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On May 18, 2016, the EPA published proposed revisions to the regulations governing the National Pollutant Discharge Elimination System (NPDES) program. One of the proposed revisions is to adjust how states grant allowances for dilution. Currently, federal regulations at 40 CFR 122.44(d)(1)(ii) allow states to account for, where appropriate, the dilution of effluent in the receiving water when preforming a reasonable potential analysis (RPA). EPA asserts that the current provision does not specify when a dilution allowance is “appropriate”. Therefore, EPA proposes that any allowance for dilution must comply with applicable dilution and mixing zone requirements and low flows established in state water quality standards (WQS) and be supported by data or analyses quantifying or accounting for the presence of each assessed pollutant or pollutant parameter in the receiving water.

The concentration or mass of a pollutant or pollutant parameter that can be safely mitigated by these various processes in the receiving water without exceeding any applicable WQS and without causing adverse effects is commonly referred to as the ‘‘assimilative capacity’’ of the receiving water. An assumption of “zero background” levels of a pollutant in an upstream water indicates that the full assimilative capacity of the water is available, while an assumption that the background concentration is at or above the applicable water quality criteria indicates that there is no remaining assimilative capacity. The selection of one of the end point values, or some value between these two extremes, is typically a matter of state policy. An assumption of zero background levels of a pollutant in an upstream water, in the absence of data or analyses to validate such an assumption, results in permit conditions that use as much as 100 percent of the receiving water’s dilution capacity to the discharging facility. Therefore, in situations where some of the pollutant is actually present in the upstream waters, an assumption of zero background concentration overestimates the available assimilative capacity of the receiving water and could result in limits that are not protective of applicable WQS.

In order to calculate a dilution allowance, any decision regarding the assimilative capacity of the receiving water must be supported by data or analyses quantifying or accounting for the presence or absence of each assessed pollutant in the receiving water. States may choose to collect data and information on the receiving water from applicants, either prior to issuance of the permit or as a condition of the permit. In limited circumstances (e.g., where ambient data are unavailable), permitting authorities may satisfy this requirement by conducting a qualitative analysis of the ambient level of a pollutant of concern; however, the analysis must be pollutant- and site-specific, supported by the available information, and documented in the record consistent with current federal requirements. Where the actual assimilative capacity of the receiving water cannot be accurately determined or predicted (e.g., by using data, models, or analyses), the permitting authority would be expected to establish effluent limits based on the application of applicable water quality criteria at the point of discharge (often referred to as ‘‘criteria end-of-pipe’’).

The proposed changes will undoubtedly impact municipal and industrial dischargers in Kentucky, especially those who have been granted a mixing zone or zone of initial dilution. Interested parties are encouraged to submit comments. All comments must be received on or before July 18, 2016.


William Shane is an Environmental Engineer at Smith Management Group. William can be reached at williams@smithmanage.com.

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A local company engaged in manufacturing imported a small amount of a chemical substance defined under TSCA. Faced with a potential EPA enforcement action with penalties assessed for noncompliance under TSCA of up to $32,500 per day per violation, the company called SMG for help.


SMG analyzed the company’s current TSCA procedures and assisted the company in developing a proactive, cost-effective compliance procedure. SMG also facilitated a training program to educate employees about TSCA.

SMG worked with the company to develop mechanisms that assured adherence with the policies that were being implemented for compliance. Procedures to promptly correct any potential violations and prevent future violations were also put into place.


SMG was able to show that the company complied with the relevant TSCA regulations and was improving their TSCA policies and procedures to assure that future issues were less likely to occur. The company was not subjected to the proposed penalties and now has mechanisms in place to maintain TSCA compliance.