Environ. Sci. Technol. 2000, 34, 812-818
Degradation as a Loss Mechanism in the Fate of r-Hexachlorocyclohexane in Arctic Watersheds P A U L A . H E L M , † M I R I A M L . D I A M O N D , * ,‡ RAY SEMKIN,§ AND TERRY F. BIDLEMAN# Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5 Canada, and Department of Geography, University of Toronto, 100 St. George Street, Toronto, Ontario, M5S 3G3 Canada, and National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario, L7R 4A6 Canada, and Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4 Canada
Water extracts of samples collected from Amituk Lake in July-August, 1994 and samples collected at Char and Meretta Lakes in July 1997 were analyzed for enantiomers and concentrations of R-HCH to estimate the extent of biodegradation in watersheds in the Canadian High Arctic. (+)/(-)-R-HCH enantiomer ratios (ERs) in three streams entering Amituk Lake ranged from racemic values of 1.01 in snow to 0.36 in meltwater. Lower ERs were promoted by warmer temperatures and increased contact with stream substrates during low streamflows, especially biologically productive substrates. Most R-HCH degradation occurred during peak runoff when ERs were 0.95-0.80, rather than later in summer when ERs reached their minimum. Approximately 7% of R-HCH in the Amituk Lake basin was enantioselectively degraded prior to entering the lake. ERs within Amituk Lake are controlled by meltwater inputs rather than within lake degradation and clearly illustrate the riverine-like nature of high arctic lakes. Differences in lake R-HCH inventory from end of summer 1993 to spring 1994 indicate that from 33 to 61% of R-HCH within the lake may have been lost via nonenantioselective microbial degradation at a rate ranging from 0.48 to 1.13 y-1.
Introduction Contamination of Canadian arctic ecosystems with industrial and agricultural chemicals has been well documented for air, water, sediments, and biota (1) and is attributed to longrange transport through air and ocean currents (1-3). Isomers of hexachlorocyclohexane (HCH) are widespread global pollutants (4), and the R-HCH isomer is the most abundant organochlorine in arctic air and surface waters. * Corresponding author phone: (416)978-1586; fax: (416)946-3886; e-mail:
[email protected]. † Chemical Engineering and Applied Chemistry, University of Toronto. ‡ Department of Geography, University of Toronto. § National Water Research Institute, Canada Centre for Inland Waters. # Atmospheric Environment Service. 812
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Contaminant fate processes have been studied in some detail in the arctic marine environment over the past few years. Declining atmospheric R-HCH concentrations in recent years (5) have resulted in the Arctic Ocean becoming a source rather than a sink for R-HCH (6, 7). In addition to air-water exchange, sedimentation and degradation are possible loss mechanisms from the water column. Sedimentation of R-HCH was found to be negligible in the Arctic Ocean (1, 8), while degradation has been observed (7, 9-11) and may be the most important loss process (11). R-HCH is a chiral compound, meaning it has two structural enantiomers that are nonsuperimposable mirror images. Separation of these enantiomers by gas chromatography and subsequent calculation of the area ratio of the (+)- and (-)- enantiomers (ER) allows for differentiation between biotic and abiotic transformation processes (4, 12). Biotic processes such as microbial degradation or biological uptake may be enantioselective, causing the observed ER to vary from the racemic value of 1.0 found in technical mixtures. Abiotic processes such as hydrolysis and photolysis are not enantioselective. Enantiomeric excesses of R-HCH have been found to occur at low concentrations in marine and freshwater environments. ER values in the North Sea ranged from 0.80 to 1.19 with variations attributed to differing microbial communities (12, 13). In arctic seawater, ER reversals occurred between the Bering and Chukchi Seas (ER > 1.00) and the Arctic Ocean (ER < 1.00) (7), and ERs decreased with depth in the Arctic Ocean (7, 10, 11). Enantioselectivity has been reported for R-HCH in freshwaters in the Yukon and Mackenzie River basins in the Canadian subArctic (14, 15) and on Cornwallis Island in the Canadian high Arctic (16). Recent studies have used ERs to investigate the role of degradation in R-HCH removal in the arctic ocean (11) and have examined some environmental influences on degradation of chiral pollutants (17). However, relatively little is known about the magnitude, rates, mechanisms and influence of field parameters on microbial degradation of R-HCH in the environment (4, 18). Contaminant processes and fluxes were studied from 1992 to 1994 at Amituk Lake on Cornwallis Island, NT in the Canadian Arctic (1, 19). Selected samples from 1992 were analyzed for R-HCH ERs (16), and results suggested ER values were inversely related to stream temperatures. This paper summarizes R-HCH ER results from more detailed sampling in the final year of the Amituk Lake study and from recent field investigations. The importance of microbial degradation as a loss mechanism for R-HCH in arctic freshwater systems is estimated from enantiomeric differences in stream loadings to Amituk Lake and over-winter degradation within the lake. Watershed factors that influence these processes are also identified. This analysis aims to identify the magnitude of enantioselective and nonenantioselective degradation of R-HCH in high arctic lakes and catchments and illustrate the use of ERs in assessing these processes.
Methods Lake Descriptions and Sampling Sites. Samples were collected from Amituk (Figure 1a), Char, and Meretta Lake watersheds (Figure 1b) on Cornwallis Island, NT. Amituk Lake (75°02′N, 93°45′W) has an area of 0.38 km2, mean and maximum depths of 19.4 and 43.0 m, respectively, and a drainage area of 26.5 km2 consisting of six subcatchments (19). Gorge, Mud, and Cave Creek basins cover 76% of this area. Char (74°42′N, 94°56′W) and Meretta Lakes (74°41′N, 10.1021/es990688j CCC: $19.00
2000 American Chemical Society Published on Web 01/27/2000
FIGURE 1. (a) Amituk Lake (75°02′N, 93°45′W) and its basin with 1997 sampling sites numbered. (b) Resolute area with Char Lake (74°42′N, 94°56′W) and Meretta Lake (74°41′N, 94°56′W) basins and numbered inlet streams. 95°02′W), located near the village of Resolute, have been well characterized (20, 21). Char Lake has an area of 0.53 km2, mean and maximum depths of 10.2 and 27.5 m, respectively, and a drainage area of 4.35 km2. Meretta Lake has an area of 0.26 km2, maximum depths of 12 and 9 m in the lower and upper basins, respectively, and a mean depth of 3.25 m in the upper basin. Water samples were collected from each of the five major inlet streams, the outflow, and depths of 3, 20, and 40 m of Amituk Lake over June, July, and August, 1994. In July 1997, water samples were collected from four locations along each of Mud and Gorge Creeks at Amituk Lake (Figure 1a). At Char Lake, samples were collected from four inlet streams, the outflow, and the lake at 10 m depth. Finally, at Meretta Lake, two inlet streams, the outlet, and the lake at 5 m were sampled (Figure 1b). Sample Collection and Extraction. Sample collection, extraction, and cleanup procedures for 1994 samples are described by Falconer et al. (16). Water sampled in 1997 was collected into stainless steel canisters from stream surfaces or from lake depths with a submersible pump. Each sample received 1.0 mL of d6-R-HCH in acetone as a surrogate standard then mixed thoroughly. Approximately 18 L of water was passed through glass fiber filters to remove particles, then through 1 g, 6 mL Isolute ENV+ cartridges (Jones Chromatography, Lakewood, CO) to extract dissolved HCHs. Used filters and cartridges were wrapped in aluminum foil, sealed in plastic bags, and stored at ∼5 °C until returning to the laboratory. Cartridges were conditioned with 5 mL of methanol and then 5 mL of deionized water before processing samples. Blanks were prepared by passing chromatographicpure water through the extraction apparatus.
HCHs were eluted from extraction cartridges with 20 mL of dichloromethane (DCM), while selected filter samples were Soxhlet extracted with DCM overnight. Extracts were exchanged into isooctane and reduced to ∼1 mL. Samples were cleaned up by column chromatography with 0.5 g of neutral alumina (6% water), topped with sodium sulfate, eluted with 10 mL of 10% DCM-petroleum ether, and transferred to isooctane. Mirex (10 µL) was added as an internal standard, and then extracts were shaken with concentrated sulfuric acid for final cleanup. Quantitative Analysis. HCHs in 1994 samples were quantified as described by Falconer et al. (16). HCHs in 1997 samples were determined using a Hewlett-Packard 5890 gas chromatograph (GC) with 63Ni electron capture detection. Samples (2 µL) were injected splitless (split opened after 30 s) onto a 60 m DB-5 column (0.25 mm i.d., 0.25 µm film thickness, J&W Scientific) using hydrogen carrier gas at 60 cm s-1 and nitrogen at 50 mL min-1 as the makeup gas. The injector and detector temperatures were each 250 °C. The temperature program was as follows: initial temperature, 90 °C; 10 °C min-1 to 160 °C; 2 °C min-1 to 200 °C; 20 °C min-1 to 270 °C; 15 min hold. Data were collected and quantified against five standards using an HP Chemstation. Chiral Analysis. Separation of the R-HCH enantiomers was performed by GC-negative ion mass spectrometry (NIMS) using a HP 5890 GC-5989B MS. The primary separation column was BGB-172 (20% tert-butyldimethylsilylated β-cyclodextrin in OV-1701, BGB Analytik AG, Switzerland) with results confirmed using a Beta-DEX 120 column (20% permethylated β-cyclodextrin in SPB-35, Supelco, USA). Each column was 30 m in length with a 0.25 mm i.d. and 0.25 µm film thickness. (+)-R-HCH eluted first on the Beta-DEX column (16), while (-)-R-HCH eluted first on the BGB-172 (10). Temperature conditions were as follows: initial temperature, 90 °C; 10 °C min-1 to 140 °C; 1 °C min-1 to 180 °C; 10 min. hold; 10 °C min-1 to 220 °C; 10 min. hold. Other GC-NIMS instrument conditions were as follows: injector temperature, 220 °C; transfer line temperature, 220 °C; helium carrier gas at 40 cm s-1; quadrupole temperature, 100 °C; source temperature, 150 °C; and methane at a nominal pressure of 1.0 Torr. Ions 255 and 257 were monitored, with target ion 255 used in calculations and ion 257 used as a qualifier.
Results and Discussion Quality Control. Analytical blank procedures and typical blank and recovery values are described by Falconer et al. (16). R-HCH concentrations for 1994 samples were blank and recovery corrected. Detectable amounts of R-HCH were not found in 1997 water sample blanks (n ) 9), thus a blank correction was not applied. Recoveries of the d6-R-HCH surrogate averaged 81 ( 7% (range 58-93%, n ) 59). Additional samples were spiked with 231 ng R-HCH, and average recovery was 90 ( 5% (n ) 6, corrected for native amounts in water samples). Reported concentrations were corrected according to individual d6-R-HCH recoveries. R-HCH was not detected in suspended sediment on analyzed filter samples (LOD of 10 pg L-1). ERs of selected 1994 and 1997 samples were confirmed by comparing results obtained on the BGB-172 and BetaDEX 120 columns. The average percent difference was 2.5 ( 3.0% between the two columns (n ) 50). Standard R-HCH ERs were 0.99 ( 0.01 (n ) 8) for the Beta-DEX 120 column and 1.01 ( 0.01 (n ) 9) on the BGB-172 column. Sample 255/257 ion ratios were within (5% of standard ion ratios. Sampling variation was estimated by taking samples in triplicate on one occasion. The average R-HCH concentration and ER for these samples had relative standard deviations of 1.4% and 2.2%, respectively. VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Median and Range r-HCH Concentrations and ERs, Stream Temperatures and Discharge, and Water Chemistry Parameters Measured at Amituk Lake June to August 1994 creeks measured parameter enantiomer ratio R-HCH concn (pg L-1) temp (°C) total discharge (×106 m3) Ca2+ (mg L-1) Mg2+ (mg L-1) conductivity (µS cm-1) alkalinity (mequiv
L-1)
pH DOC (mg
L-1)
SS (mg L-1) N-NO3
(×10-2
mg
L-1)
lake
Gorge
Mud
Cave
3m
20 m
40 m
outflow
0.72 (0.36-1.00) 419 (89-2330) 3.0 (0.0-4.0) 33.8 19.0 (9.8-21.1) 4.21 (2.52-7.60) 126 (70-166) 1.20 (0.65-1.66) 8.18 (7.81-8.27) 0.61 (0.19-1.17) 0.26 (0.11-17.76) 4.1 (1.8-8.0)
0.71 (0.64-0.88) 119 (20-307) 6.8 (2.0-10.0) 8.64 26.1 (19.5-31.1) 3.35 (2.32-5.90) 152 (117-200) 1.51 (1.16-1.91) 8.20 (8.04-8.44) 0.78 (0.33-1.14) 0.51 (0.20-6.86) 2.4 (1.2-12.8)
0.74 (0.60-0.92) 529 (293-958) 1.5 (0.0-2.0) 10.3 19.4 (14.4-21.8) 3.11 (1.69-3.37) 126 (86-141) 1.19 (0.82-1.38) 8.12 (8.02-8.28) 0.59 (0.34-0.89) 5.15 (1.10-18.10) 2.2 (1.8-3.1)
0.76 (0.73-0.90) 644 (583-944) 2.4 (1.7-3.7) 20.6 (15.4-26.0) 3.98 (2.75-4.58) 137 (104-161) 1.30 (0.96-1.51) 8.24 (8.16-8.33) 0.59 (0.46-0.76) 1.14 (0.15-2.22) 2.1 (1.4-2.8)
0.74 (0.73-0.77) 520 (469-598) 2.7 (2.4-3.7) 22.9 (20.2-25.2) 4.19 (3.92-4.70) 157 (137-158) 1.50 (1.30-1.51) 8.25 (8.20-8.34) 0.55 (0.38-0.61) 1.11 (0.25-1.19) 2.2 (1.6-2.8)
0.72 (0.66-0.74) 688 (342-727) 2.8 (2.4-3.7) 24.3 (21.1-25.9) 4.47 (3.94-5.03) 157 (136-170) 1.51 (1.32-1.62) 8.21 (8.09-8.32) 0.59 (0.48-0.77) 1.11 (0.43-2.06) 2.1 (0.8-8.4)
0.81 (0.75-0.90) 851 (514-1069) -
Enantiomeric Degradation in Arctic Streams. Table 1 and Figure 2 summarize ERs in Amituk Lake and streams that were compared with water chemistry, temperature, discharges, and R-HCH concentrations to determine where in the watershed degradation was occurring, the amount degraded, and factors affecting degradation. ERs in Amituk Lake streams decreased from a racemic value of 1.01 ( 0.01 (n ) 4) in snow to as low as 0.36 and 0.65 in Gorge and Cave Creeks in August (Figure 2, series A), respectively, indicating that (+)-R-HCH is preferentially degraded. These values agree with those of Falconer et al. (16) for the same streams sampled over a shorter time in 1992. R-HCH concentrations peaked at 2330, 307, and 958 pg L-1 for Gorge, Mud, and Cave Creeks, respectively, at the beginning of snowmelt or during peak meltwater runoff and then declined to as low as 89, 20, and 293 pg L-1 in late summer (Figure 2, series B). Preferential elution of R-HCH from the snowpack has been reported (1, 19) and is expected from model calculations (22). These concentrations are lower than values reported in 1992 (16), reflecting declines in basin snowpack R-HCH burdens (1). Stream discharges were highest in late June to early July and then rapidly decreased to a level that was maintained through August (Figure 2, series B). This is typical of arctic hydrology in which snowmelt occurs rapidly, and then streamflow is diminished in late summer but maintained by subsurface flow (23). Since enantioselective degradation of R-HCH is biologically mediated (13, 24), ERs will be influenced by factors that favor microbial activity such as stream temperature, nutrient availability and substrate contact time. A linear correlation analysis between ERs and several water chemistry parameters for Gorge Creek showed statistically significant (p < 0.05) negative correlations with temperature, conductivity, Mg2+, and alkalinity (Table 2). As found by Falconer et al. (16), ERs declined with increasing stream temperatures (Figure 2, series A). Chemical weathering parameters such as conductivity, Ca2+, Mg2+, and alkalinity increase as streamflows decline (19) as do stream volume-to-surface ratios, resulting in greater contact time between water and bed surfaces. Increasing temperatures throughout summer also deepen the active 814
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61.6 -
layer (23) leading to greater contact between meltwater and stream substrates. In addition, water from late-lying snowbanks typically flows over or through depressions and rills (23) that support the limited vegetation in the catchment. Thus, the results suggest that enantioselective degradation is enhanced by the greater contact time between the chemical in the water and stream or slope substrates, the presumed site of microbial communities. Although changes in nutrient parameters such as dissolved organic carbon (DOC) and nitrate-nitrogen (NO3-N) may impact enantioselective degradation, correlation coefficients between these and ERs were low. Gradual DOC and NO3-N concentration increases over the summer after peak melt, which support increased microbial activity, were not captured by the regression analysis. Declines of R-HCH ERs and concentrations with stream discharges indicate that stream hydrology is a major factor controlling the extent of R-HCH degradation in stream watersheds. To illustrate this, the magnitude of enantioselective degradation was calculated using ERs and concentrations in Mud, Gorge, and Cave Creek basins. Daily concentrations and ERs were estimated by polynomial regression, then daily R-HCH loadings (in mg) were calculated using measured daily stream discharges and the estimated concentrations. The amount enantioselectively degraded (Ndeg) each day is the difference between the amount of (-)-RHCH and (+)-R-HCH
Ndeg ) [ΧR-HCH/(ER + 1)] - [ER•ΧR-HCH/(ER + 1)] (1) where ΧR-HCH is total daily R-HCH loadings (mg). This calculation represents a minimum estimate of microbial degradation of R-HCH since it does not account for nonenantioselective degradation or enantioselective degradation that may affect both enantiomers but at differing rates (11, 24). For Gorge Creek, enantioselective degradation was estimated at 164 mg or a loss of 6% of total R-HCH entering the lake through the creek. Only 38 mg or 23% of this amount was degraded after mid-July when ERs were much lower. The amounts enantioselectively degraded in Mud and Cave creeks were 25 mg (13%) and 52 mg (8%), respectively, and
FIGURE 2. Trends of r-HCH ERs and stream temperatures (series A) and r-HCH concentrations and stream discharges (series B) with Julian Day for streams flowing into Amituk Lake during summer 1994.
TABLE 2. Correlation Coefficients (r 2) for r-HCH ERs and Concentrations and Stream Discharge, Temperature, and Water Chemistry Parameters for Gorge Creek in 1994a parameter
ER
concn
discharge
temp
SS
cond
Ca2+
Mg2+
alkal
NO3-N
DOC
ER concn discharge temp suspend. sed. conductivity Ca2+ Mg2+ alkalinity NO3-N DOC
1.00 0.61 0.35 0.67 0.32 0.73 0.51 0.84 0.80 0.41 0.22
1.00 0.00 0.50 0.90 0.16 0.03 0.32 0.23 0.06 0.59
1.00 0.29 0.07 0.79 0.95 0.53 0.72 0.56 0.02
1.00 0.21 0.44 0.32 0.50 0.50 0.25 0.11
1.00 0.02 0.01 0.12 0.06 0.00 0.65
1.00 0.92 0.92 0.99 0.79 0.01
1.00 0.71 0.88 0.69 0.00
1.00 0.94 0.80 0.02
1.00 0.76 0.03
1.00 0.04
1.00
a
Bold italicized type - significant to p < 0.05.
of these amounts, 2% and 26% were degraded after midJuly. Combined, approximately 7% of total R-HCH was enantioselectively degraded prior to entering Amituk Lake,
most of which occurred during peak melt. In comparison, rates of base hydrolysis of R-HCH (26) calculated using extreme pH and temperature values for Gorge, Mud, and VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Ranges of r-HCH Enantiomeric Ratios and Concentrations from Char and Meretta Lake Watersheds and from a Wetland, July 1997 site
enantiomer ratio
concn (pg/L)
temp (°C)
inlet 1 inlet 2 inlet 3 inlet 4 10 m outflow
Char Lake 0.58-0.71 64-414 0.75-0.88 279-602 0.72-0.77 437-474 0.38-0.64 619-733 0.62-0.69 839-1021 0.69-0.72 641-843
4.0-7.8 0.5-2.5 1.0-4.3 2.0-7.8 2.5-3.5 1.5-3.8
inlet 1 inlet 2 5m outflow
Meretta Lake 0.42-0.54 332-429 0.73-0.81 669-828 0.84-0.90 1205-1264 0.82-0.84 603-758
3.8-9.5 4.5-9.0 3.5-4.5 3.0-5.0
inflow centre outflow
0.85 0.69 0.63
FIGURE 3. Summer 1994 trends for r-HCH ERs at 3, 20, and 40 m depths of Amituk Lake. Representative ice thickness from 2.2 m on day 166 to ice-off on or about day 199. Outflow discharge (m3 s-1) over the sampling period.
Wetland 433 285 175
10.0 11.3 11.8
Cave Creeks (Table 1) could account for
