This bill creates a new classification for riverine impoundments that thermally stratify during some portion of the year. Currently, these waters are classified as Class C water bodies, and are limited to 4 impoundments: Gulf Island Pond, Dolby Pond, Burnham Pond and the Fort Halifax impoundment. The bill does not apply to impoundments classified as GPA.

Historically, impoundments were classified as either Class C or Class GPA depending upon the presence of upstream point sources. Impoundments downstream from point sources were classified as riverine waters, for example, Class C, because discharges were prohibited to GPA waters.

A riverine designation, however, does not accurately reflect the physical characteristics of some of these impoundments. Unlike free-flowing rivers, some riverine impoundments exhibit important lake-like characteristics, such as thermal stratification. The RI Class provides appropriate criteria reflective of these physical characteristics while, at the same time, ensuring maintenance of existing water quality.

For purposes of determining whether water quality is sufficient to support habitat for fish and aquatic life, water quality is to be measured when existing fish populations are present and at locations in the water body necessary to support all life stages of existing fish populations. In determining whether the water is of a quality to support other aquatic life, effects associated with thermal stratification, including inhibited mixing, are considered natural conditions.

This bill establishes a de minimis level for total phosphorus discharges. The de minimis level is based on the minimum detectable total phosphorus concentration using low detection limit analyses and clean sampling techniques.

This bill repeals and replaces the existing definition of "indigenous" by specifying that introduced species are not indigenous to waters of the State. A new definition of thermal stratification has been added.