Page 141 - California Stormwater Workshop Handouts

Industrial General Permit Fact Sheet

value (minimum level or reporting limit) is that these values are very low and
are unlikely to contribute to an NAL exceedance. There are statistical
methods to include low values when calculations are for numeric criteria and
limitations, however, the NALs in this General Permit are approximate values
used to provide feedback to the Discharger on site performance, and are not
numeric criteria or limitations. Therefore, it is not necessary to include these
insignificant values in the calculations for the NALs. For Dischargers using
composite sampling or flow measurement in accordance with standard
practices, the average concentrations shall be calculated in accordance with
the U.S. EPA Guidance Manual for the Monitoring and Reporting
Requirements of the NPDES Multi-Sector Storm Water General Permit.14

i. Instantaneous maximum NAL exceedance - the Discharger is required to
compare all sampling and analytical results from each distinct sample
(individual or combined) to the corresponding instantaneous maximum NAL
values in Table 2 of this General Permit. An instantaneous maximum NAL
exceedance occurs when two or more analytical results from samples taken
for any parameter within a reporting year exceed the instantaneous
maximum NAL value (for TSS and O&G), or are outside of the instantaneous
maximum NAL range (for pH).

b. Instantaneous maximum NAL analysis

In its June 19, 2006 report, the Blue Ribbon Panel of Experts (Panel) made
several specific recommendations for how to set numeric limitations in future
industrial storm water general permit(s). For sites not subject to TMDLs, the
Panel suggested that the numeric values be based upon industry types or
categories, with the recognition that each industry has its own specific water
quality issues and financial viability. Furthermore, the Panel concluded:

To establish Numeric Limits for industrial sites requires a reliable
database, describing current emissions by industry types or categories,
and performance of existing BMPs. The current industrial permit has not
produced such a database for most industrial categories because of
inconsistencies in monitoring or compliance with monitoring
requirements. The Board needs to reexamine the existing data sources,
collect new data as required and for additional water quality parameters
(the current permit requires only pH, conductivity, total suspended solids,
and either total organic carbon or oil and grease) to establish practical
and achievable Numeric Limits.

The Panel suggested an alternative method that would allow the use of the
existing Water Board dataset to establish action levels, referred to as the “ranked
percentile” method. The Panel recommended:

14 U.S. EPA. NPDES Storm Water Sampling Guidance Document. Web. July 1992.
<http://www.epa.gov/npdes/pubs/owm0093.pdf>. [as of February 4, 2014].

Order 2014-0057-DWQ 57