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 subject to increasing federal and state scrutiny since the EPA's 1994 Combined Sewer Overflow Control Policy, affecting water quality, public health, and the city's compliance with federal Clean Water Act requirements.

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 under the Clean Water Nashville program, with documented reductions in overflow events entering the Cumberland River. Despite this progress, CSO events continue to occur during major storm events and winter ice storms, and regulators have emphasized that full compliance with permit obligations remains years away. Eyewitness accounts from affected Nashville neighborhoods have described raw sewage visibly surfacing in streets and yards during overflow events, with residents reporting the smell and presence of solid waste as unmistakable signs of system failure.

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.

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 (59 Fed. Reg. 18688, 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.

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. Complete elimination of overflows required sustained investment and infrastructure transformation spanning multiple decades.

In 2013, the Metropolitan Government signed an Agreed Order with TDEC that formalized a compliance schedule and required regular reporting milestones. This agreement established binding expectations for long-term planning and capital project implementation, with specific consequences for missed 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, setting out the most detailed compliance roadmap Nashville had yet committed to.

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. 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 elevated 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.

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 has not been upgraded at the same pace as wealthier neighborhoods.

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. After more than twelve years of documented violations, residents have demanded regulatory action against a facility that leaked at least 20,000 gallons of sewage into the Harpeth River, prompting local ordinance changes and intensifying public scrutiny of regional wastewater management practices. Lebanon, Tennessee, has similarly faced state scrutiny for repeated sewer violations tied to rapid growth, illustrating a pattern of aging and overburdened sewer infrastructure across the region. These regional incidents demonstrate 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 Region 4 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 some flexibility in the schedule for capital projects, though missed milestones carry potential enforcement consequences under both state and federal law.

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 in the Cumberland River and its tributaries. These regulatory relationships have been largely collaborative, with technical assistance from state and federal agencies helping Nashville develop cost-effective solutions. Metro Water Services is required to notify the public when CSO events occur, and annual compliance reports are submitted to TDEC documenting overflow volumes, frequencies, and the status of capital improvement milestones.

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.

Environmental and Public Health Impacts

The discharge of untreated or partially treated sewage into the Cumberland River and its tributaries during CSO events poses documented risks to both aquatic ecosystems and human health. Combined sewer overflows introduce elevated concentrations of fecal coliform bacteria, biochemical oxygen demand (BOD), nutrients including nitrogen and phosphorus, and suspended solids into receiving waterways. These pollutants can depress dissolved oxygen levels, harm aquatic macroinvertebrates and fish, and create conditions that make contact recreation unsafe for days following a significant overflow event.

The Cumberland River at Nashville is monitored for water quality parameters by both Metro Water Services and the U.S. Geological Survey, which operates gauging stations that provide continuous data on river conditions. Bacterial contamination levels following CSO events frequently exceed safe limits for body contact recreation as defined by EPA standards — 126 colony-forming units of E. coli per 100 milliliters for recreational waters. Residents and visitors are advised to avoid swimming and water contact in areas near known CSO outfalls during and for 48 to 72 hours following significant rainfall events.

Mill Creek, which receives CSO discharges from South Nashville outfalls before joining the Cumberland River, has historically exhibited particularly elevated bacterial and nutrient levels tied to combined sewer contributions. The creek flows through densely populated urban neighborhoods and has limited capacity to dilute overflow volumes during wet weather. Downstream of the Mill Creek confluence, the Cumberland River can show measurably elevated turbidity and bacterial loads that persist until flows recede and natural dilution takes effect.

Beyond waterway impacts, CSO events create direct public health hazards in affected neighborhoods when overflows surface through manholes, catch basins, or low-lying yards during extreme rainfall. Residents in East Nashville and downtown have reported visible sewage surfacing in streets during major storm events, with the characteristic odor and solid waste content making the health risk immediately apparent. These surface overflows, sometimes called "sanitary sewer overflows" when they occur from manhole surcharging rather than designated outfalls, expose residents to pathogens including E. coli, Cryptosporidium, and Giardia, particularly concerning for children and immunocompromised individuals.

Long-term exposure to CSO-impaired waterways has broader public health implications for communities that rely on the Cumberland River for recreation, subsistence fishing, or simply proximity to waterfront green space. Environmental justice advocates have noted that the