Many of these chemicals are metals, some of which are naturally occurring. The greatest benefit of the Phase 2 study is that it will serve as a definitive basis for future studies. The study will also serve to set priorities for wastewater treatment upgrades, nonpoint source pollution best management practices, and other potential practices to reduce environmental impact. Criteria and screening levels were used to determine whether measured concentrations of substances had the potential to cause long-term health problems for humans and aquatic life. In addition, the Water Quality Index was used, which is an average of the effects caused by varying levels of parameters measured in a flowing body of water.
Percentiles are the relative standing of a measurement.
For example, suppose you scored an 80 on an examination and you want to know how you fared in comparison with others in your class. In specific terms, criteria refer to specific numerical based concentrations for the protection of aquatic life and human health. Screening levels are more general, and are mainly based on State and National 85 th percentiles. From a statistical standpoint then, toxic chemicals were detected more frequently in the tributaries than in the mainstem. In addition, relatively few chemicals exceeded screening levels.