Variation of dissolved phosphorus in groundwater of riparian zones in an agricultural area

Phosphorus, which is one of important factors of surface water eutrophication, has been the main issue in surface water quality management. In recent years, there has been an increasing interest in phosphorus in groundwater as well as surface water. Increasing number of studies has reported groundwater with high concentrations of phosphorus and its effect on adjacent surface water. The multi-level monitoring wells were installed in riparian zones of an agricultural area to demonstrate processes of phosphorus in groundwater. A stream in the area is largely in gaining condition, but losing condition was found in the area with extensive groundwater pumping. In this study, the processes of increasing and decreasing phosphorus concentration in groundwater under anaerobic conditions were examined with redox sensitive species. The dominant redox processes in groundwater were identified using redox sensitive parameters, which varied from oxic to sulfate reduction. Phosphorus concentrations were low in oxic and denitrification dominant condition and high in iron reducing dominant condition. This result was consistent with many recent studies. It is expected that phosphorus concentrations were reduced by precipitation of secondary iron minerals in the aerobic condition and increased by dissolution of the secondary minerals in the anaerobic condition. However, phosphorus concentration in the groundwater tended to attenuate under the more reducing condition than iron-reducing dominant condition. In this study, we tried to interpret dissolved phosphorus concentration in relation to the redox sensitive species and to understand the attenuation processes of dissolved phosphorus under strongly reducing conditions.