Storm Water Compliance Series: Permit Benchmark Data

Friday, September 11, 2015

This article is the first in a series discussing data and guidance about industrial storm water in Ohio, presented by Pat Kneip of PSARA at the 25th Annual MEC Environmental Permitting in Ohio Conference in July 2015.

The Ohio Environmental Protection Agency Ohio (EPA) developed the NPDES Multi-Sector General Permit (MSGP) because conditions affecting the presence of certain pollutants in storm water vary by industry. The current permit, effective January 1, 2012 through December 31, 2016, covers facilities in 29 industrial classifications. Those subject to the requirements of the MSGP may already be familiar with the simplest method of complying: applying for a No Exposure Certification and avoiding the permit altogether. This certification, which is facility-specific rather than company-specific, will be discussed in a subsequent article.

In the previous version of the MSGP, certain facilities were required to perform annual sampling for specific parameters. In the current version, storm water benchmark sampling requirements have been added. Facilities are required to sample for specific parameters, based on the applicable industry classification or classifications, during four of the twelve quarters during the first three years of the permit. During the fourth year of the permit, facilities must compare their results with the published benchmarks and make adjustments to achieve compliance if necessary.

There has been a steep learning curve and adjustment period to these new requirements. Labor costs associated with sampling for these benchmarks can add up. As we near the fourth year of the permit, the Ohio EPA, industry, and consultants alike are beginning to evaluate the data collected thus far, both in terms of what it means for compliance, as well as the quality of the data itself.

While the current dataset is still relatively small, PSARA requested all data through May 2015 from the Ohio PEA earlier this summer. Our initial review of the data generated several questions. For example, significant outliers were reported in most benchmark categories—so significant they led us to wonder whether lab results are being interpreted correctly, or whether there might be unit conversion issues.  While all ranges in the permit are given in mg/L, several of the parameters were reported in µg/L, which could account for some of the outliers.

Two box-and-whisker plots from our data analysis are shown below. Box-and-whisker plots have the advantage of capturing the range of data, including high and low outliers, as well as showing where the average falls, and how this average is shifted by the outliers.

The pH plot, containing 179 data points, is an example of a parameter where the data falls into the anticipated range of values, although Sector J: Mineral Mining and Dressing did have the highest outlier, with one reported pH value of 30 s.u. As pH ranges from 0.0 to 14.0 s.u., this data point is clearly questionable, but we included it to demonstrate some of the data quality issues previously mentioned. (pH value is not a benchmark per se, but rather an effluent limitation.)


The total suspended solids (TSS) plot, on the other hand, displays data that creates more questions. The high outliers are literally off the chart for most of the reporting sectors, the highest value of 5,950 mg/L reported by one facility in Sector M: Automobile Salvage Yards. Much of the data falls under the benchmark, but some data and two of the average values (pulled up by the high outliers) are over the benchmark value.

Storm Water Compliance Series: Permit Benchmark Data | Psara Psara

Our next installment will delve into our analysis of the data for hardness as well as several metals.