Page 143 - California Stormwater Workshop Handouts

Industrial General Permit Fact Sheet

The percentile approach is a non-parametric approach identified in many
statistical textbooks for determining highly suspect values. Highly suspect values
are defined as values that exceed the limits of the outer fences of a box plot.
Upper limits of the outer fence are calculated by adding three times the inter-
quartile range (25th to 75th percentiles) to the upper-end of the inter-quartile
range (the 75th percentile). The California Stormwater Quality Association
calculated an NAL value of 401 mg/L for TSS using the percentile approach
using the Water Board dataset. The State Water Board performed the same
analysis with the same Water Board dataset and calculated a slightly different
value of 396 mg/L; therefore, the instantaneous maximum NAL value for TSS of
400 mg/L was established. Appling the percentile approach to the existing O&G
data results in the instantaneous maximum NAL value for O&G of 25 mg/L.

The State Water Board compared existing sampling data to the instantaneous
maximum NAL values and concluded that seven (7) percent of the total samples
exceeded the highly suspected value for TSS and 7.8 percent of the total
samples exceeded the highly suspected value for O&G. These results suggest
that the instantaneous maximum NAL values are adequate to identify drainage
areas of concern statewide since they are not regularly exceeded. Using best
professional judgment, the State Water Board concludes that an exceedance of
these values twice within a reporting year is unlikely to be the result of storm
event variability or random BMP implementation problems, and the use of the
percentile approach is therefore appropriate.

Due to issues with the ranges of concentrations and the logarithmic nature of pH,
statistical methods cannot be applied to pH in the same ways as other
parameters. Review of storm water sampling data by the State Water Board and
other stakeholders has shown that pH is not typically a parameter of concern for
most industrial facilities. Accordingly, a range of pH limits established in
Regional Water Board Basin Plans is implemented in this General Permit for the
instantaneous maximum NAL values. Most Basin Plans set a water quality
objective of 6.0 - 9.0 pH units for water bodies, an exceedance outside the range
of 6.0 - 9.0 pH units is consistent with the water quality concerns for pH among
Regional Water Boards. An industrial facility with proper BMP implementation is
expected to have industrial storm water discharges within the range of 6.0 - 9.0
pH units.

High concentrations of TSS and O&G, or pH values outside the range of 6.0 –
9.0 pH units, in a discharge may be an indicator of potential BMP implementation
or receiving water quality concerns with other pollutants with parameters that do
not have an instantaneous maximum NAL value. The State Water Board may
consider instantaneous maximum NAL values for other parameters in a
subsequent reissuance of this General Permit, based on data collected during
this General Permit term.

The percentile approach is considered by many stakeholders to be the best
method to evaluate BMP performance and general effluent quality in a
community or population where the vast majority of the industrial facilities are
implementing sufficient pollutant control measures. The Water Board’s current

Order 2014-0057-DWQ 59