Examining nitrogen dynamics in the unsaturated zone under an inactive cesspit using chemical tracers and environmental isotopes

Author(s)

Varnier, Claudia ; Hirata, Ricardo ; Aravena, Ramon

• Document type: Journal article
• Source: APPLIED GEOCHEMISTRY; v. 78, p. 129-138, MAR 2017.
• Web of Science Citations: 3

Abstract

This study evaluates the dynamics of nitrogen compounds generated by infiltration of wastewater from an inactive cesspit in the unconfined and sedimentary Adamantina Aquifer in Urania, Brazil. A monitoring station, consisting of an 11.2 m well (1.8 m in diameter) with an array of 12 tensiometers and 12 suction lysimeters, was installed to monitor the shallow unsaturated zone from 0.5 to 9 m depth. A monitoring well was also installed below the water level to monitor the shallow aquifer. High amounts of ammonium (up to 96 mg/L NH4+-N) and nitrate (up to 458 mg/L NO3--N) were observed in the unsaturated zone porewater which is comparable to active septic systems effluents. The distribution of NOS, and Na+, typical constituents of sewage effluents, varied seasonally and spatially, which is correlated with changes in infiltration rates between the wet and dry seasons and with hydraulic Conductivity variations in interlayered sandy and clayey sediments. A detailed monitoring of porewater geochemistry demonstrated the occurrence of several important reactions affecting nitrogen dynamics in the unsaturated zone: i) oxidation of organic matter, ii) ammonification, iii) nitrification, iv) methatiogenesis, v) denitrification and likely, vi) sulfate reduction. The changes in nitrogen compound distribution and delta N-15(N03) and delta O-18(NO3) values in porewater, in association with the N2O concentration and delta N-15(N2o) and delta O-18(N2o) signatures in gas samples, indicate the occurrence of nitrificatidn and denitrification, suggesting the coexistence of reducing and oxidizing microsites in the unsaturated zone. This study indicated that cesspits can generate a significant amount of nitrate even a few years after being inactivated which can represent a potential long-term source of nitrate to groundwater in highly populated areas. (C) 2016 Elsevier Ltd. All rights reserved. (AU)