Chemical controls on dissolved phosphorus mobilization in a calcareous agricultural stream during base flow

This study explores the sources and mechanisms of dissolved phosphorus (P) mobilization under base flow conditions in a headwater stream. We characterized the relevant chemical species and processes within the watershed to investigate connections between stream sediment, surface water, and groundwater with respect to P dynamics. Waters were monitored monthly during the 2017 snow-free period for temperature, pH, dissolved oxygen, conductivity, soluble reactive P (SRP), total P, strong acid anions, strong base cations, dissolved organic carbon (DOC), Al, Fe, and Mn. Phosphorus speciation within sediment samples was determined by sequential chemical extractions. The emerging groundwater was under-saturated by up to 40% with respect to O₂, with pH = 7.24, T = 7.0 °C, and SRP = 3.0 μg L^-1. Groundwater P_CO2 was up to ~35× the ambient P_CO2 (410 ppm). Degassing of CO₂ from the emerging groundwater resulted in a significant increase in pH downstream, and an increase in the SRP concentration from 3.0 to a maximum of 40.6 μg L^-1. Laboratory experiments, using homogenized stream sediment, identified a reduction in the P adsorption capacity, and an increase in desorption of native P with increasing pH from ~7.25 (emerging groundwater) to ~8.50 (air-equilibrated surface water). These data allow us to identify the pH-dependent desorption from P-laden sediment as the most significant source of dissolved P in the headwater stream under base flow conditions.