Draft Ohio Industrial Permit, Stormwater Monitoring and Effect of Hardness on Benchmarks

The proposed stormwater monitoring requirements in the proposed General Permit for Storm Water Discharges Associated with Industrial Activity (Multi-Sector General Permit, or MSGP) include both effluent monitoring requirements for some facilities and benchmark monitoring requirements for most permitted facilities in Ohio. Consistent with the US EPA MSGP, the OEPA draft permit contains numeric effluent limitations based on Effluent Limitations Guidelines for Sectors A, C, D, E, J and O. [more information on difference between effluent limitations and benchmark concentrations]

The Ohio MSGP requires benchmark monitoring, specific for each industrial sector, for pollutant parameters which US EPA has determined to be of concern by industrial sector. The draft Ohio general permit requires that 19 of the 29 industrial sectors perform benchmark analytical monitoring. The benchmark monitoring requirements are not effluent limitations. Benchmark values represent a level to determine whether a facility’s SWP3 is effective.

The benchmark monitoring is based on a collection of 4 quarterly samples. If the average of the 4 monitoring values for any parameter does not exceed the benchmark value, the permittee has fulfilled their monitoring requirements for that parameter for the permit term. If this average exceeds the benchmark for a parameter, then the permittee will need to review their SWP3 and control measures and modify accordingly. Additional monitoring would be required. The US EPA MSGP’s primary source of benchmark concentrations is derived from EPA’s National Water Quality Criteria. For a majority of the benchmarks, USEPA used the acute aquatic life, fresh water ambient water quality criteria. USEPA believes these acute freshwater values best represent the highest concentrations at which typical fresh water species can survive exposures of pollutants for short durations, such as a storm discharge event.

[read recent comparison of benchmark concentrations to typical industrial sector monitoring data]

Ohio EPA evaluated each benchmark monitoring parameter and followed US EPA’s methodology, but used Ohio Water Quality Criteria when available. As such, some US EPA benchmark values have been changed to be consistent with Ohio Water Quality Criteria. The benchmark values of some metals are dependent on water hardness. For these parameters, permittees will need to determine the hardness of the receiving water to determine the benchmark concentration that applies to their facility. Depending on the condition of the receiving water, benchmark values for heavy metals can vary widely between facilities.

[read of summary of the revised OEPA genernal stormwater discharge permit]

Caltha LLP provides expert consulting services to public and private sector clients in Ohio & nationwide to address Stormwater Permitting & Regulatory Support, Stormwater Pollution Prevention Plans (SWP3), Stormwater Monitoring (including determination of the effects of receiving water hardness on benchmark concentrations), and Stormwater Training.

For further information contact Caltha LLP at

info@calthacompany.com or Caltha LLP Website