A one-dimensional, steady-state, dissolved-oxygen model and waste-load assimilation study for Little Laughery Creek, Ripley and Franklin counties, 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 Little Laughery Creek tributary and Little Laughery Creek was used to predict 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.

Natural streamflow during the summer and annual 7-day, 10-year low flow is zero. Headwater flow upstream from the wastewater-treatment facilities consists solely of process cooling water from an industrial discharger. This flow is usually Less than 0.5 cubic foot per second. Consequently, benefits from dilution are minimal. As a result, current and projected ammonia-nitrogen concentrations from the municipal discharges will result in in-stream ammonia-nitrogen concentrations that exceed the Indiana ammonia-nitrogen toxicity standards (maximum stream ammonia-nitrogen concentrations of 2.5 and 4.0 milligrams per liter during summer and winter low flows, respectively).

This waste-Load assimilation study is not based on a verified model. The changes in stream water quality predicted by the model represent only possible stream responses to differing effluent conditions.

Benthic-oxygen demand is probably the most significant factor affecting Little Laughery Creek and is probably responsible for the in-stream dis-solved-oxygen concentration being less than the Indiana stream dissolved-oxygen standard (5.0 milligrams per liter) during two water-quality surveys. After municipal dischargers complete advanced waste-treatment facilities, benthic-oxygen demand should be less significant in the stream dissolved-oxygen dynamics.