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Open AccessArticle

Is Flood Irrigation a Potential Driver of River-Groundwater Interactions and Diffuse Nitrate Pollution in Agricultural Watersheds?

1
Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parco Area delle Scienze 11/A, 43124 Parma, Italy
2
Department of Theoretical and Applied Sciences, University of Insubria, Via JH Dunant 3, 21100 Varese, Italy
3
Institute for Electromagnetic Sensing of the Environment, National Research Council, Via Bassini 15, 20133 Milan, Italy
4
Department of Life Sciences and Biotechnology, University of Ferrara, Via L. Borsari 46, 44121 Ferrara, Italy
5
Department of Earth and Environmental Sciences, University of Pavia, via Ferrata 1, 27100 Pavia, Italy
6
Marine Science and Technology Center, Klaipeda University, Klaipeda 92294, Lithuania
*
Author to whom correspondence should be addressed.
Water 2019, 11(11), 2304; https://doi.org/10.3390/w11112304
Received: 15 September 2019 / Revised: 26 October 2019 / Accepted: 30 October 2019 / Published: 3 November 2019
In the Po plain, northern Italy, rivers within agricultural basins display steep summer increases in nitrate (NO3) concentrations. Flood irrigation in overfertilized, permeable soils may drive such diffuse pollution, facilitating interactions between NO3-rich groundwater and surface waters. We discuss multiple, indirect evidence of this mechanism in the Adda, Oglio, and Mincio rivers. These rivers drain agricultural soils with elevated nitrogen (N) surpluses, averaging 139, 193, and 136 kg ha−1 in the Adda, Oglio, and Mincio watersheds, respectively. The three rivers cross a transitional area between highly permeable and impermeable soils, where summer NO3 concentrations may increase by one order of magnitude over short distances (8–20 km). Upstream of this transitional area, a major fraction of the river flow is diverted for flood irrigation, a traditional and widespread irrigation technique for permeable soils. We speculate that diverted water solubilizes soil N excess, recharges the aquifer, and transfers soil N surplus into groundwater, resulting in NO3 pollution. Groundwater–river interactions were estimated experimentally, via water and NO3 budgets in 0.3 to 1 m3 s−1 km−1 and in 1500 to 5400 kg NO3–N day−1. The data suggest a pronounced east–west gradient of groundwater to river diffuse water inputs among the three adjacent basins, reflecting the soil permeability and the width of the river–groundwater interaction zone. Given the large stock of NO3 in groundwater, management interventions performed at the basin scale and aimed at decreasing N excess will not produce an immediate decrease in river NO3 pollution. View Full-Text
Keywords: river; groundwater; nitrogen; diffuse pollution; flood irrigation; agricultural practices river; groundwater; nitrogen; diffuse pollution; flood irrigation; agricultural practices
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Racchetti, E.; Salmaso, F.; Pinardi, M.; Quadroni, S.; Soana, E.; Sacchi, E.; Severini, E.; Celico, F.; Viaroli, P.; Bartoli, M. Is Flood Irrigation a Potential Driver of River-Groundwater Interactions and Diffuse Nitrate Pollution in Agricultural Watersheds? Water 2019, 11, 2304.

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