Illinois Environmental Protection Agency Crab Orchard Creek Watershed TMDL
C.4.3 Carbondale City Lake Water Quality Data Summary
Water quality data for Carbondale City Lake were obtained from STORET and Illinois EPA databases.
For Year 2000, the mean annual phosphorus concentration in Carbondale City Lake was 0.048 mg/L
which is below the TP standard (0.05mg/L). Therefore, only concentrations that exceeded the standard
were considered in the model for year 2000. Mean annual total phosphorus, chlorophyll-a, and secchi
depth for Carbondale City Lake used in the BATHTUB model are summarized in Table C-27.
Table C-27. Annual Average Water Quality Parameters for Carbondale City Lake
Year Total Phosphorus
1991 0.211 85.80 11.8
1997 0.095 49.70 19.7
2000 0.074 28.30 15.7
C.4.4 Carbondale City Lake BATHTUB Modeling Results
The BATHTUB model was set up to simulate phosphorus response in Carbondale City Lake for the years
1991, 1997 and 2000 when total phosphorus data were available. A second order available phosphorus
model was used to simulate phosphorus concentrations and the phosphorus/light/ flushing rate model was
used to predict mean chlorophyll-a. Using ‘reverse’ BATHTUB, the tributary total phosphorus
concentrations were adjusted so the simulated in-lake concentrations match the observed in-lake
concentrations. A calibration factor of 1 was used for all parameters which indicate that no adjustment to
the model is needed. Table C-28 compares the simulated and observed total phosphorus concentrations in
Carbondale City Lake.
Table C-28. Simulated and Observed Phosphorus Concentrations in Carbondale City Lake
Year Simulated TP
1991 0.211 0.211
1997 0.094 0.095
2000 0.074 0.074
During the reverse BATHTUB application, several simulations were made to accurately predict total
phosphorus loading patterns. Final predicted and observed concentrations are shown in Figure C-4.
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