PCBs Cycling Among Air Water and Sediments

If released into the environment, PCBs persist for many years because they are so resistant to breakdown by chemical or biological agents. Although their solubility in water is very slight—indeed they are more likely to be adsorbed onto suspended particles in the water than dissolved in it—the tiny amounts of PCBs in surface waters are constantly being volatilized and subsequently redeposited on land or in water after traveling in air for a few days. By such mechanisms, PCBs have been transported worldwide. There are measurable background levels of PCBs even in polar regions and at the bottom of oceans. Indeed, the ultimate sinks for PCBs that are mobile are in the deep sediments of oceans and large lakes. This environmental load of PCBs will continue to be recycled among air, land, and water, including the biosphere, for decades to come, as analyzed in greater detail in Chapter 12. Only a minority of PCBs manufactured in the past are currently found in the environment or have been destroyed; much of the production lingers in storage or old electrical equipment and may ultimately be released. Recent research has found that PCB releases from older consumer products into indoor air, which then is eventually vented outside, is a major source of PCBs in urban air.

A quantitative measure of the recycling of substances within a water body is provided by the mass balance of its current annual inputs and outputs of the compounds. The PCB mass balance for a very large, relatively clean water body—Lake Superior—is illustrated in Figure 11-1. Although it is the least polluted of the Great Lakes, Lake Superiors burden of PCBs in its water and its sediments is substantial. Currently, almost all the input of PCBs occurs from the air, with relatively little added from industries or via tributary rivers (Figure 11-1). Overall, Lake Superior is now gradually "exhaling" its historical load of PCBs into the air, the output to air being much greater than the annual input from the atmosphere. Little of Lake Superior's PCB content

Direct discharges

Water

Water column total = 10,100 kg in 1986

Direct discharges

Water

Water column total = 10,100 kg in 1986

Net loss/year = 1735 (1540) kg

1 I0

Sediment t

Sediment total = 4900 kg (1986)

Notes: I. Data are for J 986; where data for 1992 differ from 1986. they are shown in parentheses. 2. Note that burial in sediment represents the small difference between large deposition and resuspension from surface sediments.

FIGURE 11-1 Mass balance of PCBs in Lake Superior, in kilograms per year, [Source: The State of Canada's Environment 1996 (Ottawa: Government of

is now being lost to sediments; about as much is redissolved from them as is deposited onto them each year.

By contrast, the mass balance of PCBs in Lake Ontario, another of the Great Lakes, is quite different from that of Lake Superior. The PCB concentration in Lake Ontario water substantially exceeds that of Lake Superior, since it is located in a much more industrialized area. In Lake Ontario, the greatest current input comes from land-based sources such as waste dumps that still leach PCBs into the lake and its tributaries. About the same quantity of PCBs are present in the water flowing out of the lake. Approximately equal amounts are lost annually to sediments and to the atmosphere; about one-third of such losses are canceled by new inputs from the sediments and the air.

PROBLEM 11-8

The PCB concentration in Lake Michigan is declining according to a firstorder rate law having a rate constant of 0.078/year. If the PCB concentration in Lake Michigan averaged 0.047 ppt in 1994, what will it be in 2010? In what year will the concentration fall to 0.010 ppt? What is the half-life period of PCBs in this lake? [Hint: Recall that for first-order processes, the fraction / of any sample that still remains after time t has passed isf= e~ktJ

Because of their persistence and their solubility in fatty tissue, PCBs in food chains undergo biomagnification; an example is shown in Figure 11-2.

FIGURE 11-2 The biomagnification of PCBs in the Great Lakes aquatic food chain. [Source: The State of Canada's Environment 1996 (Ottawa: Government of Canada, 1991).]

FIGURE 11-2 The biomagnification of PCBs in the Great Lakes aquatic food chain. [Source: The State of Canada's Environment 1996 (Ottawa: Government of Canada, 1991).]

Food Chain Pcbs

Notice that the ratio of PCBs in the eggs of herring gulls in the Great Lakes was 50,000 times that in the phytoplankton in the water at the time of these measurements. The good news is that the average level of PCBs in such eggs has fallen with time in many locations, as the data in Figure ll-3a illustrate for

1000

Observed

—•- Predicted

§ ^ f & $ i1 ii J j

> j £ £ *

'74 '76 '78 '80 '82 '84 '86 '88 '90 '92 '94 '96 '98 '00 '02

Year

4000 3500 3000 2500 2000 1500 1000 500 0

'74 '76 '78 '80 '82 '84 '86 '88 '90 '92 '94 '96 '98 '00 '02

Year

FIGURE !!-3 (a) PCB concentrations in herring gull eggs in Toronto Harbor, 1974-2002. The predicted curves correspond to exponential decay in those time periods. [Source: Dr. Chip Weseloh, Environment Canada.] (b) PCB concentrations in 65-cm coho salmon from Lake Ontario. [Source: Ontario Ministry of the Environment.)

gull colonies in Lake Ontario around Toronto. The concentrations are plotted in the figure on a logarithmic scale, and the data seem to fit two intersecting straight lines, corresponding to first-order decay sequences with half-lives of first five years and more recently seven years. This complicated behavior may arise because of continuing sources of PCBs to the system. PCB levels in fish at the top of the Lake Ontario food chain have also declined since the 1970s, but the current rate of decrease is slow and erratic (see Figure 1 l-3b).

The relative concentrations of the congeners of a PCB mixture begin to change once they enter the environment. Microorganisms in soils and sediments and large organisms such as fish both preferentially metabolize congeners having relatively few chlorine atoms. Thus the relative concentrations of the more heavily chlorinated congeners increases with time since they are degraded much more slowly. Thus, for example, between 1977 and 1993, the proportion of PC B molecules with four or five chlorine atoms decreased by 6% each in trout in Lake Ontario, whereas those with seven or eight chlorines increased by 7% and 4%, respectively. However, PCBs present in anaerobic soil are eventually dechlorinated microbially at their meta and para positions, leaving congeners that are only chlorinated at ortho positions. Aerobic degradation occurs with congeners having adjacent carbons (ortho + meta, or meta + para) chlorine-free.

Continue reading here: PCB Contamination by Furans

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