Objective: This study was undertaken to assess the contribution of a large dental treatment facility to the mercury levels (Total and Methyl) seen in wastewater.  Methods: Three ISCOŽ model 6700 series composite samplers were placed in sewer lines upstream and downstream from a large (117 chair) Navy dental treatment facility.  Composite samplers were programmed to collected 50 ml samples every 15 minutes over 24 hours.  Composited wastewater samples were retrieved daily for 30 days.  Sample Site #1 was located one-half mile upstream from the clinic.  Sample Site #2 was located directly downstream from the clinic.  Sample Site #3 was located one mile downstream from the clinic.  Dye studies were done to confirm the flow patterns from the clinic.  A total of 30 composite samples were collected and analyzed for total Hg utilizing EPA standard methods 7470A and 1631.  Six samples were analyzed for methyl mercury utilizing EPA method 1630. Results:  Results are presented in the table below:

HgT =Total Mercury, MeHg=Methyl Mercury, MDL=Method Detection Limit (0.2ppb for HgT and 0.02 ppt for MeHg), ppm=parts per million, mg/Liter, ppb=parts per billion, μg/Liter..

Conclusions:  Relative to mean upstream total mercury, the data show a 12-fold increase in total mercury leaving the dental clinic and a 3.6-fold increase in methyl mercury.  One mile downstream of the clinic the total mercury returned to upstream levels but the methyl mercury levels remained approximately the same. This study was supported by the US Navy Bureau of Medicine and Surgery and a grant from the USEPA Great Lakes National Program Office (IAG# DW17947929-01-0).

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