Water movement and isoproturon behaviour in a drained heavy clay soil: 2. Persistence and transport

Journal of

Hydrology ELSEVIER

Journal of Hydrology 163 (1994) 217-231

[3]

Water movement and isoproturon behaviour in a drained heavy clay soil: 2. Persistence and transport A.C. Johnson a'*, A.H. Haria a, C.L. Bhardwaj a, C. Vrlkner a, C.H. Batchelor a, A. Walker b alnstitute of Hydrology, Wallingford, OX10 8BB, UK bHorticulture Research International, Wellesbourne, CV35 9EF, UK

Received 22 February 1994; revision accepted 7 June 1994

Abstract In a study of isoproturon applied to winter wheat in a heavy clay soil, high concentrations of herbicide were detected in overland flow (surface runoff) water, mole drain water and field drain water. The amount ofisoproturon detected in the field drain, over two major rainfall events in 1993, was estimated to be 2.7% of that potentially available at the soil surface. The peak drain water concentration of isoproturon in the first significant drain flow event was in excess of 500 ppb. For these two rainfall events, 1% of the compound originally applied was lost to the drainage system. Comparison of isoproturon with chloride and sulphate concentrations suggests different origins for the drain water, with the majority of isoproturon being carried down to the drainage system by preferential flow from the soil surface. A residue of 5-10% of the herbicide persisted in the top soil and did not appear to be degraded in the period of May to June.

1. Introduction

Isoproturon (3-(4-isopropylphenyl)-l, 1-dimethylurea) is a residual soil-applied herbicide commonly used for pre- or early post-emergence weed control in winter cereals in the UK. Recent research has shown that, under certain conditions, a proportion of the herbicide applied using standard agricultural practice may leave the field and contaminate surrounding water courses. Isoproturon has been found in streams and rivers of agricultural catchments at concentrations reported to be * Corresponding author. 0022-1694/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)02546-N

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A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

exceeding the 0.1 ppb maximum acceptable concentration in drinking water for the European Union (Harris et al., 1991; Williams et al., 1991; Clark and Gomme, 1992). Contamination of surface water courses usually follows the initiation of drainflow after pesticide application (Harris et al., 1993). Drained heavy clay soils which exhibit preferential flow characteristics are thought to represent a particular pesticide contamination threat. When preferential flow occurs, rapidly moving water may permit the transport of the herbicide with the minimum attentuation or retardation. Therefore a moderately persistent herbicide such as isoproturon with a DTs0 (the time required for 50% of the compound to be degraded) of 30 days, (Blair et al., 1990) applied to winter cereals on heavy clay soils may represent a potentially serious contamination threat to surrounding water courses. Such soils represent up to 33% of the land area of England and Wales, a large proportion of which is involved in the production of cereals (Cannel et al., 1984). Consequently, it is important that the factors which lead to the rapid transport of pesticides in these soils are clearly understood. Some research has already been carried out to study the interactions of herbicides in underdrained heavy clay soils at Brimstone in Oxfordshire. Levels of 10-50 ppb isoproturon have been detected in the drainage from mole drained field plots when applied in winter seasons (Harris, 1991; Harris et al., 1992). In contrast to winter applications, concentrations as high as 550ppb have been detected in drainwater after spring applications (Harris et al., 1993). Total reported losses of isoproturon to drain water over drainage seasons at Brimstone have been assessed as less than 1% of the initial dose. Previous research has shown that significant losses of pesticide only occur during a few rainfall events in the season, and on these occasions important changes in water flow pathways (and hence pesticide transport) take place over a period of only an hour or less (Williams et al., 1991). The broad aim of this study was to collect the maximum amount of hydrological and hydrochemical data for the major pesticide transport events using automatically operating instruments, since from these brief events we can learn the most about the mechanisms involved. This paper presents the hydrochemical findings from the 1992-93 field season. A more detailed discussion of hydrological processes is presented in the companion paper of Haria et al. (1994).

2. Method 2.1. Field instrumentation

The field site and instruments used to study the soil hydrology are described in Haria et al. (1994). To monitor (and sample) field drain water emanating from the plot area, a pit was dug to intercept the drain at the end of the plot. The 'catchment' of this field drain prior to its interception was estimated to be 1800 m 2. This estimation was carried out by measuring the distance to the expected watershed both upslope and to the side of the field drain. The drain was cut, and the end placed into 82 mm plastic pipe. The pipe led 15 m down the slope to a flow gauge, consisting of a V-notch

A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

219

weir box which contained a pump-operated sampling tube activated by a float switch. The autosampler (Dalog Sampling Equipment, Alton, UK) is designed to collect 24 x 1 L samples when triggered. These instruments are described more fully in Haria et al. (1994). Eight suction samplers were installed at three depths (4 × 0.25, 2 x 0.5 and 2 × 0.75 m) through aluminium access tubes, to assess isoproturon concentrations in the soil pore water (in previous tests the ceramic pots of suction samplers have not shown any interaction with herbicides with a higher sorption potential than isoproturon, P. Matthieson, personal communication, 1994). The augured hole was partially filled with silica flour into which the ceramic pots of the suction samplers were bedded. Two 2 m overland flow (surface runoff) traps (manufactured by Soil Survey and Land Research Centre at Shardlow, UK) were installed and connected to 5 L plastic sampling vessels. The traps represented a steel gutter with a leading edge lip (which was pushed in just below the soil surface) and a lid to keep out the rain. The traps were installed so that they collected water running along the soil surface. Access tubes (110 mm) were installed directly over three mole drains and small glass beakers (75 mL) installed in the bottom of the moles, to act as sumps to collect drainwater for manual sampling. Care was taken to ensure that the soil immediately beside the access tubes (and suction samplers) was compressed around the tubes, to minimise water ingress down the walls from the soil surface. Filter paper discs (Whatman no. 1; 10cm diameter) were placed at equidistant intervals over a 12m distance in five groups of four (four filter paper discs were fastened onto 1 m long wooden battens to keep them above the moist soil). The discs collected the pesticide spray to estimate the initial variability in application rate. Water samples were taken manually from the field drain outflow (where it emptied into the ditch) and from the ditch at the bottom of the field. As soil coring techniques proved impossible, due to the sticky consistency of the soil, soil was collected for residue analysis only from the upper 2 cm of the soil with a spatula. At fortnightly intervals, 1 kg of soil was collected from the surface of 1 m 2 grid on a line. Samples were frozen prior to analysis. 2.2. Isoproturon, anion, total organic carbon and clay mineral analysis

The filter paper discs collected on the day of application were dried, chopped into pieces and placed in 250 mL conical flasks. The isoproturon was extracted with 90% methanol and 10% water prior to high performance liquid chromatography (HPLC) analysis. Water samples were maintained at 4°C prior to analysis (for not more than 1 month). Prior to isoproturon analysis, samples were concentrated using C18 bond elute cartridges (Sorbex) and eluted from the column with methanol. Analysis of isoproturon was by HPLC with a C8 column and acetonitrile/water eluent, with detection at 240 nm. Soil samples were analysed for isoproturon by taking four 30 g samples and extracting with 50 mL methanol prior to determination by HPLC. Prior to anion analysis a 5 mL aliquot was taken from the original water sample, filtered using 0.45 #m disposable filters (Millipore), and analysed using a Dionex ion chromatograph. The eluent used contained 1.8 mM sodium bicarbonate and 1.7 mM

220

A.C. Johnson et al./JournalofHydrology 163 (1994) 217-231

sodium carbonate. The regenerant used was 25 mM sulphuric acid. Detection was by electrical conductivity. Soil was collected for total organic carbon (TOC) analysis from the topsoil and the Bg horizon (40 cm), four replicate sub-samples of 2 g were placed in small porcelain crucibles. The soils were wetted with a solution of 4M HCI containing 30gL -~ FeC12.4H20. The acid oxidises any carbonates present and the FeCI2 was added to prevent any MnO2 present from indirectly oxidising the organic carbon. The crucibles were placed on a hot plate in a fume cupboard for 2 days to evaporate off the solution. Dried sub samples (0.2 g) were added to teracotta crucibles and then analysed with a LECO ~4a. carbon/sulphur analyser with a furnace temperature of 1400°C. Soil samples were taken from the top 5 cm and 40 cm depths of a soil pit for semi-quantitative clay mineral analysis. The equipment used comprised a B-pex goniometer with an EFG X-ray generator.

3. Results

3.1. Weather conditions Although winter wheat (Triticum aestivum cv Haven) was sown in October 1992, the first pesticide application was delayed until 10 February 1993 due to unusually wet conditions which prevented the use of machinery on the field. After application, warm (15-18°C), dry conditions prevailed until 1 April 1993. Between 10 February 1993 and 1 April 1993 (50 days after application) no measurable drain flow occurred (Haria et al., 1994). The first rainfall events after pesticide application to generate measurable drain flow events occurred between 1 and 12 April 1993.

3.2. Soil characteristics and the persistence of residues in the field The total organic carbon of the topsoil (the top 5 cm) was found to be 2.57%, and 0.48% in soil from a depth of 40cm (mean of four observations for each, SD 0.027 and 0.083, respectively). The higher clay content at depth, which is a feature of Denchworth series soils (Jarvis and Hazelden, 1982) contains high amounts of smectite which may also influence degradation through adsorption of the pesticide, making it less available to bacteria, as noted by Sims et al. (1992). Semi-quantitative clay mineral analysis showed an increase in smectite and non-expandable 14A minerals (Johnson et al., 1993) when compared with the topsoil. The distribution of pesticide sprayed over a 12m long sampling area on 10 February 1993 gave a mean of 2.45kgha -1 with a standard deviation of 0.33 and a range of 1.91-3.06 kg ha -1 . The data showed a remarkably even application of pesticide over the width of the plot. This information lends considerable confidence to the extrapolation of the soil residue data sampled in 1 m plots to the field as a whole. The measurement of soil residues showed a good agreement between the replicates (particularly beyond 35 days after application) within each square metre that was sampled (see Fig. 1). The rate of decline in the amount of isoproturon over time

A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

221

11 10

Average

0

Range of replicates

[

9

~ o ¢3. o

6 5 4

7

21

35

49

64 78 100 113 Time after application (el)

127

141

155

Fig. 1. Isoproturon residue remaining in the top 2cm of the soil. Soil collected at weekly intervals and isoproturon extracted with methanol.

indicates a DT50 in the region of 30 days, as would be expected from the literature. However, when the pesticide concentration was reduced to the 1 mg kg -1 level in the soil, it persisted for longer than the DTs0 assumption of 30 days.

3.3. Isoproturon in overland flow water Whilst measurements were made of isoproturon concentrations in overland flow water, the total loss of pesticide from the field plot by this route could not be estimated, as the overland flow traps collected water from an unknown and variable area. The data do, however, give an indication of the amounts of pesticide that could be mobilised and transported from the soil surface. The overland flow water is potentially the same water that could enter a macropore and find its way to the drainage system. Between 50 and 90 days after isoproturon application, rainfall events generated both overland flow and drainflow, but subsequent rainfall generated overland flow without drainflow (see Fig. 2). The chloride and sulphate

222

A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231 800

700

8OO

~ 500 .,~

8 4o0 2 CL 300

200

100

0

Time after isoproturon

50

56

64

75

78

go

100

106

113

120

124

127

162

=v*dandllow B

817

330

179

73

N/A

40

66

47

31

30

19

13

20

tool* dr~n ~ ]

252

183

120

67

50

44

NA

NA

NA

NA

NA

NA

NA

ap~ica~on (d)

a NA

No water available to collect

Fig. 2. A comparisonof isoproturonconcentrationsfound in overlandflowand moledrainage water (two replicates for overlandflowand one to three for the mole drains). concentrations found in the overland flow water were lower than those in the soil water (up to 50% less for chloride) collected by the suction samplers. This presumably reflected anion concentrations in the rainwater (see Table 1). Clearly, the overland flow that occurred during May to July would have been limited in extent by the large shrinkage cracks that were developing during this period. The small volumes of 100-500 mL collected by the 2 m traps during this period were probably derived from rainwater that had collected immediately in front of the traps. 3.4. Isoproturon in soil water collected by suction samplers

Data from the suction samplers showed the presence of large amounts of pesticide at depth, following the rain event which occurred 16 days after application (see Table 2). The suction sampler data must be viewed with great caution, however, as bromide tracer added on the soil surface on day 35 in the immediate vicinity around the 50 and 75 cm suction samplers was detected after only 5 days by all the samplers (data not shown). This suggested that water could enter their ceramic pots by running along the length of the suction sampler directly from the soil surface. In this case, the suction samplers themselves acted as macropores. However, the suction samplers must have interacted to some degree with the surrounding soil pore water, as anion concentrations detected were greater than those found in overland flow (see Table 1).

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A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

Table 1 Comparison of anion concentrationson day 50 found during the drain flowevent with that found in water samples taken from the overland flow traps and in suction samplers (averageof four) Source of sample

FD 08.30h (peak flow)

FD 14.00h (tail end)

Overland flow

Suction samplers

Chloride (ppm) Sulphate (ppm)

10.5 26.0

16 31

10.5 24.5

32.5 42.8

3.5. Isoproturon in mole drainage water

Three mole drains contained 7 5 m L capacity sumps, from which water was collected on a routine basis. It was difficult to establish whether the drainage water collected in the sumps reflected concentrations in the initial drainage water, in the penultimate drainage water or some mixture of the two. Comparison with the field drain isoproturon concentrations for day 50 would suggest, however, that the water in the sump reflected the 'tail' of the storm event. Water was collected from all three sumps on days 50, 56 and 64; concentrations ranged from 185-290 ppb on day 50, to 108-129 ppb on day 64 (see Fig. 2). The mole drain with the highest concentration of pesticide varied with each storm event. Water could be collected from only one or two of the three moles between days 75 and 90, and none was collected subsequently. The reduction in water reaching the mole drains (and also the field drain) from day 75 onwards was probably due to a much higher water deficit in the soil, due mainly to crop water use and soil evaporation. Pesticide concentrations in overland flow and mole drain water were very similar from days 64 to 90. 3.6. lsoproturon in main field drainage water

The results of the first rainfall event to trigger the autosampler are shown in Fig. 3, and those from the second in Fig. 4. A number of observations can be made from these storm events: (i) The rapid response of drain flow to rainfall (2 h) and the translocation of pesticide to drainage water indicates that vertical by-pass flow is the major mechanism by which water (and hence, pesticides) can enter the drainage system (Haria et al., 1994). (ii) Both events revealed high pesticide concentrations in the field drain. Only a small reduction in the maximum pesticide concentration was found in the second event, when compared with concentrations found in the first. (iii) Isoproturon concentration and drain flow velocity appeared to be closely related, although peak pesticide concentration lagged behind peak drain flow by 1 h, and pesticide concentrations declined more slowly with time. (iv) Chloride and sulphate concentrations showed an inverse relationship to both drainflow and isoproturon concentration; as isoproturon concentration and drainflow velocity increased, so chloride and sulphate concentrations decreased, and vice versa.

a

25a 25b 25c 25d 25 mean 50a 50b 50 mean 75a 75b 75 mean

19.8 18.1 NA NA 18.7 4.6 0.0 4.6 0.9 4.3 2.6

7

9.3 6.9 NA NA 8.1 5.8 NA 5.8 4.6 NA 4.6

14

73.0 NA NA NA 73.0 2.4 NA 2.4 1.5 102.0 51.7

21 350.0 500.0 15.0 NA 288.0 6.5 NA 6.5 1.3 290.0 146.0

28

Days after isoproturon application

NA, no water collected.

Su Su Su Su Su Su Su Su Su Su Su

Suction sampler

aNA NA NA NA NA 5.6 NA 5.6 1.0 210.0 105.0

35 340.0 230.0 111.0 NA 227.0 10.6 NA 10.6 1.1 175.0 88.0

40 200.0 250.0 NA NA 225.0 10.6 NA 10.6 1.1 180.0 90.0

42

Table 2 Isoproturon measured in soil water collected by the suction samplers (ppb)

80 NA NA NA 80 NA NA NA NA NA NA

49 NA NA NA NA NA 79 83 81 250 NA 250

50 170 185 220 NA 192 240 NA 240 50 NA 50

56 129 118 195 180 155 30 NA 30 67 102 84

64 151 135 126 NA 137 NA NA 0 151 29 90

71

163 122 98 136 129 59 NA 59 99 37 68

78

112 109 96 113 107 54 NA 54 30 92 61

85

44.0 61.0 NA 47.0 51.0 29.0 NA 29.0 15.6 56.0 35.8

100

22 NA NA NA 22 NA NA NA NA NA NA

113

~ ~ ~£ ~

~

~.~

t~

t,~

,4.C. Johnson et el. / Journal of Hydrology 163 (1994) 217-231

6O

600 Isoproturon

. -& ....

550 500 I'-

I

450.

A'

Chloride

Sulphate

A ....

O-.

Drainflow

X

. _~t,

5O

,r "~.~

400 ( -

4O

\,

%

~

",

,G ..~., E)_ E7 ,Z3 .-~ib= ~

300 • C

225

I

250 ;

O- - 0

:t4k.- 13o

.~ r

200

~

20

\

150 100 50 I

0700

I

I

0800

I

I

0900

I

I

1000

I

I

1100

I

I I I I~ ~ 1 I I1 1 2 0 0 1 3 0 0 1 4 0 0 1500

Time (h) Fig. 3. Comparison of solute concentrations (isoproturon -&-, chloride -A-, and sulphate -O-) analysed from water samples collected by the autosampler with drain flow (-x-) for the storm event 50 days after isoproturon application.

The field drain catchment for the plot was estimated, and together with the amount of pesticide known to be available in the soil surface at the time from the soil residue analysis (Fig. 1), and the amount of drainage water to emanate from the plot, the pesticide losses were calculated (2.9 mg kg -1, giving approximately 167 g in 1800 m2; in 8 h of rainfall on 1 April 1993 the drain efflux was 7000 L). For the first event (on day 50), a cumulative loss of 2.5 g of isoproturon is suggested, which would mean a loss of 1.5% to the drainage system in the first event. The second event, based on the

226

A.C. Johnson et al./JournalofHydrology163 (1994) 217-231 650

600

-

60

550

5O

500

450

-A. ~

#,-,It

400

40 A

"Ak.

Q. p.

350

30!

~ - dk " ik 300 ( 250 1

p-O"

%

~... q

t~

/ 2O

2OO 150 100

"~ .&.

10

50 0 1900

I

I 2000

I

I 21 O0

I

I 2200

I

I 2300

i

i 2400

i

i 0100

I

I 0200

Time (h)

Fig. 4. Comparison of solute concentrations (isoproturon -A-, chloride -zx-, and sulphate -©-) analysed from water samples collected by the autosampler with drain flow (-x-) for the storm event 52 days after isoproturon application.

same parameters, yielded 2 g of isoproturon, an equivalent of 1.2%; therefore, in the combined events 2.7% of the pesticide was lost to the drainage system in 3 days. Therefore 1% of the original isoproturon applied on 10 February was lost in these events. 3.7. lsoproturon in the ditch at the bottom of the field Water samples were taken on a routine basis (once a week on average) both from the field drain outfall and from the ditch 15 m downstream from the field drain

A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

227

outfall. Unfortunately, the field and the adjacent ditch at Wytham did not comprise a hydrologically defined catchment. A component of the water in the ditch would have come from Wytham wood nearby, and would have therefore diluted the water and pesticide from the field. However, the figures are of interest in that they reveal pesticide concentrations in a ditch which ultimately enters the River Thames. Pesticide concentrations in the ditch from spraying day (10 February 1993) until 100 days after spraying were routinely above 0.1 ppb (see Table 3). The highest concentrations of 16.8 and 23ppb corresponded to the rainfall events on, or prior to, days 42 and 50.

4. Discussion and conclusions

4.1. Isoproturon degradation

The degradation of isoproturon (deduced from the reduction of its concentration in the soil surface) was found to be similar to the results of previous studies (Blair et al., 1990). A DTs0 of 30 days was estimated from the decline in residues. The increased persistence of residues from 78 days onwards suggested that they may have been protected from biodegradation. Mudd et al. (1983) noted that a proportion (less than 4% of the original) of isoproturon persisted in a sandy loam soil for over 203 days. This may be due to a strong sorption with the organic matter fraction of the soil. However, rainfall during the 78-162 day period caused small-scale overland flow which contained isoproturon in the range of 13-60ppb. This would suggest that a proportion of the pesticide was still in the aqueous phase, and readily available for degradation. The explanation may be a decline in microbial activity. The 78-162 day period after application (late May to July) was one of increased temperatures and greater crop water use; total potentials in the soil at 10cm ranged from -20 to -80 Kpa. A decline in microbial activity may therefore have been due to moisture stress. A reduced moisture content in the soil surface is known to be an important factor in limiting biodegradation (Walker, 1991). 4.2. The relationship between soil residues and pesticide available for transport

Pesticide in the soil may be partitioned between one of three groups: aqueous, weakly sorbed and strongly sorbed phases. As yet, it is not known exactly how much pesticide falls into each category, although sorption experiments for isoproturon and Wytham soil will be carried out in due course. It may be that only the aqueous phase pesticide is involved in transport during storm events. This pool of transportable pesticide would appear to be replenished between the events by desorption from the solid phase. It is not yet clear whether desorption occurs on a significant scale during a storm event (although pesticide may also be transported whilst it is sorbed onto sediments). The amount of mixing of rainwater with soil water at the soil surface (once matrix infiltration capacity had been exceeded) must also influence the concentration of pesticides found in drainage water.

a(0.1) (0.1)

0.16 0.18

0

0.15 1.53

7 1.10 1.16

14 1.12 0.80

21 (0.08) 0.95

28 0.25 (0.08)

35 1.31 26.00

40

49

16.80 3.8 0.32 112.0

42 23 bNA

50

b NA, no determination possible. Note, for isoproturon: EC limit for drinking water is 0.1 ppb, WHO limit for drinking water is 9 ppb.

a (0.10) Below detection limit.

Ditch FD outfall

-69

Days after isoproturon application

Table 3 Isoproturon measured in ditch and field outfall water

0.81 2.10

56

71 0.43 0.24 0 . 8 3 0.86

64

0.28 0.99

78

(0.08) 0.38

85

0.14 0.92

100

(0.1) 0.09

113

t-,a

~

'~

oo

A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

229

4.3. Isoproturon entering the field drain Preliminary observations of the data from storm events at Wytham suggest that changes in pesticide and anion concentrations may be attributed to the water that entered the drains (via macropores) having different origins, for example, with the storm event on day 50: (i) water from the soil surface, which was predominantly new rainwater with a pesticide concentration of perhaps 600 ppb and low anion concentrations similar to that found in overland flow water, which is largely rainwater (see Table 1). (ii) water that moved into the macropores below the soil surface which was a mixture of 'new' rainwater and 'old' soil water from within the soil A horizon. This water contains little or no pesticide (not having penetrated to this depth in appreciable quantities) but an anion concentration similar to that found in the suction samplers. Haigh (1985) suggested that lateral water movement into macropores within the A horizon was the major component throughout a drainflow event. Fluctuations observed in solute concentrations in the two storm events (Figs. 3 and 4) may have been the result of the following sequence: (1) Following heavy rainfall, water probably initially flows down macropores from the soil surface (with a high pesticide concentration) and loses pesticide by readsorption on the macropore walls (observed by Edwards, 1991 with atrazine) (2) Salts on the macropore walls (concentrated by evaporation) go into solution (thus raising the salt concentration). This phenomenon may influence the first couple of water samples collected by the autosampler. (3) Eventually, at the height of the event, the water from the soil surface, moving to the drains, becomes the dominant component of drainwater, with a high pesticide and low salt concentration. The water moves via a combination of macropores, both lateral and vertical in the A horizon, and then down the moling fissures in the B horizon. (4) As drain flow decreases towards the end of the event, water entering vertical macropores which has had more opportunity to interact with the matrix within the A horizon probably becomes an important component, and represents a higher proportion of the drainwater than that arriving direct from the soil surface, thus introducing water with a higher salt and a lower pesticide concentration. The recently installed mole drain network may have led to the high loss of pesticide to the field drain. Isoproturon concentrations in the ditch during the April rainfall period, collected by routine manual sampling, were above both the EC limit of 0.1 ppb and the 9 ppb WHO limit for this compound in drinking water (Table 3). The water that enters the mole drain is likely to do so via a combination of fissures left by their manufacture, and biopores. Lateral interflow and overland flow also occur, but their effect on the direct contamination of the ditch is less clear. Bromide tracer evidence suggests that water can move laterally across the 'inter-mole area' either above or below the surface (see Haria et al., 1994). The evidence, therefore, from the first season of fieldwork at Wytham suggested that a recently moled heavy clay soil posed a serious threat in terms of pesticide contamination to the surrounding water courses. The results demonstrated that

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A.C. Johnson et al. / Journal of Hydrology 163 (1994) 217-231

high contamination of drainage water could occur some considerable period after pesticide application to the field. Data is being collected in the 1993-94 field season with additional effort being directed at quantifying pesticide losses via the different preferential flow pathways.

Acknowledgements The authors wish to thank NERC/AFRC for providing financial assistance for this project. The authors also wish to thank Oxford University for permission to work at Wytham farm, and in particular David Sharpe, the farm manager, whose assistance was and is vital to the success of the field experiment. The assistance of Institute of Hydrology personnel to the project in hydrochemistry, the workshop, site services and instrument sections is gratefully acknowledged. The authors are grateful for helpful discussions on the work from John Bell, Colin Neal, Richard Williams (IH), Tim Burt (Oxford University) and Andrre Carter (SSLRC).

References Blair, A.M., Martin, T.D., Walker, A. and Welch, S.J., 1990. Measurement and prediction of isoproturon movement and persistence in three soils. Crop Prot., 9: 289-294. Cannel, R.Q., Goss, M.J., Harris, G.L., Jarvis, M.G., Douglas, J.T., Howse, K.R. and LeGrice, S., 1984. A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil. 1: Background, experiment and site details, drainage systems, measurement of drainflow and summary of results, 19781980. J. Agric. Sci. Cam., 102: 539-559. Clark, L. and Gomme, J., 1992. Pesticides in a Chalk catchment in Eastern England. Hydrog~ol. 4: 169-174. Edwards, C.A., 1991. Long-term ecological effects of herbicides: Field studies. In: Proc. Crop Protection Conf., Brighton, 18-21 November, British Crop Protection Council, pp. 883-890. Haigh, R.A., 1985. Water balance and water quality studies in an underdrained clay soil catchment. PhD thesis, University of Oxford. Haria, A.H., Johnson, A.C., Bell, J.P. and Batchelor, C.H., 1994. Water movement and isoproturon behaviour in a drained heavy clay soil: 1. Preferential flow processes J. Hydrol., 163: 203-216. Harris, G.L., 1991. Soil drainage and crop management strategies to minimise pesticide leaching. In: J.A. Catt (Editor), The Brimstone experiment, Proc. of a Conf. on collaborative work by ADAS Field Drainage Experimental Unit and Rothamsted Experimental Station, Swindon. October 15-16, 1991, Lawes Agricultural Trust, Rothamsted, pp. 55-69. Harris, G.L., Bailey, S.W. and Mason, D.J., 1991. The determination of pesticides losses to water courses in an agricultural clay catchment with variable drainage and land management. In: Proc. Crop Protection Conf. Weeds, Brighton, pp. 1271-1278. Harris, G.L., Turnbull, A.B., Gilbert, A.J., Christian, D.G. and Mason, D.J., 1992. Pesticide application and deposition - their importance to pesticide leaching to surface water. Proc. Crop Protection Conf. Pests and Diseases, Brighton, pp. 477-486. Harris, G.L., Hodgkinson, R.A., Brown, C, Rose, D.A., Mason, D.J. and Catt, J.A., 1993. The influence of rainfall patterns and soils on losses of isoproturon to surface waters. Proc. Crop Protection Conf. Nothern Britain, Dundee, 22-25 March, British Crop Protection Council, pp. 247-252. Jarvis, M.G. and Hazelden, J., 1982. Soils in Oxfordshire 1, Soil Survey Record No. 77. Soil Survey of England and Wales, Rothamsted Experimental Station, Harpenden, pp. 53-64.

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