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Proposed Sulfate Water Quality Standard


On January 9, 2008, the Indiana Water Pollution Control Board (“WPCB”) will consider preliminary adoption of an amendment to 327 IAC 2-1-6 which will create a new sulfate criterion that will replace the existing criterion of 1,000 mg/L. Revision of the sulfate criterion will impact current and future holders of National Pollution Discharge Elimination System (“NPDES”) permits containing sulfate limits. The Indiana Department of Environmental Management (“IDEM”) proposed the revised sulfate criterion in order to comply with the U.S. Environmental Protection Agency’s (“U.S. EPA”) requirement that IDEM revise and readopt a sulfate criterion that is protective of aquatic life in all surface waters.

The necessity to revise the sulfate criterion arises from the WPCB’s amendment of the sulfate standard in October 2004 from 250 mg/L to 1,000 mg/L in surface waters not used as a drinking water supply. It was understood at the time of the revision that the 1,000 mg/L sulfate criterion was an interim standard that would apply until the completion of a sulfate toxicity study conducted by the Illinois Natural History Survey (“INHS”). Once the results of the INHS study became available, the IDEM indicated it may propose to further revise the sulfate standard.

Based on the results of the INHS study, the U.S. EPA determined that Indiana’s current sulfate criterion of 1,000 mg/L is not protective of aquatic life in waters with an ambient hardness value of less than 109 mg/L. In general, water “hardness” is a measure of the amount of calcium and magnesium dissolved in water. In order to comply with the U.S. EPA’s determination, the proposed sulfate criterion revision will replace the existing single numerical criterion with equations developed by the INHS. The equations will allow the sulfate criterion to vary according to the hardness and chloride concentration of receiving waterbodies. The IDEM indicates that the sulfate criterion calculated in this manner will vary from 500 mg/L in low-chloride waters to over 2,500 mg/L in hard waters with average chloride concentrations. There are a limited number of streams with low hardness in Indiana. According to the IDEM, in most cases the proposed sulfate criterion determined according to the INHS equations will be less stringent than the current criterion.

Additional information regarding the proposed revision of the sulfate criterion may be obtained from the IDEM, Office of Water Quality, at 317-232-8635.

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