Abstract Body:
Introduction: More magnesium in drinking water is associated with lower CVD risk, while more lead is associated with higher CVD risk. Rural US residents often rely on private well water, which has more variable concentrations of these substances than municipal water. Our previous work in Alabama demonstrated that more households reliant on private wells was associated with less hypertensive heart and stroke/cerebrovascular disease mortality. Although self-reported Black race is associated with higher CVD mortality, it is not a confounder for private well use, which depends on rurality of residence. However, it is unknown whether self-reported Black race may be an effect modifier for the protective associations with private well use.

Hypothesis: In Alabama, the neighborhood-level relationship between high private well use and lower CVD mortality depends on percentage of self-reported Black residents.

Methods: We used mortality data for Alabama Census block groups (CBGs) from 2016 to 2019 for 3 types of CVD: hypertensive heart disease, ischemic heart disease, and stroke/cerebrovascular disease. We estimated the rate ratio (RR) and 95% CIs for a 1 SD higher percentage of households reliant on private wells, adjusted for age group (45 - 54, 55 - 64, 65 - 74, and 75+ years) and rurality. We tested for interactions between the percentage of households reliant on private wells and the percentage of residents who reported Black only race on the 2020 Census. A p-value of 0.05 was considered statistically significant. Data were provided by the Alabama Department of Public Health, the EPA, and the US Census.

Results: Median (1^st quartile, 3^rd quartile) percentage of households reliant on private wells in a CBG (n = 3,919) was 7% (0%, 38%). The percentage of Black residents was 18% (5%, 48%). The adjusted RRs for a higher percentage of households reliant on private wells differed by percentage of Black residents for hypertensive death and ischemic death. The protective association for higher percentages of households reliant on private wells was more protective in CBGs with higher percentages of Black residents (see Figure 1).

Conclusions: The association between more private well use and hypertensive heart and ischemic heart disease mortality depended upon the percentage of Black residents. Further research is needed to understand whether some groundwater quality parameters, such as water hardness, may be more beneficial for Blacks than for Whites.