The median total dissolved phosphorus concentration (0.41 mg/L P) of groundwater from the Mississippi River Valley alluvial aquifer exceeded the U.S. Environmental Protection Agency’s stream nutrient criteria (of 0.1 mg/L P) and the national background for phosphorus in groundwater (0.02 mg/L P). A general association between elevated phosphorus and dissolved iron concentrations suggests that reducing conditions that mobilize iron in the aquifer also may facilitate transport of phosphorus. These elevated concentrations of phosphorus may indicate phosphorus in the study area may be concentrated through irrigation return flow and groundwater discharge, and may contribute to the Gulf of Mexico hypoxic zone. The data do not appear to follow any spatial, geologic, or application pattern. The research indicates that phosphorus concentration in the aquifer was primarily sourced from natural geochemical reactions within the aquifer media.