Disentangling nitrate pollution sources and apportionment in a tropical agricultural ecosystem using a multi-stable isotope model. (1st July 2023)

• Record Type: Journal Article
• Authors: Saka, David; Adu-Gyamfi, Joseph; Skrzypek, Grzegorz; Antwi, Eric Ofosu; Heng, Lee; Torres- Martínez, Juan Antonio
• Is Part Of: Environmental pollution. Volume 328(2023)
• Publication Date: 2023-07-01
• Subjects: Nitrate pollution -- Nitrogen isotopes -- Hierarchical cluster analysis -- Bayesian mixing model -- Southern Ghana; Pollution -- Periodicals; Pollution -- Environmental aspects -- Periodicals; Environmental Pollution -- Periodicals; Pollution -- Périodiques; Pollution -- Aspect de l'environnement -- Périodiques; Pollution -- Effets physiologiques -- Périodiques; Pollution; Pollution -- Environmental aspects; Periodicals; Electronic journals; 363.73
• DOI: 10.1016/j.envpol.2023.121589
• Languages: English
• ISSNs: 0269-7491

Abstract

Abstract: Fertilizers increase agricultural productivity and farmers' income. However, intensive agriculture frequently overuses fertilizers, which in turn can contaminate surface and groundwater. In this study, hydrochemical and multi-isotope ( δ 15 NNO3, δ 18 ONO3 and δ 18 OH2O ) data have been combined to identify nitrate pollution sources in Ghana's Densu River Basin, trace the Nitrogen (N) biogeochemical processes in the basin and apportion the contribution of each pollution source. Surface water NO3 − ranged from 0.3 to 10.6 mg/L (as N), while groundwater NO3 − ranged from 0.9 to 34 mg/L. Hierarchical cluster analysis classified the water samples into three spatial categories: upstream, midstream, and downstream, reflecting river and land use patterns. The multi-isotope model considered five primary NO3 − sources: atmospheric deposition, manure/sewage, NH4 + in fertilizers, other NO3 − based fertilizers and soil N. Nitrification was identified as the major biogeochemical process upstream, whereas mixing of sources and denitrification dominate the midstream to downstream sections of the basin. Nitrate source apportioning using a MixSIAR model reveal that N fertilizers (40 %) and soil N (34 %) contribute the most to nitrate pollution upstream of the river. From the midstream to downstream sections, manure/sewage (43 %) become the dominant nitrate source, reflecting the transition from agriculture to peri-urban and urban land use. This study has shown that soil erosion and runoff contribute to nitrate pollution in the Densu River, at levels comparable to N fertilizers, and groundwater across the basin is impacted mainly by manure/sewage. The multi-isotope analyses allowed the partitioning of N sources in other ways not possible using only classical hydrochemical methods. Graphical abstract: Image 1 Highlights: Densu Basin nitrate levels steady over the past decade. HCA classified basin into three groups reflecting land and river use patterns. Nitrogen isotopes identified five potential nitrate pollution sources. Nitrification dominant upstream, denitrification dominant midstream and downstream. MixSIAR apportioned N fertilizers, soil N, manure/sewage as major nitrate sources.