The Indiana State Board of Health is developing a State water-quality management plan that includes the establishing of limits for wastewater effluents discharged into Indiana streams. A digital model calibrated to conditions in Cedar Creek was used to develop alternatives for future waste loadings that would be compatible with Indiana stream water-quality standards defined for two critical hydrologic conditions, summer and winter low flows. All point-source waste loads affecting Cedar Creek are in the four incorporated municipalities of Auburn, Garrett, Huntertown, and Waterloo, in a primarily agricultural area. Avilla, because of its distance from Cedar Creek, does not significantly affect the water quality of the modeled segment.

The model indicates that the dissolved-oxygen concentration of the Auburn wastewater effluent and nitrification are the most significant factors affecting the dissolved-oxygen concentration in Cedar Creek during summer low flows. The observed dissolved-oxygen concentration of the Auburn wastewater effluent was low, and averaged 30 percent of saturation. Whether the effluent is aerated before discharge will ultimately define the waste-load assimilative capacity of Cedar Creek. Projected nitrogenous biochemical-oxygen demand loads, from the Indiana State Board of Health, for the Auburn and Waterloo wastewater-treatment facilities will result in violations of the current instream dissolved-oxygen standard (5 milligrams per liter), even with an effluent dissolved-oxygen concentration of 80 percent saturation.

Natural streamflow for Cedar Creek upstream from the confluence of Willow and Little Cedar Creeks is small compared with the waste discharge, so benefits of dilution for Waterloo and Auburn are minimal. Stream reaeration capacity is not sufficient to maintain an average dissolved-oxygen concentration of at least 5 milligrams per liter, the State's water-quality standard for streams.

The model also indicates that, during winter low flows, ammonia toxicity, rather than dissolved oxygen, is the limiting water-quality criterion in the reach of Cedar Creek downstream from the wastewater-treatment facility at Auburn and the confluence of Garrett ditch. Ammonia-nitrogen concentrations predicted for 2978 through 2000 downstream from the Waterloo wastewater-treatment facility do not exceed Indiana water-quality standards for streams.

Calculations of the stream's assimilative capacity indicate that future waste discharge in the Cedar Creek basin will be limited to the reaches between the Auburn wastewater-treatment facility and County Road 68.