persistence of endothall in the aquatic environment

P E R S I S T E N C E O F E N D O T H A L L I N T H E

A Q U A T I C E N V I R O N M E N T

G. V. SIMSIMAN and G. CHESTERS*

Dept of Soil Science and Water Resources Center, University of Wisconsin, Madison, Wis. 53706, U.S.A.

(Received 4 December, 1974)

Abstract. In a simulated lake impoundment 72 ~ of added endothall [7-oxabicyclo-(2.2.1) heptane- 2,3-dicarboxylic acid] persisted in water for 30 days due to prolonged dissolved 02 depletion following weedkill during this period. Rapid disappearance of endothall occurred only after the restoration of oxygenated and oxidizing conditions; under these conditions little or no endothall was detectable after 60 days.

Degradation of endothall was slow in weed-free and quiescent sediment-water systems containing abundant overlying water (5 ml sediment, 200 ml water), even under aerobic conditions. The slowness of degradation was evidenced by the small amount of 14CO2 evolution and the persistence of intact endothall remaining in the water after 2 too. Reducing the water volume 10-fold, i.e., 5 ml sediment: 20 ml water, resulted in a 25-fold increase in the degradation of endothall in the Lake Mendota but not the Lake Tomahawk system. Slow degradation in the latter indicated that either the sediment contained low endothall-degrading microorganisms or rapid adsorption decreased the availability of the herbicide for microbial decomposition.

1. Introduction

Endothall [7-oxabicyclo-(2.2.1)heptane-2,3-dicarboxylic acid] is an effective aquatic

herbicide which is used widely for the control of numerous submerged weed species

(Hiltibran, 1963; Walker, 1963; Yeo, 1970; Serns, 1974). In Wisconsin, it is presently

the major herbicide approved for supervised chemical control of aquatic vegetation

(Lueschow, 1972). Applied at recommended rates - 1 to 5 mg 1.2 - endothall has

been shown to have no direct adverse effect on fish and aquatic fauna (Surber and

Pickering, 1962; Hiltibran, 1967; Sanders, 1970; Yeo, 1970; Serns, 1974).

It appears that endothall is degraded rapidly in soils and aquatic environments

(Comes et al., 1961; Montgomery and Freed, 1964; Horowitz, 1966; Sikka and

Saxena, 1973). In soils, most of the herbicide dissipated to CO2 in 14 days (Montgomery

and Freed, 1964), and in aquatic systems 25% was evolved as CO2 in 10 days (Sikka

and Saxena, 1973). Microbial degradation seems to account for the relatively rapid

disappearance of endothall in natural aquatic environments. However, bioassay

(Hiltibran, 1962; Yeo, 1970; Holmberg, 1973) and gas-liquid chromatographic (Sikka

and Rice, 1973) analyses indicated a wide range - 2.5 to 30 days - of dissipation

periods of endothall from water. This suggests that variable conditions existing in

aquatic environments determine to a large extent the persistence of the herbicide.

* Presently Project Associate, Water Resources Center; Professor of Soil Science and Director, Water Resources Center, University of Wisconsin, Madison.

Water, Air, and Soil Pollution 4 (1975) 399-413. All Rights' Reserved Copyright © 1975 by D. Reidel Publishing Company, Dordreeht-Holland

4 0 0 G.V. SIMSIMAN AND G. CHESTERS

Recently, Sikka and Rice (1973) reported that a portion of the endothall removed from water is sorbed by the sediment and persists for an extended period.

Available evidence indicates that endothall is degraded readily by Arthrobacter spp. isolated from sediments. Degradation occurs by initial cleavage of the ring followed by incorporation of C-fragments mainly with glutamic acid in the tricarboxylic acid cycle and by an unknown alternate pathway (Sikka and Saxena, 1973). This metabolic scheme has yet to be demonstrated in soils and aquatic environments. The subject of this investigation is an elucidation of the factors controlling the persistence of endothall in the aquatic environment.

2. Materials and Methods

2.1. SEDIMENT

The sediment samples were obtained from Lake Tomahawk in Oneida County and the University Bay portion of Lake Mendota in Dane County. These lakes are located in northern and southern Wisconsin, U.S.A., respectively. Some of the important physical and chemical properties of the two sediment samples are shown in Table I.

TABLE I Properties of sediments

Sediment pH (wet) CEC a me (100 g)-i at, pH4 pH7

Clay, ~ Silt, ~ Organic C, ~ CaCO3,

Tomahawk 5.6 32.8 52.6 Mendota 7.1 24.1 34.3

58.9 27.2 15.0 0.0 39.1 57.5 8.4 40.8

a Cation exchange capacity in milliequivalents (100 g)-l.

The sediment sample obtained from Lake Tomahawk - an oligotrophic soft-water lake - contains high amounts of organic matter and clay. The sediment from Lake Mendota - an eutrophic hard-water lake - possesses less organic matter and clay, but a significant portion of the sediment is comprised of CaCO 3. X-ray diffraction analysis of the clay fraction indicated that the crystalline mineral composition of the two sediments was qualitatively similar. Amongst the minerals identified were mica, kaolinite, chlorite, montmorillonite, and vermiculite, but Lake Tomahawk sediments contained higher amounts of amorphous inorganic materials.

2.2. HERBICIDE

Endothall (99.9~ pure) - both labeled and nonlabeled - was obtained through the courtesy of the Pennwalt Chemical Corp., Tacoma, Washington, U.S.A. The 14C- endothall labeled in the ring C-atoms adjacent to the COOH groups had a specific activity of 2.42 mCi m M - ~

PERSISTENCE OF ENDOTHALL IN THE AQUATIC ENVIRONMENT 401

2.3. E X P E R I M E N T A L PROCEDURES

The persistence of endothall was followed in a simulated lake impoundment and closed sediment-water incubation systems.

2.3.1. Simulated Lake Impoundment

Details of the simulated lake impoundment experiment were presented in an earlier report (Simsiman et al., 1972). This experiment was conducted in a temperature- controlled plant growth room (25°C) which was provided with fluorescent and in- candescent light (305 lux at the water surface) and ventilation. The impoundments consisted of five plexiglas acrylic resin tanks (90 cm high and 30.5 cm diameter), each containing 8 1 of Lake Mendota sediment and 60 1 of water. Prior to herbicide addition, aquatic macrophytes, namely, watermilfoil (Myriophyllum spicatum L.) and elodea (Elodea canadensis Michx.), were allowed to grow in the impoundment until they produced a thick matted stand. After weed growth was established, 3 mg 1-1 endothall was added to one of the impoundments. The a4C-ring labeled endothall (2.946 #Ci = 6 5.40060 DPM) was added to monitor the degradation of the herbicide following weedkill. Water samples were collected periodically through sampling ports placed at two depths in the water column - halfway from the water surface and 0.5 cm above the sediment surface - and stored in a freezer at - 5 °C.

2.3.2. Incubation Experiments

The experimental setup of the incubation experiments (Figure 1) was similar to that of Hance and Chesters (1969), but was modified to include (1) a spout on the side of the flask provided with a septum to facilitate drawing of water for 14C analysis, and (2) a stopcock and a ground-joint between the H2SO4 and ethanol-ethanolamine traps to remove and change the 14CO2 trapping solution. The sediment-water system was prepared by adding 5 ml fresh sediment and 180 ml water to each flask. The sediment was allowed to settle to the bottom of the flask, prior to the addition to each system of a 5 0 m g l - I solution (20ml) of endothall with an activity of 1.0245 #Ci 14C (2 274 540 DPM), to provide a final concentration of 5 mg 1-1 endothall and a sediment:water ratio of 5 ml sediment: 200 ml water. The oven-dried weights of 5 ml Lakes Mendota and Tomahawk sediments were 1.302 and 0.460 g, respectively. The sediment-water-herbicide systems were incubated at ambient temperature (23_ 2 °C) in streams of 02 or N 2 to maintain aerobic or anaerobic environments, respectively. Tile gas flow in each case was approximately 2 ml m i n - 1 The effluent gases were passed through 20 ml of concentrated H2SO 4 to remove water and basic products, 20 ml 2: 1 (v: v) ethanol: ethanolamine solution to absorb CO2, and 20 ml ethanol to remove other volatile products. In the aerobic environment two additional treatments were included, namely, (1) an addition of sucrose (4~o by weight of the dried sediment) was made 1 week prior to introduction of the herbicide to stimulate the growth of the indigenous microbial population, and (2) a sediment-water-herbicide system was developed by addition of 20 ml of the ~4C-labeled herbicide directly to 5 ml sediment

402 G.V. SIMSIMAN AND G. CHESTERS

Fig. 1. Experimental setup of the incubation investigation. Note: A. gas inlet, B. erlenmeyer flask with spout containing sediment-water system, C. H2SO4 trap, D. stopcock, E. ground-joint with

clamp, F. ethanol-ethanolamine trap, and G. ethanol trap.

to provide a 5: 20 sediment: water ratio to determine the effect of different sediment:

water ratios on the degradation rates of the herbicide. All treatments were duplicated. To ascertain whether degradation of the herbicide was a microbiological process,

a sterile (autoclaved) treatment was included.

2.4. ANALYTICAL PROCEDURES

2.4.1. Analysis of Endothall Residues

Endothall residues in water were estimated using a thinqayer chromatographic (TLC) technique. Samples (200 to 500 ml) were concentrated to an appropriate volume over

a water bath (50 to 60 °C) with a stream of clean air. Aliquots were spotted on 0.25 mm SilicAR TLC 7GF plates and were developed by two solvent systems - ethyl acetate: CHC13 :formic acid (4:5: 5) and n-butanol: CH3COOH: H 2 0 (3 : 2: 2) - in a solvent- saturated chamber allowing the solvent front to move 16 cm from the origin. The ethyl acetate-CHC13-formic acid solvent provided better resolution and was used in the experiment. After drying the plates - air-drying overnight or oven-drying at 80 °C for 60 rain - spots were detected by spraying with 0.04~ bromcresol green in ethanol. Radioactivity of the spots was determined following the method of Snyder (1964)

and confirmed by co-chromatography with endothall. Sediment was separated from the water phase by filtration, and radioactivity in

the sediment was estimated by extracting the sediment with 2N HCI in ethanol for 4 h on a Soxhlet apparatus (Sikka, 1971).


2.4.2. Liquid Scintillation Spectrometry

A Packard Model 3365 liquid scintillation spectrometer was used for radioactive measurements of 14C in tracer-labeled compounds. Counting conditions for the detection of 14C-activity were as follows: window setting, 50 to 1000, and gain, 8%. External standardization was employed unless otherwise stated. Window and gain settings for counting the external 226Ra standard were 100 to 700 and 0.5%, respectively. A correction for quenching was made by plotting the counting efficiency (CPM/DPM x 100) against counts of the external standard. Counts of 10000 or more were obtained for each sample.

Aqueous solutions (1 or 2 ml) were counted in 1,4-dioxane (8 or 9 ml) and a scintillation solution (10 ml) consisting of 20 g PPO (2,5-diphenyloxazole), 0.1 g dimethyl POPOP (1,4-bis-[2-(4-methyl-5-phenyloxazolyl)]-benzene), 100 g naphthalene, and 334 ml ethylene glycol monoethyl ether diluted to 1 liter with 1,4-dioxane (Bruno and Christian, 1961).

The 14CO2 evolved was absorbed in a 2:1 (v:v) ethanol:ethanolamine solution (20 ml). The trapping solution was changed periodically. Aliquots (10 ml) of the absorbent were added to the scintillation cocktail (10 ml) which contained 6.0 g 1 - t PPO and 0.1 g 1 - t dimethyl POPOP in toluene (Skipper et aI., 1967). The activity of the solution was counted in the liquid scintillation spectrometer using 14C-toluene internal standardization to check counting efficiency.

The ~4C-contents of the H2SO4 and ethanol traps were estimated, following the procedure of Hance and Chesters (1969). The contents of the ethanol traps were added to water (10 ml), and the solution was concentrated to 2 ml and added to the scintillation cocktail. Toluene (20 ml) was added to the H2SO4 trap contents and the solution was made alkaline with 40% NaOH. (Caution: NaOH was added slowly to the solution in an ice bath to prevent violent reaction.) The mixture was shaken, and after separation the toluene layer (10 ml) was added to the scintillation cocktail.

Spots on the thin-layer chromatograms were scraped carefully with a razor blade into counting vials. The scrapings were suspended in scintillation solution (15 ml) containing 7 g PPO, 0.3 g dimethyl POPOP, 100 g naphthalene, 48 g Cab-O-Sil, diluted with 1 liter dioxane and 200 ml water (Snyder, 1964).

3. Results and Discussion

3 . 1 . L A K E I M P O U N D M E N T EXPERIMENT

The persistence and degradation of endothall in water is shown in Figure 2. Initially, the total 14C-activity in the water decreased, reaching a value of 8t% on the 16th day. Thereafter, 14C-activity increased to 88% by the 27th day and then declined rapidly to 8% by the 62nd day. The nature of the 14C present in the water was elucidated by TLC analysis. Three spots were detected but only one spot with an Rfvalue of 0.538 contained radioactivity. The radioactive spot corresponded to endothall and co- chromatographed with authentic standard ~4C-endothall. The other spots probably

404 G . V . $ I M S I M A N A N D G. C H E S T E R S

Fig. 2.

IOO

90

E t.~ ~- 80 . <

z 70

~' 60 22

50 Z t.~

~, 40 Z

o < 301

~. 20 g

I0

0;

::::,:4o:,

4 8 12 16 22 27 34 46 62 TIME, days

Persistence and degradation of 14C-endothall in water of a weed-infested simulated lake impoundment containing Lake Mendota sediment.

originated from components of the decomposing weeds. Quantification by liquid scintillation counting of the scraped radioactive spot revealed that the bulk of the ~4C-activity remaining in the overlying water resulted essentially from intact 14C- endothall. Values of 14C-endothall paralleled closely those of total 14C-activity. The amounts of endothall declined gradually from 95~ on the 1st day to 66~ on the 22nd day following herbicide application. From the 22nd day the values increased slightly to 72~ by the 27th day, and decreased rapidly thereafter until only negligible amounts were detectable on the 62nd day.

Differences between total ~4C-activity and X*C-endothall increased from 4 to 16~ in the first 27 days of the experiment. These differences could be due to formation of water-soluble degradation products. However, no spots were present on the chromatograms corresponding to this radioactivity. The only possible explanation for non-detection of these products was that volatilization occurred during the development and drying of the chromatograms, and/or the products were present in un- detectably low concentrations. Concentration of water samples by evaporation in a water bath at 50 to 60°C in a stream of clean air resulted in losses of radioactivity ranging only from 2.1 to 2.7~. Furthermore, loss of radioactivity was not found during the development and drying of chromatograms containing standard ~4C- endothall.


In the experiment 72~ of the initial 14C-endothall added persisted in water during a 27-day period, and as much as 16~o remained as water-soluble degradation products. The disappearance of the other 12~o could be attributed to either one or combinations of the following processes: (1) adsorption by the sediment, (2) absorption by the macrophytes, and (3) volatilization of metabolic products such as 14CO2. While sediment adsorption of endothall has been reported (Sikka and Rice, 1973), it may not be significant in the present study because of the matted growth of weeds. Uptake of the chemical by the weeds was possible a few days after application, as suggested by the gradual decrease of total 14C-activity and ~4C-endothall from the 1st to the 16th day, followed with an increase by the 27th day. Uptake of endothall by aquatic weeds - including elodea and watermilfoil - has been reported (Thomas, 1966; Sikka, 1971; Haller and Sutton, 1973). Elodea appears to tolerate higher concentrations of the herbicide, and the absorbed molecule is not metabolized extensively by the plant (Sikka, 1971). Weedkill in the endothall-treated lake impoundment was gradual (Simsiman et al., 1972). Watermilfoil was killed first, followed by elodea, and disintegration occurred only after 16 days. Release of absorbed endothall, including any metabolic products, may have occurred after the disintegration of the weeds, thus increasing the total 14C-activity and 14C-endothall concentration of the overlying water. Additionally, adsorption of the chemical by the dead weeds was possible and the sorbed herbicide was released following weed disintegration. Evidently, appreciable amounts of endothall persisted in water for almost 30 days despite the presence of herbicide-killed weeds. A sdemonstrated previously (Simsiman et al., 1972), the overlying water of the endothall-treated lake impoundment remained devoid of dissolved oxygen (DO) and was subsequently in a reduced state during most of the first 30 days of the experiment. Because of these conditions during this period, microbial decomposition was sluggish. Even with the presence of abundant decomposing weeds in the system, degradation of endothall was extremely slow, thereby accounting for its lengthy persistence in water. Soil and aquatic microorganisms have been shown to degrade endothall readily, but apparently most of them need O2 to function effi- ciently. An Arthrobacter sp. isolated from soils (Jensen, 1964) and sediment (Sikka and Saxena, 1973) has been implicated as the organism responsible primarily for endothall metabolism in these environments. Arthrobacters, which are aerobes, are likely to be present in Lake Mendota, particularly in the shallower portion of the lake (McCoy, 1974). Therefore, it was only after the 27th day when reoxygenated and oxidizing conditions were restored naturally that rapid degradation of the herbicide proceeded. At this time, decomposition of the weeds became more active. Simul- taneously, microorganisms in the system involved in endothall metabolism also became more active in degrading the herbicide in water presumably to volatile products. However, the DO content of the water in endothall-treated ponds at approximately 5 mg 1-1 did not retard the rapid degradation of the herbicide (Holmberg, 1973; Serns, 1974).

The results of this study demonstrate clearly that endothall - although degradable microbiologically-can persist in heavily weed-infested aquatic systems if unfavorable

406 G . V . S I M S I M A N A N D G . C H E S T E R S

conditions such as a lack of DO limit the activity of the microorganisms. This may be a significant consideration when treating heavily weed-infested ponds or lakes in toto with endothall where complete stripping of DO is likely to occur.

3.2. INCUBATION EXPERIMENTS

3.2.1. Adsorption

The disappearance of 1*C-ring-labeled endothall from water was monitored in a well- settled sediment (5 ml) : water (200 ml) system (Figure 3). It can be seen that in 2 days the initial 14C-activity in the aerobic and anaerobic Lake Mendota systems decreased by 6 to 8~ and then tended to stabilize until the 27th day. Beyond this period, ~*C- activity changed only negligibly in the anaerobic condition, while a definite decrease was evident in the aerobic system. Contrarily, in the Lake Tomahawk system a rapid decrease in a'C-activity occurred initially. Within 2 days only 65~ of the ~4C added could be detected in the water. Thereafter, values continued to decrease gradually until the 14th day when 52~o was present. From the 14th day l~C-activity increased slightly until the 35th day.

The data reveal that the two sediments affected the amount of ~4C-activity remaining in the overlying water to different extents. The initial decrease of 14C-activity a few days after application of the herbicide could be attributed to adsorption. According to Sikka and Rice (1973), the initial decrease in endothall concentration added to

ta~

Z

z

Z

c~

m

10C

90

8C

70

60

• L. Mendota, aerobic

o L. Mendote, anaerobic

• L. Tomahawk, aerobic

\

5O

0.08 2 4 7 14 21 28 35

Fig. 3.

TIME, days

The 14C remaining in the overlying water on addition of 14C-endothall to three sediment-water systems.


the water of a pond and aquaria is primarily a result of sediment adsorption. Complete adsorption in the aquaria and pond occurred after 3 and 22 days, respectively. The sediments used in the present study varied significantly in their capacity to adsorb endothall, i.e., the adsorptive capacity of the Lake Tomahawk sediment for endothall was much higher than that of the Lake Mendota sediment. Based on the oven-dried weights of the 5 ml samples of sediment used (1.302 g for Lake Mendota; 0.460 g for Lake Tomahawk), the Lake Tomahawk sediment was able to adsorb almost 10 times as much endothall per unit weight as Lake Mendota sediment during the first 2 days of the experiment. The significantly higher adsorptive capacity of Lake Tomahawk sedi~ment for endothall could be attributed to its higher organic matter and amorphous mineral contents and lower pH than the Lake Mendota sediment. Hamaker et al.

(1966) found that synthesized or natural amorphous minerals have high affinity for acidic herbicides. The extent of adsorption of acidic herbicides to organic colloids and[ amorphous minerals appears to be inversely related to pH. The sediment adsorption of endothall - applied in the presence of abundant overlying quiescent water - probably takes place in two stages. The first stage is rapid, followed by a second stage of gradual adsorption. Apparently, endothall adsorption by Lake Mendota sediment was completed in 4 days and by Lake Tomahawk sediment in 14 days. Further evi- den~ce of endothall adsorption was the presence of extractable 14C from the sediment, which is discussed in the section entitled '~4C-Distribution'.

The importance of adsorption has generally been overlooked in studying the persistence of endothall in aquatic systems. However, evidence from this study and that of Sikka and Rice (1973) indicates that adsorption of endothall by highly organic sediments may play an important role in the persistence of this herbicide in aquatic environments.

3.2.2. x4CO2 Evolution

The degradation of endothall in various sediment-water systems was monitored by measuring the evolution of ~4CO2. The t4COz evolved was attributed mainly to the microbial metabolism of the tagged herbicide. This was confirmed by the absence of 14CO2 evolution from a sterilized (autoclaved) sediment-water system.

Figure 4A illustrates the evolution of ~4COz from endothall in aerobic and anaerobic Lake Mendota sediment-water systems (5 ml sediment, 200 ml water). Greater amounts of ~4CO2 were evolved from the aerobic than from the anaerobic system. Addition of sucrose to the aerobic system 7 days prior to herbicide treatment enhanced 14CO2 evolution slightly. Degradation of endothall was comparatively slow even under aerobic conditions. After 35 days, only 2.2, 3.0 and 1.1~o of the initial 14C-endothall (5 mg 1-1) was metabolized to ~4CO2 in the aerobic, aerobic sucrose amended, and anaerobic systems, respectively. Previous investigations have pointed out that under favorable microbiological conditions endothall is readily degradable in ..soil and aquatic environments (Montgomery and Freed, 1964; Horowitz, 1966; Sikka and Saxena, 1973).

The higher amount of ~4CO 2 evolved under aerobic conditions indicates that the

4 0 8 G.V. SIMSIMAN AND G. CHESTERS

- I ¢xa O

0 t/3

60 ,,=, ...J

o 50

4O

30

20

10

0 0

(A)

• Aerobic o Aerobic, sucrose added • Anaerobic / o

o / . /

i l I !

(c) /"

~ 5:200

5:20

,~,~T ; 4 7 14 21 28

(s)

I L. Mendota ~ / ' = L. Tomah~D /

[ ]

J i i I i u i ;

~5:200

5:20

3'5 0 4 7 14 21 28 3'5 TIME, days

Fig. 4. Cumulative 14CO2 evolution from 14C-endothaU added to (A) aerobic and anaerobic Lake Mendota sediment-water systems, (B) aerobic Lakes Mendota and Tomahawk sediment-water systems,

(C) and (D) aerobic Lakes Mendota and Tomahawk sediment-water systems, respectively, at two sediment-water ratios.

microorganisms involved primarily in endothall metabolism required adequate 0 2

supply. This supports the contention made earlier that DO depletion caused by decomposition of weeds retards the decomposition of the herbicide. Thus, insufficient DO resulting from weed decomposition and/or thermal stratification may promote

persistence of endothall. The results of this experiment suggest that endothall dispersed in large volumes of

overlying water is not degraded extensively. Nearly all the a4C-activity detected in the water was due to intact 14C-endothall, as shown in the section dealing with 'a4C-


Distribution'. Presumbably, only a few microorganisms were present in the overlying water, and, owing to the quiescent nature of the system, only small amounts of particulate matter to which the microorganisms could adhere remained suspended in the water. Since the microorganisms likely were concentrated near the sediment surface where food supply was abundant, only that herbicide present at the sediment-water interface or herbicide adsorbed by the sediment was metabolized rapidly. However, only a small portion of the herbicide added was adsorbed by the sediment, leaving the bulk in the water-soluble form (Figure 3). Thus, a slow rate of endothall degradation was observed even when Oz supply was sufficient.

The influence of sediment types on the persistence of endothall in aerobic sediment: water systems (5: 200) is presented in Figure 4B. A higher rate of 14CO z production was observed in the Lake Tomahawk system than in the Lake Mendota system. Nevertheless, degradation was still slow compared to the earlier findings of Sikka and Saxena (1973). At the end of 35 days, 4.5~o of the 14C-endothall added (5 mg 1 -a) was evolved as 14CO2 from the Lake Tomahawk system, or approximately twice that of the Lake Mendota system (2.2~o).

The higher metabolism of a4C-endothall in the Lake Tomahawk system could be due to the greater sediment adsorption of endothall, as shown earlier in Figure 3. Substantial sediment adsorption of endothall provided a means of concentrating it at the surfaces of the sediment particles where microorganisms are most abundant. Degradation of adsorbed endothall was evidenced by the decrease of a4C-activity extracted from the sediment with time. These data are discussed more fully under the section entitled '14C-Distribution'.

The ratios of sediment to water affect the degradation of endothall depending on the kind of sediment used. This was demonstrated clearly in the aerobic Lakes Mendota and Tomahawk systems where two sediment: water ratios - 5:200 and 5:20 - were incubated with 1 mg of ~4C-endothall (Figures 4C and 4D). In the Lake Mendota system, 14CO 2 evolution at the 5:20 ratio was very much greater than at the 5:200 ratio at any period of incubation up to 28 days and followed a typical microbial growth curve. In 28 days over 50~o of the endothall was degraded to ~4CO2. This was approximately 25 times greater than that evolved from the system at the 5:200 ratio. In contrast, reducing the water volume 10-fold in the Lake Tomahawk system did not influence 14CO2 production.

The shorter persistence of endothall in the Lake Mendota sediment-water system containing the smaller water volume was due evidently to the greater solution concentration of the herbicide. Thus, the endothall was in closer proximity to the sediment where endothall-degrading microorganisms apparently were concentrated. Since the sediment adsorbed only a small portion of the herbicide (Figure 3), most of it was available for microbial attack. It appears that in aquatic environments degradation of endothall is enhanced markedly when the herbicide is in close contact with the sedi- me, nt or perhaps suspended particulate matter. This contention is supported by the rapid metabolism of endothall to CO2 in moist soils (Montgomery and Freed, 1964; Horowitz, 1966), where more intimate contact exists between the herbicide and the

410 C~. V. SIMSIMAN AND G. CHESTERS

soil particles. However, this was not observed in the Lake Tomahawk system despite the concentration of endothall on or near the sediment surfaces by adsorption and reduction of the water volume. This suggests that the indigenous microbial population of this sediment is less capable of degrading the herbicide. It is also possible that adsorption decreased the availability of the herbicide to aquatic microorganisms. Sikka and Rice (1973) reported longer persistence of endothall in sediment than in water, which they attributed partially to the decreased microbial availability of sediment-adsorbed endothall.

3.2.3. 14C-Distribution

A balance sheet of the 14C_endothal 1 added to various sediment-water systems (5:200) is shown in Table lI. From Table II it is seen that nearly all the 14C-activity in the water arose from intact 14C-endothall, regardless of the conditions of the sediment- water system and kinds of sediment used. In fact, only 2 to 8~ of the radioactivity was due to components other than ~4C-endothall. This difference in radioactivity may be attributed to water-soluble degradation products and/or dissolved ~4COz. These components remained undetected by TLC analysis either because of extremely low concentration or volatilization, as explained earlier. In the Lake Tomahawk system, increases in total ~4C-activity and ~4C-endothall in water were observed from the 14th to the 65th day. Desorption of adsorbed a4C-endothall or partial solubilization of metabolic ~4CO z evolved may have contributed to the enrichment of radioactivity in the water. Solubilization of CO2 generated into a sediment-water system was observed in another experiment.

After 65 days appreciable amounts of a*C-endothall still persisted in the water of the aerobic Lakes Mendota and Tomahawk systems. From 55 to 63~ of the herbicide applied remained intact. The high concentration remaining in the water after this period, accompanied by the low amounts of 14CO2 evolved, suggests that microbial breakdown of endothall in sediment-water systems occurs only at a slow rate. This slow degradation could proceed in relatively elear, non-turbulent and weed-free aquatic environments.

Extraction experiments showed the presence of radioactivity in the sediment. Considerably higher 14C-activity was recovered from the Lake Tomahawk sediment than from the Lake Mendota sediment. This corroborated the data shown in Figure 3, indicating that the Lake Tomahawk sediment has a stronger adsorptive capacity for endothall than the Lake Mendota sediment. The rapid initial disappearance of ~4C- activity in the water may have been caused primarily by adsorption since the amounts recovered from the sediment at 14 days were close to the amounts that dissipated from the water during the same period, e.g., approximately 8 and 48~ for Lake Men- dota and Lake Tomahawk, respectively. Decline of radioactivity in the sediment with time, which was particularly noticeable under aerobic conditions, indicated degradation of adsorbed 14C-endothall, or desorption of the herbicide to the water in the case of the Tomahawk system. Degradation of adsorbed l*C-endothall accounted, presumably, for most of the ~4CO2 evolved in the systems, as stated in earlier sections,

m

xl

~6

?

o

P E R S I S T E N C E OF E N D O T H A L L I N T H E A Q U A T I C E N V I R O N M E N T

I

I

~5

~ . ~ ~ N ~ .~ ~ E .~ ~ N ~

O

r'q

e-,

411

412 G.V. SIMSIMAN AND G. CHESTERS

since a lmost all the 14C-endothall in the water remained undegraded. S ikka and

Rice (1973) also noted a decline in endotha l l concentra t ions adsorbed to the sediment

with t ime in pond and aquar ia studies.

The total 14C-endothall added init ial ly could not be accounted for at later stages

of the experiment . Since n o 14C was recovered f rom the H2SO4 and e thanol t raps or

f rom the rubber s topper used, it was appa ren t tha t some 1~C fragments result ing f rom

the b r e a k d o w n of the labeled molecule may have been incorpora ted into microbia l

cells. Al ternat ively, it is also possible that f ragments o f the herbicide molecule could

have reacted with the organic components o f the sediment and fo rmed complexes

which were resis tant to ext rac t ion (Stevenson, 1972).

Acknowledgments

Research suppor ted by the College of Agr icul tura l and Life Sciences, Universi ty o f

Wisconsin, Madison , Wisconsin, under H a t c h Project No. 1915 and the Wate r Re-

sources Center under Office of W a t e r Resources Research Project No. B-016-WIS

(Agreement No. 14-01-001-1567).

References

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