Sec. 1. 38 MRSA §464, sub-§4, ¶J is enacted to read:

J.__Licensed discharges existing on or after the effective date of this paragraph that do not increase the phosphorus concentration in the receiving water body more than one part per billion are considered de minimis and any such discharges are not considered to be causing or contributing to any water quality impairment or to a failure of the receiving water body to meet the standards of classification.__If a water quality-based effluent limitation for phosphorus is necessary, the department shall allow, at a minimum, a monthly average mass loading in pounds per day of total phosphorus equal to a one part per billion increase in the receiving water body.__The monthly average mass loading must be calculated using a phosphorus concentration of one part per billion at the 7- day low flow that can be expected to occur with a frequency of once in 10 years at the point of discharge.

Sec. 2. 38 MRSA §465-D is enacted to read:

§465-D.__Standards for classification of riverine impoundments

The department shall have one standard for the classification of riverine impoundments that thermally stratify.__Impoundments of rivers that are defined as great ponds pursuant to section 480-B and are classified as GPA under section 467 remain subject to the standards in section 465-A.

1.__Class RI waters.__Class RI is the sole classification of riverine impoundments that thermally stratify.

A.__Class RI waters are of such quality that they are suitable for the designated uses of drinking water supply after treatment; fishing; recreation in and on the water; industrial process and cooling water supply; hydroelectric power generation except as prohibited under Title 12, section 403; and navigation; and as habitat for fish and other aquatic life.

B.__Discharges to Class RI waters may cause some changes to aquatic life, as long as the receiving waters are of sufficient quality to support all applicable life stages of species of fish that are indigenous to the receiving waters and maintain the structure and function of the resident biological community.__In determining whether water is of sufficient quality to support indigenous fish, water quality must support fish populations in those seasons, and in those