Nitrate sources and transformation processes in groundwater of a coastal area experiencing various environmental stressors.

In coastal salinized groundwater systems, contamination from various nitrate (NO) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO sources and identify potential NO transformtation processes. Effective field-based NO studies in coastal areas are needed to improve the understanding of NO contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δO, δH, δN, δO, and δB) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO sources and their contributions, and (ii) to describe the potential NO transformation processes. The measured NO concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO indicates that manure, soil organic matter, and sewage are the potential sources of NO, while δB values constrain the NO contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both N and O in the remaining NO, although isotopic mass balances between the measured and the theoretical δO values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management.