Many factors go into determining water quality based effluent limits for permit renewals under the WV NPDES program. First, the permit writer determines the Reasonable Potential (RP) of a pollutant to exceed the applicable water quality criteria. Permit writers use the “Reasonable Potential” Procedures of Chapter 3.3 of EPA’s Technical Support Document for Water Quality Based Toxics Control, available here.
Past analytical data will impact the RP determination. If there is large variability in the data, the RP calculation will result in the determination that Reasonable Potential exists. Another important factor for past analytical data is the method detection limit (MDL). If the parameter is below the MDL, half of the MDL will be used in the calculations. High MDLs will not work to your advantage in RP calculations.
If the permit writer determines a pollutant has a Reasonable Potential, the permit writer utilizes an Excel workbook to determine Water Quality Based Effluent Limits (WQBEL). Sometimes this spreadsheet is referred to as the “Black Box.” The Black Box determines the WQBEL based on the most stringent water quality standard.
The spreadsheet is not available on the WVDEP website, but a copy can be requested from the permit writer.
Data elements that go into the spreadsheet include the following: past analytical data, stream mixing zones/background concentration and translators for dissolved metals.
The past analytical data can impact the WQBEL calculations in a number of ways. If the past data has large variability, this will impact the WQBEL. It is important to identify potential outlier samples at the time of analysis and repeat the analysis if possible.
Again, if the analytical values are below the MDL, half the detection limit will be used in the calculations. High MDLs will not work to your advantage in WQBEL calculations.
Some metal water quality criteria are in dissolved form. Metal translators are available for aluminum, arsenic, cadmium, chromium, copper, lead, nickel, silver and zinc. Without a translator study, it is assumed that 100% of the total recoverable form of the metal is in the dissolved form. A site-specific translator can be developed for a facility. US EPA has developed a guidance document for calculating a site-specific translator, available here. Site-specific metal translators can be as low as 0.30.
Many metals have very low water quality criteria. Clean sampling techniques should be used in the collection of all samples for metals. US EPA Method 1669 provides guidance on minimizing contamination during sampling, available here.
Before a translator study is undertaken by a permit holder, a sampling plan should be developed and shared with the permit writer. Any comments from the permit writer should be incorporated into the plan.
Once the translator study is completed a permit modification can be requested.
A mixing zone is essentially a zone in the receiving stream that dilutes the discharge to meet water quality criteria. The existing background concentration of pollutant must be known immediately upstream of the discharge to grant a mixing zone. If a mixing zone is not used, the water quality standard will be applied at the end of the pipe. The WVDEP’s Water Quality Standards/Mixing Zone Implementation Guidance (June 30, 1997), available here, can aid the development of a background water quality sampling plan or a mixing zone studies.
Before a background or mixing zone study is undertaken by a permit holder, a sampling plan should be developed and shared with the permit writer. Any comments from the permit writer should be incorporated into the plan.
Once the background study is completed, a permit modification can be requested.
It should be noted that report only monitoring requirements could result in future water quality based effluent limits. The data collected will be used at the next permit renewal to determine reasonable potential for water quality criteria exceedances. The issues identified for past analytical data should be applied to report only monitoring requirements.
This article was based on the NPDES Seminar attended on September 17, 2009 by author Joyce Gentry, Acacia Environmental Group LLC. For more information on the author see here.