A one-dimensional, steady-state, dissolved-oxygen model and waste-load assimilation study for West Fork Blue River, Washington County, Indiana

The Indiana State Board of Health is developing a water-quality management plan that includes establishing limits for wastewater effluents discharged into Indiana streams. A digital model calibrated to conditions in West Fork Blue River 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.

Alternative waste loads that would permit instream dissolved-oxygen concentrations of at least 5.0 milligrams per liter for West Fork Blue River in Washington County, Ind., were determined. All point-source waste loads that potentially impact West Fork Blue River are in the town of Salem. The Salem municipal wastewater-treatment facility is the only facility in the modeled reach that has a National Pollution Discharge Elimination System permit to discharge organic waste.

In-stream dissolved-oxygen concentration averaged 96.5 percent of saturation at selected sites on West Fork Blue River during two 24-hour summer surveys. This high dissolved-oxygen concentration reflects small carbonaceous and nitrogenous waste loads, adequate dilution of waste by the stream, and natural reaeration. Nonpoint source waste loads accounted for an average of 53.2 percent of the total carbonaceous biochemical-oxygen demand and 90.2 percent of the nitrogenous biochemical-oxygen demand.

Waste-load assimilation was studied for both summer and annual 7-day, 10-year low flow. Natural streamflow for these conditions was zero, so no benefit from dilution was provided. The projected reaeration capacity of the stream was not sufficient to maintain at least 5 milligrams per liter dissolved oxygen in the stream with current waste-discharge restrictions. Consequently, during low flows, advanced waste treatment (nitrification-denitrification) or some other form of ammonia removal may be necessary to meet current Indiana stream water-quality standards for ammonia and dissolved oxygen. During winter low flow, ammonia toxicity, rather than dissolved-oxygen concentration, was the limiting water-quality criterion downstream from the Salem wastewater-treatment facility.