 1995. Flow period
Stagnant period Period
week 1
week 2
week 3
week 4
May
13 “C
14 “C
20 “C
19 “C
June
15 “C
16 “C
20 “C
24 “C
July
23 “C
24 “C
23 “C
23 “C
After the introduction of flow into the ditches, the rate of disappearance of linuron from the water was more rapid than in the stagnant period. For the flow period, an average half-life could be calculated of 3.8 days. After three weeks of flowing the ditches, less than 2% of the nominal concentration was left in the water compartment. Calculated half-lives of linuron in water in our experiment ( 7.2 - 11.8 days) are somewhat shorter than those (16 - 40 days) found for linuron in pond enclosures [ 131.The half-lives calculated for our experiment, however, are based on a period of six days after application, whereas the results obtained by Stephenson and Kane [ 131 were based on a period of 23 to 42 days after application. The difference in nominal linuron concentrations may constitute an additional explanation. The nominal linuron concentration at the start of the pond enclosure experiment was approximately 1000 pg.dme3,which is 20-fold higher than the highest nominal concentration used in our experiment. The concentration dependence of the disappearance rate of linuron in the water compartment is in accordance with results obtained in indoor microcosm
Table 4. Average pH values of ditch water for treatment levels of 0, 0.5, 5, 15 and 50 pg.dme3 linuron during the stagnant period (7 days) after each application. Maximum and minimum pH values are given in parentheses. pH at treatment level Period May
June
July
0
0.5
5
15
50
9.1
8.6
7.9
7.7
7.2
(7.9 - 9.8)
(8.0 -9.0)
(7.5-8.5)
(7.5 -7.8)
(7.0 - 7.4)
8.3
9.2
8.0
8.1
7.2
(7.6 - 8.8)
(9.1 - 9.4)
(7.5 - 8.7)
(7.5 - 8.6)
(7.0 - 7.4)
8.5
9.1
8.5
8.5
7.3
(8.0 - 9.2)
(8.9 - 9.3)
(7.9 - 8.9)
(8.2 - 8.7)
(7.2 - 7.3)
2185 experiments in which chronic levels of linuron (0.5; 5; 15; 50 and 150 pg.dn?) were applied [14]. In these indoor microcosms half-lives for the disappearance of linuron from the water column were calculated to range from 11 to 49 days, and were found to be strongly dependent on the level of treatment and the pH of the overlying water (which was lower at higher treatment levels). The finding of a faster linuron disappearance from the water in the warmer treatment period is in agreement with results reported by Cserhati et al.[ 151, who studied the decomposition of linuron in aqueous solutions in the laboratory and found it to be faster at higher temperatures.
Linuron
in macrophytes.
Linuron showed a relatively high sorption rate onto macrophytes (Fig. 5).
Three days after application of the highest dose, an average content of 7.3 ug.g-’linuron based on dry mass was found for the three application periods. In laboratory microcosms, linuron residues in stems and roots of sorghum plants (Sorghum sp.) have been found to range from 12 to 44 mg.kg-’ after application of linuron to the sediment [16]. The dose of linuron used in the latter experiment [ 161, was about fifteen times as high as that used in our study. In contrast with our results, linuron could not be detected in aquatic macrophytes (Potamogeton
pedoliatus
L. and Myriophyllum
spicatum L.) at the end of a microcosm
experiment containing estuarine sediment and water [17]. In this context it is striking that, in spite of the absence of linuron from the macrophytes, effects were found on the photosynthesis of the macrophytes in these estuarine microcosms.
5
+ Water
pgldrrr3 -t- Sediment
I 7
@di3*
I 14
Time after application
I 21
28
(d)
Figure 5. Linuron concentrations in the water and sediment compartments per dm3, and in the macrophytes per g dry weight, averaged for the ditches treated with the highest dose and for the three application periods.
2186 During the flow period, linuron concentrations The half-life
for the disappearance
in the macrophyte
compartment
of linuron from the macrophyte
decreased
compartment
in this period was
estimated to be 6.5 d. This half-life is somewhat longer than that in the water compartment the flow period, indicating
a relatively slow desorption
gradually.
(3.8 d.) during
of linuron from macrophytes.
Linuron in the sediment. Linuron showed a relatively high sorption rate to the sediment (Fig. 5). The maximum amount of linuron sorbed to the sediment was found on day 6 after application.~During period the linuron concentration disappearance
in the sediment compartment
the flow
decreased gradually, with a half-life for the
of linuron from the sediment calculated at 4.6 d. This half-life is slightly longer than that
calculated for the water compartment
(3.8 d.) during the flow period, indicating that desorption
of linuron
from sediment seems to be a relatively fast process. The concentrations
of linuron measured in the sediment decreased
with depth (Fig. 6). Less than 1%
of the linuron sorbed to the sediment was found in the sediment at depths exceeding 4 cm. Approximately 80% of the linuron in the sediment compartment
was sorbed to the sediment, while the remaining fraction
was present in the pore water. From day 1 onwards, the concentration
of linuron in the pore water of the
upper layer (O-l cm) was approximately
in the overlying water. Only seven
hours after application overlying
water. Apparently,
concentration 50
the same as the concentration
(day 0.3) the linuron concentration the equilibrium
in the pore water was incomplete ml
150
50
100
between
in the pore water was lower than that in the the linuron
sorbed
to the sediment
and the
at that time. 150
50
Concentration (mgm”) 100 150 50
150
I
1
3 5
I 3 5
Depth (cm)
Sorbed linuron
0
Dissolved
linuron
Figure 6. Linuron concentrations in the sediment compartment as a function of the depth, averaged for the three application periods. The fractions of linuron sorbed to the sediment and dissolved in the pore water are indicated separately.
Low residues (1.4 ug.kg-‘) of linuron have been found at the end of an experiment estuarine microcosms
[ 173. At that time, the average concentration
these results were based on averaged concentrations
using sediment of
in water was 100 pg.dme3. However,
for a sediment column with a length of 8 cm. When
2187 we calculated
the average concentration
based on the dry mass of a sediment column with a length of
8 cm, we found values of approximately
10 ug.kg-‘, with a corresponding
40 ug.dmm3. This relatively high concentration
concentration
in the sediment compartment
in the water of
of our study is probably due
to the relatively high organic matter content of the upper layer of the sediment (20 - 25%).
Material
balance.
Seven hours after the application
was found in the water compartment quantitatively
compartment
for linuron. After the stagnant periods, about
was found in the water compartment
During the flow periods, a large proportion discharge
about 95% of the linuron dose
(Fig. 7). During all application periods, the overlying water remained,
speaking, the main environmental
60% of the nominal concentration
of the herbicide,
of linuron could be estimated
of all ditches.
of the linuron dose was discharged
by multiplying
from the ditches. The
the linuron concentrations
measured
before the
water discharge pipe and the registered volumes of water discharged from the ditches. The total discharge of water in the application period of July was slightly lower (125 m3) than that in May and June (140 m3). Approximately
60% of the linuron dose was discharged
In comparison,
only 4 1% of the linuron dose was discharged
in discharge
can be explained
during the May and June flow period (Fig. 7). during the July flow period. The difference
from the lower water discharge
in July in comparison
application periods. In addition, the half-life for the disappearance during the stagnant period in July was significantly
lower (ANOVA, p < 0.01) than the half-lives
other application periods (Table 2). A higher rate of disappearance less linuron is available to be discharged.
with the other
of linuron from the water compartment in the
of linuron from the water implies that
At the end of the flow periods, approximately
2% of the linuron
dose was left in the ditches treated with the highest dose. In the ditches treated with the highest dose, the macrophyte a sink for linuron (Fig. 7). Three days after the applications of the linuron macrophytes.
dose of 0.6%, 0.7% and l.l%,
compartment
in May, June and July, maximum proportions
respectively,
were found to be associated
The literature provided no other data on the role of macrophytes
In a chlorpyrifos by macrophytes of macrophytes
study [IO] approximately
only played a minor role as
40% of the chlorpyrifos
in the partitioning of linuron.
dose applied was found to be sorbed
(mainly Elodea nutfullii and Chara globularis). In this chlorpyrifos was considerably
is more lipophilic.
1 010 and 50 000, respectively
study, the biomass
higher (0.35 kg.m-‘) than that in our linuron study (0.02 - 0.11 kg.m-‘).
In addition, the sorption of linuron to macrophytes because chlorpyrifos
with the
can be expected to be lower than that of chlorpyrifos,
The partition coefficients
(K,,,) of linuron and chlorpyrifos
are
[6].
The sediment was found to contain maximum proportions of 7.3%, 6.7% and 6.0% of the linuron dose, respectively, involving
six days after the applications
the application
chlorpyrifos had expected
of chlorpyrifos
dose in the sediment. a greater difference
in May, June and July (Fig. 7). In a previous experiment in the same ditches,
Based on the difference
we found approximately
10% of the
in solubility of linuron and chlorpyrifos,
in the amounts of the two pesticides
[lo]
sorbed to the sediment.
we
That this
2188
I Start of flow 100 al -3 %
75
& 2
5o
c k
25
0
25
0
7
14
21
28
0 0
7
14
Time after application
I
0
7
Time
14
after application
21
21
28
(d)
I
28
(d)
Figure 7. Linuron as a percentage of the dose in the water, macrophyte and sediment compartments of
the ditches treated with 50 pg.dmm3,as well as the percentage of the dose discharged from the ditches during the flow period. Results are shown for the application periods of May (A), June (B) and July (C).
was not the case can be partly explained from the lower biomass (0.02 - 0.11 kg.m-*) of macrophytes during the linuron experiment compared to the chlorpyrifos experiment (0.35 kg.m-*). It has been shown that in microcosms devoid of macrophytes a relatively large proportion of chlorpyrifos is sorbed by the sediment, compared to the situation in macrophyte-dominated microcosms [lo]. Finally, the average organic matter content for the two upper sediment layers( O-1 cm and 1-2 cm) was approximately 20 to 25% during the linuron experiment (Table l), while the organic matter content of the sediment in the ditches during the chlorpyrifos experiment was only approximately 5% [lo]. A relatively high degree of sorption of linuron to sediment richer in organic matter is very likely.
Acknowledgements
This study was supported by the Dutch Ministry of Agriculture, Nature Management and Fisheries. We are indebted to Edwin Koopman for his assistance in the sampling and analyzing of the macrophytes. We thank Carolien van Rhenen, Jan Drent and Kees Kersting for providing us with the data on the environmental conditions. Last but not least, we are indebted to Paulien Adriaanse, Bram ten Cate, Jan Klerkx, Minze Leistra, Rene van Wijngaarden and the reviewers for their helpful comments on this paper.
2189
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