Thursday, March 5, 2009

Turbidity Effluent Limits for Stormwater Discharges

In December 2008, US EPA published proposed effluent limits for stormwater discharges from construction sites. The proposed rules included a numeric effluent limit of 90 NTU. NTUs are a standard measurement of turbidity in water and of suspended material in the water.

[Read more about proposed stormwater effluent standards]

Benchmark values and previous stormwater effluent limits have always been expressed as Total Suspended Solids (TSS). Turbidity is related to TSS, however the relationship between turbidity and TSS is not always direct, and will be affected by a number of different factors.

Stormwater benchmark concentrations for TSS usually range from 100 to 250 mg/L. Using a few typical conversions between NTU and TSS, it is predicted that an effluent limit of 90 NTU will be roughly equivalent to 45 to 65 mg/L expressed as TSS. Therefore, it is projected that the proposed stormwater effluent limit is about one-half the lowest current benchmark concentration for TSS.

[Read more about the use of stormwater benchmark values]

Caltha LLP provides expert consulting services to public and private sector clients nationwide to address water quality standards, wastewater permitting and assessing potential impacts of chemicals in the aquatic environment.

Caltha LLP Aquatic Toxicology / WQ Standards Services Website