AWWA ACE71096

A summary of the occurrence of N-nitrosodimethylamine (NDMA) in Metropolitan's treated waters and within its distribution system from February 2006 to November 2008 is presented. Based on quarterly data, NDMA concentrations ranged from not detected (ND, < 2.0 ng/L) to 8.3 ng/L in the effluents from Metropolitan's five treatment plants, with median values for each plant ranging from ND to 2.5 ng/L. Within Metropolitan's distribution system, NDMA concentrations ranged from ND to 10 ng/L. At some sampling sites within Metropolitan's treated water distribution system, NDMA levels did not appear to change significantly from those measured at the treatment plant effluent. This may have been due to the destruction of NDMA precursors during pre-oxidation at the treatment plants and/or short detention times within the distribution system (where NDMA is slow to form). At other sites, significantly greater NDMA levels have been measured, which has been of concern. Higher NDMA concentrations may be associated with longer detention times and/or the practice of supplemental chlorine addition at finished-water reservoirs within Metropolitan's distribution system. Includes 11 references, figures, table.