Changing aldicarb residue levels in soil and groundwater, eastern Long Island, New York

Journal of Contaminant Hydrology, 2 (1987) 73-91 73 Elsevier Science Publ ishers B.V., Amsterdam - - Printed in The Nether lands

C H A N G I N G A L D I C A R B R E S I D U E L E V E L S IN SOIL A N D G R O U N D W A T E R , E A S T E R N LONG I S L A N D , N E W YORK

STEVEN PACENKA 1, KEITH S. PORTER 1, RUSSELL L. JONES 2, YEMANE B. ZECHARIAS 1 and HENRY B.F. HUGHES 1

~ Center for Environmental Research, CorneU University, Hollister Hall, Ithaca, NY 14853, U.S.A. ~Union Carbide Agricultural Products Company, Inc., Research Triangle Park, NC 27709, U.S.A.

(Received July 16, 1987; revised and accepted March 18, 1987)

ABSTRACT

Pacenka, S., Porter, K.S., Jones, R.L., Zecharias, Y.B. and Hughes, H.B.F., 1987. Changing aldicarb residue levels in soil and groundwater, eastern Long Island, New York. J. Contam. Hydrol., 2: 73-91.

A study tha t involved repeated groundwater and soil-core sampling in four representat ive potato-growing areas of Long Island, New York was carr ied out between 1980 and 1984 to inves- t igate the impact of the pesticide aldicarb on groundwater quality. Aldicarb residue concentra- t ions in both the sa tura ted and unsa tura ted zones were analyzed together with hydrologic data from the study sites. Results showed tha t the t iming of ini t ial residue delivery to groundwater was dependent on water-table depth. After arr ival at the sa tura ted zone, the ra te of residue advance within the aquifer depended on the rate of groundwater movement, which is strongly associated in this area with location relat ive to regional groundwater divides. Residue concentra t ions in groundwater peaked earl iest and highest in areas closest to the aldicarb applicat ion sites and later and lower in areas fa r ther downgradient.

Based on the observed pat terns of residue distr ibution, the impact of the pesticide on the aquifer as a whole will be shor ter in durat ion than some previous estimates. The cont inuing processes of groundwater flow, residue degradation, and dispersion will drop aldicarb residue concentra t ions in groundwater to below the New York State dr inking-water guideline of 7 #g/L long before the associated groundwater leaves the aquifer th rough na tu ra l discharge.

INTRODUCTION

The systemic pesticide aldicarb was first used on Long Island potato fields in 1975 to control the Colorado potato beetle and golden nematode. The pesticide worked quite effectively and was widely used from 1976 to 1979 on nearly all of the more than 8,000 hectares of potato farms on Long Island. In August 1979, aldicarb residues 1 were detected in water from private wells in eastern

Copyright 1986 by Cornell Univers i ty 1 As a shor thand, "a ld icarb residues" will be used when reference is made to the total of the parent compound, aldicarb, and its two toxic degradat ion products, aldicarb sulfone and aldicarb sulfoxide. All dr inking water and envi ronmenta l s tandards and guidelines refer to the sum of these total toxic residues.

0169-7722/87/$03.50 © 1987 Elsevier Science Publ ishers B.V.

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Suffolk County. Subsequently, an intensive sampling survey of over 8,000 wells was undertaken. Chemical analyses of the samples revealed that more than 2,000 wells contained detectable traces of aldicarb residues and that concentrations in over 1,000 of them exceeded the New York State Health guideline of 7ttg/L (Zaki et al., 1982). As a result of these findings, aldicarb use on Long Island was discontinued after 1979.

These findings, together with findings in other States (particularly Wiscon- sin) led to a major effort to improve the understanding of aldicarb residues' behavior in the subsurface. The general model that emerges from the accu- mulated body of field and laboratory research is that aldicarb's t ransport and fate are governed by the following processes (Jones, 1986):

(1) Motion of residues with water flowing through the vadose and saturated zones, with insignificant retardance due to adsorption (except in soils which have high organic matter contents):

(2) Microbially-mediated oxidation which converts the parent aldicarb compound to aldicarb sulfoxide, and a]dicarb sulfoxide to aldicarb sulfone; and

(3) Additional degradation, part icularly by soil-catalyzed chemical hydrolysis, converting each of the above three compounds to low-toxicity substances.

Several local investigations have attempted to forecast the eventual fate of aldicarb residues which leached into groundwater beneath Long Island's pota- to fields. The most important of these, a recent s tudy by the Suffolk County Department of Health Services (Suffolk County Dept. of Health Services, 1982; Baier and Robbins, 1983), considered a t ransect on the North Fork of Long Island in an area where aldicarb had been heavily used. Sampling results indicated that by 1980 aldicarb residues had only reached the upper portion of the aquifer. The study estimated that it would take over 100 years for the water containing aldicarb residues to be flushed completely from the aquifer in that area; if aldicarb residues were entirely conservative in groundwater (a worst- case assumption), then significant residue concentrat ions might persist in the aquifer for a very long period. Earl ier conditional contaminant t ransport calculat ions by INTERA Environmental Consultants (1980, 1982) suggested that residue concentrat ions would fall below New York's drinking-water guideline between 1987 and 2010 depending on the degradation and dispersion assumptions made.

A number of laboratory studies have examined the degradation of aldicarb residues. Chapman and Cole (1982), Hansen and Spiegel (1983), and Lemley and Zhong (1983) considered the degradation of aldicarb residues in water, confirm- ing earlier findings that chemical hydrolysis is the major mechanism which converts aldicarb residues into compounds of low toxicity, and isolating pH and temperature as major controlling factors. Higher pH (when above 5) and higher temperature both lead to faster degradation. Recent work has indicated that the presence of soil in the experiments also makes a major difference. Lightfoot et al. (1987) found that distilled-water experiments without soil generally predict much slower degradation than do more realistic experiments in which soil is present and natural groundwater is used. Based on these major

75

factors, a value of roughly three years has been suggested as a reasonable degradation half-life for Long Island's groundwater conditions, in which pH is nominally 6.0 and temperature is about 15°C (Jones, 1986). However, this value has yet to be confirmed under Long Island field conditions. (See Lightfoot et al., 1987, for a general discussion of the aldicarb degradat ion process and a review of re levant laboratory and field experiments.)

Because of the desire to narrow the range of residue-life forecasts varying from the year 1987 to 2100, a study was under taken to improve the base of field knowledge upon which estimates of the persistence of aldicarb residues in Long Island groundwater can be based. Selected areas whose soil and groundwater were originally sampled in 1980 were resampled in 1983 and 1984 to determine temporal trends and detailed spatial pat terns of aldicarb residue levels. The additional groundwater and soil data acquired in the study were combined with earlier sampling data to determine concentra t ion pat terns over space and time.

STUDY AREA

The present field study concentra ted on a few areas where potable well sampling indicated the presence of aldicarb residues in groundwater. The sampling areas were selected based on two criteria, besides the known presence of residues in groundwater:

(1) Each sampling site had to be downgradient from potato fields where aldicarb had been used, so that observed residues could be t racked backward to specific areas of application; and

(2) Together, the sites had to span most of the range of unsa tura ted and saturated zone travel times in the region.

[ ] Groundwater Sampling Area J , ~ 0 Soil-core Sampling Area / 1[" ^ q

'% 0

'~i ~waolng i-t,ver Research Lab / , , .

,...alverto n . ~ . . . . . . . . . . . " - '

~ _ ~ "

Fig. 1. Study-site locations

76

Together these criteria were intended to foster the selection of areas where known causes (aldicarb use on farms, local groundwater flow patterns) could be associated with the observed effects (residue levels). Also, the set of sites selected would be generally representative of the range of eastern Long Isand's groundwater conditions where aldicarb residues have already appeared or could be expected to appear in the future.

Site selection

The location, distribution, depth, and aldicarb residue concentration of the wells included in the 1980 sampling were reviewed to locate areas which would best satisfy the foregoing criteria. On the basis of this review, four eastern Long Island communities (see Fig. 1) were selected as study sites. Wading River, with a water-table depth of about 30 m, was selected to represent potato- growing areas with deeper water tables. Cutchogue and Calverton represented areas with medium-depth water tables (about 10m). Bayview and part of Cutchogue, with a water-table depth of about 3 m, represented potato-growing areas tha t are underlain by shallow groundwater. (Water-table depths were derived from U.S. Geological Survey topographic maps and annual water-table elevation maps prepared by the Suffolk County, N.Y., Dept. of Health Services (SCDHS), Hauppauge, N.Y.)

On the basis of water-table depth alone, aldicarb residues in the Wading River area could take three to ten times longer to travel from the land surface to the water table compared to the corresponding travel time in the Bayview and Cutchogue areas.

Groundwater hydrologic setting

In all four study sites the unconfined surficial aquifer consists of sand and gravel with occasional clay beds, material originally deposited as either glacial moraines or outwash. This aquifer, which extends over nearly all of Long Island, lies atop the deeper Magothy aquifer in much of eastern Long Island (Suter et al., 1949). In the Wading River and Calverton areas, this Upper Glacial aquifer is about 60m thick and overlies the 150-m-thick Magothy (McClymonds and Franke, 1972). The topmost aquifer materials in'the Wading River area are part of a terminal moraine, and the Calverton area to the south-southeast of Wading River is an outwash plain.

Crandell (1963) describes the geology of the North Fork of Long Island which includes the Cutchogue and Bayview study areas. The SCDHS (1982) studied the Upper Glacial aquifer in detail along a transect of the North Fork in the Cutchogue area. They describe the aquifer here as having a saturated thick- ness of about 30m and as being underlain by a layer of clay from 26 to over 60 m thick. Salt-water wedges bound the fresh water in the aquifer near both the north and south shorelines. The Bayview area is a small peninsula near Cutch- ogue and, al though the fresh water lens is not as thick, the characteristics of

77

the Upper Glacial aquifer here are probably similar to those encountered in the Cutchogue area.

The sand and gravel materials of the Upper Glacial have very high hydraulic conductivit ies and exhibit significant vertical-to-horizontal anisotropy. SCDHS' analysis on the North Fork employed a figure of about 100 m/day for horizontal conductivity, and a 1:5 vertical-to-horizontal conductivity ratio. Franke and Cohen's (1972) Long Island summary suggested a general Upper Glacial aquifer conductivity figure of 90 m/day (horizontal) with an average anisotropy ratio of 1:10 vertical-to-horizontal.

Local soils have high infiltration rates, and most of the land surface is relatively fiat. Consequently, rainfall results in little or no runoff generation (Baskin, 1977, Appendix C) and the resulting recharge, averaging about 0.5 m/ yr (Baskin, 1977; Steenhuis et al., 1985), takes place over nearly all of the land surface. Groundwater eventually discharges along the shorelines, there being forced to flow upward by the denser salt-water wedges. Major groundwater divides exist along the longitudinal axes of the main Island and both the North and South Forks. The resulting flow pattern is dominantly vertical near the divides and coasts and largely horizontal in between (Baier and Robbins, 1983).

The latest SCDHS annual water-table evelation map that was available during this study was one for 1982, and was used to determine the horizontal direction of groundwater flow and the depth to the water table at each site. Although water-table elevations change seasonally and from year to year, the horizontal component of groundwater flow in the study area is known to remain quite stable.

DATA COLLECTION AND CHEMICAL ANALYSES

Water samples

Once the study sites were delineated, the 1980 sampling data were again examined to evaluate wells that could be included in the present sampling program. Out of the 8,000 wells sampled for aldicarb residues in 1980, a subset of 188 private wells was selected within the four sites for resampling in 1983 and 1984. The selected wells provided a wide vertical and lateral distribution of sampling points with respect to the upgradient potato fields.

The subset consisted of 58 deep wells located at Wading River, 78 medium- depth wells at Cutchogue and Calverton, and 52 shallow wells at Bayview and Cutchogue. Of these 188 wells, 169 could be accessed in both 1983 and 1984, giving three years of data when the original 1980 results are included.

The Union Carbide Agricultural Products Company, Inc. laboratory in North Carolina tested all of the water samples collected in this study for aldicarb residue concentrations. The Suffolk County Department of Heal th Services laboratory also analyzed portions of many of the 1983 and 1984 water samples as part of a quali ty assurance effort (see following section). The analyses of the 1983 and 1984 samples at both laboratories made use of an HPLC

78

procedure involving post column reactions and fluorescence detection similar to that described by Krause (1979). Samples collected in 1980 were analyzed by a gas chromatography procedure involving oxidation of residues with peracet- ic acid to aldicarb sulfone, extraction with methylene chloride, and measurement with a flame photometric detector with a sulfur filter. Analytical sensitivity was l~g /L for both laboratories and all sampling periods.

Soil cores

To obtain information on any a]dicarb residues that were still in transi t in the unsaturated zone, the well sampling discussed in the preceding section was supplemented by soil sampling. Soil cores extending from the land surface to the water table were obtained from potato fields at four sites. One soil core had been drilled in 1980 in a potato field at the Long Island Horticultural Research farm. During the 1983 sampling period a total of 10 soil cores were obtained: three cores at Bayview, five at Cutchogue and two at Wading River. In 1984, two additional cores were drilled at the Hort icul tural Research farm and another two at Wading River. The Bayview and Cutchogue soil cores were collected using a bucket auger technique described elsewhere (Jones et al., 1986). Soil cores at Wading River and the Hort icultural Research farm were collected using a drilling rig with a split spoon sampler placed inside a hollow- stem auger.

Samples from successive depths of each core were analyzed for aldicarb residues by the North Carolina laboratory. Each soil sample was extracted (with water in 1983 and 1984 and with a water-acetone mixture in 1980) and the extract was analyzed using the previously described procedures for analysis of water samples. The sensitivity of the analytical method used for determining a]dicarb content of the soil cores was 5 t~g/kg for the 1980 core and 1 pg/kg for the 1983 and 1984 cores.

Quality control

To determine the reliability of the concentrat ion determinations, several quality control tests were carried out. The most important test was one that evaluated differences between analyses of samples split for analysis by the two laboratories mentioned previously. All of the water samples for 1983 and 10 percent of those collected in 1984 were analyzed by both laboratories. The two sets of concentrat ion values were compared against each other for the 139 samples that were found to contain measurable amounts of aldicarb at or above 1 ~g/L. (Both laboratories reported 107 additional split samples to contain no detectable aldicarb residues, and in a further 7 samples one of the laboratories reported no detectable residues while the other reported 1 or 2 pg/L.)

The comparison revealed tha t the mean of the differences in concentrations reported for the split samples was not significantly different from zero (confidence level over 99.9%), using a paired t-test; the mean and standard deviation

79

of the differences were 3.5 and 6.8 pg/L, respectively. Since the reported aldicarb residue concentra t ions in these split samples ranged from 1 to 217 pg/L, the relat ively small values of the mean and standard deviation of the between- lab differences indicate a very good agreement between the two laboratories ' results, hence establishing the consistency of the analyt ical methods and procedures used.

As an additional quali ty-control test, 10% of the water samples were analyzed twice at the North Carolina laboratory in order to obtain an estimate of repeatabili ty. Differences between analyses were within 5%.

RESULTS AND DISCUSSION

Aldicarb residues in the unsaturated zone

The eight cores drilled in Cutchogue and Bayview in late 1983 were in areas where the depth to groundwater is less than 11 m. Only one of these cores had a detectable t race of aldicarb residues. This core, which was obtained from the Cutchogue area, contained 1 #g/kg aldicarb residues (just at the limit of detection) at a depth of 7.6 m below ground surface and no detectable residues at any other level.

The depth to groundwater is about 24 m at the Long Island Hort icul tura l Research farm and about 30 m at the sampled field in Wading River. The soil core from the Hor t icul tura l Research farm in November, 1980 contained aldicarb residues at and over the 5 pg/kg detection limit for most of the distance from 4.5 m below the land surface down to the water table. Above 4.5 m, only one sample contained detectable residues, at a 6 #g/kg level. Nei ther of the 1984 cores obtained from the Hort icul tura l Research farm site contained residues at or over the 1/tg/kg detect ion limit above the water table, but residue levels of 1 and 13 pg/kg were measured just below the water table.

0

Depth ~ ~ de tec ted a m o u n t

b e l o w 5 . ..,,-:3 i~ ,~ ,~ , l no res idue de tec ted

surface -,-.:1. (meters) 3:;3

10 -,,.-:1 i i

15

2O

25

30 . . . . . . . . . . . . . . . . . . . . . . . . . . . wa te r tab le . . . . . . . .

i i IS i 0 5 10 1 20

res idue con ten t (p.g/kg)

Fig. 2. Aldicarb residue d is t r ibut ion in a Wading River soil core (1984).

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In the 1983 Wading River soil-cores, residues were detected from depths of l l m in one and 17m in the other down to the water table. In both 1984 cores obtained from the same site, aldicarb residues were detected from a depth of 14m down to the water table. Figure 2 shows the vertical distribution of residues in one of the Wading River cores.

The results of the soil-core analyses show that aldicarb residues have disap- peared from the upper 10m of the unsaturated zone, presumably due to a combination of degradation and downward movement. Residues still remain in some of the areas where the water table is deeper than 10 m.

Amount of aldicarb residues The field at the Horicultural Research farm was treated with aldicarb ap-

plied at a rate of 3.3 kg/ha (3 lb/acre) of active ingredient in 1975 and 5.6 kg/ha (5 lb/acre) from 1976 through 1979 - - a total of 26 kg/ha for the entire 1975-1979 period. Aldicarb application rates at the field in Wading River were 3.3 kg/ha in 1975 and 1976, and 4.5-5.6 kg/ha in 1977 through 1979 adding up to about 22 kg/ha for the period during which the pesticide was in use.

Using the estimated aldicarb residue contents from the soil core samples discussed above, the approximate total mass of aldicarb residues present in the cores was calculated (Table 1). (A high amount of variabili ty between the cores makes these estimates relatively poor; the 95% confidence ranges shown in the table were estimated from the replicate cores at Wading River in 1983 and 1984, assuming a log-normal distribution.) At the Hort icul tural Research farm the amount present in the unsaturated zone decreased from a 1980 figure of about 10% of the cumulative applied aldicarb to an undetectable amount. For Wading River the total mean aldicarb residue present in the unsaturated zone in 1983 and 1984 remained at approximately 10% of the applied amount.

Rate of downward movement The soil-core data were also used to obtain a rough estimate of the rate of

downward residue migration in the unsaturated zone. Using the depth below

TABLE 1

Total mass of aldicarb res idues remain ing in the u n s a t u r a t e d zone

Locat ion Aldicarb residue con ten t (kg/ha)

Year Core 1 Core 2 Mean 95% confidence interval

Long Is land Hor t i cu l tu ra l 1980 1.8 - 1.8 1.2 3.2 Research farm 1984 nd* nd nd

Wading River 1983 2.5 1.3 1.9 1.5-3.0 1984 1.9 1.5 1.7 1.3-2.8

*nd = none detected.

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which residue contents in successive cores equaled or exceeded 5gg/kg (the 1980 detection limit) and the associated elapsed time, the approximate rate of downward residue motion was 4.6 m/year at the Hort icultural Research farm. The corresponding value for the Wading River field was at most 4.3 m/year for 1979-1983 and about 3.7 m/year for 1983-1984. Since these estimates are based on the successive positions of the trailing end of a residue plume, these rates underestimate the averages for the plume as a whole. Dispersion generally causes the front of a plume to advance faster than the centroid advances, and causes the tail of the plume to advance at an apparently slower rate than the centroid. If the movement of the centroid is approximated by the ratio of the nominal recharge rate (0.5 m/yr) to the nominal water-holding capacity of these materials (5% by volume), the resulting water movement rate of roughly 10m/yr is about double the apparent movement of the trailing edges of the residue plumes.

The fields at the Horticultural Research farm are comparable to those at Wading River with regard to groundwater depth, the cumulative amount of aldicarb applied, and (probably) the average amount of recharge. However, residue contents of the soil cores as well as computed rates of aldicarb movement are quite different for the two fields. One cause of these differences could be the differing water holding capacity of the subsoils. The measured water content of the soil cores from the Wading River field (3.8% by weight) was higher than at the Horticultural Research farm (2.8% by weight), indicating that less movement of water would occur at Wading River from a given amount of recharge.

Aldicarb residues in groundwater

Results from the 1980, 1983, and 1984 samples from each of 169 sampled wells at the four study sites were examined in a number of ways to assess patterns of residue distribution in time and space. The primary questions considered were:

- - How have general residue concentrations within each site changed over time?

- - How have the spatial distributions of residues changed at each site? and - - How do the observed patterns compare to the patterns to be expected

based on idealized models of solute plume transport?

Changes in overall concentration levels Considering the water tapped by wells at a given site and time to be a

statistical population, the residue concentrat ion data for wells were subjected to the Wilcoxon matched-pairs signed-ranks test (see, for example, Daniel, 1978) to evaluate the significance of the change in median concentrations between sampling periods. A nonparametric test was chosen for several reasons:

(1) Quite a few of the sampling results were "non-detects", which are only semiquantitative values which do not lend themselves to the types of computa-

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tions needed in tests relying heavily on specific numerical values; (2) The distributions of the residue data and changes over time were not

expected to be normal, ruling out the use of more powerful tests like the paired-sample t-test; transforming data to achieve normali ty would be ham- pered by the non-detect problem;

(3) Most parametric tests are very sensitive to outliers in data, and occasional anomalously high or low results were expected.

The Wilcoxon test handles problems 1 and 3 very well, working with ranks of data rather than the direct values. The test does require that the underlying population be symmetric, which at least is a less str ingent requirement than normality. The less powerful signs test, which is similar to the Wilcoxon test but which ignores the magnitude of differences entirely, was considered and rejected since it throws away too much of the information content of the data being evaluated.

A basic problem with all standard statistical tests applicable to this situation is that they assume that the data are independent. Since some of the sampled wells are close together, they are tapping essentially the same groundwater. Results from two adjacent wells would thus be equivalent to two samples from one of the wells. The practical effect of this problem is that the number of true degrees of freedom that should be used in significance tests is less than the number of samples taken. Interpretations in face of nonindepen-

TABLE 2

Aldicarb residue concentrat ion statistics for deep water-table areas in Wading River

Statistics 1980 1983 1984

1. Number of wells sampled

2. Arithmetic mean of aldicarb residue concentrat ion for all wells (pg/L)

3. Number of wells with aldicarb residues

4. Number of wells with aldicarb residue concentrat ions exceeding 7,ug/L

5. Number of wells in the range of Over 200 pg/L:

151-200 pg/L: 101-150 pg/L: 51-100 pg/L:

8-50 #g/L: 1-7 pg/L:

Not detected:

50 50 50

0 14 16

6 35 35

1 23 23

0 0 0 0 0 0 0 0 1 0 4 1 1 19 21 5 12 12

44 15 15

6. Results of Wilcoxon test for significance of change in median aldicarb residue concentrat ion at 95% confidence level:

1980-1983: significant increase 1983-1984: no significant change

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dent data thus must be conservat ive about statements of significance. In all of the significance tests done in this project, a 95% confidence level was used instead of 90% to help compensate for this problem.

The Wilcoxon test when applied to this case involved computing changes in concentra t ions at each well between a pair of sampling periods, ranking the absolute values of the changes (omitting all changes of zero), and forming two sums of the computed ranks: one for wells where concentra t ions declined and one for wells where concentra t ions increased. If the difference between the "decreases" rank sum and the " increases" rank sum is significant when compared to a table of Wilcoxon test critical values, then one can reject the null hypothesis being tested. A typical null hypothesis would be "concent ra t ions increased or remained the same" for a case where concentra t ions were expected to be declining.

Upon close examinat ion of which wells at each site contained residues, it was determined that most of the wells for which no aldicarb residues were detected during any of the three samplings were outside the local aldicarb plume. Such wells ei ther lay off the path of the groundwater tha t recharged through upgradient potato fields, or are too far downgradient for the plume to have reached them by 1984. Therefore, wells which had no detectable residues in any year were excluded from the time-trend analyses described below.

(a) Deep water table - - W a d i n g River . According to the Wilcoxon test, a statist ically significant increase (95% confidence) in aldicarb residue concentrat ions occurred in the groundwater at this site between 1980 and 1983 (Table 2). From a limited occurrence in 1980 when only six wells contained residues at or above the detection limit of I gg/L, the aldicarb residues reached a much larger segment of the aquifer by 1983 with 70 percent of the 50 sampled wells showing detectable concentrat ions.

The number of wells in which aldicarb residues were detected was 35 both in 1983 and 1984 with 27 of them containing residues in both years. In contrast to the 1980-1983 period, there was no statist ically significant change in concentrat ions during the 1983-1984 period.

(b) M e d i u m depth water table - - Cutchogue-Calver ton . Table 3 summarizes the sampling results from these sites. The change in concentra t ions that occurred in this group of wells between 1980 and 1983 was not statist ically significant. On the other hand, a small but statist ically significant decrease in concentra t ions occurred between 1983 and 1984 during which the mean concentra t ion for the wells dropped from 37 #g/L to 28 ttg/L.

(c) S h a l l o w water table - - B a y v i e w - C u t c h o g u e . A close examinat ion of the data for 1980 and 1983 reveals that at this site aldicarb residue concentra t ions seemed to have decreased in many wells located close to the potato fields and increased in those tha t are far ther downgradient of the fields (see Table 4). However, not enough wells were sampled far ther away from the fields to allow a stat ist ically meaningful comparison.

The sampling results for wells in this group indicated a decrease in aldicarb

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TABLE 3

Aldicarb residue concentration statistics for medium depth water-table areas in Cutchogue and Calverton

Statistics 1980 1983 1984

1. Number of wells sampled 72 72 72

2. Arithmetic mean of aldicarb residue concentration for all wells (pg/L) 21 23 17

3. Number of wells with aldicarb residues 37 39 34

4. Number of wells with aldicarb residue concentrations exceeding 71~g/L 25 27 26

5. Number of wells in the range of Over 200 t~g/L: 0 2 1

151-200 t~g/L: 2 0 0 101-150 ~g/L: 1 5 2 51-100 pg/L: 7 3 4

8-50 pg/L: 15 17 19 1-7 t~g/L: 12 12 8

Not detected: 35 33 38

6. Results of Wilcoxon test for significance of change in median aldicarb residue concentration at 95% confidence level:

1980-1983: no significant change 1983-1984: significant decrease

residue concen t r a t i ons when compar ing 1980 and 1984. The Wi lcoxon test showed a s ignif icant decrease in c o n c e n t r a t i o n to have occur red be tween 1980 and 1983 if five wells t ha t were inaccessible in 1984 were included in the 1980/83 comparison. The decrease between 1983 and 1984 was also s ta t i s t ica l ly significant.

The pa t t e rns at the different sites appear to be most re la ted to the water-

table depth, which cont ro ls the residue t ravel t imes from the land sur face at the o r ig ina t ing po ta to fields to the wate r table. Areas close to po ta to fields and under la in by shal low g r o u n d w a t e r should be expected to exhibi t the ear l ies t a r r ivals and c o n c e n t r a t i o n peaks, as they do in the sha l low water- table areas at Bayview and par t of C u t c h o g u e (peaks a round 1980, r ecovery s tar ted before 1983). Residues should ar r ive and peak somewha t la te r in areas with an inter- media te t ravel time from field to well, as evidenced at Ca lve r ton and the remainder of the Cu tchogue site (peaks be tween 1980 and 1983, r ecover ing after 1983). Final ly , residues should take the longes t to ar r ive and peak at areas with the longest t ravel t ime from field to well, such as Wad ing River (peaks af ter 1983, r ecovery may be s ta r ted by 1984).

In addi t ion to the differences re la t ing to t iming of res idue arr ival , two o ther expected consequences of deeper wa te r tables are g rea te r dispersion and degrada t ion . Both processes should lower peak values at wells in areas with

85

TABLE 4

Aldicarb residue concentra t ion stat is t ics for shallow water-table areas in Bayview and Cutchogue

Stat ist ics 1980 1983 1984

1. Number of wells sampled 47 47 47

2. Ari thmetic mean of aldicarb residue concentra t ion for all wells (pg/L) 33 20 9

3. Number of wells with aldicarb residues 31 28 22

4. Number of wells with aldicarb residue concent ra t ions exceeding 7 pg/L 21 22 11

5. Number of wells in the range of Over 200 gg/L: 0 0 0

151-200 pg/L: 3 1 0 101-150 ~g/L: 3 2 0 51-100 #g/L: 6 1 3

8-50pg/L: 9 18 8 1-7 pg/L: 10 6 11

Not detectable: 16 19 25

6. Resul ts of Wilcoxon test for significance of change in median aldicarb residue concentrat ion at 95% confidence level:

1980-1983: no significant change* 1983-1984: significant decrease

*If six additional wells sampled in 1980 and 1983 but not 1984 are included, the Wilcoxon test finds a significant decrease in concentrat ion between 1980 and 1983.

deeper water tables. This is consistent with the field data (summarized by water-table depth in Table 5) in which the early-arrival wells showed the highest peak values. The intermediate-arr ival wells peaked at significantly lower values, and the farthest-removed wells peaked at the lowest concentrations. (Concentrat ions at a number of the last-arrival wells may have peaked after the end of the final sampling period.)

Extent of residues in groundwater Additional information can be derived by examining the changing spatial

pat terns within the residue plumes. Due to a non-ideal distribution of wells and the short time between the first and final samples, only quali tat ive interpretations can be made.

For each of the study sites a groundwater flow direction map was drawn based on the water-table contour map, and the sampled wells were plotted on the map together with their respective aldicarb residue concentrat ions. The potato fields upgradient of the wells (and where the observed aldicarb residues probably originated) were also located on the maps. These maps were used to develop for each site a depth-versus-distance plot (Figs. 3 and 4) on which each well was represented using the elevation of its screen and its distance from the

86

TABLE 5

Comparison of aldicarb residue concentrat ions in the three groups of wells*

Water table Number depth group

Arithmetic mean and median concentrat ion (~g/L)

1980 1983 1984

Mean Median Mean Median Mean Median

Deep water table 45 1 nd** 16 7 17 7 (> 18m) Medium water table 44 35 10 38 14 28 12 (6-18 m) Shallow water table 32 49 29 30 18 13 4 (<6m)

*wells in which aldicarb residues were detected in any of the three sampling years, **none detected, at a detection limit of 1 #g/L.

A. 1980

Elevation AMSL (meters)

t I Values shown are in pg/L; 4 nd = "none detected"

-4'

-8"

-12 •

• 50

O" • 97

• 3 .30 • 149

27• 29 •

83•

I"B7 • nd

approximate water table

nd •

nd nd nd nd , nd

"nO ~d

nd

B. 1984


O'

-4'

-8'

-12-

100

4,

89 "3

"10 • 70

"43

96.

200 300 400 500 Distance from potato fields (meters)

600

~o

• 30


• 12 n d nd nd .nd • nd

• ; ,d nd

nd

100 200 300 400 500 600 Dis tance from potato fields (meters)

Fig. 3. Aquifer cross-sections showing residue concentrat ion distribution (Cutchogue site).

87

upgradient potato fields as coordinates. This simple, approximate depiction of concentra t ion data assumes a lateral ly uniform flow velocity and direction as well as a source-area boundary that is normal to this direction.

An unfor tunate consequence of relying entirely on preexisting, private wells is that there were quite a few wells whose screen depths were unknown. Such wells could not be included in this stage of the analysis. This prevented any additional in terpre ta t ion of pat terns at the shallow water-table area in Bay- view, since no residue-containing wells had recorded depths.

The Cutchogue cross-sectional plot (Fig. 3) shows that in 1980 aldicarb residues had moved 180m downgradient of the potato fields. Despite the additional elapsed time, the downgradient extent of the residues in 1983 (not shown) remained the same as in 1980. However, in 1984 the maximum extent of residues in the groundwater increased to about 270 m from the potato fields (see Fig. 3B). The apparent ly changing rate of plume advance could be an art ifact of the location of wells near the leading edge. Some of the difference can be accounted for by changing groundwater velocities over time. Groundwater levels, and hence gradients, were highest in 1980 and 1984, and lowest during the 1981 1983 period which would change the water flow rate correspondingly.

The aldicarb-residue plume moved more rapidly at Wading River than at Cutchogue, despite a later delivery of residues at Wading River due to a much deeper water table. As shown in Fig. 4, only 5 of 25 wells contained residues in 1980, all at levels under 5~g/L and all within 250-350m downgradient of the potato fields. The distance over which aldicarb residues were detected increased to 600m by 1983 (not shown), 16 of 25 wells contained detectable residues, and concentra t ions were as high as 39 ~g/L. The water samples collected in this area in 1980 and 1983 were all from wells within 600m of the upgradient potato fields. Even the wells farthest from the fields contained aldicarb residues in 1983, and it is possible that wells more than 600m away in the general direction of groundwater flow also contained residues.

To check this possibility, 20 additional wells more than 600 m downgradient from local potato fields were sampled in 1984. Of these, 8 contained detectable concentra t ions of aldicarb residues. Three samples had concentra t ions above the 7gg/L New York State guideline and the maximum concentra t ion was 141~g/L. Residues were detected in wells as far as about 1200m, but the wells with concentra t ions above 7 pg/L were all within 750 m of a potato field.

The difference in plume movement rates between the Wading River and Cutchogue areas suggests a large difference in water flow velocities. At Wading River, the water-table gradient in 1980 was about 3.6 x 10 -3 versus a gradient of only 4.9 x 10 -4 at Cutchogue. Using a porosity of 35% and a conductivi ty of 90m per day (SCDHS, 1982), the horizontal component of water velocity would be about 0.93m/day at Wading River and 0.13m/day at Cutchogue.

While it was originally intended that plots like Figs. 3 and 4 would assist in determining the depth of vertical penetrat ion of residues into the aquifer, at both sites the residues had penetrated to the deepest sampled wells by the time

88

A. 1980

Elevation AMSL (meters}

- 8

-16

-24


nd " nd nd 3 "

• nd nd"

,rid . I • n d nd• *nd

4" , 2

1 " nd • nd

• n'd nd* "nd ,;d

• n d

n d •

• n d

• n d

• n d

B. 1984


-8

-16

-24

300 400 500 600 700

Distance from potato fields (meters)


27 1- " 2

25" • nd

• n d "32 nd• * 25 18" . 8

. 6 39

7 4 . •nd

- 1 4

. 5

300 400 500 600 700

Distance from potato fields (meters}

Fig. 4. Aquifer cross-sections showing residue concentration distribution (Wading River site).

the main body of the residue plume had arrived. Thus the data only show the minimum depth to which the residues have penetrated. (This is another problem related to sampling on]y existing drinking-water wells.)

Evidence for residue degradation As described in the first section of this paper, under soil and water con-

ditions similar to those found on Long Island, aldicarb residues degrade to substances of much lower toxicity. While the data collected in this study do not in themselves allow quantitative estimates of rates of degradation to be made, some of the residue distribution patterns are best explained by the occurrence of residue degradation. However, because this study was regional in scope rather than a detailed, site-specific, mechanistic investigation, other factors such as variations in groundwater velocities, vertical movement into unsam-

89

pled portions of the aquifer, and residue dispersion cannot be ruled out as contributing to apparent loss of residues.

The decline in residue levels in these areas with shallow groundwater is supportive of a degradation half-life in the order of two to three years. Al- though movement of the residue plumes resulted in decreasing concentrat ion levels in wells near potato fields, these decreases were not completely offset by increasing residue levels in downgradient wells.

In the areas with medium-depth water tables, the continuing movement of aldicarb residues into groundwater after 1980 does not permit the use of residue data from the saturated zone to estimate degradation rates. In these areas the movement of the plumes does appear to be only 70-80% of the estimated groundwater velocities. Acknowledging the imprecision of these estimates, there still appears to be retardance of the plumes relative to the movement of groundwater. Since aldicarb residues will not adsorb significantly to the coarse sand and gravel subsoils containing essentially no organic matter (Felsot and Dahm, 1979; Bromilow and Leistra, 1980), this apparent retardance is unlikely to be related to adsorption processes. Degradation of residues to below detectable levels in a diffuse plume front could result in apparent retardance, as could vertical dispersion.

CONCLUSIONS

The data acquired and analysis performed in this study have allowed a general assessment to be made of patterns of aldicarb residue concentrations in the unsaturated zone and groundwater in study sites spanning most of the range of conditions found in the potato-farming area of eastern Long Island. The assessment should be viewed in light of some unavoidable limitations of the present study.

The most important l imitation was the distribution of sampling points. The sampling program had to rely on existing household wells and this, together with the requirement that selected wells be accessible for repeated sampling over a relatively long period, imposed a severe constraint on the distribution of wells. Thus, al though a wide vertical and areal distribution of wells was achieved, the distribution was not uniform within an area. Sampling-point density higher than was possible in this study is needed to make quanti tat ive evaluations of individual processes that act to reduce residue concentrations in groundwater.

With these cautions in mind, the following conclusions are drawn from the results of this investigation:

(1) In shallow water-table areas, practically all aldicarb residue movement into groundwater had been completed by 1980. In areas where the water table is as deep as 30 m, most movement of aldicarb residues into groundwater had taken place by the end of 1984. However, in a few of these areas, limited amounts of residues may continue to move into groundwater for an additional 3 to 5 years.

90

(2) Residues tha t cross the water table tend to move away from the input area in the direction of groundwater flow. The pat tern of groundwater flow (hence aldicarb residue movement) varies depending on location relat ive to the shoreline. In general, the movement is away from wells adjacent to pota to fields toward wells tha t are far ther (vertical or laterally) downgradient of the fields. Consequently, most wells close to potato fields will recover within a short time after residue input to the groundwater ceases. Wells tha t are fa r ther from the fields may require a longer time to recover.

(3) While this study could not provide quant i ta t ive estimates of the time when groundwater in eastern Long Island will be free of aldicarb residues, its results suggest tha t the residue levels in parts of the aquifer tha t are now used for drinking water will fall below the State 7 ~g/L guideline in significantly less time than the 100 or more years suggested as upper bounds by ear l ier studies. A combination of groundwater t ranspor t and apparent degradat ion and dispersion of the residues has significantly reduced residue levels in areas with moderate to shallow water tables, where most incidences of aldicarb residues in drinking water have occurred.

ACKNOWLEDGMENTS

This work was funded by the Union Carbide Agricul tural Products Com- pany, Inc., with addit ional support provided by the Cornell Universi ty College of Agricul ture and Life Sciences. Donations of services by the Suffolk County Depar tment of Heal th Services, the Suffolk County Cooperative Extension Association, and the Long Island Hort icul tura l Research farm are grateful ly acknowledged. Ear l ier work funded by the U.S. Environmenta l Pro tec t ion Agency in several projects provided much of the background data.

Valuable assistance was received from Thomas Martin, James Pike, Terry Marquardt , R.R. Romine, Elizabeth Snowdon, James Hansen, Ann Lemley, Joseph Baler, Jef fWagenet , Mary Anderson, and Mat thew Lorber. The authors also thank the homeowners and farmers whose wells and fields were sampled.

REFERENCES

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Baskin, L.B., 1977. A groundwater recharge model for Nassau and Suffolk Counties, New York. Masters Thesis, Cornell University, Ithaca, NY.

Bromilow, R.H. and Leistra, M., 1980. Measured and simulated behaviour of aldicarb and its oxidation products in fallow soils. Pestic. Sci., 11: 389-395.

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ERM-Northeast Engineers and Camp Dresser and McKee, 1983. North Fork Water Supply Plan, Suffolk County, New York. Plainview, NY.

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Hansen, J.L. and Spiegel, M.H., 1983. Hydrolysis studies of aldicarb, aldicarb sulfoxide and aldicarb sulfone. Environ. Toxicol. Chem., 2: 147-153.

INTERA Environmental Consultants, Inc., 1980. Mathematical simulation of aldicarb behavior on Long Island: unsaturated flow and groundwater transport. Contract 80-6876-02. U.S. Environ- mental Protection Agency, Office of Pesticide Programs, Washington, D.C., 130 pp.

INTERA Environmental Consultants, Inc., 1982. Effects of varying input load and degradation rates on simulated regional movement of aldicarb in groundwater in Long Island. Report to the Union Carbide Agricultural Products Company, Inc., 19 pp.

Jones, R.L., 1986. Field, laboratory, and modeling studies on the degradation and transport of aldicarb residues in soil and groundwater. In: W.Y. Garner, R.C. Honeycutt, and H.N. Nigg (Editors), Symposium on Evaluation of Pesticides in Groundwater. Am. Chem. Soc. Syrup. Ser., 315: 197-218.

Jones, R.L., Hansen, J.L., Romine, R.R. and Marquardt, T.E., 1986. Unsaturated zone studies on the degradation and movement of aldicarb and aldoxycarb residues. Environ. Toxicol. Chem., 5: 361-372.

Krause, R.T., 1979. Resolution, sensitivity, and selectivity of a high performance liquid chromato- graphic post column fluorometric labeling technique for determination of carbamate insecticides. J. Chromatogr., 185: 615-625.

deLaguna, W., 1963. Geology of Brookhaven National Laboratory and vicinity, Suffolk County, New York. U.S. Geol. Surv., Bull. 1156-A, 35 pp.

Lemley, A.T. and Zhong, W.Z., 1983. Hydrolysis of aldicarb, aldicarb sulfoxide and aldicarb sulfone. J. Environ. Sci. Health, B18: 189-206.

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. En- viron. Toxicol. Chem., 6: 377-394.

McClymonds, N.E. and Franke, O.L., 1972. Water transmitting properties of aquifers on Long Island, New York. U.S. Geol. Surv., Prof. Pap. 627E., Washington, D.C., 65 pp.

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Suter, R., deLaguna, W. and Perlmutter, N.M., 1949. Mapping of geologic formations and aquifers of Long Island, New York. New York Water Power and Control Commission, Bull. GW-18, Albany, NY.

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Changing aldicarb residue levels in soil and groundwater, eastern Long Island, New York