Monday, December 29, 2008

EPA Proposed Stormwater Eflluent Limits - Comparison To Benchmark Concentration

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

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.

Caltha LLP provides expert technical support to dischargers subject to State and EPA stormwater discharge permit, including permitting, SWPPP training, stormwater monitoring, site inspections, and overall compliance program development.

For further information contact Caltha LLP at
Caltha LLP Website

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