Screening assessment report for chlorinated naphthalenes: appendix A

Appendix A

Persistence in sediments

Tri- to hepta-chlorinated naphthalenes (CNs) can also be shown to meet the criterion under Persistence and Bioaccumulation Regulations of the Canadian Environmental Protection Act, 1999 (CEPA), for persistence in sediment (half-life in sediment of one year or more). Gevao et al. (2000) measured CNs in a sediment core from Esthwaite Water, a semi-rural lake in northwest England. Using the lowest homologue group concentration found in the sediments of this lake--0.619 µg hexa-CNs/kg (dated from 1962 and measured in 1995)--and assuming a half-life of one year, one can calculate the concentration of CNs that would have had to have been deposited in the sediments in 1962 for this concentration to currently be there. Using a first-order decay scenario, the following calculation is made:

C(t) = C_oe^-kt

• C(t) is the concentration, measured in 1995, from sediments dated from 1962 (0.619 µg/kg);
• C_o is the original concentration deposited in 1962;
• k is the reaction rate, which is equal to 0.693, assuming a half-life of one year (t_1/2 = ln 2/k); and
• t is the number of years from 1962 to 1995: 33.

Therefore,

0.619 = C_oe^{-(0.693) (33)}
C_o = 5.32 x 10⁹ µg/kg
 = 5.32 kg/kg

Therefore, 5.32 kg/kg of hexa-CNs would have had to have been deposited in the sediments in 1962 for a concentration of 0.619 µg/kg to exist in 1995, assuming a half-life of one year. Since that amount of deposition was not possible, it shows that the half-life of CNs in sediment must be longer than one year. Similar calculations were also carried out for the other homologue groups Gevao et al. (2000) had measured (tri- to hepta-CNs). Higher C_o values were estimated for each of these homologue groups, suggesting that these groups also have half-lives of longer than one year, thus meeting the CEPA criterion for persistence in sediment.

Persistence in soils

Using the data from Meijer et al. (2001), who measured the concentration of CNs over time in archived agricultural soil from the U.K., it is possible to show that CNs meet the criterion for persistence in soil specified in the Persistence and Bioaccumulation Regulations of CEPA (half-life equal to or greater than 182 days (0.5 year) in soil; Canada 2000). Sewage sludge containing 245 ng/g dry wt. of CNs was applied to the subject agricultural soil in 1968 and mixed in to a depth of 15 cm. The soil received no other inputs of CNs over the years, other than atmospheric deposits. An archived sample of the soil from 1972 was found to contain 0.4061 ng/g dry wt. of penta-CNs. In 1990, a sample from this same soil was found to contain 0.2853 ng/g dry wt. penta-CN. The half-life of CNs in this soil can be calculated using the first-order decay equation:

C(t) = C_oe^-kt

• C(t) is the concentration of 0.2853 ng/g, measured in 1990;
• C_o is the concentration of 0.4061 ng/g, measured in 1972;
• k is the reaction rate; and
• t is the number of years from 1972 to 1990: 18.

Therefore, to solve for k,

k = [ln (C(t) / C_o )] / -t
k = [ln (.2853/.4061)] / -18
k = 0.0196

t_½ = ln 2/k
t_½ = ln 2/0.0196
t_½ = 35.33

Therefore, the half-life of penta-CNs in the soil is 35.33 years, demonstrating that the penta-CN homolgue group meets the CEPA criterion for persistence in soil (half-life in soil of 182 days of more).

Based on the data of Meijer et al. (2001), who analyzed 39 CN congeners or groups of congeners (ones that could not be analytically separated) in the soil samples, the hexa- and hepta-CN congeners 66/67, 64/68, 69, 71/72, 63, 65, 73 and 74 were found to be very persistent, since their concentrations did not change significantly over the 18-year period from 1972 to 1990. All of the other homologue groups analyzed, which included tri-, tetra- and penta-CNs, were also found to meet the CEPA criterion for persistence in soil, with half-lives in the soil ranging from 7 to 9.5 years for the tri-CNs and from 10.6 to 18 years for the tetra-CNs, and with a half-life of 35.3 years for the penta-CNs as a group.