Salt Creek TMDL- Appendix H
Draft Report for USEPA Approval 30 July 2004
reference, there is no indication how that information may have been used in the TMDL analysis.
Having sound, measured SOD numbers provides much more reliability in calibrating realistic
oxidation and nitrification rates, and greatly reduces the possibility of a false conclusion in
determining the relative sources of oxygen demand, such as represented in figure 5-6.
The SOD data are discussed in the response to comment #72. The NIPC 208 study was
referenced, but the data were not used because the report is over 25 years old.
83. 5.3.1 - Diurnal Variation of DO Due to Algae and Photosynthesis: The report notes that QUAL2E
cannot represent time-varying flow and pollutant loads. It notes several other shortcomings, including
the sample period used for diurnal calibration (e.g., significant flow variability during the period,
inability to represent attached algae). It ends up concluding that the model is not capable of
simulating the full extent of the diurnal variation of DO. As previously pointed out, it is puzzling why
HSPF was not used instead. In particular, HSPF does not have the limitations of QUAL2E in
representing diurnal and flow-varied changes in DO and algal concentrations, can also represent
attached algae, and was successfully applied to Salt Creek and a range of other stream and river
conditions during the 208 process. A consequence of limitations of the selected model may be mis-representation
of critical factors, such as phosphorus and other nutrients, that contribute to observed
DO problems.
Please refer to the response to comment #80 concerning why HSPF was not used to model
diurnal variation.
84. It is reported that a future-conditions model run with increased point source loadings shows improved
DO conditions in the creek. This model result, and its explanation – “flow augmentation” – seem
counterintuitive and inconsistent with previous modeling results (e.g., NIPC and others). It also raises
further questions regarding the previously mentioned relationships between SOD, instream BOD and
ammonia, and diurnal algal effects, and the adequacy of their representation in a model that is
admittedly constrained in its ability to represent complex instream phenomena.
The improved DO concentration with increased flow was a very small increase and it did not
make a difference in the load allocations.
85. 6.4 - Dissolved Oxygen: This section notes that chlorophyll a concentrations in Salt Creek “did not
show any obvious eutrophication problem.” It is therefore concluded that the steady-state QUAL2E
model was appropriate for developing the DO TMDL. This seems to directly contradict both the
observed significant diurnal variations in DO (figure 4.14) and the simulated diurnal variability
(figure 5.8), albeit with a model that admittedly has limited ability to represent actual diurnal
variations. As a result, any results coming out of such steady state modeling that does not represent
algal-induced diurnal variations is suspect, at best, and likely to substantially underestimate the actual
degree of dissolved oxygen violations in the creek. It also leads directly to a likely erroneous
conclusion that there is no need to evaluate factors (i.e., phosphorus) that contribute to algal growth.
There is diurnal variation and higher chlorophyll a concentrations in Salt Creek. However,
according to IEPA staff, the chlorophyll a concentrations in Salt Creek are lower than in
streams impaired for DO in the state. The steady-state application of QUAL2E was
appropriate to use for this application not only based on this statement by IEPA, but also by the
modeling results shown in Figure 5-6. This figure shows that modeling algae has a small
