International Journal of Environmental Studies,Vol. 62, No. 2, April 2005, 235–248

International Journal of Environmental StudiesISSN 0020-7233 print: ISSN 1029-0400 online © 2005 Taylor & Francis Group Ltd

http://www.tandf.co.uk/journalsDOI: 10.1080/0020723042000257745

Natural attenuation of metabolites of a chlorinated pesticide in soil

T.F. GUERIN*

40 Forman Street, Glenwood NSW 2768, AustraliaTaylor and Francis LtdGENV041064.sgm

(Received in final form 7 June 2004)10.1080/0020723042000257745International Journal of Environmental Studies0020-7233 print/1029-0400 onlineOriginal Article2005Taylor & Francis Group Ltd620000000March 2005Dr Tu[racute]laughF.GUERIN40 Forman StreetGlenwood NSW 2768Australiaturlough.guerin@shell.com

Monitoring the fate of metabolites (of contaminants) provides a further line of evidence that contami-nants can be naturally attenuated in soil. Endosulfan diol and endosulfan sulfate, the two major degra-dation products of the widely used insecticide endosulfan, were aerobically degraded in soil understerile and non-sterile conditions. The trial demonstrated that endosulfan diol and endosulfan sulfateare both degraded to CO2, demonstrating that they are not ultimate degradation products of the parentendosulfan compound in soil. Endosulfan diol was transformed to 14CO2 to a greater extent thanendosulfan sulfate. Of the originally applied endosulfan diol, 6.5% was mineralized over the 9 weektrial under non-sterile conditions. This was approximately 20 times greater than that for endosulfansulfate.

Keywords: Metabolites; Soil; Attenuation; Endosulfan diol; Endosulfan sulfate

1. Introduction

Insecticides pose a challenge for remediation technologists and practitioners. While there arereports on the remediation of insecticide contaminated sites and their remediation, these arefew in comparison with the detailed studies on hydrocarbons and industrial chemicals.Furthermore, much of the contamination associated with insecticides and other agriculturalchemicals is diffuse in its distribution, requiring remediation strategies to encompass wideareas of land. Endosulfan is one such insecticide and several reports have described the dissi-pation of the environmentally important insecticide, endosulfan, when applied to soil. Thereare, however, few studies in the literature which have reported the soil degradation (andspecifically biodegradation) of the two major degradation products of endosulfan I and II,endosulfan sulfate and endosulfan diol [1–9,14].

Endosulfan has been in use for many years in most countries for the control of insects in awide range of crops. Under common field conditions, endosulfan acts selectively on arthro-pods as a contact and stomach poison. At higher temperatures, it may also act as a respiratorypoison via the gaseous phase. It is commonly used to control populations of chewing and suck-ing insects, as well as mites of the families Eriophyidae and Tarsonemidae and a host of otherplant pests. One of the most important applications of endosulfan in Australian agriculture,particularly in cotton growing, has been its use in the Insecticide Resistance Strategy (IRS). It

*Email: turlough.guerin@shell.com

236 T. F. Guerin

was chosen because of its selective nature in control of insect pests. Endosulfan, though highlytoxic to moths and various mites that attack crop plants, does not adversely affect the survivalof insect parasite and predator populations to any significant extent. One benefit of the IRS inAustralia has been that its use has reduced substantially over the period of 1991 to 2001 withapplication rates falling from 2.5 kg/ha to <1kg/ha [7].

Endosulfan is applied to crops in a number of different ways in various formulations. Someof the names of the various technical formulations are presented in table 1. Endosulfan, whichis comprised of two isomers, endosulfan I (or α) and endosulfan II (or β), has five well char-acterized degradation products. These are endosulfan sulfate, endosulfan diol, endosulfanether, endosulfan hydroxyether and endosulfan lactone [10]. These compounds are differentto the parent isomers in terms of their chemical, physical and physico-chemical properties.The degradation products are considerably more water soluble than the parent isomers, withthe exception of endosulfan sulfate. Endosulfan is related to the classic cyclodienes, aldrin anddieldrin, indicated by the presence of a hexachloronorbornene ring in its structure. This chlo-rinated ring structure generally attributes to cyclodienes, low water solubility, high volatilityand recalcitrance in the environment. However, unlike the classic cyclodienes such as aldrinand dieldrin, endosulfan is relatively labile or unstable in the environment. This characteristiccan be attributed to the presence of a cyclic sulfite diester group in its structure which makesthe molecule highly reactive. Specifically, this group imparts to the parent endosulfanisomers, susceptibility to hydrolysis in water and in alkaline solutions and the possibility offurther breakdown in the environment, particularly through biological degradation [14].

Endosulfan, if not used with care and vigilance may cause environmental problems. Driftof endosulfan to wooded areas occupied by wildlife, fish-bearing waters and other land areasnot intended for treatment are the main problems associated with the current use of endosul-fan. There are other areas of concern with regard to endosulfan use, namely the problems ofrun-off or wash-off by rain from treated areas. Further problems arise where applications aremade too often or in excess of the recommended amounts, or where they are made at thewrong times of the growing season. Operator carelessness is one of the major problems asso-ciated with the pollution of the environment with endosulfan and related chemicals. Table 2shows how endosulfan sulfate concentrations in river water tends to increase throughout acotton growing season where endosulfan has been used. The endosulfan compounds in theriver have come from run-off from nearby cotton growing areas.

The parent isomers of endosulfan are toxic to a wide range of organisms [14]. There is,however, only limited information in the literature regarding the toxicity of the endosulfan

Table 1. Common names and trade names of technical formulations of endosulfan

Benzoepin MalixBeosit Niagara 5462BIO 5462 OM 570Chlorthiepin PhaserEndocel SD 4314Endosan ThimulEndosulfan 35EC ThiodanEndosulphan ThionexEndotaff ThiotoxENT 23979 TiovelHidan TionexHOE 2671

Endosulfan metabolites 237

degradation products. From the data available, it appears that insects, birds and mammals aremuch less sensitive to endosulfan than fish (table 3). It is apparent that the degradation prod-ucts endosulfan diol, endosulfan ether, endosulfan hydroxyether and endosulfan lactone areconsiderably less toxic than the parent compounds. From the limited number of toxicity stud-ies conducted, it appears that the degradation products have considerably lower toxicities,with one exception, endosulfan sulfate. Endosulfan sulfate is reported to be even more toxicto mammals, than either of the parent isomers. However, with regard to fish, the sulfate andparent isomers have a similar toxicity (table 3). Endosulfan sulfate, on the other hand, has asimilar toxicity to the parent compounds and is reported to be equally (or even more) toxic tomammals, than either of the parent isomers (table 4). In this regard, it is considered to be a co-contaminant of endosulfan and is often reported (as a residue) along with the parent isomers.

The aim of the current laboratory study was to determine the fate of endosulfan sulfate andendosulfan diol in an unamended clay soil; representative of typical cotton growing soils inAustralia that contain these compounds. A further hypothesis tested was that endosulfan dioland endosulfan sulfate could be mineralized to 14CO2. This work follows on from a relatedstudy on the natural attenuation of the parent compound, endosulfan [11].

2. Materials and methods

2.1. Chemicals and soil preparation

[5a,9a -14C]-endosulfan sulfate (specific activity of 252.3 MBq/g) and [2,3-14C]-endosulfandiol (specific activity of 1794.5 MBq/g) (Hoechst AG Melbourne and Frankfurt) was

Table 2. Endosulfan and endosulfan sulfate in river water during cotton season

Sampling datea Endosulfan (µg/L) Endosulfan sulphate (µg/L)

15/10 ND 0.00223/11 0.011 0.07114/1 0.01 0.1394/2 0.004 0.07125/2 0.022 0.19925/3 ND 0.04723/4 ND 0.01520/5 ND NDb

Notes: aSamples of water were taken from a river adjacent to a cotton crop (Otton 1991 citedin Guerin 1993). bND means concentrations <0.01 µ/L.

Table 3. Acute toxicity of endosulfan and its degradation productsa

Toxicity LD50 (mg.kg−1)

Compound Insects Fish Birds Mammals

Endosulfan I 5.5 0.001–0.01 26–1000 9.4–40Endosulfan II 9.0 0.001–0.01 26–1000 177Endosulfan sulfate 9.5 0.001–0.01 – 8–76Endosulfan diol >500 1–10 – >1500

Note: aAnonymous [7], Guerin [14] and references cited therein. Values for fish are LC50 for 24–96 tests.

238 T. F. Guerin

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Endosulfan metabolites 239

dissolved in 100 % methanol (1000 mg/ml). Both compounds were radiolabeled in non-chlorinated ring carbons (figure 1). The solution was added to subsamples of soil (1–2 g) togive starting concentrations of 2 mg/kg (equivalent to 0.78 and 3.36 MBq/g soil, for endosul-fan sulfate and endosulfan diol, respectively). The radiopurity of the compounds were 99%.Chemical properties of endosulfan and the major metabolites are listed in table 5. The soilwas collected from a cotton farming area in northern New South Wales, Australia (table 6).The soil were then prepared as previously described, using 250 ml glass jars [11]. One set ofsterile (autoclaved 3× and NaN3 added at 1% w/w) controls was used. At the beginning and

Figure 1. Summary degradation scheme for the endosulfan compounds under (a) low oxygen conditions and (b)oxygenated conditions (asterix indicates position of 14C label).

Figure 2. Extraction

240 T. F. Guerin

end of the trial, microbiological plate counts were conducted on soil extract agar, in both thesterile and non-sterile treatments. The gravimetric moisture content (θg) of the soil in each ofthe duplicate incubation vessels, was maintained at θg = 0.3 g/g during the course of theexperiment.Figure 1. Summary degradation scheme for the endosulfan compounds under (a) low oxygen conditions and (b) oxygenated conditions (asterix indicates position of 14C label).Figure 2. Extraction

2.2. Analytical procedures

Glass vials containing 1 ml of 2 M NaOH were placed on the soil surface inside the flasks.These were removed and replaced with fresh solutions at each of the sampling times. Theamount of 14CO2 captured was determined by adding the alkali solution into a liquid scintilla-tion (LS) fluid, trade name Hionic Fluor® (Canberra Packard, Australia) and counted in a LScounter (LSC) as described in the following section. The detailed methodology for detecting14CO2 has been described elsewhere [12].

After any soil-bound 14CO2 was checked for using the method described [12], soil samples(1–2 g) were placed into 250 ml ground glass sealed flasks and analysed as previouslydescribed [11]. The soil was extracted on an orbital shaker at 160 rpm for 2–3 h with 40 mlhexane:acetone (3:1). The extraction regime is illustrated in figure 1. The radioactivity ineach of the three phases (HAE, ME and WRE) (1–2 ml in 8–9 ml Packard 299®) was deter-mined by counting the samples for 10 min using a United Technologies 4000 series liquidscintillation counter. Background counts and counts from each treatment and control weredetermined (recorded as counts per minute). The corresponding disintegrations per minute(DPM) were then determined from the quench correction curve. Endosulfan degradationproducts in the HAE, ME and WRE phases, were analysed using the methods previouslydescribed [13,14]. Total heterotrophic populations were determined by a plating technique.The solid growth medium was a soil extract yeast mannitol agar previously described [15].Representative soil samples were taken at weeks 0 and 9 from the sterile and non-sterile

Table 5. Liquid-phase physico-chemical properties of endosulfan and its major degradation products

Compound Solubility in water (S) ppmc Log Kow v.p. (Pa)a,c Hb

Endosulfan I 0.51 3.6 4 × 10−4 0.72Endosulfan II 0.45 3.83 8.0 × 10−5 0.04Endosulfan sulfate 0.48 3.66 3.7 × 10−5 0.03Endosulfan diol 300 3.68 2.3 × 10−6 0.00013

Notes: aVapor pressure in units of Pa. bHenry’s constant (H) = v.p./S in units of Pa.m3.mol−1, calculated from the v.p. & S datareported in this table. cValues reported are from the PhysProp and DATALOG Databases from Syracuse Research Corporation whereavailable [22]. Values for endosulfan diol are from elsewhere [14, 23]

Table 6. Characteristics of the soils used in the studya

Fraction (%)

Soil sample Moistureb (g.g−1) OC(%) OM(%) Cc Sd FSe CSf

Cotton farming soil 0.09 1.09 1.91 62.4 20.4 14.3 2.9

Notes: aOC = organic carbon, OM = organic matter. bMoisture content of the soils at the time of sampling. cC = clay fraction. dS =sand fraction. eFS = fine sand fraction. fCS = coarse sand fraction.

Endosulfan metabolites 241

microcosms and serial dilutions in 0.9% saline were prepared from soil-water extracts. Plateswere incubated at 30°C for 3–5 days prior to counting.

3. Results and discussion

3.1. Dissipation of endosulfan sulfate and endosulfan diol from soil

The radioactivity in the HAE phase, which contained the majority of the original radiolabel,decreased in soils treated with both endosulfan sulfate and endosulfan diol (figure 3(a) and3(b)). This decrease in the radioactivity in the HAE phase was significantly faster (p < 0.05)

Figure 3. Decrease in radioactivity in the HAE from cotton farming soil with applied (A) 14C-endosulfan sulfateand (B) 14C-endosulfan diol.

242 T. F. Guerin

in the non-sterile treatments, indicating that there was a relatively important contribution tothe dissipation by the indigenous soil microflora. After 9 weeks of incubation in the nonster-ile treatments, these losses were 50 and 38% of the originally applied endosulfan sulfate andendosulfan diol, respectively. The corresponding values in the sterilized treatments wereapproximately 5 and 10%, respectively. The relatively large differences between the steril-ized and non-sterilized treatments suggested that endosulfan sulfate and endosulfan diol werelargely biodegraded under these conditions.Figure 3. Decrease in radioactivity in the HAE from cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.In the endosulfan sulfate treated soil, the ME radioactivity remained constant in the steril-ized treatments (figure 4(a)). In the non-sterile soil, there was a substantial decrease in the

Figure 4. Changes in the distribution of radioactivity in the ME from cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.

Endosulfan metabolites 243

radioactivity, suggesting that microorganisms had contributed to the decrease. However, inthe endosulfan diol treated soil, the ME radioactivity under non-sterile conditions increasedduring the course of the trial. This suggested a microbiological contribution to the formationof methanol-soluble forms of endosulfan diol (figure 4(b)).Figure 4. Changes in the distribution of radioactivity in the ME from cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.The radioactivity in the WRE phase from the soils treated with endosulfan sulfate, did notincrease in either the sterilized or non-sterilized treatments (table 7). However, in theendosulfan diol treated soil, the radioactivity in the WRE phase increased significantly in themicrobiologically active treatments (p < 0.05).

In the treatments containing endosulfan diol in non-sterile soil, ME phase assays indicatedthat microorganisms increased the extent of conversion of endosulfan diol to less hydropho-bic forms. Endosulfan sulfate, conversely, did not undergo further transformation to metha-nol and water soluble forms in either the sterile or non-sterile treatments.

Large amounts (62%) of endosulfan sulfate were converted to forms which were notrecoverable in the current study. This was particularly observed in the non-sterile treatments.This suggested that this compound was adsorbed or incorporated into the soil matrix over thetrial period (table 7). Further analysis of the ME and WRE revealed that 40–50% of the orig-inally applied radioactivity from endosulfan sulfate and endosulfan diol was converted tounidentified forms, and some limited evidence was obtained that these compounds wereacidic [14]. In the endosulfan sulfate treated soil, a small proportion of the endosulfan sulfate(<10%) was converted to endosulfan diol over the 9 week period. From the studies recentlyreviewed [7], the extractability of endosulfan compounds are shown to typically declineduring incubation in non-sterile soils, indicating that endosulfan and its residues becomeincreasingly bound to the soil with time. It is recommended that any further work in this areashould address the identity of these unrecovered forms of endosulfan, including the nature oftheir interaction with the soil.

Previous laboratory and field studies have reported the formation (and subsequent dissipa-tion) of endosulfan sulfate, in soil and water [7]. Endosulfan sulfate is typically found in soilsand sediments with the parent isomers, typically 10–50 days after application of the parent

Table 7. Endosulfan degradation product mass balance study in cotton farming soil

Sterilized Non-sterilized

Treatment Extract/Phase 0 weeks 9 weeks 0 weeks 9 weeks

Endosulfan diol a HAE 72.15 62.64 70.89 44.20ME 5.06 6.33 4.43 15.83WRE 1.52 2.49 1.14 3.8814CO2 0 1.67 0 6.56Unrecoveredc 21.27 26.87 23.54 29.54

Endosulfan sulfate b HAE 44.4 37.2 47.22 23.8ME 22.2 22.2 21.1 13.3WRE 0.22 2.0 0.11 0.514CO2 0 0.12 0 0.32Unrecoveredc 33.2 38.4 31.5 62.0

Notes: aThe amount of radioactivity added to the soil as endosulfan diol was 3.36 MBq/g soil (w/w) (dry weight); standard errors (%)between replicates were 3.5, 14.6 and 13.2 for HAE, ME and WRE, respectively. bThe amount of radioactivity added to the soil asendosulfan sulfate was 0.78 MBq/g soil (w/w) (dry weight); the average standard errors (%) between replicates were 14.3, 8.5 and 33for HAE, ME and WRE, respectively. cThe unrecovered fraction was calculated by difference.

244 T. F. Guerin

isomers to soil or a crop. Endosulfan diol, on the other hand, is not always reported in routinestudies of endosulfan in soil. This is probably due to the relative difficulty in extracting andanalysing this compound compared with the parent isomers or endosulfan sulfate [14]. Inprevious studies there has been only limited evidence that endosulfan sulfate and diol arefurther metabolized to CO2 [7]. Martens [19] has demonstrated that 18% of soil appliedendosulfan diol can be converted to 14CO2 over a period of a year.

Studies on the specific contribution of microorganisms to the degradation of endosulfanare limited. Miles and Moy [16] have reported that endosulfan sulfate and endosulfan diolhad half-lives of 14 and 11 weeks respectively, in a mixed liquid culture of soil micro-organisms isolated from a sandy loam. In the same study, endosulfan hydroxyether, endosul-fan ether and endosulfan lactone demonstrated half-lives of 8 weeks, 6 weeks, and 5.5 hrespectively. Katayama and Matsumura [17] claim that the common soil fungus, Tricho-derma harzianum, can degrade the parent isomers, as well as endosulfan sulfate and endosul-fan diol, also in liquid culture. These researchers have provided evidence that both parentisomers are first converted to endosulfan sulfate and then subsequently to endosulfan diolunder aerobic conditions. A recent study has clearly demonstrated that the parent isomers ofendosulfan can be biodegraded under conditions of low oxygen such as in water logged soilsor sediments [18].

3.2. Mineralization of endosulfan diol and endosulfan sulfate

Mineralization was highest in the nonsterilized soil treated with endosulfan diol. There weresignificant differences (p < 0.05) observed in 14CO2 release between the sterilized and non-sterilized treatments when the radioactive degradation products of endosulfan were added tothe soil (figure 5). Although the absolute amounts of 14CO2 released from the endosulfansulfate treated soils were low, the rate of mineralization was approximately twice that foundin the biologically active (non-sterile) treatment compared with the sterile treatment. In thisbiologically active treatment, 6.5% of the 14C-endosulfan diol applied, was converted to14CO2 over the trial period. These results demonstrated that endosulfan diol and endosulfansulfate can be mineralized, at least to an extent, by indigenous soil micro-organisms.Figure 5. Release of radioactive carbon dioxide from the cotton farming soil with applied (A) 14C-endosulfan sulfate and (B) 14C-endosulfan diol.Studies have been conducted on endosulfan diol, also radiolabelled in the ring at the twonon-chlorinated positions, with the aim of determining the mineralization rate of the bicyclicring. Endosulfan diol was added to a loamy soil (pH 5.3) at a level of 1 mg/kg and incubatedat 22°C for a year, with evolved 14CO2 monitored on a fortnightly basis. The main degrada-tion products found in the soil were endosulfan lactone and a polar unknown. Small quanti-ties of the hydroxyether and ether were also detected. Evolution of 14CO2 reachedapproximately 18% over the year. At the end of the study, the soil contained ∼ 40% each ofextractable and residual radioactivity [19].

3.3. Mechanisms for dissipation of endosulfan sulfate and endosulfan diol

Although there are reports describing the formation of endosulfan diol and endosulfan sulfatefrom the degradation of the parent endosulfan compounds in soils and water, there have beenvery few studies that describe the subsequent fate of these compounds. It is now well estab-lished that in soils treated with technical grade endosulfan, there is a formation and subse-quent (and often gradual) disappearance of the degradation product endosulfan sulfate [14]and references cited therein. The mechanisms for dissipation of endosulfan diol and endosul-fan sulfate in soil are, however, largely unknown, though micro-organisms are likely to play

Endosulfan metabolites 245

a role. Losses of endosulfan sulfate due to leaching from a soil profile are unlikely because ofits reported high binding affinity to soil and sediment particles [20,21] and from leachingstudies conducted on the parent compounds [7]. Volatilization is unlikely to be a route due tothe increased water solubility of these compounds (table 4) compared with the parentcompounds (although there is no readily available data on the volatility of either degradationproduct). There is no direct evidence in the literature to indicate that endosulfan sulfate isdegraded either biologically or chemically. It is possible, however, that endosulfan sulfatemay undergo hydrolysis, under certain environmental conditions, to form endosulfan diol(Anonymous 1998). Katayama and Matsumura [17] have proposed that the common soilfungus, T. harzianum, can hydrolyse endosulfan sulfate to endosulfan diol under laboratory

Figure 5. Release of radioactive carbon dioxide from the cotton farming soil with applied (A) 14C-endosulfansulfate and (B) 14C-endosulfan diol.

246 T. F. Guerin

conditions in liquid culture, but there is relatively little known about this reaction. One otherstudy, using mixed aerobic cultures from an agricultural soil, has suggested that endosulfanmay be completely dissipated to forms that are not detected by ECD gas chromatography[16]. This does not mean that the parent isomers of endosulfan are readily degraded to thediol and sulfate forms since this study did not control nonbiological losses.

Endosulfan diol does not contain the cyclic sulfite structure and therefore is not as readilysubject to hydrolysis as are the parent isomers. Endosulfan diol is also more mobile in soilthan endosulfan sulfate. Endosulfan sulfate, which contains a cyclic sulfate structure, hasbeen reported to be significantly more recalcitrant than the parent isomers [7,10,16] predom-inantly due to it being less susceptible to hydrolysis. This attribute may account for its slowerrate of loss under sterile conditions, compared to endosulfan I, in the current trial.

The slower rates of loss of endosulfan sulfate and endosulfan diol under sterile conditionsreflects their low volatility and very low susceptibility to alkaline hydrolysis. This is unlikethe faster rates of degradation due to chemical losses and volatilization previously observedwith the principal parent isomer, endosulfan I [7].

3.4. Monitoring of microbial populations

At the beginning of the trial, in the nonsterile treatments, the number of heterotrophic micro-organisms (per gram of oven dry soil) were ∼ 108. This value did not significantly changeduring the course of the incubations. In the sterile treatments, the number of heterotrophs was<102 (per gram of oven dry soil). This also was the case at both the beginning and end of thetrial period, indicating that microbial numbers were kept suppressed in the sterile treatments.

4. Conclusion

It is apparent that both chemical and biological mechanisms cause the further dissipation ofendosulfan sulfate and diol in soil. This study has demonstrated that microbial processes areresponsible for their degradation. Biodegradation is therefore put forward as the main mecha-nism for the dissipation of these major degradation products in the soil studied. Analyses onthe HAE, ME and WRE indicate that water soluble degradation products are formed and theextent of this formation varied between 10–30% of the originally applied endosulfan diol andendosulfan sulfate. Further research would be required to identify the nature of the biodegra-dation products of both endosulfan sulfate and endosulfan diol.

There were marked differences between rates of dissipation of endosulfan sulfate andendosulfan diol in sterilized and non-sterilized cotton farming soil (table 8). These resultsdemonstrated that micro-organisms contribute to loss of these degradation products in thesoil studied. This result suggests that both of these degradation products are likely to befurther biodegraded in the soil profile, after they are formed from the parent compounds.Based on the findings here, the degradation products of endosulfan will have half-lives of60–85 days in biologically active soils, whereas in sterile soils, these values increase to240–260 days.

Endosulfan diol underwent substantial mineralization. Endosulfan sulfate also released14CO2, but at lower rates when compared with endosulfan diol.

The major degradation products of the chlorinated insecticide endosulfan, endosulfan dioland endosulfan sulfate, can therefore be further degraded in laboratory scale studies, in soilsin which they commonly occur. This indicates that these compounds are unlikely to be the

Endosulfan metabolites 247

ultimate degradation products of endosulfan, and that they are intermediates only in the over-all degradation of endosulfan in soil.

References

[1] Stewart, D.K.R. and Cairns, K.G., 1974, Endosulfan persistence in soil and uptake by potato tubers. Journal ofAgricultural and Food Chemistry 22(6), 984–986.

[2] Doelman, P., Loonen, H. and Vos, A., 1988, Ecotoxicologisch onderzoek in met endosulfan verontreinigdegrond: toxiciteit en sanering (RIN [88/39]) (unpublished report).

[3] Doelman, P., 1990, Microbial degradation of hexachlorocyclohexane isomers in mineral soil and of endosulfanisomers in organic soil in connection with soil and water quality: ecotoxicological research. Report of theNetherlands Organisation for Applied Scientific Research (TNO) 23, 73–91.

[4] Van Dyke, L.P. and Van der Linde, A., 1976, Persistence of endosulfan in soils of the Loskop Dam irrigationarea. Agrochemophysica 8, 31–34.

[5] Van Dyk, L.P. and Greef, C.G., 1977, Endosulfan pollution of rivers and streams in the Loskop Dam cotton-growing area. Agrochemophysica 9, 71–76.

[6] Guerin, T.F., 1995, In: R.E. Hinchee, G.S. Douglas, A.D. Little and S.K. Ong (Eds) Monitoring and Verifica-tion of Bioremediation (Columbus, Ohio: Battelle Press).

[7] Anonymous, 1998, Review of Endosulfan (Canberra: National Registration Authority for Agricultural andVeterinary Chemicals). Available online at: http://www.apvma.gov.au (accessed July 2004).

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Table 8. The estimated half lives of endosulfan diol and endosulfan sulfate in the cotton farming soila

Half life (days)

Compound Treatment Current study Other studies

Endosulfan diol Sterile 255 –b

Non-sterile 83 –Endosulfan sulfate Sterile 240 –

Non-sterile 60 100–150c

Notes: aDetermined from the decrease in radioactivity in the HAE, using an exponential decay equation. bNot Reported. cNRA(1998) [7].

248 T. F. Guerin

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