Dissolved oxygen modeling of effluent-dominated macrophyte-rich Silver Bow Creek

A dissolved oxygen (DO) mass balance was developed for a 5.9 km reach of the heavily eutrophied and macrophyte-rich wastewater effluent dominated Silver Bow Creek, MT, USA, to assess water-quality changes following an upgrade to the municipal wastewater facility. Surveys of diurnal DO were completed in 2015 and 2016, and various mass transfer rates were quantified including macrophyte photosynthesis and respiration, macrophyte density, sediment oxygen demand, and reaeration. A DO mass balance model was then developed in QUAL2K, where oxygen equivalents of macrophytes were incorporated as closely spaced diurnally varying point sources (sinusoids). Overall, diurnal DO swings and longitudinal oxygen profiles were reasonably replicated in the pre- and post-upgrade synoptic surveys and models, and the facility upgrade greatly enhanced DO concentrations in the stream. Improvements, however, were not enough to meet the minimum DO criterion for the waterbody. The results above have important implications for assessing DO responses and impairment status in macrophyte-rich streams. Moreover, they suggest wastewater treatment upgrades may not always achieve desired results and that modeling should be conducted a priori to understand receiving-water responses.