In the United States, this typically takes the form of a consent decree from the US Environmental Protection Agency (USEPA) ordering the reduction or elimination of CSO discharge volumes ( US Environmental Protection Agency 1994). When CSOs harm a community or surrounding environment, government regulators may respond by taking enforcement actions requiring CSS municipalities to reduce CSO discharges or otherwise act to improve water quality. Indeed, emerging research on predicting CSO occurrence suggests CSO events will worsen in urban areas under climate change ( Semadeni-Davies et al. In these regions, urbanizing areas with combined sewers are particularly vulnerable, as increasing urbanization, absent mitigation of additional flow volumes, also increases the frequency and volume of CSO discharges ( Semadeni-Davies et al. 2017) and northern Europe ( Christidis & Stott 2022) are expected to face more rainfall and worsening wet extremes. Though climate change is forecast to reduce rainfall events and wet extremes in some areas ( Christidis & Stott 2022), much of North America ( Easterling et al. 2012 Fortier & Mailhot 2015 Mahaut & Andrieu 2019 Botturi et al. Changing precipitation patterns may lead to more severe and more frequent incidents of combined sewer overflow (CSO), releases of untreated raw sewage mixed with stormwater from CSS ( Astaraie-Imani et al. Thousands of CSOs occur each year in the United States ( US Environmental Protection Agency 2022a), Europe ( Environmental Agency 2020 Scottish Water 2020), and elsewhere around the world. These stresses are particularly acute in combined sewer systems (CSS), in which stormwater is collected in the same pipes as domestic and/or industrial wastewater. 2017) stress aging wastewater and stormwater infrastructure systems designed for the climate of the past ( Rudberg et al. Together, this indicator and modeling approach can help CSS communities assess the performance of their CSS over time, especially to evaluate the effectiveness of efforts to reduce CSOs.Ĭlimate changes including more frequent and extreme wet weather events ( Easterling et al. Using the city of Cumberland, Maryland as a case study, we use public data on CSOs and precipitation over a span of 16 years to identify a new critical rainfall intensity threshold that triggers likely CSO incidence, and a multiple linear regression model to predict CSO volume using rainfall event characteristics. Yet, CSS communities lack the tools needed, such as performance indicators, to assess CSS performance. CSOs pose risks to humans and the environment, and as such, CSS communities are under regulatory pressure to reduce CSOs. Climate change makes CSOs worse in many locales as the frequency and severity of wet weather events increases, so do the frequency and volume of CSO events. Combined sewer overflows (CSOs) occur when untreated raw sewage mixed with rainwater, runoff, or snowmelt is released during or after a storm in any community with a combined sewer system (CSS).
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