Assessing fluoride concentrations in Iowa's groundwater and drinking water: implications for public health and water management

Abstract

This study investigates the occurrence and distribution of fluoride in Iowa's groundwater and drinking water. Fluoride, added to community water supplies to prevent dental caries, can pose health risks at high concentrations. The U.S. Public Health Service recommends an optimal fluoride concentration of 0.7 mg L⁻¹, while the EPA sets a maximum contaminant level (MCL) at 4 mg L⁻¹ and a secondary MCL at 2 mg L⁻¹. This research analyzes fluoride data from various sources, including the Iowa Department of Natural Resources and the US Geological Survey, covering 9011 raw groundwater samples from 1931 and 2017 and 26 280 treated drinking water samples from 1934 to 2021. Fluoride concentrations in Iowa's groundwater ranged from <0.1 mg L⁻¹ to 11.2 mg L⁻¹, with an average of 0.65 mg L⁻¹ and a median of 0.35 mg L⁻¹. Approximately 69% of untreated raw source groundwater samples fell below the recommended 0.7 mg L⁻¹, while 7% exceeded the secondary MCL of 2 mg L⁻¹. Higher fluoride levels are associated with deeper wells and specific aquifers, such as the Cambrian-Ordovician and Mississippian. Treated public drinking water showed an average fluoride concentration of 0.87 mg L⁻¹, indicating a higher average of 0.24 mg L⁻¹ (mean) compared to untreated groundwater due to fluoridation practices. Fluoride concentrations in treated water peaked between 1980 and 1999, then declined slightly after 2000 and more so when systems began aligning with the 2015 recommendation to lower the optimal level to 0.7 mg L⁻¹. This pattern reflects how regulatory guidance and water source management have influenced fluoride levels over time. This study highlights significant regional variability in fluoride levels, influenced by aquifer lithology, well depth, and water chemistry. Anthropogenic sources also contribute to fluoride concentrations. The findings underscore the need for tailored water management strategies to balance the benefits of fluoridation with the risks of excessive fluoride intake. This research provides valuable insights for public health agencies, water suppliers, and residents, aiming to optimize fluoride levels in Iowa's drinking water to ensure safety and efficacy.