Agricultural and urban delivered nitrate pollution input to Mediterranean temporary freshwaters

Nitrate dual stable isotopes (delta N-15(NO3) and delta O-18(NO3)) have proven to be a powerful technique to trace nitrate sources and transformations in freshwater systems worldwide. However, most studies have focused on perennial systems, and less is known about intermittent ones. The impacts of intensive agricultural practices and wastewaters in Donana (SW Spain), an iconic Mediterranean temporary wetland protected as a UNESCO World Heritage Site, were quantified using stable isotope mixing models in a Bayesian framework under different denitrification scenarios. We aimed to identify the main nitrate sources and transformation processes in surface waters of interconnected temporary streams, ponds and marshes, and link them with the main human pressures in the watershed (e.g. intensive fruticulture, urban wastewaters). We measured nitrate (NO3-) concentrations and stable isotopes (delta N-15(NO3) and delta O-18(NO3)) in water samples collected during different periods over two years (2015-2016). Most sites showed coupled increases of nitrate isotopic values (delta N-15(NO3) and delta O-18(NO3)), which were higher than reference values of any possible sources (e.g. synthetic/organic fertilizers and wastewaters), indicating fractionations typical of denitrification processes. The main nitrate sources to the watershed were linked to agricultural practices and the use of synthetic fertilizers, but further investigations in other transformation processes that occur simultaneously should be evaluated. These results highlight an important nitrate removal capacity (i.e. denitrification) of the system, which may positively contribute to natural resilience against eutrophication. However, given the high intra and interannual hydrological fluctuations of Mediterranean aquatic systems, future studies on the relative contribution of nitrate sources and processes should increase spatio-temporal resolution of water sampling, and include measurements of groundwater and interstitial water as well as surface water.