Risk in quality violations under uncertainty

Violation of water quality conditions occurs whenever levels of a contaminant exceed an established standard. Preventive measures are frequently introduced to reduce frequency of failures. This paper applies reliability analysis to examine stream water quality conditions. Satisfactory water quality condition means tolerating failure at an acceptable rate. Failures are random events, and to achieve long-term environmental benefit one would expect observed reliability to be higher than the required reliability. Likewise, an average time between failures should be several times the required average time between failures