Persistence of aldicarb residues in the sandstone aquifer of Prince Edward Island, Canada

Journal of Contaminant Hydrology, 6 (1990) 21-35

21

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

PERSISTENCE OF ALDICARB RESIDUES IN THE SANDSTONE AQUIFER OF PRINCE EDWARD ISLAND, CANADA

R.E. JACKSON 1'*, J.P. MUTCH 2 and M.W. PRIDDLE 1

1National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ont. L7R 4A6 (Canada) 2Groundwater Studies Group, Department of Civil Engineering, University of New Brunswick, Fredericton, N.B. E3B 5A3 (Canada) (Received July 25, 1989; revised and accepted J a n u a r y 16, 1990) ABSTRACT Jackson, R.E., Mutch, J.P. and Priddle, M.W., 1990. Persistence of aldicarb residues in the sandstone aquifer of Prince Edward Island, Canada. J. Contam. Hydrol., 6: 21-35. Aldicarb residues were found in the shallow groundwaters of the fractured, sandstone aquifer of Prince Edward Island, Canada, more t h a n two years after the last application of this pesticide. Furthermore, the concentrations of aldicarb measured were relatively constant with time. The chemical and hydrogeological mechanisms by which such persistence occurs are discussed. It is deduced t h a t the detoxifying abiotic transformation (hydrolysis) of aldicarb is inhibited by the low pH and temperature of the soil and groundwater, the former being partly due to the pH-buffering effects of ammonium fertilizer oxidation. Aldicarb residues remain c o n s t a n t and relatively high because of t h e i r storage within the sandstone matrix and subsequent diffusion back into the fractures of this dual porosity system. Attempts to simulate the observed persistence of aldicarb in this hydrogeologic environment using a one-dimensional, solute t r a n s p o r t simulation code were unsuccessful, probably because of the three-dimensional nature of the matrix diffusion process. The simulations suggested t h a t the overall half-life for aldicarb in the till-sandstone system approaches 150 days.

INTRODUCTION

Prince Edward Island (PEI) is situated in the Gulf of St. Lawrence (Fig. 1) and is composed of Permo-Pennsylvanian sandstone red-beds. The fractured sandstone constitutes the sole-source aquifer of this the smallest province of Canada (5660km 2, 127,000 population). The annual precipitation is l l 2 0 m m with average mean temperatures of - 7 ° C in J a n u a r y and 18°C in July. Surveys of domestic wells on PEI in 1983 and 1984 indicated that aldicarb contamination of groundwater was widespread; however, levels of contamination were low (< 6#g L '). This pesticide, which is the active ingredient of Temik lOG ®, has been widely used to control aphids and the Colorado potato beetle on the PEI potato crop. A research project was begun in 1985 to *To whom all correspondence should be addressed.

0169-7722/90/$03.50

© 1990 Elsevier Science Publishers B.V.

22

R.E. JACKSON ET AL.

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determine the fate of the aldicarb in order to learn more about the vulnerability of groundwaters to pesticides. Knowledgde gained from this project is to provide advice to regulatory agencies within the Government of Canada. Three field sites were established in 1985 and 1986, one of which is described in this paper. The sites were chosen on evidence from the 1983 and 1984 surveys, which indicated that Temik ® had been used on nearby fields and would therefore provide suitable study sites. Aldicarb was applied to the field in question at Augustine Cove, PEI, with the seed potatoes in 1983 and 1986. The field was in hay or clover in the intervening and subsequent years. It will be shown t h a t aldicarb has persisted in PEI groundwaters due to both chemical and hydrogeologic mechanisms. Aldicarb is a very toxic carbamate pesticide with an LD~0 = 0.9mgkg -I (rat) and an aqueous solubility of 6000mgL -1 (Moye and Miles, 1988). Following application in granular form, aldicarb undergoes rapid dissolution and subsequent microbially-catalyzed oxidation, first to aldicarb sulfoxide (LDs0 = 0.9mgkg -1 (rat) and an aqueous solubility -- 330,000 mg L-l), then to aldicarb sulfone (LDs0 = 24 mg kg-1 (rat) and solubility = 8000mgL 1). The degradation pathway of these three aldicarb species is shown in Fig. 2. Aldicarb is detoxified by hydrolysis to the respective oxime species, a process which is strongly temperature- and pH-dependent (Moye and Miles, 1988). The Canadian drinking-water guideline for the total of these three toxic species, referred to hereafter as total aldicarb, is 9 # g L 1. METHODS OF SAMPLINGAND ANALYSIS Groundwater monitoring was conducted using polyvinylchloride (PVC) piezometers with 1- or 1.5-m screens that were installed using an air rotary drilling rig. Samples were collected from these 5-cm monitoring wells using either an all Teflon ® or a Teflon®-stainless steel bladder pump (QED

PERSISTENCEOF ALDICARBRESIDUESIN A SANDSTONEAQUIFER

ALDICARB

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OXlMEALDICARB

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ALDICARB SULFOXIDE

ALDICARB SULFOXIDE OXIME

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Fig. 2. Simplified schematic of aldicarb degradation pathways.

Systems ®, Ann Arbor, Michigan, U.S.A.). These pumps were not dedicated and had to be cleaned by flushing with distilled water between wells. After about two well volumes were removed from each well, flow cell analyses were begun. These included Eh and pH using combination platinum and glass electrodes, respectively (Orion ®, Cambridge, Massachusetts, U.S.A.). An Orbisphere ® model 2606 oxygen meter was used to measure dissolved oxygen. Samples were also t a k e n for a t i t rat i on to determine alkalinity (as HCO3) and in some cases to measure ammonia with an Orion ® gas-sensing electrode. Conductivity and temperature were also measured in the field. After field analyses were completed, samples for aldicarb were filtered (0.45#m), acidified to pH 5 (H3PO4) and stored in amber glass bottles with no headspace at 4°C. All aldicarb samples were analysed within 10 days of collection. Field blanks of distilled water and water from the pump cleaning procedure were t a k e n and stored in a similar fashion. Samples for inorganic parameters were collected in plastic bottles and stored at 4°C until analysis. Aldicarb analysis was initially conducted using the method of Chaput (1986). Since only aldicarb sulfoxide and aldicarb sulfone were detected in the samples from 1985 and 1986, a modified procedure was used in 1987 and 1988. An isocratic elution method similar to t ha t of Lesage (1989) was used to separate these two compounds in < 19 min. HYDROGEOLOGY The site at Augustine Cove, PEI, was established in 1985, and has been continually expanded since t h a t time. It now has twenty-five 5-cm piezometers, three Solinst ® multilevel bedrock monitors, and a continuously recording observation well. The sandstone aquifer is usually overlain by 2-3 m of sandy till.

24

R.E. JACKSON ET AL.

The hydraulic properties of the aquifer are controlled by the spacing, orientation and aperture of the fractures. The bulk hydraulic conductivity of the sandstone varies from 4.10 -5 m s- 1in the shallow bedrock to 5.10 -6 m s- 1below the first 5m (Lapcevic and Novakowski, 1988). By contrast, Francis (1981) measured hydraulic conductivity values of 5 . 1 0 - S m s ~ in the matrix of the sandstone, i.e. that part of the sandstone unaffected by fracturing, together with a matrix porosity ranging from 12% to 20%. Lapcevic and Novakowski (1988) estimated the transmissivity of the bedrock to be of the order of 6" 10 4m2s-~ with a storativity of 2.10 -4. The groundwater flow pattern at the Augustine Cove site is shown in Figs. 3 and 4 at two different periods of time. Fig. 3 shows the flow pattern during June 1987, at which time the water table was at a relatively low level. Recharge to the groundwater flow system came from both the upland area around the farm house and from the losing stream. Discharge from the cross-section occured through a fracture zone, normal to the plane of the figure, around piezometers 5 and 6. Fig. 4 shows the flow pattern during a period with a high water table in May 1988. Recharge occurs in the upland area with discharge occurring to the stream. Because of the high hydraulic gradients at the site during May 1988, and the very low fracture porosities (perhaps 1%), groundwater velocities may have reached as high as 3 m day-1. It appears probable that the high water-table regime occurs each spring, but that the low watertable regime describes discharge at other times of the year. GROUNDWATERQUALITYMONITORING Groundwater quality samples collected from the first 10m of saturated bedrock generally contain measurable dissolved oxygen and abundant nitrate with a wide range of pH-values (Table 1). Figs. 5 and 6 show the time series of nitrate and total aldicarb, respectively, for the period 1985-1988 in four monitoring wells whose locations are shown in the upper part of Fig. 5. Also shown are the aldicarb and fertilizer inputs. There are three significant features in these figures: (1) There is a pronounced drop in both nitrate and total aldicarb levels in 1985 following the drilling and instrumentation of the boreholes. This is probably due to infiltration of shallow, highly contaminated groundwaters into the new boreholes during drilling and to the rapid flushing of this water during the following month. (2) From 1986 to 1988, these time series have displayed relatively constant concentrations of both nitrate and total aldicarb despite that fact that only one application of aldicarb was made during this period, ~ 2 k g h a -1 in mid-May 1986. Irrespective of the solubility and mobility of both contaminants, the time series show remarkable persistence in both. (3) Short-term variations are minor. Continual sampling of the four monitoring wells shown in Figs. 5 and 6 during May 1988 showed little day-today variation in either nitrate or aldicarb concentrations. The relative

25

P E R S I S T E N C E OF A L D I C A R B R E S I D U E S I N A S A N D S T O N E A Q U I F E R

JUNE 1987

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VERTICAL EXAGGERATION X5

Fig. 3. Groundwater flow patterns at Augustine Cove, PEI, during June 1987, a period of low groundwater table. The upper figure shows the water-table contours; the lower figure shows equipotential lines of hydraulic head through piezometer nests A, B, C and D. Hydraulic heads are measured in metres above mean sea level (a.s.1.). s t a n d a r d d e v i a t i o n ( = s t a n d a r d d e v i a t i o n × 100 d i v i d e d by t h e m e a n ) o f t o t a l a l d i c a r b at e a c h o f t h e f o u r m o n i t o r i n g w e l l s r a n g e d f r o m 6% t o 19% d u r i n g t h e c o u r s e o f t h e m o n t h w i t h a n a v e r a g e o f ~ 10% for all f o u r w e l l s . Fig. 7 s h o w s t h e t i m e s e r i e s o f t h e r a t i o o f a l d i c a r b s u l f o n e t o t o t a l a l d i c a r b .

26

R.E. JACKSON ET AL.

AUGUSTINE COVE, RE.I. MAY 1988

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Fig. 4. Groundwater flow patterns at Augustine Cove, PEI, during May 1988, a period of high groundwater table. The upper figure shows the water-table contours; the lower figure shows equipotential line of hydraulic head through piezometer nest A, B, C and D. Hydraulic heads are measured in metres above mean sea level (a.s.1.). In this case, t o t a l a l d i c a r b is the sum of the sulfoxide and sulfone species only, due to the r a p i d o x i a t i o n of the p a r e n t aldicarb. This time series also shows r e l a t i v e l y c o n s t a n t ratios. F u r t h e r m o r e , t h e r e was little c h a n g e in the sulfone/ t o t a l a l d i c a r b r a t i o with time d u r i n g the intensive sampling period in M a y

PERSISTENCE OF ALDICARBRESIDUESIN A SANDSTONE AQUIFER

27

TABLE1 G r o u n d w a t e r quality d a t a from A u g u s t i n e Cove, PEI Parameter

Unit

AC-4

AC-6

AC- 7

AC-9

(V) ( m g L 1) (pS c m - 1) (mg L - 1) ( m g L 1) ( m g L 1) ( m g L 1) (mg L 1) ( m g L 1) ( m g L -1) ( m g L 1) ( g g L 1)

7.5 0.47 9.0 390 104 12.0 6.0 2.8 27.4 4.3 6.5 1.2 0.2 -

6.3 0.54 2.8 340 31 15.0 8.0 5.0 20.0 3.2 8.7 1.2 3.9 0.72

6.4 0.49 9.0 360 91 17.0 12.0 11.0 29.6 3.2 8.5 1.2 5.9 0.76

109 22.0 21.0 14.0 46.7 3.3 10.5 1.1 12.3 0.72

(V) ( m g L 1) (ttS cm 1) ( m g L 1) ( m g L 1) ( m g L 1) ( m g L 1) (mg L 1) (mgL 1) (mgL 1) (mg L - 1) (#g L 1)

7.0 0.43 8.7 310 101 14.2 15.3 7.6 43.0 4.9 7.6 0.8 3.6 0.70

6.4 0.42 5.7 270 49 17.7 20.0 8.4 35.0 4.5 9.8 0.9 7.1 0.77

5.6 0.48

6.6 0.48

280 51 18.0 17.2 8.0 36.0 3.5 0.5 1.0 4.7 0.79

440 94 24.6 25.2 13.6 66.0 4.3 9.4 1.1 12.8 0.73

7.1 0.55 10.0 300 66 16.6 14.9 10.0 41.0 4.0 7.8 0.7

6.3 0.56 9.0 270 34 18.1 21.8 12.0 29.7 4.9 10.0 1.1

66 0.51 8.0 290 57 17.0 17.2 10.0 39.0 3.9 8.1 0.9

7.5 0.47 6.0 450 113 23.1 23.1 13.0 68.7 4.7 9.0 0.9

September 1985: pH En D.O. S.C. HCO~ C1 SO 4 NO3-N Ca Mg Na K Total aldicarb Ratio

September, 1986: pH EH D.O. S.C. HCO 3 C1 SO 4 NQ-N Ca Mg Na K Total aldicarb Ratio

June~July 1987". pH EH D.O. S.C. HCO3 C1 SO 4 NO 3-N Ca Mg Na K

(V) ( m g L 1) (/~S cm 1) ( m g L 1) ( m g L 1) ( m g L 1) (mg L 1) ( m g L 1) (mg L - 1) ( m g L -I) ( m g L 1)

28

R.E.JACKSONETAL.

TABLE 1 (continued) Parameter

Unit

AC-4

Total aldicarb Ratio

(pgL ~)

AC-6

3.1 0.79

AC-7

6.9 0.74

AC-9

3.7 1.00

15.0 0.71

E H = measured Pt electrode potential referred to normal hydrogen electrode; D.O. = dissolved oxygen; S.C. = specific electrical conductance; Ratio = aldicarb sulfone/total aldicarb; = not measured.

Augustine Cove Observation Wells --0--4 14. --[3--6 ---e--7 .....A..,.,,9 12-

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Fertilizer 21 and Aldicarb / application ( k g / h a ~ _ Aldicarb 12.0 ~2J r ~ . . . . . . . . . . . . . . 2.0 . . . . . . . . . • NH4 NO3 (as N) 1190[/'~1 I/.~ 210 (NH4)2 H P04 ~as _~'.~ 54~A urea ~as N) L ~ . u ) ~ ] , ~ 1983 1984 1985 1986 DATE

N)J_

l

-- . . . . . . . . . . . . . . . . . 52 1987

1988

Fig. 5. Time series of nitrate-nitrogen concentrations in four observation wells from Augustine Cove, PEI. Fertilizer and pesticide application inputs are identified along the abscissa. Well locations are shown in Fig. 3.

1988; t h e a v e r a g e r e l a t i v e s t a n d a r d d e v i a t i o n for a l l f o u r m o n i t o r i n g w e l l s d u r i n g M a y 1988 w a s o n l y 2%. I t is n o t e w o r t h y t h a t t h e o t h e r t w o i n s t r u m e n t e d s i t e s o n P E I s h o w s i m i l a r persistence in both the c o n c e n t r a t i o n a n d ratio d u r i n g the same period, a l t h o u g h a b s o l u t e v a l u e s for n i t r a t e a n d t o t a l a l d i c a r b differ.

PERSISTENCE OF ALDICARBRESIDUES IN A SANDSTONE AQUIFER

29

Augustine Cove Observation Wells

14.

12-

AVERAGECONCENTRATIONS OF 9 SAMPLES TAKEN THROUGHOUTMAY1988 (.*lSTD DEV)

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1988

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Fig. 6. Time series of total aldicarb concentrations in four observation wells from Augustine Cove, PEI. Fertilizer and pesticide application inputs are identified along the abscissa. Well locations are shown in Fig. 3.

10-

/ /@1

--0--4 --{3--6

MOLE RATIO ASO2 ASO + ASO2

Augustine Cove Observation Wells

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0.2Fer til izer i and Aldicarb application (kg/ha~ A,dicarb '2~07~]--~I . . . . . . . . . . . . . '22100. . . . NH4 NO3 (as N ) '190.~ ~ I (NH4)2 HPO4 /as N~J ~4 54 [~ . . . . . U r e a /as F z l L ~ , ~ I 1983 1984 1985 1986 DATE

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1988

Fig. 7. Time series of the ratio of aldicarb sulfone to total aldicarb in four observation wells from Augustine Cove, PET. Fertilizer and pesticide application inputs are identified along the abscissa. Well locations are shown in Fig. 3.

30

R.E JACKSON ET AL.

SIMULATION OF ALDICARBTRANSPORT Attempts were made to simulate the persistence of aldicarb, as shown in Figs. 6 and 7, to determine whether it would have been possible, a priori, to have predicted the fate of the applied aldicarb using the published soil properties for the relevant soil series and an advanced transport code. The code chosen was LEACHMP, which was developed by Wagenet and Hutson (1986) especially for aldicarb transport. It is a one-dimensional code simulating water and solute transport through unconsolidated porous media under transient conditions and which allows for the relevant oxidation and hydrolysis reactions shown in Fig. 2. The simulations were based upon a 4.4-m soil profile depth with the water table initially set at 3.0 m for J a n u a r y 1, 1983. The soil profile was assumed to be ~ 54% sand, ~ 36% silt and ~ 10% clay, for which the hydraulic properties were taken from the Canada Department of Agriculture report for PEI soils (MacDougall et al., 1981). The dispersivity was set at 10 cm so that no aldicarb residues would remain in the first metre of the simulated soil profile, as was observed in the field. Daily climatic data from nearby meteorological stations were used as input to the model. The oxidation and hydrolysis rate constants were taken from Zhong et al. (1986), corrected for temperature to 10°C, and then further adjusted in the calibration step in an attempt to replicate the observed field values of total aldicarb (4-12 #g L -1) and the sulfone/total aldicarb ratio

(7~80%). Fig. 8 shows a typical output of the LEACHMPsimulations. While reasonable predictions of the sulfone aldicarb ratio are produced throughout the period of simulation, the model suggests that pulses of aldicarb infiltrate to the water table then drain away or degrade very quickly, and that the water-table aquifer then appears to become uncontaminated. Therefore, the model displays none of the persistence or dampened response noted in the field data (Figs. 6 and 7) in which high concentrations persist in the shallow bedrock for several years. Furthermore, the calibrated values of the half-lives for the various oxidation and hydrolysis reactions are about three times the temperature-adjusted values of Zhong et al. (1986) indicating that aldicarb degradation rates appear to be very slow in PEI soils (Mutch, 1989). We estimate the overall half-life of aldicarb in PEI soils and groundwaters to be on the order of 150 days, as opposed to the 100-day half-life estimated by Jones et al. (1986) for aldicarb in Maine (U.S.A.) potato fields. DISCUSSION The longer half-lives predicted for the oxidation and hydrolysis reactions and the failure of the model to reproduce the persistence of the field data demonstrate that: (1) aldicarb is not readily degraded in the hydrologic system; and (2) some in situ storage mechanism is retaining undegraded aldicarb species and slowly releasing them over time, thereby maintaining relatively high concentrations in the monitoring wells. There are several possible

PERSISTENCE OF ALDICARE RESIDUES IN A SANDSTONE AQUIFER

284Rate constants I/~ Total Jas half-lives I ] ~u,-~Pesticide

2 4 ~ 1

T Range total Pesticide | concentrations | observed in the field on the selected dates

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Fig. 8.A. Predicted values of total aldicarb, aldicarb sulfoxide ( A S O ) and aldicarb sulfone (ASO._,) at a depth of 3.25m from LEACHMP simulations. Also s h o w n are the mean and range of total pesticide c o n c e n t r a t i o n s observed in the field during sampling campaigns. Oxidative and hydrolytic rate constants e m p l o y e d in the modeling are s h o w n as first-order half-lives in which the symbols refer to the following: K I * = aldicarb oxidation: K 2 * = aldicarb sulfoxide oxidation: K 1 = aldicarb hydrolysis; K 2 ~ aldicarb sulfoxide hydrolysis: K 3 = aldicarb sulfone hydrolysis. B. Predicted values of the aldicarb sulfone/tota] aldicarb ratio at a depth of 3.25 m from LEACHMP simulations. A l s o s h o w n are the mean and range of ratios observed in the field.

mechanisms which may play a role: (1) sorption of aldicarb and its slow release; (2) downslope migration of aldicarb within the flow system; (3) inhibition of degradation; (4) slow infiltration through the unsaturated till; and (5) matrix diffusion within the sandstone and slow release. Due to the high solubility of the aldicarb species, it is very unlikely that significant sorption of aldicarb takes place. Laboratory column tests, using a methodology similar to that described by Zhong et al. (1986), showed that aldicarb and aldicarb sulfoxide had retardation factors of 1.25 and 1.26, respectively. That is, they migrate at an average of 80% of the groundwater velocity. Therefore, this process alone cannot explain the persistence of aldicarb in the PEI soils and aquifer. Similarly, the downslope migration of aldicarb from further up the ground-

32

R.E.

JACKSON

ET

AL.

water flow system does not explain the problem of persistence. Generally, the concentrations of total aldicarb in the monitoring wells from the upland recharge area (e.g., AC-4 in Table 1) are significantly lower than the concentrations in the wells along the slope of the flow system (e.g., AC-6, -7, -9). Furthermore, the groundwater pH upslope tends to be neutral or even higher (see Table 1), thus resulting in relatively rapid degradation of aldicarb species. Therefore, downslope migration of aldicarb would have to be coupled with a much higher continuous flux of aldicarb percolating from the vadose zone in the direct vicinity of the downslope wells if the observed persistence is to be explained. Therefore, this process does not explain the persistence of the aldicarb. In previous papers (Priddle et al., 1987, 1988) it has been argued that aldicarb hydrolysis, and therefore detoxification, is strongly inhibited because of the chemical environment within which migration is taking place. Fig. 9 shows that aldicarb hydrolysis is very dependent upon temperature and pH and that 1000.'~ iiiiiiii:i:i:~:!i!i!iiiii:..~

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5

6 pH

Fig. 9. Hydrolytic degradation of aldicarb sulfoxide and aldicarb sulfone in distilled water as a function of pH (from Lightfoot et al., 1987). The shaded region indicates the pH zone of greatest total aldicarb persistence.

PERSISTENCEOF ALDICARBRESIDUESIN A SANDSTONEAQUIFER

33

a l d i c a r b species a r e m o s t p e r s i s t e n t in the p H r a n g e of 5-6. B e c a u s e s p r i n g r a i n s a n d s n o w m e l t in P E I a r e acidic ( < p H 5) a n d cold, a n d b e c a u s e the soil itself is acidic, a l d i c a r b is i n h e r e n t l y p e r s i s t e n t in this e n v i r o n m e n t and long half-lives m i g h t be expected. A d d i t i o n a l l y , t h e a c i d i t y of t h e s o i l - w a t e r env i r o n m e n t is m a i n t a i n e d by the p H - b u f f e r i n g effects of the o x i d a t i o n of a m m o n i u m fertilizers: 0 . 5 N H ; + 02 ~

0.5NO3- + H ÷ + 0.5H20

(1)

for w h i c h t h e o p t i m u m m i c r o b i a l a n d c h e m i c a l c o n d i t i o n s a r e r e a d i l y satisfied in P E I soils. G i v e n t h a t t h e r e is a b u n d a n t o x y g e n a n d e v e n some dissolved a m m o n i u m r e m a i n i n g ( m e a n = 0 . 1 4 m g N H ; - N L -1) in P E I g r o u n d w a t e r s , r e a c t i o n (1) p r o b a b l y p l a y s a n i m p o r t a n t role in i n h i b i t i n g a l d i c a r b d e g r a d a tion p r o c e s s e s in t h e v a d o s e zone. T h e r e c a n be little d o u b t t h a t slow i n f i l t r a t i o n t h r o u g h the u n s a t u r a t e d till c a n c r e a t e a r e s e r v o i r of a l d i c a r b t h a t slowly l e a c h e s a l d i c a r b species to the w a t e r table. E v e n in m u c h m o r e p e r m e a b l e sediments, s u c h as t h o s e on L o n g I s l a n d ( P a c e n k a et al., 1987), l a r g e a m o u n t s of a l d i c a r b were r e t a i n e d a n d w e r e b e i n g slowly l e a c h e d t h r o u g h the t h i c k v a d o s e zone some five y e a r s a f t e r the l a s t a p p l i c a t i o n of aldicarb.

2-3 m

table

Fig. 10. Schematic of aldicarb transport under the hydrologic regime of Prince Edward Island. While aldicarb transport may be relatively rapid through the processes act to store undegraded aldicarb species in a low-pH-low-temperature environment within the sandstone blocks. Subsequent back-diffusion into the fracture systems maintains aldicarb concentrations at relatively constant levels that are measured in samples from the observation well network.

34

R.E. JACKSON ET AL.

Finally, it has been shown by both Grisak and Pickens (1980) and Neretnieks (1980) t h a t diffusion of relatively high concentrations of solutes from rock fractures into the porous matrix of a bedrock can result in the storage of the solute over time (Fig. 10). When hydrologic conditions cause the water in the fractures to have a lower solute concentrations than that in the matrix, diffusion will produce a net flux from the rock matrix to the fractures that will restore that aldicarb levels of groundwater in the fractures. Clearly, this could account for aldicarb storage in the PEI sandstone in which intergranular porosities average 16% (Francis, 1981). Naturally, a one-dimensional, porous media-based model, such as LEACHMP, could not be expected to mimic the retention of solutes by matrix diffusion. However, the simulations were helpful in indicating what hydrologic processes are unlikely to be occurring and what magnitude of reaction constants are required to produce the observed total aldicarb and aldicarb speciation data. SUMMARY AND CONCLUSIONS Aldicarb residues (up to 16 gg L 1) persist in the shallow groundwater of the fractured sandstone aquifer of Prince Edward Island well over two years after the last application of the pesticide. The persistence appears to be the consequence of two processes: one chemical, the other hydrological. Aldicarb species are not rapidly transformed by hydrolysis to much less toxic species due to the low pH and temperature of the soil-water environment. Low pH levels are probably maintained by the oxidation of ammonium fertilizers which may well buffer the pH in the 5 ~ range and therefore inhibit the hydrolytic degradation of aldicarb sulfoxide and sulfone. Given the relatively high groundwater velocities at the field site, high concentrations of aldicarb species would have to be stored in situ to prevent their discharge from the flow system. Such in situ storage could well occur due to matrix diffusion from fractures to the integranular matrix. Reverse diffusion could then maintain relatively constant levels of aldicarb species and nitrate in the fracture waters sampled by the monitoring wells and displayed in Figs. 5 and 6. In addition, slow infiltration of aldicarb through the thin till would provide additional storage of solute and would act to produce the dampened monitoring well response of aldicarb to the pulse inputs to the system. The inability of the solute transport code LEACHMPto simulate the persistent behaviour of aldicarb in this hydrogeologic environment points out the need for detailed field testing of mobile pesticides in the principal areas of their use. Furthermore, three-dimensional versions of the model will be required if the behaviour of such pesticides is to be modeled in dual-porosity hydrogeologic systems. ACKNOWLEDGEMENTS The authors t h a n k J.L. Hutson of Cornell University and R.L. Jones of RhSne-Poulenc, for their assistance and critique of this study over the past

PERSISTENCEOF ALDICARBRESIDUESIN A SANDSTONEAQUIFER

35

s e v e r a l y e a r s . T h e r e c e n t s u g g e s t i o n s a n d a d v i s e b y S. K u n g o f t h e U n i v e r s i t y of W i s c o n s i n and P.S.C. R a o of t h e U n i v e r s i t y of F l o r i d a are m u c h a p p r e c i a t e d .

REFERENCES Chaput, D., 1986. On-line trace enrichment for the determination of aldicarb species in water, using liquid chromatography with post-column derivitization. J. Assoc. Off. Anal. Chem., 69: 98M989. Francis, R.M., 1981. Hydrogeological properties of a fractured porous aquifer, Winter River Basin, Prince Edward Island. M.Sc. Thesis, University of Waterloo, Waterloo, Ont., 154 pp. Grisak, G.E. and Pickens, J.F., 1980. Solute transport through fractured media, 1. The effect of matrix diffusion. Water Resour. Res., 16:719 730. Jones, R.L., Rourke, R.V. and Hansen, J.L., 1986. Effect of application methods on movement and degradation of aldicarb residues in Maine potato fields. Environ. Toxicol. Chem., 5: 167- 173. Lapcevic, P.A. and Novakowski, K.S., 1988. The hydrogeology of a sandstone aquifer underlying a field near Augustine Cove, Prince Edward Island: Preliminary results. Natl. Water Res. Inst., Burlington, Ont., RRB-88-65, 38 pp. Lesage, S., 1989. Solid-phase sample collection for the analysis of aldicarb residues in groundwater. Liq. Chromatogr./Gas Chromatogr., 7: 268-271. Lightfoot, E.N., Thorne, P.S., Jones, R.L., Hansen, J.L. and Romine, R.R., 1987. Laboratory studies on mechanisms for the degradation of aldicarb, aldicarb sulfoxide and aldicarb sulfone. Environ. Toxicol. Chem., 6: 377-394. MacDougall, J.I., Veer, C. and Wilson, F., 1981. Soils of Prince Edward Island. Min. Supply Serv., Ottawa, Ont., LRRI Contrib. No. 141. Moye, H.A. and Miles, C.J., 1988. Aldicarb contamination of groundwater. Rev. Environ. Contam. Toxicol., 105: 99-146. Mutch, J.P., 1989. Fate and migration of aldicarb in an unsaturated saturated flow system in Prince Edward Island. M.Sc. Thesis, University of New Brunswick, Fredericton, N.B., 184 pp. Neretnieks, I., 1980. Diffusion in the rock matrix: An important factor in radionuclide retardation? J. Geophys. Res., 85(B8): 4379-4397. Pacenka, S., Porter, K.S., Jones, R.L., Zecharias, Y.B. and Hughes, H.B.F., 1987. Changing aldicarb residue levels in soil and ground water, eastern Long Island, New York. J. Contain. Hydrol., 2: 73 91. Priddle, M.W., Jackson, R.E., Novakowski, K.S., Denhoed, S., Graham, B.W., Patterson, R.J., Chaput, D. and Jardine, D., 1987. Migration and fate of aldicarb in the sandstone aquifer of Prince Edward Island. Water Pollut. Res. J. Can., 22: 173-185. Priddle, M.W., Jackson, R.E., Crowe, A.S. and Mutch, J,P., 1988. Aldicarb and nitrogen residues in a sandstone aquifer. In: Agricultural Impacts on Ground Water. Natl. Water Well Assoc., Dublin, OH, pp. 191 210. Wagenet, R.J. and Hutson, J.L., 1986. Predicting the fate of nonvolatile pesticides in the unsaturated zone. J. Environ. Qual., 15: 315-322. Zhong, W.Z., Lemley, A.T. and Wagenet, R.J., 1986. Quantifying pesticide adsorption and degradation during transport through soil to ground water. In: W.Y. Garner, R.C. Honeycutt and H.N. Nigg (Editors), Evaluation of Pesticides in Ground Water. Am. Chem. Soc. Symp. Ser., No. 315, Am. Chem. Soc., Washington, D.C., pp. 61 77.