Groundwater phosphate dynamics in a river riparian zone: effects of hydrologic flowpaths, lithology and redox chemistry

This study examines the influence of riparian zone hydrology, lithology and redox chemistry on groundwater phosphate dynamics. Patterns of soluble reactive phosphorus (SRP), dissolved oxygen (DO) and ferrous iron (Fe(2+)) in combination with hydrologic data and sediment characteristics were studied in a forested floodplain connected to a large upland sand aquifer in an agricultural region of southern Ontario, Canada. Groundwater discharge from the upland aquifer flowed laterally beneath peat in a 2-4 m thick zone of permeable sands across the floodplain to the river. Within the sands, low SRP concentrations (<25 microgram L(-1)) occurred in a plume of groundwater with DO concentrations >3 mg L(-1) and Fe(2+) concentrations <0.2 mg L(-1) which extended for a horizontal distance of 100-140 m across the riparian zone. High SRP concentrations (50-950 microgram L(-1)) were associated with low DO and high Fe(2+) concentrations which exceeded 1 mg L(-1) in buried channel sediments near the river bank. Sediment P fractionation indicated that the buried channel sediments contained a much higher pool of total P and Fe + Al-P than the sands. Groundwater SRP concentrations at the river bank were 25-80 microgram L(-1) compared to <10 microgram L(-1) in river water indicating that the floodplain was a source of SRP to the river. Areas of elevated SRP and Fe(2+) within the floodplain expanded in August when DO concentrations in groundwater were lower than in late spring or autumn. These data suggest that the microbial reduction of Fe(3+) to soluble Fe(2+) in anaerobic conditions influences groundwater SRP concentrations in the riparian zone. This study shows that well-organized patterns of groundwater SRP concentrations occur in riparian zones which reflect the interaction of hydrologic flowpaths and environments of different redox state. Internal sources of P associated with buried channel sediments can also influence subsurface SRP transport and release to streams.