DISSOLVED PHOSPHATE CONCENTRATIONS IN IOWA SHALLOW GROUNDWATER

Regional groundwater phosphorus (P) concentrations are rarely reported and it is important to develop a better understanding of background concentrations in shallow groundwater to help develop strategies to mitigate environmental risks. In this study, results collected from 17 different Iowa-based studies conducted from 2006 to 2019 and a total of 210 discrete locations of water table dissolved phosphate (DPO₄^3-) measurements were summarized to:1) assess the occurrence, range and statistical distribution of groundwater DPO₄^3- concentrations in Iowa; and 2) evaluate state-wide patterns of DPO₄^3-concentrations related to land use/land cover and landscape position. DPO₄^3- concentrations ranged from 0.02 to 1.56 mg L^-1, averaged 0.15±0.19 mg L^-1 with a median value of 0.10 mg L^-1 (95% CI of 0.08 to 0.11 mg L^-1). Variations were observed among land cover class and landscape position, but concentrations exhibited remarkable uniformity across the state, likely attesting to the legacy of P from historical agricultural management. Median concentrations are higher than typical water quality criteria used to assess risk to surface water systems, implying that simply discharging groundwater DPO₄^3-- to streams, rivers and lakes would be sufficient to cause environmental degradation.