Fluoride occurrence in United States groundwater

Area of Science:

• Environmental Science
• Hydrogeology
• Public Health

Background:

• Fluoride (F) occurrence in untreated U.S. groundwater is not fully characterized.
• Domestic wells are typically not monitored for fluoride quality.
• Optimal fluoride for dental health is 0.7 mg/L, EPA SMCL is 2 mg/L, and EPA MCL is 4 mg/L.

Purpose of the Study:

• To characterize fluoride occurrence in U.S. groundwater from domestic wells.
• To identify factors controlling fluoride concentrations.
• To estimate populations potentially exposed to varying fluoride levels.

Main Methods:

• Analysis of data from 38,105 wells, including 11,032 domestic wells.
• Statistical comparisons of fluoride concentrations and controlling factors (pH, TDS, alkalinity, well depth, Ca/Na ratio, precipitation).
• Case studies using groundwater age, isotopes, and temperature to illustrate spatial variability.

Main Results:

• 10.9% of domestic wells exceed 0.7 mg/L F; 2.6% exceed 2 mg/L; 0.6% exceed 4 mg/L.
• The primary concern nationally is under-consumption of fluoride for dental health.
• Elevated fluoride is regionally important, especially in the western U.S.
• High fluoride concentrations correlate with higher pH, TDS, alkalinity, and well depth, and lower Ca/Na ratios and precipitation.

Conclusions:

• While most domestic wells have low fluoride, elevated levels pose regional risks.
• Water chemistry and hydrogeological factors significantly influence fluoride occurrence.
• Millions of people may be served by domestic wells with fluoride levels below, at, or above health benchmarks.