Modeling the changes of geochemical conditions under municipal sewage discharge on the example of Obskoye fen (Western Siberia)

The study aims to create a numerical model of chemical element speciation and mineral precipitation from fen water and sewage and fen water and sewage mixing model. The mixing model was created to assess changes in the geochemical conditions of the Obskoye fen under the influx of municipal wastewater. The Obskoye fen near the Nashchekovo village was taken as a geochemical background to study the fen water chemical composition. It is located approximately 2 km from the sewage discharge of the communal services of Melnikovo settlement up the slope. The simulation was performed in the HCh software package. The developed thermodynamic models consider the metal complexation with organic ligands since this mechanism can significantly affect secondary mineral formation in the fen environment. In its turn, it affects the accumulation of pollutants entering the fen with the sewage. The differences in chemical element speciation in the fen water and sewage are mainly due to a slight increase in the pH value of the sewage relative to the fen water of the background area. These differences are expressed in a decrease in the proportion of hydroxo complexes and free metal ions with a simultaneous increase in the proportion of carbonate complexes in the sewage. The proportional content of sulfate and chloride complexes in the sewage slightly increases comparing with the fen water due to the increased concentrations of SO4^2- and Cl^- in the wastewater. However, chloride and sulfate ions predominantly remain as free SO4^2- and Cl^- not associated with metal cations even in the sewage. Among the studied metals, calcium and iron show affinity for organic acids. Fulvic acid binds almost all iron under the studied conditions. Calcium has an affinity to humic acid. This is equally typical for both the fen water and sewage. In the sewage, an increase in the iron concentration balances with an increase in the fulvic acid content. Calcite and goethite are potentially deposited from the fen water. The proportion of kaolinite and apatite among the potential secondary minerals of the fen background area does not exceed 0.5 %. Calcite, apatite, and goethite are precipitated from the sewage, according to the results of the simulation; kaolinite is precipitated in a smaller amount. It should be noted that the proportion of apatite among the secondary minerals precipitated from the sewage is two orders of magnitude higher than the ones precipitated from the fen water. The proportion of calcite slightly decreases, and the proportion of goethite slightly increases in the sewage compared with the fen water. According to the mass ratio, almost two times less mass of the secondary minerals is precipitated from the fen water than from the sewage. The mixing of the sewage with fen water has a significant effect on the precipitation of the secondary minerals, particularly apatite. The apatite content significantly increases when the sewage is added to the natural fen water. The mass of apatite increases by almost 19 times compared to the fen water of the background area at the first simulation step, which corresponds to the ratio of fen water/sewage equal to 10/1. The mass of other precipitated secondary minerals (calcite, goethite, kaolinite) also increases.

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