xceptions
Total coliforms are not a perfect indicator of the actual or potential presence of harmful organisms. Some disease-producing organisms, especially protozoa such as Giardia and Cryptosporidium, are able to withstand treatment which kill the total coliform. These two protozoa are often found in surface waters (the principle carriers of these organisms) contaminated by human sewage or wildlife. However, for the majority of PWSs this is not a significant threat since most PWSs obtain their water from wells rather than surface-water sources such as rivers and lakes. For those PWSs that use surface water, a combination of coagulation, filtration, and disinfection has been successful and is recommended to reduce the risk of Giardia or Cryptosporidium contamination.