Agricultural practices are known to contribute to the excess loading of phosphorus (P) in surface waters, promoting accelerated eutrophication. This study explores the sources and mechanisms of phosphorus (P) mobilization in the watershed of Amsden Brook, Maine, USA, which is drained by a second order stream. The watershed land use is primarily agricultural; the surficial geology is calcareous till on calcareous sedimentary rock. Recharge to the stream is largely from spring water issuing from the till-bedrock surface. The stream and spring water were monitored monthly during the snow-free period of 2017 for temperature, pH, dissolved oxygen, conductivity, P, strong acid anions, strong base cations, DOC, Al, Fe, and Mn. P speciation within soil and sediment was determined by sequential chemical extractions. These data enabled us to identify sources and sinks of P, and the movement of P in the watershed. Emerging groundwater was under-saturated by 30-40% with respect to O2, with pH = 7.1-7.2, T = 7.0 °C, and P = 2.4-5.0 µg L-1. The groundwater PCO2 was up to ca. 35X &gt; ambient PCO2. The air-equilibrated groundwater pH was ca. 8.4, because of degassing of CO2. Upon discharge and mixing of groundwater into the stream, the stream CO2 concentration reached equilibrium with the overlying atmosphere via degassing within approximately 500 m. The pH increase leads to the precipitation of CaCO3, evident by an increase in its saturation index that ranged from 1.5 to 9, and % sediment total carbonate in the downstream direction. Also, within this distance, the dissolved P concentration increased from 5.0 to 41 µg L-1. We hypothesize that P mobilization in the stream is due to pH-controlled desorption from the P-laden stream sediment eroded from agricultural fields. Laboratory experiments with homogenized stream sediment are being conducted to test this hypothesis, and determine P adsorption behavior and capacity.

Controls on phosphorus export from an agricultural watershed with calcareous bedrock and surficial geology

255th National Meeting (2018)

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