Association between combined sewer overflow events and gastrointestinal illness in Massachusetts municipalities with and without river-sourced drinking water, 2014-2019

Abstract

Background Combined sewer overflow (CSO) events release untreated wastewater into surface waterbodies during heavy precipitation or snowmelt. Combined sewer systems serve approximately 40 million people in the United States, primarily in urban and suburban municipalities in the Midwest and Northeast. Predicted increases in heavy precipitation events driven by climate change underscore the importance of quantifying potential health risks associated with CSO events. Objectives The aims of this study were: to 1) estimate the association between CSO events (2014-2019) and emergency department (ED) visits for acute gastrointestinal illness (AGI) among Massachusetts municipalities that border a CSO-impacted river, and 2) determine if associations differ by municipal drinking water source. Methods A case time series design was used to estimate the association between daily cumulative upstream CSO discharge in the prior four days and ED visits for AGI, adjusting for temporal trends, temperature, and precipitation. Associations between CSO events and AGI were also compared by municipal drinking water source (CSO-impacted river vs. other sources). Results Extreme upstream CSO discharge events (>95th percentile by cumulative volume) were associated with a cumulative risk ratio (CRR) of AGI of 1.22 (95% CI: 1.05, 1.42) over the next four days for all municipalities, and the association was robust after adjusting for precipitation (1.17 [0.98, 1.39]). In municipalities with CSO-impacted drinking water sources, the adjusted association was somewhat less pronounced following 95th percentile CSO events (1.05 [0.82, 1.33]). The adjusted CRR of AGI was 1.62 in all municipalities following 99th percentile CSO events (95% CI: 1.04, 2.51) and not meaningfully different across strata defined by drinking water source. Discussion In municipalities bordering a CSO-impacted river in Massachusetts, extreme CSO events are associated with higher risk of AGI within four days. The largest CSO events are associated with increased risk of AGI regardless of drinking water source.