As a former geological engineer working in the environmental remediation business 16 plus years ago, I was responsible for design and construction of groundwater remediation systems at old landfills. Old landfills are notorious as many were just old gravel pits which were filled with garbage. In this garbage would be all sorts of hazardous waste from a time when nobody cared about such things. When looking at Mass Spectroscopy plots I would see hundreds of peaks representing some sort of organic chemical. Many of these could be identified as a pollutant, like benzene, but many were “unknowns”. As long as we met the Maximum Contaminant Levels (MCLs) established by the EPA all was fine, except the water would still contain many “unknowns”. And even if we had identified all the “unknowns” since no MCL was established we were still good-to-go.
The treated groundwater had to go somewhere, either re-infiltrated off-site or discharged to a nearby stream or river. Again we had to meet discharge limits, which we did. But everyone knew full well that there were pollutants still in the water. Now what will happen in the future when one of these “unknowns” or unregulated chemicals become regulated? Good question.