Alabama has the highest age-adjusted CVD mortality in the US. Despite this, our group showed that lower rates of hypertensive heart disease, ischemic heart disease, and stroke/cerebrovascular deaths were associated with higher percent private well use at the census block group (CGB) scale. The mechanism driving this association is unknown. Dietary nitrate has been associated with cardiovascular health benefits such as vasodilation and a decrease in myocardial infarction. Nitrate is also a common component in private well water. Therefore, we tested the hypothesis that CBGs with high percent private well use will have greater protective associations for CVD when median groundwater nitrate concentration is also high. The outcomes were hypertensive heart disease, ischemic heart disease, and stroke/cerebrovascular deaths from 2016 - 2019 in Alabama for age groups 45 - 54, 55 - 64, 65 - 74, and 75+ aggregated to the CBG scale (n = 3,924). Percent private well use and median nitrate concentration in each CBG were predictor variables in Poisson regression models, with additional adjustment by age group and rurality. We obtained percent private well use from the US Environmental Protection Agency’s domestic well dashboard. The United States Geological Survey (USGS) produced a 1km raster of shallow groundwater nitrate concentrations, and we calculated median nitrate concentration for each Alabama CBG. Results revealed that the protective associations between high percent well use and hypertensive heart disease and stroke/cerebrovascular mortality did not statistically significantly depend on median nitrate concentration. For example, the RR (95% CI) for a higher percentage of well users at the average nitrate level was 0.89 (0.83 to 0.95) for hypertensive heart disease, and the RR at the nitrate level 1 SD higher than the mean was 0.90 (0.85 to 0.95). Our findings may be due to low concentrations of nitrate included in our study (0.03 – 6.20 mg/L nitrate as N across all CBGs) compared to previous clinical studies on the blood pressure lowering effects of nitrates contained in beet root juice (~45 mg/L nitrate as N). In summary, in Alabama groundwater nitrate concentrations are likely not driving the protective association between percent private well use and hypertensive heart disease and stroke/cerebrovascular death in the state of Alabama. Future work will expand the study area to include geographic areas that have larger exposure ranges for nitrate.
Moylan et al. (Tue,) studied this question.