(c) If separate TMDLs are prepared for different, but overlapping, segments of the same
watershed and the separate TMDLs each include WLAs for the same toxic substances for 1 or
more of the same point sources, then WQBELs for that toxic substance for the point source or
sources shall be consistent with the most stringent of the WLAs to ensure attainment of all
applicable water quality standards.
(d) Each TMDL shall include an MOS sufficient to account for technical uncertainties in
establishing the TMDL and shall describe the manner in which the MOS is determined and
incorporated into the TMDL. The MOS may be provided by leaving a portion of the loading
capacity unallocated or by using conservative modeling assumptions to establish WLAs and LAs.
The MOS may also include reserved allocation for future growth.
(e) Where appropriate and where sufficient data or sediment criteria are available, TMDLs
shall prevent the accumulation of the toxic substance in the sediment to levels injurious to
designated or existing uses and shall reflect contributions to the water column from the sediments.
(f) Where appropriate and where sufficient data are available, TMDLs shall reflect
loadings of toxic substance discharges resulting from wet- weather events
(g) The representative background concentration of toxic substances to develop TMDLs
and WLAs calculated in the absence of a TMDL shall be established as follows:
(i) "Background" represents all toxic substance loadings as specified by the following:
(A) Flow from upstream waters into the specified watershed, water body, or water body
segment for which a TMDL or WLA in the absence of a TMDL is being developed.
Enter the specified watershed, water body, or water body segment through
atmospheric deposition, chemical reaction, or sediment release or resuspension.
(ii) When determining what available data are acceptable for use in calculating
background, the department shall use its best professional judgment, including consideration
of the sampling location and the reliability of the data through comparison, in part, to
detection and quantification levels. When data in more than 1 of the data sets or categories
described in paragraph (iii) of this subdivision exists, best professional judgment shall be used
to select the data that most accurately reflects or estimates background concentrations. Toxic
substance degradation and transport information may be considered when using toxic
substance loading data to estimate a water column concentration.
(iii) The representative background concentration for a toxic substance in the specified
watershed, water body, or water body segment shall be established as the geometric mean of
acceptable water column data or water column concentrations estimated through the use of
acceptable or projected toxic substance loading data. When determining the geometric mean of
the data for a toxic substance that includes values both above and below the detection level,
values less than the detection level shall be assumed to be present at 1/2 of the detection level if
the detection level is less than the lowest water quality value for that substance. If all of the
acceptable data in a data set are below the detection level for a toxic substance, then all the data
for the toxic substance in that data set shall be assumed to be zero. If the detection level of the
available data is greater than the lowest water quality value for the substance, then the background
concentration will be determined by the department on a case-by-case basis after considering all
representative data, including acceptable fish tissue data.
(h) If the margin of safety does not include a reserved allocation, then any increased
loadings of the toxic substance for which the TMDL was developed that are due to a new or
expanded discharge shall not be allowed unless the TMDL is revised in accordance with these
procedures to include an allocation for the new or expanded discharge.