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Rate of abiotic formation of 1,1-dichloroethylene from 1,1,1-trichloroethane in groundwater
Journal of Contaminant Hydrology, 1 (1987) 299-308
299
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
RATE OF ABIOTIC FORMATION OF 1,1-DICHLOROETHYLENE FROM 1,1,1-TRICHLOROETHANE IN GROUNDWATER
TIMOTHY M. VOGEL and PERRY L. McCARTY
Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, CA 94305, U.S.A. (Received May 23, 1986; revised and accepted September 16, 1986)
ABSTRACT Vogel, T.M. and McCarty, P.L. 1987. Rate of abiotic formation of 1,1-dichloroethylene from 1,1,1-trichloroethane in groundwater. J. Contam. Hydrol., 1: 299-308. The abiotic transformation of the halogenated aliphatic compound, 1,1,1-trichloroethane (TCA), to 1,1-dichloroethylene (1,1-DCE) in groundwater was measured in the laboratory at 20°C and neutral pH. The measured pseudo first-order disappearance rate constant for TCA was 0.11 + 0.16yr -1 (95% confidence interval). The formation rate constant for 1,1-DCE could be determined with greater precision, and was found to be 0.040 + 0.003yr ~. These results indicate the 95% confidence interval for TCA half-life at 20°C and pH 7 in homogeneous solution is 2.8 to 19yrs. While these results confirm that 1,1-DCE is a product from the abiotic transformation of TCA, other products such as acetic acid are also possible, although they were not measured. INTRODUCTION
The solvent 1,1,1-trichloroethane (TCA) is one of the most common contaminants in groundwaters used as a source of drinking water supply in the United States (Westrick et al., 1984). Since groundwater moves slowly and residence times are in the order of years to centuries, the eventual fate of TCA in g ro u n d water is of environmental interest. Under reducing environmental conditions, TCA can be biologically converted by reductive dehalogenation (Bouwer and McCarty, 1983; Gossett, 1985; Parsons and Lage, 1985), with 1,1-dichloroethane (1,1-DCA) being formed as an intermediate product (Gossett, 1985; Parsons and Lage, 1985). The abiotic transformation of TCA in water has also been reported. Several abiotic studies carried out at elevated temperatures (from 55° to 160°C under pressure) indicated TCA was transformed to acetic acid (Britton and Reed, 1932; Walraevens et al., 1974; Mabey et al., 1983). Rates of abiotic TCA transformation at lower temperatures were estimated by extrapolation. Such extrapolation has resulted in suggested half-lives for TCA at neutral pH and 25, 20, and 10°C of 0.6, 1.4, and 7 yrs, respectively (Mabey et al., 1983). Two studies were carried out at temperatures more characteristic of groundwaters. Dilling et al. (1975) reported a TCA half-life of 0.5 yrs at 25°C in deionized water, but transformation products were not measured. Pearson and McConnell (1975)
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300 reported a TCA half-life of 0.8 yrs at 10°C and pH 8 in seawater, and indicated 1,1-DCE was the major product. Thus, the c u r r e n t information indicates abiotic TCA transformation results in two possible products, one from elimination and the ot her from hydrolysis: ko / C H 2 : C C 1 2
+ H ~ + C1
Elimination
CH~CCI3
(1) +2H~o - ' ~ C H 3 C O O H + 3H* + 3C1
Hydrolysis
where k = ke + kh
(2)
In this example, the overall rate constant k is assumed to equal the sum of the hydrolysis rate constant (kh) and the elimination rate constant (ke). In the long-term laboratory study reported here, the rate of abiotic transformation of low concentrations of TCA added to groundwater was measured in the laboratory at 20°C and neutral pH, conditions that more resemble an actual contaminated groundwater environment. MATERIALS AND METHODS Two studies were conducted. In the first, groundwater was obtained from a freshwater aquifer in San Jose, California, by pumping from an extraction well. The water was supplemented with TCA, and its fate under both biotic and abiotic conditions was monitored over several months. In the second study, a more defined media (BOD dilution water) was used to evaluate the biotic and abiotic fate of TCA over several months in order to obtain additional information to confirm or refute the results of the first study. In the first study, the groundwater was divided into three fractions, each of which was supplemented with about 1800 pg/L TCA. The first fraction was used without further addition. The second was supplemented with about 10mg/L each of acetone and 2-propanol, solvents that are sometimes found as groundwater contaminants and can serve as substrates for bacterial growth. The third fraction was also supplemented with acetone and 2-propanol, but in addition, 10 mg/L HgC12 was added to serve as a biological inhibitor. Each of the above fractions was added to a series of 240-mL bottles, which were then capped without headspace using teflon-coated seals. The bottles were kept at 20°C in the dark, and a set was periodically sacrificed over a 198-day period of analysis for acetone, 2-propanol, a range of halogenated volatile organics, and on occasion, pH and dissolved oxygen (DO). The organic analyses were conducted by a certified commercial laboratory (Stoner Laboratories, Santa Clara, California). Two gas chromatographic (GC) procedures were used for the chlorinated compounds: (1) purge-and-trap using a Hall electrolytic conductivity detector (HECD) and a 240cm × 2mm Carbopack B/1% SP-1000 column; (2) liquid-liquid extraction using pre-tested
301 i s o - o c t a n e a n d an e l e c t r o n c a p t u r e d e t e c t o r (ECD) with a 240cm x 2 m m C a r b o p a c k B / l % SP-1000 c o l u m n as well as a 180cm z 2 m m n - o c t a n e / P o r a s i l C column. A c e t o n e and 2-propanol were d e t e r m i n e d by distilling sample portions u s i n g a B a r r e t t distilling r e c e i v e r (Corning 3622). Distillate, collected in the r e c e i v i n g tube s u r r o u n d e d by cold packs, was examined by GC using a 240 cm x 2 m m C a r b o p a c k B/3% SP-1500 c o l u m n and a flame i o n i z a t i o n detector. The r e p o r t e d limit of d e t e c t i o n for a c e t o n e and 2-propanol was 20 l~g/L. In the second study, 60 ml s e r u m bottles were filled with BOD dilution w a t e r c o n t a i n i n g dissolved o x y g e n and b a c t e r i a l seed (APHA, 1980) to w h i c h a b o u t 130pg/L of T C A ( B a k e r A n a l y t i c a l , B a k e r Chem. Co.) was added. M e r c u r i c chloride (10 mg/L) was added to some samples to serve as abiotic controls. The samples were stored in the d a r k at 20°C for up to 380 days. In this study, a n a l y s e s were c o n d u c t e d at the S t a n f o r d U n i v e r s i t y W a t e r Q u a l i t y C o n t r o l R e s e a r c h L a b o r a t o r y , u s i n g t h r e e different m e t h o d s for c o n f i r m a t i o n of products. First, a modified liquid-liquid e x t r a c t i o n p r o c e d u r e (after H e n d e r s o n et al., 1976) was used. U s i n g glass syringes (Hamilton) and 20 G needles, isooct a n e (1.0mL) was added to a serum bottle with s i m u l t a n e o u s removal of water. After a d d i n g an i n t e r n a l s t a n d a r d ( b r o m o c h l o r o p r o p a n e ) , the bottles were v i g o r o u s l y mixed for 3 0 m i n on a s h a k e r table, 5 p L of the i s o o c t a n e was injected into a Grob i n j e c t o r and onto a 200cm x 2.5mm 10% s q u a l a n e C h r o m o s o r b W / A W c o l u m n in an i s o t h e r m a l (50°C) GC (Tr~cor) with an ECD. The second m e t h o d i n v o l v e d use of a 5-mL sample with a purge-and-trap system (Tekmar) c o n n e c t e d to a GC with c o l u m n similar to the above followed by a H E C D with C o u l s o n modification. The t h i r d m e t h o d involved closed-loop stripping followed by GC/mass s p e c t r o m e t r y ( F i n n i g a n 4000 with Incos d a t a system) as described by G r a y d o n et al. (1983). This p r o c e d u r e was used to confirm t r a n s f o r m a t i o n p r o d u c t identification.
TABLE 1 pH, and dissolved oxygen (DO) in TCA-amended groundwater samples (20°C) Day of Study
Control DO (mg/L)
0 9 16 23 30 38 44 51
8.0 7.9 7.0 6.4 7.1 2.7 7.0 5.7
Acetone/2-propanol pH
7.4 7.5 7.5 7.4 7.3 7.4 7.4
DO (mg/L)
pH
5.8
7.6 7.4 7.4 7.4 7.3 7.2 7.3 7.3
<0.1 <0.1 <0.1 <0.1 <0.1 <0.1
Acetone/2-propanol HgC12 DO (mg/L)
pH
3.7
7.5 6.6 6.5 6.5 6.5 6.6 6.6 6.7
5.1 1.8 1.5 3.4 6.2 3.5
302 [2
-¸
E Z o
P
6
z w 4 z (D cD z 0
/ '
~ '
) '
~
~
~ w i t
4'0 . . . . 80 . .
h0u1':gCI 2
~ - 120 ' ~--'
~ ~'160
~
~
'
'~ 200
TIME (days) Fig. 1. A c e t o n e (filled symbols) a n d 2-propanol (unfilled symbols) c o n c e n t r a t i o n s v e r s u s time for groundwater fractions; acetone and 2-propanol without HgC12 (m, O) a n d a c e t o n e / 2 - p r o p a n o l with HgC12 (o, ~).
RESULTS
Measurements of dissolved oxygen (DO) and pH during the first 51 days of the first study are given in Table 1. The initial DO concentration was lower in the samples with acetone and 2-propanol, probably resulting from organic biological oxidation during the few hours before measurement and I-IgC12 addition. Measureable DO concentrations remained in the first and third fractions, but decreased to zero in the biologically active second fraction supplemented with acetone and 2-propanol. The 2-propanol was removed rapidly in this second fraction, and acetone disappeared after about three months TABLE 2 S u m m a r y of statistical analysis for T C A p s e u d o first-order decay rate constant (k) in groundwater s a m p l e s (20°C)
Fraction
Number of data n
TCA zero-time intercept (~g/L) a
TCA average conc. (pg/L) b
k (yr 1)a
Control Acetone/isopropanol
19 19 19 57
1800 ± 140 1470 _+ 100 1410 + 130
1810 _+ 140 1450 + 170 1350 ± 130
-0.026 0.097 0.26 0.11
Acetone/isopropanol/HgCI
All d a t a (normalized)
2
;' + v a l u e s r e p r e s e n t 95% confidence i n t e r v a l s . b + v a l u e s represent one standard deviation.
± + ± ±
0.25 0.27 0.36 0.16
303
(Fig. 1). Since acetone and IPA were not consumed in fraction 3 (Fig. 1) and DO remained in these samples, inhibition of biological activity by the added HgC12 is indicated. TCA c o n c e n t r a t i o n as a function of time in the three fractions is shown in Fig. 2. The initial TCA concentrations, based upon the zero-time intercepts and 95% confidence intervals from a least-squares regression analysis of loge of concentrations (Table 2), indicates TCA c o n c e n t r a t i o n in the first fraction was significantly higher t han in the other two. The difference probably resulted from losses during the initial preparation of the samples for storage. Assuming th at TCA transformation can be modeled as a first-order reaction, the following equations were applied:
C/Co tl/2
= e k,
(3)
0.69/k
(4)
=
where C = c o n c e n t r a t i o n of TCA at time t; Co = initial TCA concentration; k = pseudo first-order r a t e constant, and tl/2 is the half life for TCA. The negative slope of the regression lines for IOge of TCA concent rat i on versus time would give the pseudo first-order decomposition rate constants. The calculated rate constants to g ethe r with 95% confidence intervals are listed in Table 2. The calculated decay rates for TCA are not significantly different t han zero for any of the samples. When data from the three fractions are normalized by division of c o n c e n t r a t i o n by the initial concentrations listed in Table 2 for the respective samples, the three data sets were found not to be significantly different from one another. These normalized data were then pooled to obtain better precision on the decay rate constant and its 95% confidence interval, resulting 2.40 -
2.00 <[ 0 t,-
1.60
-
U_
o z 0 I..< a: t-.z LLI 0 Z 0 £..)
1.20 -
0.80-
0.40 -
0.00 0
'
'4'o'
'
'
.
TiME
.
.
.
. . 120
.
.
.
. . 160
200
(days)
Fig. 2. TCA concentration versus time for three groundwater fractions; water only (Q), water plus acetone and 2-propanol (~>), and water plus acetone and 2-propanol with HgC12 (o).
304 TABLE 3 Concentrations of volatile organic compounds (except TCA and 1,1-DCE) measured in groundwater samples (20°C) over period of first study (pg/L)a
Freon Trichloroethylene Tetrachloroethylene Xylene + Ethylbenzene 1,1-DCA~
No. of analyses
Control
Acetone/ 2-propanol
Acetone/ 2-propanol/ HgC12
19 19 19 19 8
3.2 i 0.4 < 0.5 7.5 + 0.4 <5 11 _+ 1
2.9 i 0.3 < 0.5 6.5 _+ 0.5 <5 11 _+ 2
2.8 < 0.5 6.0 21 8
_+ 0.5 _+ 0.6 _+ 5 _+ 1
a _+ values represent standard deviation. bData were not obtained over first 120 days of study. i n v a l u e s a l s o l i s t e d i n T a b l e 2. T h e u p p e r b o u n d a r y f o r t h e d e c a y r a t e c o n s t a n t using the 95% confidence interval can be obtained from the pooled data with a s i n g l e - t a i l e d a n a l y s i s , y i e l d i n g a v a l u e o f 0.25 y r - l , c o r r e s p o n d i n g t o a m i n i mum half-life for abiotic transformation of TCA under the experimental cond i t i o n s u s e d h e r e o f 2.8 y r s . T h e a c t u a l h a l f - l i f e is l i k e l y t o b e l o n g e r . V o l a t i l e o r g a n i c c o m p o u n d s o t h e r t h a n T C A a n d 1,1-DCE w e r e f o u n d present in the original samples because the groundwater used was contamin a t e d ( T a b l e 3). H o w e v e r , t h e o n l y c o m p o u n d f o r w h i c h c o n c e n t r a t i o n c h a n g e w a s s t a t i s t i c a l l y s i g n i f i c a n t d u r i n g t h e c o u r s e o f t h e s t u d y w a s 1,1-DCE ( F i g . 3). T C A w a s t h e o n l y c h l o r i n a t e d c o m p o u n d p r e s e n t i n s u f f i c i e n t l y h i g h c o n -
40 =L UJ 0 I
50
h_
(D Z 0 F<
20
Z i,i (D Z 0 0 i
0
40
80 TIME
120
i
160
,
i
200
(days)
Fig. 3. 1,1-DCE concentration versus time for three groundwater fractions; water only (D), water plus acetone and 2-propanol (O), and water plus acetone and 2-propanol with HgC12 (o).
3O5 c e n t r a t i o n from w h i c h the i n c r e a s e in 1,1-DCE c o n c e n t r a t i o n could h a v e resulted. Thus, 1,1-DCE was assumed to be formed from abiotic d e h y d r o h a l o g e n a tion of TCA as r e p o r t e d by P e a r s o n and M c C o n n e l l (1975). T h e r a t e c o n s t a n t of TCA t r a n s f o r m a t i o n to 1,1-DCE is given by ke, w h i c h is assumed e q u i v a l e n t to the 1,1-DCE f o r m a t i o n r a t e constant: d[1,1-DCE] dt
=
ke [TCA]
(5)
B e c a u s e the TCA c o n c e n t r a t i o n d e c r e a s e d v e r y little (about 6%) d u r i n g the s t u d y period, the following e q u a t i o n was found a p p r o p r i a t e for d a t a analysis: [ 1 , 1 - D C E ] t - [1,1-DCE]0 [TCA]
=
(6)
ket
In o r d e r to d e t e r m i n e ko, the initial c o n c e n t r a t i o n of 1,1-DCE, [1,1-DCE]0, in e a c h sample was needed. This was o b t a i n e d by a least-squares e x t r a p o l a t i o n of the d a t a to time equals zero (Table 4). The d a t a sets were e a c h n o r m a l i z e d by s u b t r a c t i n g the initial 1,1-DCE v a l u e s from the m e a s u r e d 1,1-DCE c o n c e n t r a tions [1,1-DCE]t, and dividing by the a v e r a g e 1,1,1-TCA c o n c e n t r a t i o n , [TCA] (Table 2). A least-squares r e g r e s s i o n analysis of the left-hand side of eqn. (6) versus t was m a d e for e a c h d a t a set in o r d e r to d e t e r m i n e ke (Table 4). The ke v a l u e s differed from an a v e r a g e v a l u e by less t h a n 10% for samples with or w i t h o u t a n a c t i v e b a c t e r i a l population, dissolved oxygen, or HgC12, and t h u s the t r a n s f o r m a t i o n of TCA to 1,1-DCE appears to be abiotic. Since the results for the sets did not differ significantly from one a n o t h e r , the normalized d a t a were pooled to d e t e r m i n e ke with g r e a t e r precision, r e s u l t i n g in a v a l u e of 0.040 + 0.003yr 1 (Table 4). In o r d e r to confirm the finding t h a t 1,1-DCE was formed from dehydroc h l o r i n a t i o n of TCA, the second s t u d y was c o n d u c t e d using TCA-supplemented BOD d i l u t i o n w a t e r at pH 7 and i n c u b a t e d at 20°C. No 1,1-DCE was d e t e c t a b l e i n i t i a l l y in these samples. After 380 days of i n c u b a t i o n , 4 samples of dilution TABLE 4 Summary of statistical analysis for 1,1-DCE first-order formation rate constant (ke)~ in groundwater samples (20°C) Sample
Number of data n
1,1-DCE zero-time intercept (#g/L)
ke (yr- 1)
Control Acetone/2-propanol Acetone/2-propanol/HgC12 All data (normalized)
19 19 19 57
15.5 _+ 1.8 13.6 ± 2.6 12.3 ± 1.8
0.040 0.037 0.044 0.040
a ± values represent 95% confidence intervals.
+ 0.004 _+ 0.006 ± 0.006 _+ 0.003
306 water without HgC12 and 2 with HgC12 added were analyzed for a range of chlorinated products. TCA was found to decrease from an average initial value of 132 ttg/L to an average of 109 rig/L, and an average of 4.3 ttg/L of 1,1-DCE was formed. No other chlorinated products were detected by any of the three GC techniques used. All three procedures, including GC/MS, indicated the product formed was 1,1-DCE. The calculated rate constant, ke, for 1,1-DCE formation from TCA was 0.04 yr -1, which is in agreement with the previous study results. A mass balance for TCA and 1,1-DCE did not account for 74% of the TCA concentration decrease during the 380 days of incubation. This loss might be associated either with formation of decomposition products other than 1,1DCE, e.g., acetic acid (T. Mill, pers. commun., 1986), with analytical errors, or with loss of TCA and 1,1-DCE from the sample bottles during the long storage period. In any event, the mass balance is consistent with a half-life from TCA in excess of 3yrs under the experimental conditions used. This is also in agreement with the initial study results. DISCUSSION This study confirms that 1,1-DCE is formed from the abiotic dehydrochlorination of TCA in dilute aqueous solutions at neutral pH and 20°C, conditions common in groundwater environments. From the extension study of groundwaters in the United States (Westrick et al., 1984), 1,1-DCE was found in detectable concentrations about one-third as frequently as TCA. In addition, a co-occurrence study of these results indicated that TCA was also observed in 67% of the groundwater samples where 1,1-DCE was found (Price, 1985). This study indicates some of the 1,1-DCE detected may have resulted from TCA transformation. Although the formation rate of 1,1-DCE is relatively low, residence times of groundwaters are relatively long. Within a few years, significant concentrations of 1,1-DCE could result from TCA abiotic transformation. 1,1-DCE might not be the only product resulting from abiotic transformation of TCA. While no other confirmed products were detected in this study, analyses were not conducted for acetic acid, which has been reported as a product (Britton and Reed, 1932; Walraevens et al., 1974; Mabey et al., 1983). If 1,1-DCE were the only product of abiotic TCA transformation under the conditions studied, then the TCA half-life could be inferred from the measured constant ke. This would be 17 yrs based on an average ke value of 0.040 yr- i. Considering the 95% confidence intervals for the pooled 1,1-DCE data, the shortest half-life would be 16 yrs and the longest 19 yrs. If TCA decomposed to form products in addition to 1,1-DCE, such as acetic acid (eqn 1), then the overall rate of TCA decomposition would be greater than that given by ke alone. The TCA concentration measurements and time scale of the experiments were not adequate to obtain a precise direct measurement of TCA loss. However, they were sufficient to put bounds on the rate. Using the outer bound indicating the most rapid decomposition rate at the 95% confidence limit for the pooled data, a minimum half-life for TCA under the experimental conditions studied would be 2.8 yrs.
307 A t the o t h e r extreme, if 1,1-DCE were the o n l y product, t h e n the m a x i m u m half-life for T C A w o u l d be i n d i c a t e d by the 95% confidence i n t e r v a l a s s o c i a t e d with the m i n i m u m f o r m a t i o n r a t e of 1,1-DCE, This half-life is 19 yrs. Thus, the half-life for abiotic d e c o m p o s i t i o n of TCA from this s t u d y lies b e t w e e n 2.8 and 19 yrs in dilute a q u e o u s s o l u t i o n s at n e u t r a l pH and 20°C. The T C A half-life r e p r e s e n t e d by the r a n g e d e t e r m i n e d in this s t u d y is l o n g e r t h a n the half-life of 1.4 yrs t h a t has been estimated (at 20°C) from e x t r a p o l a t i o n of h i g h e r - t e m p e r a t u r e e x p e r i m e n t a l d a t a (Mabey et al., 1983), and is also c o n s i d e r a b l y h i g h e r t h a n v a l u e s of 0.5 yrs at 25°C in distilled w a t e r r e p o r t e d by Dilling et al. (1975) and 0 . 8 y r s in sea w a t e r at 10°C and pH 8 r e p o r t e d by P e a r s o n and M c C o n n e l l (1975). R e a s o n s for the d i s c r e p a n c i e s are n o t evident a l t h o u g h e x p e r i m e n t a l p r o c e d u r e s differed significantly. In addition, the abiotic t r a n s f o r m a t i o n r a t e s in g r o u n d w a t e r e n v i r o n m e n t s m a y differ from those m e a s u r e d in h o m o g e n e o u s solutions. Little is k n o w n a b o u t the effects of sorption on m i n e r a l surfaces on t r a n s f o r m a t i o n rates. While sufficiently precise k i n e t i c m e a s u r e m e n t s are difficult to m a k e for r e a c t i o n s t h a t are very slow, it is n e v e r t h e l e s s i m p o r t a n t t h a t t h e y be o b t a i n e d for e n v i r o n m e n t a l c o n d i t i o n s of i m p o r t a n c e . These d a t a are n e c e s s a r y in o r d e r to u n d e r s t a n d the dimensions of a g r o u n d w a t e r c o n t a m i n a t i o n problem, and to develop t e c h n i c a l l y and e c o n o m i c a l l y s o u n d c o n t r o l strategies. REFERENCES American Public Health Association (APHA), 1980. Standard Methods for Examination of Water and Wastewater 15th ed. Washington, D.C. pp. 1134. Bouwer, E.J. and McCarty, P.L. 1983. Transformations of 1- and 2-carbon halogenated aliphatic organic compounds under methanogenic conditions. Appl. Environ. Microbiol., 45: 1286--1294. Britton, E.C. and Reed, W.R. 1932. Hydrolysis of 1,1,1-trichloroethane. Chem. Abstr., 26: 5578. Dilling, W.L., Tefertiller, N.B. and Kallos, G.J., 1975. Evaporation rates and reactivities of methylene chloride, chloroform, 1,1,1-trichloroethane, trichloroethylene, tetrachloroethylene, and other chlorinated compounds in dilute aqueous solutions. Environ. Sci. Technol., 9:833 838. Gossett, J.M., 1985. Anaerobic degradation of C1 and C2 chlorinated hydrocarbons. Air Force Engineering and Services Center. ESL-TR-85-38.Final Report. Graydon, J.W., Grob, K., Zuercher, F. and Giger, W., 1983. Determination of highly volatile organic water contaminants by the closed-loop gaseous stripping technique followed by thermal desorption of the activated carbon filters. J. Chromatogr., 285: 307-318. Henderson, J.E., Peyton, G.R., and Glaze, W.H., 1976. A convenient liquid-liquid extraction method for the determination of halomethanes in water at the parts-per-billion level. In: L.H. Keith (Editor), Identification and Analysis of Organic Pollutants in Water. Ann Arbor Science Publishers, Inc., Ann Arbor, MI, pp. 105-112. Mabey, W.R., Barich, V. and Mill, T., 1983. Hydrolysis of polychlorinated ethanes. Presented, Division of Environmental Chemistry, 186th Annu. Mtg. of Am. Chem. Soc., Washington, D.C., September 1983. Parsons, F. and Lage, G.B., 1985. Chlorinated organics in simulated groundwater environments. J. Am. Water Works Assoc., 77: 5, 52-59. Pearson, C.R. and McConnell, G., 1975. Chlorinated C~and C~hydrocarbons in the marine environment. Proc. R. Soc. London, Ser. B., 189:305 332. Price, P.S., 1985. VOC Degradation. Memo of U.S. Environmental Protection Agency, Office of Water, Washington, D.C. August 21, 1985.
308 Walraevens, R., Trouillet, P. and Devos, A., 1974. Basic elimination of HC1 from chlorinated ethanes. Int. J. Chem. Kinet. 6: 777-786. Westrick, J.J., Mello, J.W. and Thomas, R.F., 1984. The groundwater supply survey. J. Am. Water Works Assoc., 76: 5, 52-59.
299
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
RATE OF ABIOTIC FORMATION OF 1,1-DICHLOROETHYLENE FROM 1,1,1-TRICHLOROETHANE IN GROUNDWATER
TIMOTHY M. VOGEL and PERRY L. McCARTY
Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, CA 94305, U.S.A. (Received May 23, 1986; revised and accepted September 16, 1986)
ABSTRACT Vogel, T.M. and McCarty, P.L. 1987. Rate of abiotic formation of 1,1-dichloroethylene from 1,1,1-trichloroethane in groundwater. J. Contam. Hydrol., 1: 299-308. The abiotic transformation of the halogenated aliphatic compound, 1,1,1-trichloroethane (TCA), to 1,1-dichloroethylene (1,1-DCE) in groundwater was measured in the laboratory at 20°C and neutral pH. The measured pseudo first-order disappearance rate constant for TCA was 0.11 + 0.16yr -1 (95% confidence interval). The formation rate constant for 1,1-DCE could be determined with greater precision, and was found to be 0.040 + 0.003yr ~. These results indicate the 95% confidence interval for TCA half-life at 20°C and pH 7 in homogeneous solution is 2.8 to 19yrs. While these results confirm that 1,1-DCE is a product from the abiotic transformation of TCA, other products such as acetic acid are also possible, although they were not measured. INTRODUCTION
The solvent 1,1,1-trichloroethane (TCA) is one of the most common contaminants in groundwaters used as a source of drinking water supply in the United States (Westrick et al., 1984). Since groundwater moves slowly and residence times are in the order of years to centuries, the eventual fate of TCA in g ro u n d water is of environmental interest. Under reducing environmental conditions, TCA can be biologically converted by reductive dehalogenation (Bouwer and McCarty, 1983; Gossett, 1985; Parsons and Lage, 1985), with 1,1-dichloroethane (1,1-DCA) being formed as an intermediate product (Gossett, 1985; Parsons and Lage, 1985). The abiotic transformation of TCA in water has also been reported. Several abiotic studies carried out at elevated temperatures (from 55° to 160°C under pressure) indicated TCA was transformed to acetic acid (Britton and Reed, 1932; Walraevens et al., 1974; Mabey et al., 1983). Rates of abiotic TCA transformation at lower temperatures were estimated by extrapolation. Such extrapolation has resulted in suggested half-lives for TCA at neutral pH and 25, 20, and 10°C of 0.6, 1.4, and 7 yrs, respectively (Mabey et al., 1983). Two studies were carried out at temperatures more characteristic of groundwaters. Dilling et al. (1975) reported a TCA half-life of 0.5 yrs at 25°C in deionized water, but transformation products were not measured. Pearson and McConnell (1975)
0169-7722/87/$03.50
~ 1987 Elsevier Science Publishers B.V.
300 reported a TCA half-life of 0.8 yrs at 10°C and pH 8 in seawater, and indicated 1,1-DCE was the major product. Thus, the c u r r e n t information indicates abiotic TCA transformation results in two possible products, one from elimination and the ot her from hydrolysis: ko / C H 2 : C C 1 2
+ H ~ + C1
Elimination
CH~CCI3
(1) +2H~o - ' ~ C H 3 C O O H + 3H* + 3C1
Hydrolysis
where k = ke + kh
(2)
In this example, the overall rate constant k is assumed to equal the sum of the hydrolysis rate constant (kh) and the elimination rate constant (ke). In the long-term laboratory study reported here, the rate of abiotic transformation of low concentrations of TCA added to groundwater was measured in the laboratory at 20°C and neutral pH, conditions that more resemble an actual contaminated groundwater environment. MATERIALS AND METHODS Two studies were conducted. In the first, groundwater was obtained from a freshwater aquifer in San Jose, California, by pumping from an extraction well. The water was supplemented with TCA, and its fate under both biotic and abiotic conditions was monitored over several months. In the second study, a more defined media (BOD dilution water) was used to evaluate the biotic and abiotic fate of TCA over several months in order to obtain additional information to confirm or refute the results of the first study. In the first study, the groundwater was divided into three fractions, each of which was supplemented with about 1800 pg/L TCA. The first fraction was used without further addition. The second was supplemented with about 10mg/L each of acetone and 2-propanol, solvents that are sometimes found as groundwater contaminants and can serve as substrates for bacterial growth. The third fraction was also supplemented with acetone and 2-propanol, but in addition, 10 mg/L HgC12 was added to serve as a biological inhibitor. Each of the above fractions was added to a series of 240-mL bottles, which were then capped without headspace using teflon-coated seals. The bottles were kept at 20°C in the dark, and a set was periodically sacrificed over a 198-day period of analysis for acetone, 2-propanol, a range of halogenated volatile organics, and on occasion, pH and dissolved oxygen (DO). The organic analyses were conducted by a certified commercial laboratory (Stoner Laboratories, Santa Clara, California). Two gas chromatographic (GC) procedures were used for the chlorinated compounds: (1) purge-and-trap using a Hall electrolytic conductivity detector (HECD) and a 240cm × 2mm Carbopack B/1% SP-1000 column; (2) liquid-liquid extraction using pre-tested
301 i s o - o c t a n e a n d an e l e c t r o n c a p t u r e d e t e c t o r (ECD) with a 240cm x 2 m m C a r b o p a c k B / l % SP-1000 c o l u m n as well as a 180cm z 2 m m n - o c t a n e / P o r a s i l C column. A c e t o n e and 2-propanol were d e t e r m i n e d by distilling sample portions u s i n g a B a r r e t t distilling r e c e i v e r (Corning 3622). Distillate, collected in the r e c e i v i n g tube s u r r o u n d e d by cold packs, was examined by GC using a 240 cm x 2 m m C a r b o p a c k B/3% SP-1500 c o l u m n and a flame i o n i z a t i o n detector. The r e p o r t e d limit of d e t e c t i o n for a c e t o n e and 2-propanol was 20 l~g/L. In the second study, 60 ml s e r u m bottles were filled with BOD dilution w a t e r c o n t a i n i n g dissolved o x y g e n and b a c t e r i a l seed (APHA, 1980) to w h i c h a b o u t 130pg/L of T C A ( B a k e r A n a l y t i c a l , B a k e r Chem. Co.) was added. M e r c u r i c chloride (10 mg/L) was added to some samples to serve as abiotic controls. The samples were stored in the d a r k at 20°C for up to 380 days. In this study, a n a l y s e s were c o n d u c t e d at the S t a n f o r d U n i v e r s i t y W a t e r Q u a l i t y C o n t r o l R e s e a r c h L a b o r a t o r y , u s i n g t h r e e different m e t h o d s for c o n f i r m a t i o n of products. First, a modified liquid-liquid e x t r a c t i o n p r o c e d u r e (after H e n d e r s o n et al., 1976) was used. U s i n g glass syringes (Hamilton) and 20 G needles, isooct a n e (1.0mL) was added to a serum bottle with s i m u l t a n e o u s removal of water. After a d d i n g an i n t e r n a l s t a n d a r d ( b r o m o c h l o r o p r o p a n e ) , the bottles were v i g o r o u s l y mixed for 3 0 m i n on a s h a k e r table, 5 p L of the i s o o c t a n e was injected into a Grob i n j e c t o r and onto a 200cm x 2.5mm 10% s q u a l a n e C h r o m o s o r b W / A W c o l u m n in an i s o t h e r m a l (50°C) GC (Tr~cor) with an ECD. The second m e t h o d i n v o l v e d use of a 5-mL sample with a purge-and-trap system (Tekmar) c o n n e c t e d to a GC with c o l u m n similar to the above followed by a H E C D with C o u l s o n modification. The t h i r d m e t h o d involved closed-loop stripping followed by GC/mass s p e c t r o m e t r y ( F i n n i g a n 4000 with Incos d a t a system) as described by G r a y d o n et al. (1983). This p r o c e d u r e was used to confirm t r a n s f o r m a t i o n p r o d u c t identification.
TABLE 1 pH, and dissolved oxygen (DO) in TCA-amended groundwater samples (20°C) Day of Study
Control DO (mg/L)
0 9 16 23 30 38 44 51
8.0 7.9 7.0 6.4 7.1 2.7 7.0 5.7
Acetone/2-propanol pH
7.4 7.5 7.5 7.4 7.3 7.4 7.4
DO (mg/L)
pH
5.8
7.6 7.4 7.4 7.4 7.3 7.2 7.3 7.3
<0.1 <0.1 <0.1 <0.1 <0.1 <0.1
Acetone/2-propanol HgC12 DO (mg/L)
pH
3.7
7.5 6.6 6.5 6.5 6.5 6.6 6.6 6.7
5.1 1.8 1.5 3.4 6.2 3.5
302 [2
-¸
E Z o
P
6
z w 4 z (D cD z 0
/ '
~ '
) '
~
~
~ w i t
4'0 . . . . 80 . .
h0u1':gCI 2
~ - 120 ' ~--'
~ ~'160
~
~
'
'~ 200
TIME (days) Fig. 1. A c e t o n e (filled symbols) a n d 2-propanol (unfilled symbols) c o n c e n t r a t i o n s v e r s u s time for groundwater fractions; acetone and 2-propanol without HgC12 (m, O) a n d a c e t o n e / 2 - p r o p a n o l with HgC12 (o, ~).
RESULTS
Measurements of dissolved oxygen (DO) and pH during the first 51 days of the first study are given in Table 1. The initial DO concentration was lower in the samples with acetone and 2-propanol, probably resulting from organic biological oxidation during the few hours before measurement and I-IgC12 addition. Measureable DO concentrations remained in the first and third fractions, but decreased to zero in the biologically active second fraction supplemented with acetone and 2-propanol. The 2-propanol was removed rapidly in this second fraction, and acetone disappeared after about three months TABLE 2 S u m m a r y of statistical analysis for T C A p s e u d o first-order decay rate constant (k) in groundwater s a m p l e s (20°C)
Fraction
Number of data n
TCA zero-time intercept (~g/L) a
TCA average conc. (pg/L) b
k (yr 1)a
Control Acetone/isopropanol
19 19 19 57
1800 ± 140 1470 _+ 100 1410 + 130
1810 _+ 140 1450 + 170 1350 ± 130
-0.026 0.097 0.26 0.11
Acetone/isopropanol/HgCI
All d a t a (normalized)
2
;' + v a l u e s r e p r e s e n t 95% confidence i n t e r v a l s . b + v a l u e s represent one standard deviation.
± + ± ±
0.25 0.27 0.36 0.16
303
(Fig. 1). Since acetone and IPA were not consumed in fraction 3 (Fig. 1) and DO remained in these samples, inhibition of biological activity by the added HgC12 is indicated. TCA c o n c e n t r a t i o n as a function of time in the three fractions is shown in Fig. 2. The initial TCA concentrations, based upon the zero-time intercepts and 95% confidence intervals from a least-squares regression analysis of loge of concentrations (Table 2), indicates TCA c o n c e n t r a t i o n in the first fraction was significantly higher t han in the other two. The difference probably resulted from losses during the initial preparation of the samples for storage. Assuming th at TCA transformation can be modeled as a first-order reaction, the following equations were applied:
C/Co tl/2
= e k,
(3)
0.69/k
(4)
=
where C = c o n c e n t r a t i o n of TCA at time t; Co = initial TCA concentration; k = pseudo first-order r a t e constant, and tl/2 is the half life for TCA. The negative slope of the regression lines for IOge of TCA concent rat i on versus time would give the pseudo first-order decomposition rate constants. The calculated rate constants to g ethe r with 95% confidence intervals are listed in Table 2. The calculated decay rates for TCA are not significantly different t han zero for any of the samples. When data from the three fractions are normalized by division of c o n c e n t r a t i o n by the initial concentrations listed in Table 2 for the respective samples, the three data sets were found not to be significantly different from one another. These normalized data were then pooled to obtain better precision on the decay rate constant and its 95% confidence interval, resulting 2.40 -
2.00 <[ 0 t,-
1.60
-
U_
o z 0 I..< a: t-.z LLI 0 Z 0 £..)
1.20 -
0.80-
0.40 -
0.00 0
'
'4'o'
'
'
.
TiME
.
.
.
. . 120
.
.
.
. . 160
200
(days)
Fig. 2. TCA concentration versus time for three groundwater fractions; water only (Q), water plus acetone and 2-propanol (~>), and water plus acetone and 2-propanol with HgC12 (o).
304 TABLE 3 Concentrations of volatile organic compounds (except TCA and 1,1-DCE) measured in groundwater samples (20°C) over period of first study (pg/L)a
Freon Trichloroethylene Tetrachloroethylene Xylene + Ethylbenzene 1,1-DCA~
No. of analyses
Control
Acetone/ 2-propanol
Acetone/ 2-propanol/ HgC12
19 19 19 19 8
3.2 i 0.4 < 0.5 7.5 + 0.4 <5 11 _+ 1
2.9 i 0.3 < 0.5 6.5 _+ 0.5 <5 11 _+ 2
2.8 < 0.5 6.0 21 8
_+ 0.5 _+ 0.6 _+ 5 _+ 1
a _+ values represent standard deviation. bData were not obtained over first 120 days of study. i n v a l u e s a l s o l i s t e d i n T a b l e 2. T h e u p p e r b o u n d a r y f o r t h e d e c a y r a t e c o n s t a n t using the 95% confidence interval can be obtained from the pooled data with a s i n g l e - t a i l e d a n a l y s i s , y i e l d i n g a v a l u e o f 0.25 y r - l , c o r r e s p o n d i n g t o a m i n i mum half-life for abiotic transformation of TCA under the experimental cond i t i o n s u s e d h e r e o f 2.8 y r s . T h e a c t u a l h a l f - l i f e is l i k e l y t o b e l o n g e r . V o l a t i l e o r g a n i c c o m p o u n d s o t h e r t h a n T C A a n d 1,1-DCE w e r e f o u n d present in the original samples because the groundwater used was contamin a t e d ( T a b l e 3). H o w e v e r , t h e o n l y c o m p o u n d f o r w h i c h c o n c e n t r a t i o n c h a n g e w a s s t a t i s t i c a l l y s i g n i f i c a n t d u r i n g t h e c o u r s e o f t h e s t u d y w a s 1,1-DCE ( F i g . 3). T C A w a s t h e o n l y c h l o r i n a t e d c o m p o u n d p r e s e n t i n s u f f i c i e n t l y h i g h c o n -
40 =L UJ 0 I
50
h_
(D Z 0 F<
20
Z i,i (D Z 0 0 i
0
40
80 TIME
120
i
160
,
i
200
(days)
Fig. 3. 1,1-DCE concentration versus time for three groundwater fractions; water only (D), water plus acetone and 2-propanol (O), and water plus acetone and 2-propanol with HgC12 (o).
3O5 c e n t r a t i o n from w h i c h the i n c r e a s e in 1,1-DCE c o n c e n t r a t i o n could h a v e resulted. Thus, 1,1-DCE was assumed to be formed from abiotic d e h y d r o h a l o g e n a tion of TCA as r e p o r t e d by P e a r s o n and M c C o n n e l l (1975). T h e r a t e c o n s t a n t of TCA t r a n s f o r m a t i o n to 1,1-DCE is given by ke, w h i c h is assumed e q u i v a l e n t to the 1,1-DCE f o r m a t i o n r a t e constant: d[1,1-DCE] dt
=
ke [TCA]
(5)
B e c a u s e the TCA c o n c e n t r a t i o n d e c r e a s e d v e r y little (about 6%) d u r i n g the s t u d y period, the following e q u a t i o n was found a p p r o p r i a t e for d a t a analysis: [ 1 , 1 - D C E ] t - [1,1-DCE]0 [TCA]
=
(6)
ket
In o r d e r to d e t e r m i n e ko, the initial c o n c e n t r a t i o n of 1,1-DCE, [1,1-DCE]0, in e a c h sample was needed. This was o b t a i n e d by a least-squares e x t r a p o l a t i o n of the d a t a to time equals zero (Table 4). The d a t a sets were e a c h n o r m a l i z e d by s u b t r a c t i n g the initial 1,1-DCE v a l u e s from the m e a s u r e d 1,1-DCE c o n c e n t r a tions [1,1-DCE]t, and dividing by the a v e r a g e 1,1,1-TCA c o n c e n t r a t i o n , [TCA] (Table 2). A least-squares r e g r e s s i o n analysis of the left-hand side of eqn. (6) versus t was m a d e for e a c h d a t a set in o r d e r to d e t e r m i n e ke (Table 4). The ke v a l u e s differed from an a v e r a g e v a l u e by less t h a n 10% for samples with or w i t h o u t a n a c t i v e b a c t e r i a l population, dissolved oxygen, or HgC12, and t h u s the t r a n s f o r m a t i o n of TCA to 1,1-DCE appears to be abiotic. Since the results for the sets did not differ significantly from one a n o t h e r , the normalized d a t a were pooled to d e t e r m i n e ke with g r e a t e r precision, r e s u l t i n g in a v a l u e of 0.040 + 0.003yr 1 (Table 4). In o r d e r to confirm the finding t h a t 1,1-DCE was formed from dehydroc h l o r i n a t i o n of TCA, the second s t u d y was c o n d u c t e d using TCA-supplemented BOD d i l u t i o n w a t e r at pH 7 and i n c u b a t e d at 20°C. No 1,1-DCE was d e t e c t a b l e i n i t i a l l y in these samples. After 380 days of i n c u b a t i o n , 4 samples of dilution TABLE 4 Summary of statistical analysis for 1,1-DCE first-order formation rate constant (ke)~ in groundwater samples (20°C) Sample
Number of data n
1,1-DCE zero-time intercept (#g/L)
ke (yr- 1)
Control Acetone/2-propanol Acetone/2-propanol/HgC12 All data (normalized)
19 19 19 57
15.5 _+ 1.8 13.6 ± 2.6 12.3 ± 1.8
0.040 0.037 0.044 0.040
a ± values represent 95% confidence intervals.
+ 0.004 _+ 0.006 ± 0.006 _+ 0.003
306 water without HgC12 and 2 with HgC12 added were analyzed for a range of chlorinated products. TCA was found to decrease from an average initial value of 132 ttg/L to an average of 109 rig/L, and an average of 4.3 ttg/L of 1,1-DCE was formed. No other chlorinated products were detected by any of the three GC techniques used. All three procedures, including GC/MS, indicated the product formed was 1,1-DCE. The calculated rate constant, ke, for 1,1-DCE formation from TCA was 0.04 yr -1, which is in agreement with the previous study results. A mass balance for TCA and 1,1-DCE did not account for 74% of the TCA concentration decrease during the 380 days of incubation. This loss might be associated either with formation of decomposition products other than 1,1DCE, e.g., acetic acid (T. Mill, pers. commun., 1986), with analytical errors, or with loss of TCA and 1,1-DCE from the sample bottles during the long storage period. In any event, the mass balance is consistent with a half-life from TCA in excess of 3yrs under the experimental conditions used. This is also in agreement with the initial study results. DISCUSSION This study confirms that 1,1-DCE is formed from the abiotic dehydrochlorination of TCA in dilute aqueous solutions at neutral pH and 20°C, conditions common in groundwater environments. From the extension study of groundwaters in the United States (Westrick et al., 1984), 1,1-DCE was found in detectable concentrations about one-third as frequently as TCA. In addition, a co-occurrence study of these results indicated that TCA was also observed in 67% of the groundwater samples where 1,1-DCE was found (Price, 1985). This study indicates some of the 1,1-DCE detected may have resulted from TCA transformation. Although the formation rate of 1,1-DCE is relatively low, residence times of groundwaters are relatively long. Within a few years, significant concentrations of 1,1-DCE could result from TCA abiotic transformation. 1,1-DCE might not be the only product resulting from abiotic transformation of TCA. While no other confirmed products were detected in this study, analyses were not conducted for acetic acid, which has been reported as a product (Britton and Reed, 1932; Walraevens et al., 1974; Mabey et al., 1983). If 1,1-DCE were the only product of abiotic TCA transformation under the conditions studied, then the TCA half-life could be inferred from the measured constant ke. This would be 17 yrs based on an average ke value of 0.040 yr- i. Considering the 95% confidence intervals for the pooled 1,1-DCE data, the shortest half-life would be 16 yrs and the longest 19 yrs. If TCA decomposed to form products in addition to 1,1-DCE, such as acetic acid (eqn 1), then the overall rate of TCA decomposition would be greater than that given by ke alone. The TCA concentration measurements and time scale of the experiments were not adequate to obtain a precise direct measurement of TCA loss. However, they were sufficient to put bounds on the rate. Using the outer bound indicating the most rapid decomposition rate at the 95% confidence limit for the pooled data, a minimum half-life for TCA under the experimental conditions studied would be 2.8 yrs.
307 A t the o t h e r extreme, if 1,1-DCE were the o n l y product, t h e n the m a x i m u m half-life for T C A w o u l d be i n d i c a t e d by the 95% confidence i n t e r v a l a s s o c i a t e d with the m i n i m u m f o r m a t i o n r a t e of 1,1-DCE, This half-life is 19 yrs. Thus, the half-life for abiotic d e c o m p o s i t i o n of TCA from this s t u d y lies b e t w e e n 2.8 and 19 yrs in dilute a q u e o u s s o l u t i o n s at n e u t r a l pH and 20°C. The T C A half-life r e p r e s e n t e d by the r a n g e d e t e r m i n e d in this s t u d y is l o n g e r t h a n the half-life of 1.4 yrs t h a t has been estimated (at 20°C) from e x t r a p o l a t i o n of h i g h e r - t e m p e r a t u r e e x p e r i m e n t a l d a t a (Mabey et al., 1983), and is also c o n s i d e r a b l y h i g h e r t h a n v a l u e s of 0.5 yrs at 25°C in distilled w a t e r r e p o r t e d by Dilling et al. (1975) and 0 . 8 y r s in sea w a t e r at 10°C and pH 8 r e p o r t e d by P e a r s o n and M c C o n n e l l (1975). R e a s o n s for the d i s c r e p a n c i e s are n o t evident a l t h o u g h e x p e r i m e n t a l p r o c e d u r e s differed significantly. In addition, the abiotic t r a n s f o r m a t i o n r a t e s in g r o u n d w a t e r e n v i r o n m e n t s m a y differ from those m e a s u r e d in h o m o g e n e o u s solutions. Little is k n o w n a b o u t the effects of sorption on m i n e r a l surfaces on t r a n s f o r m a t i o n rates. While sufficiently precise k i n e t i c m e a s u r e m e n t s are difficult to m a k e for r e a c t i o n s t h a t are very slow, it is n e v e r t h e l e s s i m p o r t a n t t h a t t h e y be o b t a i n e d for e n v i r o n m e n t a l c o n d i t i o n s of i m p o r t a n c e . These d a t a are n e c e s s a r y in o r d e r to u n d e r s t a n d the dimensions of a g r o u n d w a t e r c o n t a m i n a t i o n problem, and to develop t e c h n i c a l l y and e c o n o m i c a l l y s o u n d c o n t r o l strategies. REFERENCES American Public Health Association (APHA), 1980. Standard Methods for Examination of Water and Wastewater 15th ed. Washington, D.C. pp. 1134. Bouwer, E.J. and McCarty, P.L. 1983. Transformations of 1- and 2-carbon halogenated aliphatic organic compounds under methanogenic conditions. Appl. Environ. Microbiol., 45: 1286--1294. Britton, E.C. and Reed, W.R. 1932. Hydrolysis of 1,1,1-trichloroethane. Chem. Abstr., 26: 5578. Dilling, W.L., Tefertiller, N.B. and Kallos, G.J., 1975. Evaporation rates and reactivities of methylene chloride, chloroform, 1,1,1-trichloroethane, trichloroethylene, tetrachloroethylene, and other chlorinated compounds in dilute aqueous solutions. Environ. Sci. Technol., 9:833 838. Gossett, J.M., 1985. Anaerobic degradation of C1 and C2 chlorinated hydrocarbons. Air Force Engineering and Services Center. ESL-TR-85-38.Final Report. Graydon, J.W., Grob, K., Zuercher, F. and Giger, W., 1983. Determination of highly volatile organic water contaminants by the closed-loop gaseous stripping technique followed by thermal desorption of the activated carbon filters. J. Chromatogr., 285: 307-318. Henderson, J.E., Peyton, G.R., and Glaze, W.H., 1976. A convenient liquid-liquid extraction method for the determination of halomethanes in water at the parts-per-billion level. In: L.H. Keith (Editor), Identification and Analysis of Organic Pollutants in Water. Ann Arbor Science Publishers, Inc., Ann Arbor, MI, pp. 105-112. Mabey, W.R., Barich, V. and Mill, T., 1983. Hydrolysis of polychlorinated ethanes. Presented, Division of Environmental Chemistry, 186th Annu. Mtg. of Am. Chem. Soc., Washington, D.C., September 1983. Parsons, F. and Lage, G.B., 1985. Chlorinated organics in simulated groundwater environments. J. Am. Water Works Assoc., 77: 5, 52-59. Pearson, C.R. and McConnell, G., 1975. Chlorinated C~and C~hydrocarbons in the marine environment. Proc. R. Soc. London, Ser. B., 189:305 332. Price, P.S., 1985. VOC Degradation. Memo of U.S. Environmental Protection Agency, Office of Water, Washington, D.C. August 21, 1985.
308 Walraevens, R., Trouillet, P. and Devos, A., 1974. Basic elimination of HC1 from chlorinated ethanes. Int. J. Chem. Kinet. 6: 777-786. Westrick, J.J., Mello, J.W. and Thomas, R.F., 1984. The groundwater supply survey. J. Am. Water Works Assoc., 76: 5, 52-59.