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 (DPO ) measurements are summarized (a) to assess the occurrence, range, and statistical distribution of groundwater DPO concentrations in Iowa and (b) to evaluate statewide patterns of DPO concentrations related to land use or land cover and landscape position. The DPO concentrations ranged from 0.02 to 1.56 mg L and averaged 0.15 ± 0.19 mg L with a median value of 0.10 mg L (95% confidence interval of 0.08-0.11 mg L ). Although minor variations were observed among land cover class and landscape position, concentrations exhibited 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 DPO to streams, rivers, and lakes would be sufficient to cause environmental degradation.