A one-dimensional, steady-state, dissolved-oxygen model and waste-load assimilation study for Sand Creek, Decatur County, Indiana

The Indiana State Board of Health is developing a State water-quality management plan that includes establishing limits for wastewater effluents discharged into Indiana streams. A digital model calibrated to conditions in Sand 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. The Greensburg wastewater-treatment facility is the only point-source waste Load affecting Sand Creek in the vicinity of Greensburg. Nonpoint, unrecorded waste loads seemed to be significant during three water-quality surveys done by the Indiana State Board of Health.

Natural streamflow in Sand Creek during the summer and annual 7-day, 10-year low flow is zero, so no benefit from dilution is provided. Current (1978) ammonia-nitrogen and dissolved-oxygen concentrations of effluent from the Greensburg wastewater-treatment facility will not meet Indiana stream water-quality standards for ammonia toxicity and dissolved oxygen.

Model simulations indicate that benthic-oxygen demand, nitrification, and the dissolved-oxygen concentration of the wastewater effluent are the most significant factors affecting the in-stream dissolved-oxygen concentrations during summer low flows. The model predicts that with a benthic-oxygen demand of 1.5 grams per square meter per day at 20° Celsius the stream has no additional waste-load assimilative capacity.

The model also indicates that if the deoxygenation rate for nitrogenous biochemical-oxygen demand ranges from 0.2 to 1.0 day^-1 at 20° Celsius and the in-stream ammonia-nitrogen toxicity standard is not exceeded, the minimum dissolved-oxygen concentration of the stream is primarily controlled by the dissolved-oxygen concentration of the wastewater effluent and the benthic-oxygen demand. If the deoxygenation rate for nitrogenous biochemical-oxygen demand is 3.5 day^-1 or greater at 20° Celsius, then even with an effluent total ammonia-nitrogen concentration of 2.5 milligrams per liter the minimum 24-hour average in-stream dissolved-oxygen concentration of at least 5 milligrams per liter cannot be met during summer low flows.

Current carbonaceous biochemical-oxygen demand loads from the Greensburg wastewater-treatment facility are sufficient to maintain an average dissolved-oxygen concentration of 5 milligrams per liter, the State's water-quality standard for streams during winter low flows.