Research considerations for more effective groundwater monitoring.

Since numerous pathogens occur in feces, water is monitored for fecal contamination using indicator organisms rather than individual pathogens. Although this approach is supported by health effects data in recreational waters, it is questionable when used for drinking water. Most outbreaks in groundwater occur in systems that have not violated the US EPA's maximum contaminant limit (MCL) for total coliforms within 12 months before the outbreak. Additionally, environmentally stable viruses and parasites are often detected in drinking water samples with no detectable indicators. Recent detections of Escherichia coli O157:H7 and Campylobacter jejuni in groundwaters in the apparent absence of indicators also cast some doubt on the worth of indicators for fecal bacterial pathogens. Individual pathogen monitoring is now technically achievable but currently unreasonable due to the number of possible pathogens and the costs involved. Several alternatives to pathogen monitoring could significantly reduce the frequency at which pathogens occur in waters testing negative for indicators: (i) increasing sample volumes for indicators, (ii) increasing monitoring frequency, (iii) using a suite of indicators, (iv) using a more conservative polymerase chain reaction (PCR) method, (v) sampling when fecal contamination is most likely present or (vi) any combination of these options.