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 the Mississinewa River 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 hydrology of the Mississinewa River downstream from Gas City is controlled primarily by two factors: low slopes, typical of the Tipton Till Plain, and a 10-foot dam at river mile 35.90 in Marion. All point-source waste loads affecting the modeled segment of the Mississinewa River are in the four incorporated municipalities of Fairmount, Jonesboro, Gas City, and Marion, in a primarily agricultural area.

Model simulations indicate that algal photosynthesis and nitrification are the most significant factors affecting the dissolved-oxygen concentration of the Mississinewa River during summer low flows. Natural reaeration, without photosynthesis, is not sufficient to maintain an average dissolved-oxygen concentration of at least 5 milligrams per liter in the stream, the State's water-quality standard.

Projected carbonaceous and nitrogenous biochemical-oxygen demand loads, from the Indiana State Board of Health, for Owens-Illinois, Inc., and the Gas City and Marion wastewater-treatment facilities will result in violations of the in-stream dissolved-oxygen standard. Fairmount and Jonesboro, because of their distance from the Mississinewa, do not significantly affect the water quality of the modeled segment.

Model simulations also indicate that, during winter low flows, ammonia toxicity, rather than dissolved oxygen, is the limiting water-quality criterion in the Mississinewa River downstream from the Gas City wastewater-treatment facility.

Calculations of the stream's assimilative capacity indicate that future waste discharge in the Mississinewa River basin will probably be limited to the reach downstream from the Marion dam (river mile 35.90).