Identifying the source of mercury
What do you do when mercury is detected in your waste stream? Ideally, it was removed before it enters, but if mercury is found then the first step is to resample where it was found to make sure this was not a one-time event. It is difficult to identify the source of a single mercury hit; however, if repeat sampling shows elevated mercury levels, then the hunt for the contributor begins.
A proper Mercury Minimization Plan (MMP) will identify sampling requirements when mercury is detected, as well as list the known and potential contributors to investigate first. Typical known and potential contributors include dentist offices, photo developers, hospitals, schools, auto body shops or manufacturing plants in your community. If you have an Industrial Pretreatment Plan (IPP), you should sample their discharges for mercury levels. If not, then you need to set up sample points throughout the community to determine the general area where the mercury could be coming from.
Once you determine the general area of origination, then you begin sampling upstream of the location of the last elevated mercury sample. This can become very cumbersome and expensive, but to remain in compliance with your National Pollutant Discharge Elimination System (NPDES) permit, it must be done. Most NPDES permits restrict mercury discharges to 1.3 ng/l (parts per trillion).
This is why a well-written IPP, MMP or Sewer Use Ordinance could save your community significant time and money. If the mercury contributor is found, then education is the best method to eliminate the problem in the future. Substances that contain mercury can be disposed of using methods other than flushing it down the drain.
Local businesses may not be the cause of the problem. Another source of mercury contamination is Mother Nature. Atmospheric concentrations of mercury may be high due to air pollution and/or evaporation over large bodies of water. Mercury is released from the atmosphere is through precipitation which typically contains up to 40 ng/l of mercury. If your sewer system has substantial infiltration, mercury can enter the sewer system every time it rains or during spring snow melt. Mercury may also enter the sewer through inflow of groundwater, which can contain up to 25 ng/l of mercury.
Will you ever be able to eliminate all mercury from your sewer system? No. But you can eliminate many of the known contributors through good planning, ordinance enforcement and routine sewer repair.
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