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27. Effect of ebullition and groundwater temperature on estimated dinitrogen excess in contrasting agricultural environments

Author

Nicolò Colombani, Micòl Mastrocicco, Giuseppe Castaldelli, Fabio Vincenzi, Elisa Soana, Simona Castaldi, Mastrocicco, M., Soana, E., Colombani, N., Vincenzi, F., Castaldi, S., and Castaldelli, G.

Subjects
Environmental Engineering, Denitrification, Dissolved gasses, Denitrification, Exsolution, Groundwater temperature, Gas partial pressure, 010504 meteorology & atmospheric sciences, Dissolved gasses, Water table, Hydrostatic pressure, Drainage basin, 010501 environmental sciences, 01 natural sciences, chemistry.chemical_compound, Nitrate, Dissolved gasse, Environmental Chemistry, Organic matter, Exsolution, Waste Management and Disposal, 0105 earth and related environmental sciences, chemistry.chemical_classification, Hydrology, geography, geography.geographical_feature_category, Brackish water, Ambientale, Groundwater temperature, Pollution, Gas partial pressure, chemistry, Environmental science, Groundwater
Abstract

Denitrification is a key microbial-mediated reaction buffering the impact of agriculturally-derived nitrate loads. Groundwater denitrification capacity is often assessed by measuring the magnitude and patterns of dinitrogen excess, although this method can be biased by dissolved gasses exsolution and ebullition. To address this issue, shallow groundwater was sampled in two field sites via nested mini-wells on a monthly basis over an entire hydrological year and analysed for dissolved gasses, nitrate and physical parameters. Both sites are located in lowland areas of the Po River basin (Italy) and are characterized by intensive agriculture. The GUA site, a freshwater paleo-river environment, with a low content of organic matter (SOM) and oxic sub-oxic groundwater. The BAN site, a reclaimed brackish swamp environment, with abundant SOM and sulphidic-methanogenic groundwater. Groundwater samples evidenced a general deficit of dinitrogen and Argon concentrations, because of ebullition induced by a total dissolved gasses pressure exceeding the hydrostatic pressure. Ebullition was recorded only during winter at the reclaimed brackish soil and was triggered by methane exsolution. While in summer both sites were affected by ebullition because of the water table drawdown. Denitrification evaluated using dinitrogen excess via dinitrogen-Argon ratio technique, was not only affected by gas exsolution, but also by groundwater temperature fluctuations. In fact, the latter induced large biases in the calculated N2 excess even in the freshwater paleo-river environment. For these reasons, dinitrogen excess estimate with standard methods resulted to be unreliable in both lowland environments and a modified method is here presented to overcome this issue.

Published
2019

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