FACTORS CONTROLLING RELEASE AND MOBILITY OF PHOSPHORUS NEAR THE GROUNDWATER-STREAM INTERFACE

Agricultural practices can cause excess inputs of phosphorus (P) to freshwater catchments, which can lead to eutrophication and harmful algal blooms. Typically, surface runoff and tile drains are considered the major pathways delivering P to agricultural streams. However, groundwater and hyporheic discharges may also play an important role in P delivery (e.g. by affecting the timing and P form (particulate vs dissolved)). These discharges may contribute to the limited success of prior efforts to reduce P loads in agricultural streams. There is a need to better understand the hydrological and geochemical processes governing P fate near groundwater-stream interfaces and the influence of changing conditions (e.g. seasonally) to better quantify P inputs from groundwater to streams. The objective of this study is to evaluate how spatio-temporal variability in groundwater-stream interactions affect P delivery to a headwater agricultural stream.

Field investigations were conducted along a 40-m stream reach in an agricultural area of the Thames watershed, Ontario, Canada. Streambed temperature mapping and vertical hydraulic gradient measurements provided initial identification of groundwater upwelling and downwelling zones. Groundwater, tile drain and surface water samples were analyzed for soluble reactive phosphorus (SRP), total P, and parameters that influence P mobility including oxidation-reduction potential (ORP), pH, iron, aluminium, and manganese. This sampling was complemented with continuous ORP measurements in streambed sediments at select locations along the reach. Preliminary results indicate low porewater SRP concentrations (~32 μg/L) in the high groundwater discharge zones at each end of the reach. In contrast, the middle area of the reach is characterised with low groundwater discharge and high porewater SRP (>1000 μg/L). The SRP porewater concentrations are negatively correlated with ORP, and concentrations were found to be higher in summer compared with winter. Future work will focus on identifying factors contributing to the mobilization of the high streambed SRP. The study findings are needed to quantify the role of groundwater-stream interactions in delivering P to agricultural streams and potentially guide improved management practices to address this pathway.