Using a Bayesian isotope mixing model to assess nitrate sources in groundwater: A case study of Granvillebrook and Kingtom dumpsites, Sierra Leone

Abstract

The stable isotope mixing model in R (SIMMR) proves to be a robust method for quantifying the relative contributions of multiple sources to a mixture. In this study, we integrated the stable isotopes of ${\text{NO}}_3^{-}$ and hydrochemical data into the SIMMR model to assess the proportional sources of ${\text{NO}}_3^{-}$ in groundwater samples at the Granvillebrook and Kingtom dumpsites. The bi-plot analysis of δ¹⁵N versus δ¹⁸O-${\text{NO}}_3^{-}$ revealed that the ${\text{NO}}_3^{-}$ in the dumpsites predominantly originated from manure and leachate (MAL), soil organic-N (SON) and domestic wastewater (DW) confirming the prevalence of denitrification and ammonia volatilization. Furthermore, SIMMR analysis of δ¹⁵N versus δ¹⁸O-${\text{NO}}_3^{-}$ provided detailed insights. At the Granvillebrook dumpsite, SON emerged as the primary contributor (43.6%) to ${\text{NO}}_3^{-}$ in groundwater, followed by MAL (31%) and DW (17%). In the Kingtom area, significant contributions came from MAL (32.2%), SON (28.0%), and DW (26.3%). Additionally, SIMMR analysis of δ¹⁵N versus dissolved oxygen (DO) indicated that MAL played a predominant role (61.1%) in contributing ${\text{NO}}_3^{-}$ to the groundwaters of the Granvillebrook dumpsite, followed by SON (23.8%) and DW (15.1%). In the Kingtom dumpsite, DW (55.0%), SON (29.1%), and MAL (15.9%) were identified as the major contributors to ${\text{NO}}_3^{-}$ in groundwater. The study underscores that coupling δ¹⁵N and DO in the SIMMR model is a more efficient method for quantifying the contributions of ${\text{NO}}_3^{-}$ sources from MAL, SON, and DW. This research bridges a knowledge gap by providing novel insights into ${\text{NO}}_3^{-}$ source analysis, serving as a foundational resource for future ${\text{NO}}_3^{-}$ isotope studies in groundwaters near dumpsites. Assessing the temporal variations and sources of ${\text{NO}}_3^{-}$ can support the development of improved agricultural management practices and sewage disposal programs, to sustain water quality in wells near dumpsites.