Combined Sewer Overflows and Gastrointestinal Illness in Atlanta, 2002-2013: Evaluating the Impact of Infrastructure Improvements

Abstract

Background: Combined sewer overflows (CSOs) discharge untreated sewage into surface and recreational water, often following heavy precipitation. Given projected increases in frequency and intensity of precipitation due to climate change, it is important to understand the health impacts of CSOs and mediating effects of sewerage systems.

Objectives: In this study we estimate associations of CSO events and emergency department (ED) visits for gastrointestinal (GI) illness among City of Atlanta, Georgia, residents and explore how these associations vary with sewerage improvements.

Methods: We estimate associations using Poisson generalized linear models, controlling for time trends. We categorized CSOs by overflow volume and assessed effects of CSO events prior to ED visits with 1-, 2- and 3-wk lags. Similarly, we evaluated effects of weekly cumulative precipitation greater than the 90th percentile at the same lags. We also evaluated effect modification by ZIP Code Tabulation Area (ZCTA)-level poverty and infrastructure improvement period using interaction terms.

Results: Occurrence of a large volume CSO in the previous week was associated with a 9% increase in daily ED visits for GI illness. We identified significant interaction by ZCTA-level poverty, with stronger CSO-GI illness associations in low than high poverty areas. Among areas with low poverty, we observed associations at 1-wk and longer lags, following both large and lower volume CSO events. We did not observe significant interaction by infrastructure improvement period for CSO- nor precipitation-GI illness associations; however, the number of CSO events decreased from 2.31 per week before improvements to 0.49 after improvements.

Discussion: Our findings suggest that CSOs contribute to acute GI illness burden in Atlanta and that the magnitude of this risk may be higher among populations living in areas of low poverty. We did not find a protective effect of sewerage system improvements. Nonetheless, observed reductions in CSO frequency may lower the absolute burden of GI illness attributable to these events. https://doi.org/10.1289/EHP10399.

Figure 1.

Distribution of poverty by ZIP code tabulation area (ZCTA) and combined sewer overflow (CSO) outfall locations in Atlanta, Georgia. ZCTA-level poverty determined by percent of residents in a ZCTA living below the federal poverty line during 2007–2011 ($\text{median}=14.7\%$) and dichotomized as above vs. below the median. CSO outfall locations as documented by City of Atlanta, Department of Watershed Management. Inset figure shows the relative location of Atlanta in the United States. Figure 1 was produced using ArcGIS software (Esri; Version 10.3).

Figure 2.

Time-series plot of combined sewer overflow (CSO discharges by volume (kgals) and number within a week, 2002–2013. Event volumes are plotted on a logarithmic scale, with the thick horizontal line indicating discharges above the large event threshold. The number of days with an event at any outfall location within a given week are plotted on a secondary axis. All events prior to 2006 fall in the period before infrastructure improvements, denoted by the vertical line on the left. Events in 2009 and later fall in the period after infrastructure improvements, as indicated by the vertical line on the right. Summary data can be found in Table 1. Note: CSO, combined sewer overflow.

Figure 3.

Percent of combined sewer overflow (CSO) events following various levels of weeklong precipitation. Events $<25\text{th}$ percentile of volume ($\mathrm{6,183}\text{\hspace{0.17em}kgal}$) are defined as small volume, events 25th–74th percentile of volume are defined as medium volume, and events $\ge 75\text{\hspace{0.17em}th}$ percentile of volume ($\mathrm{56,548}\text{\hspace{0.17em}kgal}$) are defined as large volume. Precipitation data collected at Hartsfield Jackson Airport, Atlanta, Georgia, 2002–2013. Summary data are found in Table S2.

Figure 4.

Time-series plot of daily ED visits for GI illness, Atlanta, Georgia, 2002–2013. The plot is normalized to the total number of ED visits at participating metropolitan Atlanta hospitals to account for changes across the study period in the number of hospitals contributing to the study, changes in population, and overall ED usage. Note: ED, emergency department; GI, gastrointestinal.

Figure 5.

Comparison of rate ratios (RRs) and 95% confidence intervals for the association between ED visits for GI illness and CSO events of various volumes in the prior week, adjusted and unadjusted for weekly sum of precipitation, Atlanta, Georgia, 2002–2013. Comparison group for all models is no event in the prior week. Any is a CSO event of any volume. Large volume events are defined as an event $\ge 75\text{\hspace{0.17em}th}$ percentile of volume for CSO events in Atlanta during the study period. Summary data can be found in Table 3. Note: CSO, combined sewer overflow; ED, emergency department; GI, gastrointestinal.

Figure 6.

Comparison of rate ratios (RRs) and 95% confidence intervals for the association between ED visits for GI illness and CSO events of various volumes of effluent in the specified week allowing for effect modification by area-level poverty, Atlanta, Georgia, 2002–2013. All estimates adjusted for weekly sum of precipiation, time trends and hospital participation. Any is a CSO event of any volume. Large volume events are defined as an event $\ge 75\text{\hspace{0.17em}th}$ percentile of volume. “Med” volume events are defined as an event 25th–74th percentile of volume, and small volume events are defined as an event $<25\text{\hspace{0.17em}th}$ percentile of volume. No event in the specified week (no) is the comparison group for all models. See Table S4 for numerical presentation of results. Note: CSO, combined sewer overflow; ED, emergency department; GI, gastrointestinal; Med, medium.