『Abstract
　Concentrations of dissolved orthophosphate (ortho P) in the unsaturated zone, groundwater, tile drains, and groundwater/stream water interfaces were assessed in five agricultural watersheds to determine the potential for subsurface transport. Concentrations of iron oxides were measured in the aquifer material and adsorption of ortho P on oxide surfaces was assessed by geochemical modeling. Attenuation of ortho P in these aquifers was attributed primarily to sorption onto iron oxides, and in one location onto clay minerals. Only one location showed a clear indication of phosphorus transport to a stream from groundwater discharge, although groundwater did contribute to the stream load elsewhere. Subsurface ortho P movement at a site in California resulted in a plume down gradient from orchards, which was attenuated by a 200 m thick riparian zone with natural vegetation. Iron oxides had an effect on phosphorus movement and concentrations at all locations, and groundwater chemistry, especially pH, exerted a major control on the amount of phosphorus adsorbed. Groundwater pH at a site in Maryland was below 5 and that resulted in complete sequestration of phosphorus and no movement toward the stream. Geochemical modeling indicated that as the surfaces approached saturation, groundwater concentrations of ortho P rise rapidly.

Keywords: Agriculture; Hydrochemistry; Groundwater protection; Groundwater/surface-water relations; hydrochemical modeling; Orthophosphate』

1. Introduction
2. Study areas
3. Methods
　3.1. Field methods
　3.2. Analytical methods
　3.3. Geochemical modeling
4. Results and discussion
　4.1. Phosphorus concentrations and water chemistry
　4.2. Contents of phosphorus and iron in aquifer and unsaturated zone solids
　4.3. Geochemical modeling
5. Summary and conclusion
References