Breast Cancer Incidence and Groundwater Contamination in Southwestern Ohio: An Ecological Analysis of Communities Overlying the Great Miami Buried Valley Aquifer.

Background Regional variation in breast cancer incidence may reflect environmental factors in addition to established risk factors. Southwestern Ohio overlies the Great Miami Buried Valley Aquifer (GMBVA), a sole-source aquifer that has experienced long-term contamination by volatile organic compounds (VOCs) and documented detections of per- and polyfluoroalkyl substances (PFAS). This ecological study examined breast cancer incidence in counties overlying portions of the GMBVA with the most extensively documented groundwater contamination. Contamination was defined at the county level using publicly available environmental records, including federal Superfund site designations, US Geological Survey (USGS) PFAS detections, and Department of Defense inventories of PFAS response sites. County location over the aquifer was used strictly as a contextual population-level proxy rather than as a measure of individual exposure. Methods Age-adjusted invasive female breast cancer incidence rates for 2017-2021 and 2018-2022 were obtained from the Surveillance, Epidemiology, and End Results (SEER) Program, the National Cancer Institute State Cancer Profiles, and the Ohio Cancer Incidence Surveillance System for all ages, women aged 50 years and older, and women aged 65 years and older. Rates were compared with the corresponding Ohio statewide and US reference rates from the same reporting periods. Age-adjusted incidence rate ratios (IRRs) were computed by dividing the Montgomery County age-adjusted rate by the corresponding Ohio statewide and US reference rates, with 95% CIs derived by log transformation of the published rate standard errors. Publicly available data from the USGS, US Environmental Protection Agency (EPA), Ohio EPA, Miami Conservancy District, and US Air Force were used to characterize groundwater contamination and remediation in the region. Results Ohio's statewide rate was 132.3 per 100,000 (95% CI: 131.2-133.4), compared with 130.8 nationally (95% CI: 129.9-131.7). Montgomery County, which overlies heavily contaminated sections of the GMBVA, had a rate of 141.4 per 100,000 (95% CI: 135.7-147.3), approximately 8% above the national benchmark (standardized incidence ratio (SIR): 1.08, 95% CI: 1.04-1.13 vs. the US; SIR: 1.07, 95% CI: 1.02-1.11 vs. Ohio). Relative differences appeared somewhat larger in the older age strata (SIRs: approximately 1.08-1.13), although the ecological design precludes interpreting age as a proxy for cumulative exposure duration or timing. PFAS were detected in 11 of 23 GMBVA wells (USGS, 2019-2020); VOC concentrations at Montgomery County Superfund sites historically exceeded maximum contaminant levels by orders of magnitude. Descriptively, several core industrialized counties appeared to have higher incidence rates than less-industrialized counties in the region. Conclusions This ecological, hypothesis-generating analysis identified descriptive county-level patterns of modestly elevated age-adjusted invasive female breast cancer incidence in parts of southwestern Ohio overlying historically contaminated portions of the GMBVA. The ecological design, severe potential for exposure misclassification, lack of adjustment for confounding, qualitative rather than formal spatial comparison, and inability to assess residential history, water source, exposure timing, or latency preclude causal inference or individual-level interpretation. These findings should be interpreted strictly as contextual and descriptive.