Occurrence and distribution of ammonium in Iowa groundwater.

Excess ammonium in groundwater may lead to nitrification or loss of chlorine residuals in public drinking water supplies. In Iowa, where groundwater supplies nearly 75% of all drinking water used in the state, naturally occurring ammonium is found in all major aquifers, including two unconsolidated units and five bedrock aquifers. An evaluation of ammonium concentrations in 841 municipal water supply wells indicated highest concentrations were found most often in Quaternary wells. More than one-half of all Quaternary wells sampled in Iowa showed ammonium concentrations greater than 2 mg/L, and more than 5% had concentrations greater than 5 mg/L. Nearly 5% of all bedrock wells used in this study showed ammonium concentrations greater than 2 mg/L. Aquifers overlain by carbon-rich Pennsylvanian strata or glacial drift seemed most vulnerable to elevated ammonium. Alluvial aquifers were least vulnerable to elevated ammonium concentrations in Iowa municipal water supplies. Ammonium concentrations tended to be higher in wells that screened groundwater containing more dissolved solids, including sulfate, chloride and iron.