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For more information, contact David Eppstein by email at
deppstein@masco.harvard.edu, or by calling 617-632-2860.

 
6.0 MERCURY LOADINGS CALCULATIONS

We prepared two spreadsheets that list the uncensored minimum and maximum half-year mercury concentration averages for each sampling location along with the associated discharge flow estimates. Fiscal Year 1995 mercury loadings are presented in one spreadsheet identified as Table C-1 in Appendix C. FY 1996 mercury loadings are presented in the other spreadsheet identified as Table C-2 in Appendix C.

In the spreadsheets, we calculated mercury discharge loadings for each sampling location by multiplying mercury concentration averages by the associated flow estimates. As detailed above, we inserted the applicable minimum and maximum half-year Group Concentration Averages (GCA's) into the spreadsheets for locations that did not have any mercury discharge concentration data.

As shown on Page 4 of Table C-1, for example, 22 Clinical sampling locations had mercury concentration data for the first half-year of FY 1995 out of 64 total Clinical sampling locations (about 34 percent). Because the other sampling locations had no test data, we used the appropriate Clinical GCA’s to estimate their mercury discharge loadings. For the second half-year of FY 1995, the Clinical GCA’s were based upon 30 of the overall 64 Clinical locations (about 47 percent). Since the numbers of locations contributing to these GCA’s were statistically significant, we expect this overall loadings estimating method to be valid.

Page 4 of Table C-1 also shows the total Clinical daily discharge flow and half-year minimum and maximum mercury loadings in milligrams per day, pounds per day, and pounds per year. Based upon the total Clinical discharge flow and mercury loadings, equivalent discharge mercury concentrations are shown. For insight into the effects of the highest individual discharge loadings, the table also shows for each half-year period:

- the fractions contributed by the maximum mercury loadings expressed as percentages of the overall Clinical loadings, and

- overall Clinical loadings in pounds per day after subtracting the individual maximum loadings.

As shown on Page 11 of Table C-1, only about 15 percent of the Other study group (13 to 14 locations of the entire 85 Other locations) had mercury test data in FY 1996. This group had the smallest percentage of locations having mercury test data. In general, the absence of extensive mercury test data is the result of Sewer Discharge Permits that do not require frequent discharge sampling and measurement of mercury concentrations. The resulting loadings estimates can be considered as less reliable for the locations and half-year periods having high percentages of locations where we had to use GCA’s instead of specific actual mercury concentration data.

Thus, it is possible that the reliability of the overall Other group loadings estimates was lower than that of the other groups. Overall, however, we regard the GCA’s as representative of the minimum and maximum half-year mercury discharge concentrations of each respective study group.

We have summarized the estimated mercury discharge loadings in Table 3 for FY 1995 and Table 4 for FY 1996. The discharge loadings are also presented graphically in Figure 1 for FY 1995 and Figure 2 for FY 1996.

As we examine these discharge loadings tables and figures, we see that the Research group discharged the highest mercury loadings during FY 1995. In the second half-year, the Research group loadings significantly increased. As shown in the summary section for Research facilities in Table C-1, one Research location contributed about 35 to 40 percent of the total group mercury loadings for that period. In turn, the overall Research group was responsible for more than 80 percent of the total mercury discharge loadings of all five study groups.

Mercury discharge loadings of the Clinical and Other groups were comparable in FY 1995 and both were higher in the second half-year. The Incinerator group, consisting of only three sampling locations, contributed larger mercury loadings than the Laundry group with ten sampling locations.

The results for FY 1996, however, were quite different. Overall, mercury discharge loadings from all groups were higher than those for FY 1995, except the second half-year loadings of the Research group. In FY 1996, the Laundry group was responsible for peak loadings in the first half-year that were higher than 0.03 pounds per day. In this case, however, one Laundry location was responsible for more than 95 percent of the group mercury loadings. As shown on Page 11 of Table B-2, two samples of a total of six samples collected at that Laundry location produced an average mercury test concentration of nearly 400 µg/L (ppb).

FIGURE 1

TABLE 3
FY1995
Total Mercury Loadings

FIGURE 2

TABLE 4
FY1996
Total Mercury Loadings

Note: In FY 1995, there were 10 Laundry sampling locations.

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08/16/2006

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