6 POLLUTANT LOAD ESTIMATION AND TMDL ALLOCATION
Once the HSPF model was calibrated and validated, the models were used to estimate pollutant loads and
determine TMDL allocations for the Fox River, Olney East Fork Lake, and Borah Lake. The allocations
were based on estimating seasonal pollutant loads during both the critical period from May to October
and from November to April. Specifically, the HSPF model was used to determine loads of BOD and
NH3-N to the listed Fox River segment, TP and BOD loads to Olney East Fork Lake, and TP loads to
Borah Lake. The seasonal loads were computed using the calibrated HSPF model based on available data
from 1998 to 2000. Percent reductions needed for compliance were calculated by comparing existing
seasonal loads and the maximum allowable loads needed to achieve compliance with the water quality
This chapter discusses the estimation of loads to the listed water bodies and determination of load
reductions needed to comply with the water quality standards.
6.1 Fox River
This section discusses the load estimation and allocation for listed Fox River segment.
6.1.1 Load Estimation
To calculate seasonal loads to the Fox River, the calibrated HSPF model was run based on available data
from 1998 through 2000, which is also the period for model calibration and validation. This period was
selected because it represents a typical wet year, dry year, and intermediate year. The information
required for continuous simulation were records of precipitation, wind, solar radiation, temperature, and
potential evaporation at hourly time intervals. These data were obtained from the USGS gauge station
located at the north end of the Fox River watershed (see Figure B.2-2). The HSPF model generated
hydrographs and pollutographs for 1998 through 2000 for the Fox River, Olney East Fork Lake, and
Borah Lake that were used to determine seasonal average flows and concentrations.
The mean seasonal and daily loads were calculated based on the average seasonal loads from 1998 to
2000. The period from May to October was considered a critical season because periodic water quality
monitoring conducted in the Fox River and HSPF model simulation indicate that DO concentrations
during this period were below the 6-mg/L standard most of the time. For each modeling year, the
seasonal and daily loads for BOD and NH3-N were computed by multiplying seasonal predicted mean
flows and concentrations. Table B.6-1 summarizes the seasonal BOD and NH3-N loads in the listed Fox
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