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Lower Mill Creek Partial Bioassessment-Follow up to Sewage Releases of March 2023

In March 2023, the Mill Creek wastewater treatment plant (WWTP) suffered a complete power loss that resulted in releases of raw and partially treated sewage to the lower reaches of Mill Creek over six (6) days. To evaluate for any long term impacts, MSD tasked MBI with assessing the status of aquatic life and water quality in the lower 5.1 miles of the Mill Creek mainstem between Salway Park and the mouth of the Mill Creek during the seasonal index period of June 15-October 15, 2023.
Fish Sampling

10 Minute Read

Last Updated: May 13th, 2026

Technical Report Author

Chris O Yoder

The Metropolitan Sewer District of Greater Cincinnati’s (MSDGC) Mill Creek WWTP lost all electrical power on March 5, 2023 following the catastrophic failure of a new high-voltage transformer installed at the Mill Creek electric substation as part of a $17.5 million upgrade project. Without power to the WWTP, untreated and partially treated wastewater was discharged into lower Mill Creek and the Ohio River. Temporary power was restored on March 8, 2023 using diesel-powered generators and the resulting overflows ended on March 10th. On March 16, 2023, full power was restored after reconnecting to the Duke Energy electrical grid.

The releases occurred to the lower reach of Mill Creek via multiple combined sewer overflows and a manhole cover that persisted for six (6) days. The releases extended approximately 3.5 miles upstream on Mill Creek to the West Fork confluence. While there were no immediate impacts such as fish kills, the flow of raw and partially treated sewage during the power outage was visible (Figure 1). Concern about the extent and lasting effects was expressed by an important stakeholder group, the Mill Creek Alliance (MCA). MCA requested that a biological survey be conducted in the Mill Creek mainstem that was directly affected by the power outage related releases. MSDGC agreed and funded the sampling of nine (9) sites for fish and six (6) sites for macroinvertebrates consistent with the CSO permit biological monitoring that has been conducted by MSDGC since 2011[1].

[1] Find the 2011-2021 MSDGC bioassessments at: http://www.msdgc.org/initiatives/water_quality/index.html.

The most recent biological survey of Mill Creek was conducted in 2021 and it was preceded by surveys in 2011, 2013, and 2016. Trend analysis was conducted for each survey year and included prior Ohio EPA surveys dating back to 1992. The established methods of trend assessment were applied to the 2023 results in order to determine if any significant declines occurred as a possible result of the unintended releases due to the power outage. Incremental improvements have been observed since 2011 in the Mill Creek mainstem biota so any interruptions in this progress are of major concern. As with all prior MSDGC sponsored biological surveys, the 2023 survey was performed by Level 3 Qualified Data Collectors under a Biological and Water Quality Project Study Plan (MBI 2023) approved by Ohio EPA under the specifications of the Ohio Credible Data Law.

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mbi-msdgc-tech-memo-2023-lower-mill-creek-bioassessment-20240717-final.pdf