10.1 Effects
PCBs have a strong affinity for soils. The clay and organic matter contents of the soil, as well as the chlorine content and hydrophobicity of the individual PCB isomers, tend to increase PCB adsorption to soils (Fairbanks and O'Connor, 1984).
The most frequently encountered PCBs in soils are similar in composition to Aroclor 1254 and Aroclor 1260 (Richardson and Waid, 1979). The major means of loss of PCBs (e.g., Aroclor 1254) from soils is by volatilisation. Sewage sludge amendments have been shown to reduce volatilisation while increasing the degradation of PCBs (Fairbanks et al.,1987).
Plants grown in PCB-amended soils have been shown to accumulate these compounds in their aerial parts (Sawhney and Hankin, 1984; Suzuki et al., 1977). In a recent study, PCB concentrations of 0.01 to 0.2 µg/g (fresh weight) were detected in leaves from purple loosestrife (Lythrum salicaria ) growing in a contaminated soil (0.12 µg PCBs/g); the major route of entry was via the roots (Bush et al., 1986). However, the mechanism of PCB translocation from soil to the plant foliar tissue is controversial. Several investigators have concluded that the level of PCBs in the foliar tissues of soybean, broad bean, tomato, and cucumber plants is due to vapour transport from the soil, rather than to translocation through the plant (Fries and Marrow, 1981; Buckley, 1982; Bacci and Gaggi; 1985).
The toxicity of PCBs to terrestrial plants has not been studied extensively. Since PCBs are lipophilic and less mobile, their effects are difficult to monitor in the whole plant. A few studies have shown that PCBs in soils can interfere with the growth of the plant when applied at a concentration much higher than observed in the environment. For instance, Strek et al. (1981) noted a significant decrease in height, fresh top weight, and inhibition of cumulative water use in soybean and beet root at 1000 µg/g Aroclor 1254; however, corn and sorghum plants were unaffected.
Livestock can become exposed to PCBs applied to land by ingestion of contaminated soil when grazing. Soil consumption by grazing dairy cows can be as high as 14% of dry-matter intake when the amount of available forage is low and no supplemental feed is used (Healy, 1968; Fries, 1982). Using a worst-case scenario, Fries (1982) noted that 1.0 µg/g of PCBs in surface soil could cause milk-fat residues of 0.7 µg PCBs/g in dairy cattle, and tissue (edible) residues of 0.23 µg PCBs/g in beef cattle. It was assumed that the steady-state milk-fat concentrations are about five times the diet concentrations (dry weight) in dairy cattle, and that the steady-state body fat concentration will be similar in non-lactating animals. Note that the administrative guideline established by Health and Welfare Canada for milk or dairy products and beef is 0.2 µg/g (fat basis) (Grant, 1983).
10.2 Criteria from the Literature
Criteria for PCBs in water used for irrigation were not found in the literature. On the other hand, various jurisdictions have guidelines for cleanup of soils contaminated with PCBs from spills (Table 21). The guidelines shown in Table 21 suggest that the urgency for remediation is a function of PCB concentration in contaminated soils. Furthermore, it would appear that PCBs in soil would pose a minor threat to the environment as long as their concentration is <1.0 µg/g.
10.3 Recommended Criteria
It is recommended that the total PCB concentration in irrigation water should not exceed 0.5 µg/L.
10.4 Rationale
The PCB criterion for irrigation water was designed to limit undesirable accumulation of PCBs in soils in an agricultural environment. The information presented in Table 21 and the results obtained by Fries (1982) in his study concerning the ingestion of PCB-contaminated soil and its effect on livestock formed the basis of the criterion. A PCB concentration of 0.3 µg/g-soil (dry weight) was assumed to be the safe level in this document.
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TABLE 21 PCB Criteria for Contaminated Soil Cleanup |
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|
Criteria |
Criteria Values
|
Jurisdiction |
Date |
Reference |
|
Level considered to be contaminated;
|
> 5.0 |
Quebec |
1985 |
Bernier (1985) |
|
Target level for cleanup |
< 5.0 |
Saskatchewan |
1985 |
Chan (1985) |
|
guideline for further investigation
of contamination; target level for cleanup of residential area |
1.0 10.0 1.0 to 5.0 |
Holland Holland Holland |
1983 1983 1985 |
NMHPE (1983) NMHPE (1983) Beaulieu (1985) |
|
Investigation level (level A) for residential, recreational, & agricultural land use; Remediation level (level B) for residential, recreational, & agricultural land use; Remediation level (level c) for commercial or industrial land use |
0.1 5.0 50.0 |
British Columbia |
1989 |
BC MOE (1989) |
|
guideline for further investigation; guideline for remediation; guideline for urgent remediation |
1.0 5.0 10.0 |
France |
1985 |
Beaulieu (1985) |
|
TSCA regulation for cleanup of spills <1 lb PCBs; TSCA regulation for cleanup of high-conc. spill or low-conc. spill of 1 lb PCBs in outdoor electrical substation; TSCA regulation for cleanup of high-conc. spill of 1 lb PCBs in restricted access areas; TSCA regulation for cleanup of
high-conc. spill |
< 1.0 25.0 or 50.0 + notice 25.0 10.0 + excavation of top 25 cm;
|
U.S. EPA |
1987 |
U.S. EPA (1987) |
Given (i) the bulk density of a soil to be 1500 kg/m3, (ii) the concentration of PCBs in irrigation water at 0.5 µg/L , (iii) the irrigation rate of 1.0 m3/m2/a, and (iv) the PCBs in irrigation water to be retained in the top 0.15 m of the soil, the soil in question will accumulate PCBs at the rate of:
= (0.5 µg PCBs/L) x (1.0 m3/m2/a) x (1/0.15 m) x (1 m3/1500 kg-soil) x (1000 L/m3) = 2.2 µg PCBs/kg-soil/a;
provided there is no loss of PCBs from the soil. At this rate it will take at least (300 µg PCBs/kg-soil) / (2.2 µg PCBs/kg-soil/a) = 136.4 years for the soil to accumulate PCBs to the assumed safe level of 0.3 µg/g.
In practice, PCB levels of this magnitude (i.e., 0.5 µg/L) will not be encountered in irrigation water, and for the majority of cases the much lower criteria for the protection of aquatic life (0.1 ng/L) would apply to waters used for irrigation.
Note that the degradation of PCBs in the soil was not considered in the above calculations. This lack of consideration for degradation combined with the fact that PCB levels in ambient waters will generally be determined by the much lower aquatic life criterion, will result in a period, for PCBs to accumulate in soil to a safe level, much longer than that shown above (i.e., 136.4 years).