Sources and Fates of Aquatic Pollutants - American Chemical Society

14 Fate of Some Chlorobenzenes from the Niagara River in Lake Ontario B. G. Oliver

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Environmental Contaminants Division, National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario L7R 4A6, Canada Pollution of the Niagara River with chemical wastes has led to severe contamination of Lake Ontario. This chapter discusses the behavior of five chlorobenzenes in the Niagara River, in the river plume, and in Lake Ontario. The importance of sedimentation and volatilization on chemical pathways in the lake has been assessed. Bottom sediments are shown to contain the bulk of the chlorobenzenes that remain in the lake. The physical processes that affect the concentration distribution of chlorobenzenes in bottom sediments are explained, together with processes that affect the desorption rates of chlorobenzenes from sediments and biouptake of chlorobenzenes by benthic invertebrates. The contamination of fish and other biota is reported, and contaminant trends in sediment cores and biota are discussed.

THE CONTAMINATION OF LAKE ONTARIO BY THE NIAGARA RIVER has

been well documented. The papers presented at the 1982 Niagara River Symposium at the Great Lakes Conference have been published as a special volume (2) and accurately present the current status of the Niagara River pollution problem. In addition to direct discharges from chemical manufacturers along the river, more than 200 chemical waste dumps can be found in the river's vicinity (2). Many of these waste dumps leak into the river. Elder et al. (3) demonstrated the presence of high concentrations of chlorobenzenes, chlorotoluenes, polychlorinated biphenyls (PCBs), and many other chlorinated and fluorinated chemicals in sediments from creeks near dumpsites along the river. Oliver and Nicol (4) showed elevated concentrations of chlorobenzenes in water samples below dumpsites and chemical manufacturing discharges along the river. Yurawecz (5) identified some unusual chlorotoluenes 0065-2393/87/0216-0471$06.00/0 © 1987 American Chemical Society

Hites and Eisenreich; Sources and Fates of Aquatic Pollutants Advances in Chemistry; American Chemical Society: Washington, DC, 1987.


472

SOURCES AND FATES OF AQUATIC POLLUTANTS

and chlorinated trifluorotoluenes in fish from the river. These chemicals are produced in the river's vicinity. In addition to direct contamination of the river, considerable evidence indicates that the sediments of Lake Ontario have been severely polluted. Elevated concentrations of organochlorine insecticides and PCBs (6), mirex (7), chlorobenzenes (4), and octaehlorostyrene (8) have been observed in Lake Ontario sediments. Unusual fluorinated compounds, which seem to result from the chemical reaction of wastes in the dumpsites, have also been shown to be dispersed widely throughout the lake (9). In this chapter, the pathways of several chlorobenzenes in Lake Ontario will be discussed for which the Niagara River appears to be the major source. These compounds were chosen for discussion because they are present in significant concentrations in all compartments of both the river and the lake. They also are chemicals that exhibit a wide range of physical and chemical properties. Some of the properties of chlorobenzenes are presented in Table I and some useful characteristics of Lake Ontario are listed on page 473. A diagram of the general study area is shown in Figure 1. Analytical methods, dual column capillary gas chromatography with electron-capture detectors, and extraction and cleanup procedures were described previously (JO). Table I. Chlorobenzene Properties

Chemical

1,4-Dichlorobenzene 1,2,4-Trichlorobenzene 1,2,3,4,-Tetrachlorobenzene Pentachlorobenzene Hexachlorobenzene

Abbreviation

log Kow (Ref.)

Water Solubility at 25 °Ca (mg/L)

1,4-DCB 1,2,4-TCB 1,2,3,4-TeCB QCB HCB

3.4 (11) 4.0 (12) 4.5 (13) 4.9 (11) 5.5 (12)

90 30 4.3 0.56 0.005

°Data are from reference 14.

The Niagara River The Niagara River is one of North America's larger rivers (flow is 6400 m3/s), and the presence of Niagara Falls and the Whirlpool Rapids below the falls makes much of (he river unnavigable. Thus, sampling along many of the river reaches is difficult if not impossible. A comparison of the concentration of five chlorobenzenes at site 1 (Figure 1), the start of the river (Fort Erie); at site 2, below a chemical dumpsite and a chemical manufacturing discharge just above Niagara Falls, New York (a helicopter-collected sample); and site 3, a sample at the mouth


14.

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Fate of Some Chlorobenzenes

Lake Ontario

from the Niagara River

473

Characteristics


Water Area: 18,500 km 2 Length: 311 km Mean depth: 86 m Maximum depth: 244 m Volume: 1640 km 3 Niagara R. inflow: 6400 m 3 /s St. Lawrence R. outflow: 6700 m 3 /s Mean residence time: 7.8 years Sedimentation Basins and Their Areas Niagara: 1600 km 2 Mississauga: 2700 km 2 Rochester: 3800 km 2 Kingston: 560 km 2 of the river [ Niagara-on - the- Lake (NOTL)], is shown in Table II. As can be seen from the table, significant sources of chlorobenzenes are entering the river. Contributions from both chemical dumpsite leachates and direct discharges appear to be the major sources (15). The turbulence in the river produces a reasonably homogeneous water mass at the river's mouth. Transect sampling at N O T L has shown virtually the same concentrations of organic chemicals on the Canadian side, in the middle, and on the American side of the river. On the basis of this data, a water quality monitoring station has been established at N O T L (16). The intake line for this station is located 30 m from the Canadian shore, about 6 m above the river bottom, and 13 m below the river surface. Originally, samples of suspended solids from the river were collected by centrifugation and analyzed for organic compounds in an attempt to estimate loadings to Lake Ontario (16). This estimation was done on the basis of the assumption that most hydrophobic organic chemicals such as PCBs would be found mainly in the particulate phase. However, subsequent studies have shown that this procedure vastly underestimated chemical loadings (in the low turbidity river, suspended solids concentrations were 3-10 mg/L) because the bulk of the organic compounds was found to be present in the dissolved phase (17). Subsequently, weekly 16-L whole-water samples were collected from the river at the N O T L station for a 2-year period (1981-1983) to estimate concentrations, to assess concentration variability, and to approximate loading for several chlorinated organic compounds (18).



Figure

1. Map of the study area including surficial sediment sampling sampling sites (M) in Lake Ontario.

sites (Φ) and sediment


core

g H

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Fate of Some Chlorobenzenes from the Niagara River

475

Table II. Chlorobenzene Concentrations in the Niagara River in 1981 Site

1,4-DCB

1,2,4-TCB

1,2,3,4-TeCB

QCB

HCB

1 2 3

1.7 94 29

0.5 110 12

0.06 130 3.8

0.05 22 1.2

0.05 1 0.6


NOTE: All values are in units of nanograms per liter.

Figure 2 shows a plot of the concentration of 1,2- and 1,4-dichlorobenzenes (1,2- and 1,4-DCB, respectively) over this 2-year period. A fairly steady background concentration was observed in the river, likely due to steady leaching from waste disposal sites along the river. Superimposed on this background are large concentration spikes that are likely due to direct industrial discharges to the river. To illustrate the magnitude of the concentration spikes, on May 17, 1982, the concentration of 1,2-DCB was measured to be 240 ng/L compared to 29 ng/L recorded the previous week. The 1,2-DCB remained at an elevated concentration for 4 weeks. The estimated total quantity of chemical in this slug was about 2000 kg, or about 50% of the total 1982 river loading for this chemical. Many other chlorinated chemicals showed similar concentration profiles to these chlorobenzenes, although concentration spikes for the other chemicals were not as large (28). A crude estimate of loadings for some chlorobenzenes to Lake Ontario from the river can be made by using median river concentrations from this 2-year study. The loadings are 5800 kg/year for 1,4-DCB, 2400 kg/year for 1,2,4-trichlorobenzene (1,2,4-TCB), 760 kg/year for 1,2,3,4-tetrachlorobenzene (1,2,3,4-TeCB), 240 kg/g for pentachlorobenzene (QCB), and 120 kg/year for hexachlorobenzene (HCB). Earlier studies provided chlorobenzene concentrations in some sewage treatment plant (STP) effluents flowing into Lake Ontario (4). By using this data and a total STP discharge of 109 mVyear (19), the loadings of these chlorobenzenes from STPs are roughly 660 kg/year for 1,4-DCB, 11 kg/year for 1,2,4-TCB, 2 kg/year for 1,2,3,4-TeCB, 1 kg/year for QCB, and 2 kg/year for H C B . Thus, STPs appear to be only minor contributors of chlorobenzenes (with the possible exception of 1,4-DCB) to Lake Ontario. Also, sediment samples near the mouths of other major rivers entering Lake Ontario were found to contain very low concentrations of chlorobenzenes. Thus, the dominant chlorobenzene loading to the lake appears to originate in the Niagara River. Niagara

River Plume

and Near-River

Lake

Ontario

The distinct plume of the Niagara River has been shown to extend 10-15 km into Lake Ontario. The direction of the plume can vary from


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SOURCES A N D FATES O F AQUATIC POLLUTANTS


400 π

Figure 2. Concentrations at NOTL (Reproduced

of 1,2-DCB and 1,4-DCB in the Niagara River with permission from reference 18. Copyright 1984, Elsevier.)

northerly towards Toronto (Figure 1 ) to easterly depending mainly on wind direction and speed ( 2 0 ) . Because of the prevailing wind direction, the most frequent direction of the plume is bent over to the east along the south shore of the lake where it is caught up in the strong eastward coastal current {21). The plume can be tracked by using physical measurements such as temperature, turbidity, and conductivity. Because of the higher surface-to-volume ratio of Lake Erie compared with Lake Ontario, for much of the year the water temperature in the plume will be higher than that in Lake Ontario. Another method that has been used to track the plume is with radiotransmitting drogues. These drifters, which are propelled by underwater sails (2.4 X 3 m), move in the



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477

direction of the underwater currents and are minimally affected by surface winds (21). In addition to tracking by physical techniques, some measurements have also been made on contaminant concentrations in the plume. These measurements were made at a 1-m depth on centrifuged, large-volume (200-L) samples over a period of 24 h. Sampling occurred in the same water mass as indicated by the drogues. The concentrations of chlorobenzenes and other chlorinated contaminants were virtually constant (within experimental error) over the course of the sampling. This constant level indicates that processes that influence the concentration of these chemicals occur over a much longer time frame. Contaminant concentrations in surrounding lake water were much lower than in the plume, so a region with a concentration gradient between the plume and the lake water must exist. In addition to following the chlorobenzene concentration in the water column in the plume, the partitioning between suspended sediments and the water was also studied. The sediment-water partition coefficient (K ) normalized to organic carbon is defined as oc

Koc =

C S S / (CH2O/OC)

(1)

where C s s and C H 2 o are the chemical concentrations in the suspended sediment and water phases, respectively, and foe is the organic carbon fraction of the suspended sediments. In the plume on September 22, 1984, the suspended sediment concentration averaged 3 mg/L, and the mean organic carbon content of the sediments was 6.6$ (/oc = 0.066). In Table HI, the range and mean log Koc values for the five samples in the plume are compared to values calculated with Karickhoff s empirical equation (22), which relates K o c to K „ w (the octanol-water partition coefficient). K o c = 0.411 C

(2)

The Koc values were fairly consistent within the plume and varied by a maximum of a factor of 3. With the exception of 1,4-DCB, for Table III. Sediment-Water Partition Coefficients in the Niagara River Plume and from Karickhoffs Empirical Equation log K O C

Value

log K O C (range) log K O C (mean) Calculated Koc

1,4-DCB

1,2,4-TCB

1,2,3,4-TeCB

5.3-5.6 5.5 3.0

4.8-5.3 5.0 3.6

4.9-5.4 5.1 4.1

QCB 5.5-5.9 5.7 4.6


HCB 6.0-6.5 6.3 5.1


478


which we suspect analytical difficulties in the suspended solids fraction, the Koc value increases systematically with K o w as predicted theoretically. But the absolute values of the field Koc values are more than 1 order of magnitude greater than those predicted by Karickhoff s empirical equation. Recent studies show that the partition coefficient for organic compounds changes considerably with changing suspended sediment concentration (23). Weber et al. (24) showed that partition coefficients increased by 1 order of magnitude for each 2.5 order-ofmagnitude decrease in solids concentration. The suspended sediment concentrations used by Karickhoff to generate equation 2 were about 1000 mg/L (25). This phenomenon of changing partition coefficient with changing suspended solids concentration is the likely explanation for the apparent discrepancy between the field samples (suspended solids concentration was 3 mg/L) and the empirical equation. This result shows that considerable care must be taken before applying laboratoryderived measurements to field situations. The amount of chemicals entering the lake that were sequestered to settling particles and eventually became bottom sediments was estimated by placing three sediment traps around the edge of the plume (26). The sediment trap assembly consisted of five plexiglass tubes (7 cm in diameter and 106 cm in length) fitted at the bottom with removable cups (27). Samples in the traps were collected every month during the field season from May to November in 1981 and in 1982. The mean contaminant values in the down-fluxing material were determined. At the same time, the same contaminants and suspended solids levels were quantified in weekly samples of Niagara River water. The loading of contaminants to the bottom sediments was calculated from the mean concentration in the sediment trap material by assuming that all the suspended sediments in the Niagara River were down-fluxing to the bottom of Lake Ontario. The data for several chlorobenzenes is shown in Table IV. The data in the table clearly show that only a very small percentage of the chlorobenzene input from the river deposits to the bottom sediments. As expected, the compounds with higher K

R

OUTPUT ST LAWRENCE RIVER

PHOTOLYSIS ? BIODEGRADATION ?

SEDIMENTS Figure 3. Block diagram of major loss processes for Lake

OntaHo.


480


Table V. Yearly Chlorobenzene Inputs and Losses from Lake Ontario Source

1,4-DCB

1,2,4-TCB

1,2,3,4-TeCB

QCB

HCB

Niagara R. loading Sediment loading St. Lawrence R. loading Unaccounted material

5800 76 180 5500

2400 25 74 2300

760 16 15 730

240 10 6 220

120 18 6 96


NOTE: All values are in units of kilograms per year.

(29). Also, sensitized photodechlorination and photoisomerization were demonstrated with acetone (29). These studies were conducted at much higher concentrations than are encountered in the environment. Because the light absorbance of these chemicals in the solar region is very weak, indirect photosensitized photoreactions may be more important than direct photolysis in the environment. But the occurrence and significance of photodegradation of chlorobenzenes in the environment has not been demonstrated. These considerations lead to the conclusion that volatilization is probably the major mechanism of loss of chlorobenzenes from Lake Ontario. This result is in good agreement with earlier studies on Lake Zurich, Switzerland (30), where volatilization was shown to be the major process for loss of 1,4-DCB from the lake. Volatilization has also been shown to be important for 1,4-DCB and 1,2,4-TCB in sea water mesocosms (31). This process may provide a partial explanation for the widespread distribution of chlorinated contaminants such as PCBs and H C B in rainwater (32). During the last few years, samples from most compartments of the Lake Ontario ecosystem have been analyzed. The map in Figure 1 shows the surficial sediment sampling network in the sedimentation basins of the lake and the location of the three cores (one from each major basin) that have been collected and analyzed. From these data, a crude estimate can be made of the masses of the various chlorobenzenes in the lake's bottom sediment (33). Similarly, by analysis of the lake water from many diverse lake locations in the fall after turnover and in the spring before stratification, an estimate of the mass of chlorobenzenes in the water column of the lake was made. The approximate mass of contaminants in suspended solids in the lake was estimated by analysis of material collected with a continuous flow centrifuge and with sediment traps throughout the lake. A very crude estimate of the mass of chlorobenzenes in biota was made by using biomass estimates (34) combined with analysis of algae, zooplankton, benthic organisms, and fish from the lake. The masses of chemicals in the lake compartments are shown in Table VI. The bulk of chlorobenzenes remaining in Lake Ontario are present in bottom sediments. Surprisingly, very little


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Table VI. Masses of Chlorobenzenes in Lake Ontario Compartments Compartment Bottom sediments Lake water Suspended sediments Biota


NOTE:

1,2,4-TCB 11,000 700 10 2

1,2,3,4-TeCB 3300 210 4 2

QCB

HCB

4100 90 4 2

8500 90 9 8

All values are in units of kilograms.

mass of chlorobenzenes is found in other lake compartments. Fish and other biota contain significant concentrations of chlorobenzenes and other chlorinated chemicals, and this problem is serious. But, because of the low biomass in the lake ( « 1 0 g dry weight/m 2 ), the total mass of chlorobenzenes in this compartment is small. Because of the importance of the bottom sediments as both a sink and a possible future source for contaminants, the sediments in the lake will be discussed in detail in the next section. Sediment

Processes

Three main sedimentation basins occur in Lake Ontario: the Niagara, the Mississauga, and the Rochester, which are separated by sils. One minor basin occurs, the Kingston (35) (Figure 1). These basins are the only areas in the lake where net accumulation of sediment occurs over the year. Outside the basins, a net erosion of sediment occurs. The sampling grid in Figure 1 shows that samples were collected only in the sedimentation basins because past measurements showed that the concentration of chemicals in erosional-zone bottom material is near zero. The mean total chlorobenzene concentrations (di- through hexa-) are 610 M g / k g dry weight in the Niagara Basin, 560 Mg/kg in the Mississauga Basin, 480 ppb in the Rochester Basin, and 120 Mg/kg in the Kingston Basin. The data for individual samples as well as the basin means show that no strong plume of contaminated sediment extends from the Niagara River. The major basins have roughly the same chlorobenzene concentrations, and somewhat lower values occur in the Kingston Basin. This observation contrasts with studies on other rivers and lakes (e.g., reference 36, where elevated sediment concentrations are observed near point sources). To explain these observations, several sediment traps were placed at various locations and depths in the lake. A series of sediment traps at roughly 20-m-depth intervals were used at each site; the lowest trap was placed 2 m from the bottom. Trap catches for a typical year for an offshore site near the center of the Niagara Basin are shown in Table VII. One observation that can be made from this table is that the


482


Table V I I . Sedimentation Rate from May 1981 to May 1982 in the Middle of the Niagara Basin Trap Depth (m)


20 40 60 80 90 98

May

June

July

Aug.

Sept.

Oct.

Nov.

(Dec. to May)

2.80 2.78 2.84 2.74 2.91 3.18

2.29 2.09 2.05 2.03 1.65 2.07

3.12 2.76 3.06 3.25 3.54 4.98

1.65 1.53 1.56 1.65 2.07 2.81

1.11 1.23 1.22 1.41 1.80 2.75

0.94 0.91 1.32 2.34 3.67 5.73

0.35 0.74 0.67 1.41 1.83 3.53

2.60 2.99 3.17 4.29 4.30 5.71

NOTE: All sedimentation rates are in units of grams per square meter per day.

bottom traps (particularly the trap 2 m from the bottom) collect more material than the higher traps throughout much of the year. The largest differences between the bottom and upper traps occur when the lake is stratified in the summer and early fall. The higher catches in the lower trap indicate that significant sediment resuspension occurs even in the deepest basins of Lake Ontario. This result agrees with optical measurements in the lake that have shown the presence of a nepheloid layer (a layer of high turbidity) near the lake bottom (37). The thickness of the layer, which was present over the entire lake, was variable but could extend up to 45 m from the bottom (37). The analysis of contaminant profiles in the sediment trap material, Niagara River suspended sediments, and Lake Ontario surficial bottom sediments provided interesting evidence as to the source of the additional material in the bottom traps. Mirex in bottom sediments of the Niagara Basin averaged 48 Mg/kg, whereas recent suspended sediments from the Niagara River at N O T L contained about 5 μg/kg mirex. Also, the ratio of 1,2,4,5-TeCB to 1,2,3,4-TeCB in bottom sediments was 1.64, whereas this ratio was 0.54 in recent suspended sediments from the river. These contaminant concentration changes appear to be due to reduced loading of mirex and 1,2,4,5-TeCB in recent years (38). The mirex concentration and the ratio of 1,2,4,5-TeCB to 1,2,3,4-TeCB in bottom trap material were much higher than that in the 20- and 40-mdepth traps. This observation indicates that the major source of the additional catch in the lower traps was probably resuspended bottom sediments. Another observation that can be made from Table VII is that the trap catches are much higher in early spring and in winter when the lake is isothermal. During this time of the year, violent storms occur on the lake, and wind-driven currents penetrate deeply (because of isothermal conditions) and cause significant sediment resuspension. The data in Table VII and information generated from traps near



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483

the Niagara River indicate that sediments and detritus from the river adsorb contaminants from the river water and settle temporarily in the vicinity of the river. Currents in Lake Ontario, especially during spring and winter, resuspend the material and redistribute it to the various sedimentation basins in the lake. A strong counterclockwise circulation pattern occurs in the lake, and current speeds average 5 cm/s in the winter and 2 cm/s in the summer (39). The Kingston Basin is out of the main circulation pattern of the lake; this fact probably explains why the sediment chlorobenzene concentrations are lower in this basin. The sediment chlorobenzene concentrations in the other three basins are similar because of the dynamic nature of sedimentary processes within the lake's major circulation regime. The mean total (di- through hexa-) chlorobenzene concentration in Great Lakes surficial sediments is 560 Mg/kg for Lake Ontario, 25 M g / k g for Lake Erie, 33 M g / k g for Lake Huron, and 5.5 M g / k g for Lake Superior. Thus, Lake Ontario sediments are more than 1 order of magnitude more contaminated than the other three Great Lakes located partially in Canada. We have no data for Lake Michigan. What is the significance of this sediment contamination? Sedimentassociated contaminants can influence the concentrations in both the water column and in lake biota if they are desorbed or are available to benthic organisms. Some of the chlorobenzenes in Lake Ontario bottom sediment would be desorbed if the sediments were resuspended (40). Laboratory studies showed that chlorobenzene desorption half-lives in clean lake water averaged about 60 days at 4 ° C , 40 days at 20 ° C , and 10 days at 40 ° C (40). Temperature was a much more important variable than chemical structure in governing desorption rates. Field observations in Lake Ontario have indicated that a sediment layer about 1 mm thick in the sedimentation basins ( « 8 7 0 0 km 2 ) is in a constant state of flux. This sediment is almost continuously being resuspended and then resettled to the bottom. The near-bottom temperature in the lake is 4 ° C , and the active layer contains about 2$ solids. By using these approximations, the half-life data above, and the mean chlorobenzene concentrations in lake surficial sediments, a crude estimate of loading rates from suspended sediments to the water column can be made. These loadings are compared to loadings from the Niagara River (Table VIII). The table shows that, for chemicals with large active sources such as the lower chlorinated chlorobenzenes, the contribution from sediment desorption is low. But, for compounds with low current loadings such as H C B , desorption from sediments could play a significant role in controlling lake water concentrations. Biological

Component

Chemicals in bottom sediments may also be taken up directly from sediments and detritus by benthic organisms. Laboratory studies (41) Hites and Eisenreich; Sources and Fates of Aquatic Pollutants Advances in Chemistry; American Chemical Society: Washington, DC, 1987.


484

Table VIII. Loadings to Lake Ontario Water Column from Suspended Sediments and the Niagara River Loadings

Source

Resuspended sediments Niagara River

1,4-DCB 11 5800

1,2,4-TCB 19 2400

1,2,3,4-TeCB

QCB

HCB

7 760

7 240

19 120


NOTE: All values are in units of kilograms per year.

have demonstrated that oligochaete worms, which are found in sig nificant numbers in Lake Ontario sediments, bioaccumulate many chlorinated compounds from these sediments. Field studies near the Niagara River have shown a strong correlation between H C B concentra tion in oligochaete worms and in the sediments in which they lived (42). These benthic organisms are at the lower end of the food chain and can influence chemical concentrations in sport and commercial fish through this food chain link. Benthic organisms also enhance the rate of release of pollutants from sediments by the process of bioturbation. Karickhoff and Morris (43) showed that sediment reworking by oligochaetes enhanced the flux of QCB and H C B from a bed of sediment in a laboratory microcosm by a factor of 4-6. This process was mitigated to a certain degree by fecal pelletization of the sediment by the worms, which actually caused a reduction in the desorption rates of the chemicals from the sediment particles (43). Worms feed at depths of 8-10 cm and defecate this material at the surface (44). By this mechanism, the worms can recycle more contaminated deeper sediments (in locations where control measures have been implemented) to the sediment surface. Thus, the presence of benthic organisms may increase the time required, after implementation of controls, to observe dramatic reductions in con taminant levels in the ecosystem. This induction period will be greater in locations that have low sedimentation such as Lake Ontario [sedimen tation rates are 1-5 mm/year (45)]. The chlorobenzene concentrations in biota, such as algae and zooplankton, were studied briefly in plankton net (125 μπι) catches from the Niagara River (42). From purely physical considerations for inert material, the smallest particles would be predicted to have the highest concentration because of their higher surface-mass ratio. But in general, chlorobenzene concentrations in plankton net catches were higher in the larger size fractions. Qualitative examination of the material showed that smaller size fractions (

Sources and Fates of Aquatic Pollutants - American Chemical Society

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