2 Agricultural Impacts on Hydrology and Water Quality in the Midwest I

Monday, 16 April 2018
8:00 AM - 12:00 PM
Room 250/252

THE ROLE OF SEDIMENT IN DRIVING PHOSPHORUS LOADING AND DYNAMICS IN THE HIGHLY ALTERED AGRICULTURAL LE SUEUR RIVER BASIN

Paper number: 2-9
Program: Theme Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

The Minnesota River Basin is undergoing rapid hydrologic and geomorphic change as a result of anthropogenic pressures including climate change and intensive agricultural land use, leading to sediment and nutrient loading downstream. The Le Sueur River, a tributary to the Minnesota River, is one of the largest producers of excess sediment and phosphorus (P). To improve our ability to target mitigation for reduction of P loading to the Le Sueur, we build upon a fine-sediment budget to generate a mass-balance for sediment-derived P. Sediments were collected from erosional source areas including alluvial streambanks, glacial till bluffs, ravines, agricultural fields and agricultural ditch-banks; and were analyzed for P content and sorptive properties. These data were used to determine the extent to which sediment serves as a source of P to the Le Sueur River, and to explore the role of in-stream equilibrium processes between sediment and dissolved P in driving watershed scale P dynamics.

Mass balance reveals that, despite the very high sediment loads and high background P concentration of sediment, only 24% of total P load measured at the outlet can be attributed directly to source sediment, with 23% in particulate form and < 1% in dissolved form. Dissolved orthophosphate comprises 37% of the measured average annual total P load at the Le Sueur outlet. Incorporation of data describing sorptive properties of sediment into this budget suggests that 2-24% of the total P exiting the basin may be particulate P that formed via sorption by sediment in the water column, and that the true dissolved P load may be masked by sorption of dissolved P to sediment, potentially reducing the fraction of total P observed as dissolved load by as much as 39%.

While it is frequently assumed that reducing erosion will control P loss in agricultural landscapes, these findings indicate that even if we were able to control all sediment inputs, we would only reduce P loads by less than one quarter of the total exported. Furthermore, conversion of P from dissolved to particulate form via in-stream sorption processes may mute the signal of dissolved P inputs to the channel network. Thus, management practices which address dissolved P source will be equally as important as those targeting erosion in reducing P loading to this system.

Geological Society of America Abstracts with Programs. Vol. 50, No. 4

doi: 10.1130/abs/2018NC-313171

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