Four groundwater sources used as drinking water in Illinois were treated using the Fenton reaction (hydrogen peroxide + Fe(II)) to oxidize As(III) to As(V) before adsorption of the arsenic to the iron precipitate produced during iron removal by aeration/filtration. For all four waters used, the arsenic concentration could be reduced to below the 10 microgram/liter maximum contaminant level using relatively inexpensive doses of iron and peroxide, despite the need to add iron (Fe(II) or Fe(III) were both tried) in all cases to completely adsorb the arsenic. The waters differed in their treatability, but in each case, the least expensive configuration used Fe(III) rather than Fe(II). It was shown that consumption of reactive species by reaction with Fe(II) was a very significant sink for Fe(II) at higher iron concentrations, and was primarily responsible for the difference in efficiency using Fe(II) and Fe(III). Peroxide doses greater than 20-27 micromolar improved arsenic removal in Monticello and Dwight water, but not in Danvers water. In most cases, the treatability of the waters did not appear to correlate negatively with the alkalinity, phosphate, silicate, or organic content of the waters. A correlation was expected because of the known competition between arsenic and the other natural solutes for both sorption sites and reactive species generated during the Fenton reaction. Chemical addition in the range of 3-6.4 milligrams per liter Fe(III) and 20-45 micromolar (0.68-1.5 milligrams/liter) peroxide was adequate to reduce the total arsenic to the maximum contaminant level in all four waters. The estimated chemical cost was $0.04 to $0.07 per thousand gallons in the most favorable cases for each water.