Subsurface Mobilization of Phosphorus in an Agricultural Riparian Zone in Response to Flooding from an Upstream Reservoir

Abstract

Riparian wetlands can act as both phosphorus (P) sources and sinks depending upon a range of factors that affect hydrological and biogeochemical processes that govern P mobilization. Stream flow, groundwater levels and water chemistry (total P (TP), soluble reactive P (SRP)) were measured in a series of nested piezometers along three transects located in a riparian zone prior to and throughout a flood event resulting from the release of water from an upstream reservoir. Results of the study show that the stream was influent on all sampling dates, and groundwater flow through the riparian zone was longitudinal to the channel, rather than transverse to the stream. This drainage pattern affected riparian zone biogeochemistry. The riparian zone was a source of TP and SRP to the shallow groundwater system under both pre-flood and flood conditions, as P levels were higher in piezometers at the downstream end of the riparian zone (p <0.001). Flooding induced a brief increase in TP concentrations in shallow groundwater due to mixing with surface runoff following overbank flooding; however, these concentrations quickly returned to pre-event levels. In contrast, SRP concentrations in shallow groundwater decreased during flooding, likely resulting from mixing with oxygen-rich stream water. A large pulse of TP (12,000 g L1) was observed in the creek on the peak flood date. This P did not originate from the reservoir and was more likely due to the mobilization of P from the riparian zone surface when overbank flooding occurred. The results indicate that autumn flooding of riparian zones downstream from impoundments may mobilize phosphorus if overbank flooding occurs, thereby reducing the nutrient retention potential of riparian zones in some settings.

Les milieux humides riverains, sous l'influence des facteurs hydrologiques et biochimiques, responsables de la mobilisation du phosphore (P), peuvent agir en tant que source et puits de P. Dans le cadre de cette tude, des donnes sur la position de la nappe phratique, le dbit, ainsi que la chimie de l'eau (P total (TP), P ractif soluble (SRP)), furent collectes avant et pendant une inondation cause par le relchement des eaux d'un rservoir. La collection des donnes fut effectue sur la zone riveraine en aval du bassin, et ce l'aide d'une srie de pizomtres installs le long de trois transects. Les rsultats rvlent l'influence du cours d'eau pour chaque date dchantillonnage, ainsi qu'un coulement des eaux souterraines de faon longitudinale au canal plutt que transversale, affectant ainsi la biogochimie du milieu. L'observation de la concentration de P dans les instruments en aval fut plus leve avant et pendant l'inondation (p<0.001), indiquant que cette zone est source de PT et PRS pour les eaux souterraines peu profondes. Ce dbordement provoqua aussi une brve augmentation des concentrations de TP en eaux souterraines peu profonde, rsultat de l'union de ces eaux et du ruisslement de surface en provenance des rives inondes. Ces concentrations sont toutefois rapidement revenues leur niveau initial. En revanche, le niveau de SRP en eaux souterraines peu profondes diminua pendant l'inondation, probablement d au mlange de l'eau riche en oxygne du cours d'eau. Une augmentation momentane du TP (12,000 g L1) fut aussi observe dans le ruisseau la pointe des crues. Ce P ne provenant pas du rservoir, il est fort possiblement le rsultat de la mobilisation du P en provenance de la surface de la zone riveraine au moment de l'inondation. Nos rsultats indiquent que les inondations automnales, en aval du rservoir causeraient la mobilisation du P, rduisant, dans certaines circonstances, le potentiel de rtention de nutriments en zones riveraines.

Acknowledgements

The authors acknowledge NSERC (JSP), Ducks Unlimited Canada and the University of Waterloo for funding and logistical support. Jamee DeSimone, Bobby Katanchi and Karen Edmundson are acknowledged for field and laboratory assistance. The Hamilton Conservation Authority is gratefully acknowledged for the provision of flow data from the Valens Reservoir and for conducting the controlled flood.