Laboratory testing on drinking water is performed for analysis of chemical, radiological, and biological contaminants. Although biological testing by definition focuses on microorganisms, there is overlap with chemical analysis because many toxins occur naturally. The existing infrastructure for chemical analysis is more established than that for biological monitoring, because water contamination owing to pesticides, pollutants, or even metals has been monitored for decades and because chemical analysis has simpler, less expensive, and more routine regulatory compliance reporting requirements. Regardless of the high numbers of naturally occurring biological contaminants, as well as the costs associated with their identification, all public water suppliers must, by law, test their water for biological organisms (L. Lindsay, personal communication). To determine the prevalence of many of these naturally occurring organisms, a study in the early 1990s tested the source waters of 66 treatment plants. The study found that 97% of the tests came back positive for Cryptosporidium and/or Giardia species.
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