Combined Sewer Overflow in Nashville
```mediawiki Combined Sewer Overflow (CSO) in Nashville refers to the environmental and public health challenge posed by the city's integrated wastewater and stormwater system, which can discharge untreated sewage and stormwater into waterways during heavy precipitation events. Nashville's combined sewer system, which serves portions of the metropolitan area, was constructed in the late 19th and early 20th centuries when separate storm and sanitary sewer infrastructure was not yet standard practice. When rainfall exceeds the capacity of treatment facilities, the system is designed to overflow directly into nearby waterways including the Cumberland River, Mill Creek, and other tributaries. This practice, while historical in origin, has become a significant environmental and regulatory concern in the 21st century, affecting water quality, public health, and the city's compliance with federal Clean Water Act requirements.[1]
Metro Water Services (MWS) and the Metropolitan Government have undertaken comprehensive planning and infrastructure investments to address CSO issues as part of broader urban water management initiatives. Metro Water Services has renewed more than 200 miles of sewer pipe and over 5,300 manholes, with documented reductions in overflow events entering the Cumberland River.[2] Still, CSO events continue to occur during major storm and ice events, and regulators have emphasized that full compliance remains years away.
History
Nashville's combined sewer system traces back to the late 1800s, when the city's infrastructure was rapidly expanding to accommodate growth following the Civil War. Combined sewers, which carried both sanitary wastewater and stormwater in a single pipe, seemed like an efficient engineering solution at the time, as they minimized construction costs and land disturbance. Most of the city's early sewer infrastructure was installed between 1880 and 1930, establishing this combined system throughout downtown Nashville and surrounding developed areas.
The federal Clean Water Act of 1972 (33 U.S.C. §1251 et seq.) introduced new requirements that would eventually reshape how cities managed their older sewer systems. As late 20th-century environmental regulations took hold, the limitations of combined sewers became increasingly apparent. The law and subsequent amendments created requirements for municipalities to control combined sewer overflows and improve water quality, establishing the regulatory framework within which Nashville has operated for decades.[3]
During the 1980s and 1990s, Nashville conducted multiple combined sewer overflow studies and assessments required by federal environmental agencies. The U.S. Environmental Protection Agency (EPA) and the Tennessee Department of Environment and Conservation (TDEC) worked with the city to develop plans for addressing CSO issues. In 1994, Metro Water Services implemented its first comprehensive CSO Control Plan, which identified approximately 54 outfall locations where combined sewers discharged into the Cumberland River, Mill Creek, and other waterways. This document became foundational for long-term planning and established baseline conditions against which future improvements could be measured.
That same year, the EPA issued its national CSO Control Policy in the Federal Register (Vol. 59, No. 75, April 19, 1994). Nashville and hundreds of other cities were required to implement nine minimum controls immediately and to develop Long-Term Control Plans demonstrating compliance with Clean Water Act water quality standards.[4]
Over subsequent decades, incremental improvements were made. Storage facilities were constructed, treatment plants were upgraded, and collection systems were modified. Nashville's combined sewer infrastructure covers approximately 380 miles of combined sewer pipes serving areas including downtown, East Nashville, and portions of South Nashville.[5] Complete elimination of overflows required sustained investment and infrastructure transformation. Not without cost.
In 2013, the Metropolitan Government signed an Agreed Order with TDEC that formalized a compliance schedule and required regular reporting milestones. The 2020 Long-Term Control Plan Update, which superseded earlier planning documents, identified over $2 billion in capital projects necessary to significantly reduce CSO events over a 20-year period.
Nashville's "Clean Water Nashville" program became the primary vehicle for implementing sewer rehabilitation and renewal projects across the combined sewer service area. Developed in coordination with CDM Smith and other engineering contractors, the program has made measurable progress. By the mid-2020s, it had renewed more than 200 miles of deteriorating sewer pipe and rehabilitated over 5,300 manholes, producing documented reductions in the volume of sewage overflowing into the Cumberland River and its tributaries during storm events.[6] These outcomes represent a concrete shift from the baseline conditions documented in the 1994 plan. Still, regulators and advocates have emphasized that significant work remains before Nashville achieves full compliance with its permit obligations.
Geography
Nashville's combined sewer system is concentrated in areas of the city that experienced urbanization before mid-20th century standards were established. The primary service area encompasses much of downtown Nashville, portions of East Nashville along the Cumberland River, areas surrounding Mill Creek in South Nashville, and neighborhoods adjacent to Sulphur Fork Creek. The Cumberland River, which flows through the heart of Nashville, is the primary receiving waterway for most CSO events and has received particular environmental monitoring and management attention. CSO outfalls are distributed throughout this urban core, with the highest concentrations located near the confluence of Mill Creek and the Cumberland River, an area that has historically been subject to both urban runoff and sanitary sewer contributions during precipitation events.
Nashville's rolling terrain and relatively high annual precipitation averaging 47 inches create conditions where stormwater management is a persistent infrastructure challenge. During light to moderate rainfall events, the combined sewer system typically functions as designed, conveying flows to treatment plants. During significant precipitation events, commonly defined as those exceeding 0.5 inches in 24 hours, the system's capacity is exceeded and overflow mechanisms activate. The Cumberland River's receiving capacity, water quality characteristics, and ecological sensitivity have made it a focal point for CSO management efforts. Downstream communities, including areas along the Cumberland River in Cheatham and Robertson counties, can be affected by increased turbidity, bacterial loading, and nutrient levels following CSO events in Nashville.
The geographic distribution of older infrastructure means that neighborhoods in East Nashville and downtown face the most direct exposure to CSO-related water quality degradation. These are predominantly lower-income and historically Black communities dealing with environmental justice concerns.[7]
The EPA's 2004 Report to Congress on the Impacts and Control of CSOs and SSOs documented that this pattern is widespread. Older urban communities of color across the United States disproportionately bear the burden of combined sewer infrastructure that hasn't been upgraded at the same pace as wealthier neighborhoods.[8]
Nashville's CSO challenges exist within a broader Middle Tennessee context. Neighboring Williamson County has faced its own sewage infrastructure failures, including a long-running dispute over a failing sewage treatment plant that discharged into the Harpeth River. Repeated sewage leaks into Cartwright Creek prompted local ordinance changes.[9] These regional incidents show that Nashville's CSO management efforts take place within a broader watershed context where multiple municipalities share responsibility for the health of interconnected waterways.
Regulatory Framework and Compliance
Nashville's approach to combined sewer overflow management has been shaped by significant regulatory interactions with federal and state environmental agencies over more than three decades. The EPA's 1994 CSO Control Policy, published in the Federal Register at 59 Fed. Reg. 18688, established the national framework. It required municipalities to implement nine minimum controls immediately and to develop Long-Term Control Plans demonstrating compliance with the Clean Water Act's water quality standards.
Nashville operates under National Pollutant Discharge Elimination System (NPDES) permits administered jointly by the EPA and TDEC, which set specific limits on the frequency and volume of permitted overflow events at each outfall location. In 2013, the Metropolitan Government signed an Agreed Order with TDEC regarding CSO compliance, establishing specific milestones and timelines for infrastructure improvements. This agreement formalized expectations for long-term planning and implementation while providing flexibility in the schedule for capital projects.
TDEC and EPA oversight of Nashville's progress has included regular reporting requirements, environmental monitoring, and assessment of whether the city's management efforts are achieving measurable water quality improvements. These regulatory relationships have been largely collaborative, with technical assistance from state and federal agencies helping Nashville develop cost-effective solutions.
The 2020 Long-Term Control Plan Update represented Nashville's most comprehensive regulatory commitment to date. It lays out a 20-year roadmap for reducing overflow events to levels that protect water quality in the Cumberland River and its tributaries. Progress under this plan is tracked through annual reports submitted to TDEC, and Metro Water Services publishes monitoring data for each active outfall location as part of its public transparency obligations. Whether Nashville achieves the plan's targets on schedule will depend on sustained capital investment, continued construction of storage and conveyance infrastructure, and the effectiveness of green infrastructure programs in reducing the volume of stormwater entering the combined sewer system during storm events.[10]
Economy
The economic implications of combined sewer overflow management in Nashville extend across multiple sectors of the local economy and municipal budget. Metro Water Services has allocated hundreds of millions of dollars toward CSO abatement infrastructure improvements. The 2020 Long-Term Control Plan Update identified over $2 billion in capital projects necessary to significantly reduce CSO events over a 20-year period, with costs distributed across ratepayers through water and wastewater utility charges. This substantial capital investment has affected residential and commercial customers as utility rates have increased to fund infrastructure improvements.
Small businesses in areas with older combined sewer infrastructure, particularly in downtown and East Nashville, may experience variable impacts depending on their water usage and stormwater management practices. Water quality concerns in the Cumberland River, while not prohibitive to recreation, have influenced public perception, and some residents may be deterred from swimming and water-based activities in areas downstream of significant CSO outfalls. Water quality improvements resulting from CSO control investments have been identified as economic benefits that support downtown redevelopment, riverfront attractions, and public health.
Local environmental organizations and advocates argue that CSO control investments produce long-term economic returns through improved public health, ecosystem restoration, and enhanced property values in waterfront areas. The intersection of CSO management with broader sustainability initiatives has created economic opportunities in green infrastructure, including permeable pavements, green roofs, and bioswales that reduce stormwater runoff. Contracts for CSO control project implementation have supported construction and engineering employment, though the total economic burden of compliance remains significant for the municipal budget and ratepayers.[11]
Rate increases tied to CSO compliance haven't gone without controversy. In early 2025, the Nashville Metro Council voted down legislation that would have offered a water rate discount program to lower-income residents. Advocates criticized the decision, arguing that the cost of CSO remediation was falling disproportionately on households least able to absorb utility increases.[12] The tension between the fiscal demands of long-term infrastructure compliance and the affordability of utility services for residents represents one of the more contested dimensions of Nashville's ongoing CSO management program.
Current Infrastructure and Ongoing Projects
Clean Water Nashville is the primary mechanism for delivering the capital improvements required under Nashville's Long-Term Control Plan. Metro Water Services launched the program in partnership with engineering and construction contractors, involving systematic inspection, assessment, and rehabilitation of the city's aging combined sewer collection system, with particular focus on pipes and manholes experiencing significant infiltration and inflow. Groundwater and stormwater enter the system through cracks and defects, adding to flows that can trigger overflow events even during relatively modest rainfall.
By the mid-2020s, the program had completed renewal of more than 200 miles of sewer pipe and rehabilitation of over 5,300 manholes throughout the combined sewer service area. These improvements produced documented reductions in overflow volumes entering the Cumberland River and represent a measurable return on the hundreds of millions of dollars invested in collection system rehabilitation.[13] Work has continued in phases, prioritizing areas with the greatest rates of infiltration and those closest to sensitive receiving waters.
Nashville has also invested in green infrastructure as part of its CSO control strategy. Bioswales, rain gardens, permeable pavement installations, and green roof projects in the combined sewer service area are designed to capture and retain stormwater before it reaches the sewer system, reducing peak flows during storm events. These distributed approaches complement the larger gray infrastructure investments, such as storage tunnels and treatment plant capacity expansions, that form the backbone of Nashville's long-term compliance strategy.
Metro Water Services has engaged community groups and neighborhood organizations in the installation of small-scale green infrastructure, creating opportunities for public participation in water quality improvement. Residents and local organizations have become partners in the effort to reduce stormwater entering the combined sewer system, extending the reach of the construction program into individual blocks and properties across the affected service area. Nashville.gov publishes a running list of active water services maintenance and infrastructure projects, which includes ongoing combined sewer work across the metropolitan area.[14]
Current Challenges and Ongoing Overflow Events
Despite years of investment, CSO events in Nashville haven't stopped. During significant weather episodes, including winter ice storms and multi-inch rainfall events, combined sewer overflows continue to discharge into the Cumberland River and its tributaries. WZTV FOX 17 News has reported on overflow events occurring during recent severe weather, showing that the system's vulnerability to high-flow conditions remains an active concern rather than a resolved one.[15]
The scale of remaining work is substantial. The 2020 Long-Term Control Plan calls for over $2