Abstract In Groundwater-Dependent Terrestrial Ecosystems (GWDTEs), atmospheric nitrogen (N) inputs have often been studied in isolation from terrestrial groundwater and surface water inputs. We describe for the first time the development and application of a combined atmospheric and terrestrial N source apportionment methodology, able to identify contributing catchment and N loadings to GWDTEs. We combined all N inputs using a site-specific conceptual model supported by 12 months’ monitoring for a Chalk-fed GWDTE at Newbald Becksies, East Yorkshire. We discuss implications for effective catchment management, wetland protection and development of a source apportionment methodology. Potential sources of nitrate include: atmospheric deposition, mineralization, leaching from agricultural soils, manure heaps, septic tanks, sewer and mains water leakage. Atmospheric deposition was calculated from measurements of ammonia and nitrogen dioxide concentrations together with rainfall inputs of ammonium and nitrate. Quantification of agricultural sources used the FarmScoper modelling tool to estimate nitrate leaching in the groundwater catchment. Comparison between modelled nitrate concentrations in leachate (15–17 mg N l −1 ) and observed groundwater nitrate concentrations (12.3–19.8 mg N l −1 ) are good. The majority of nitrate is leached from arable land. FarmScoper allows mitigation scenarios to be tested, supporting measures to reduce nitrate within a groundwater catchment.

Abstract Gullies are widespread morphological features on Mars for which current changes have been observed. Liquid water has been one of the potential mechanisms to explain their formation and activity. However, under present-day Martian conditions, liquid water is unstable and should only be transiently present in small amounts at the surface. Yet little attention has been paid to the mechanisms by which unstable water transports sediment under low atmospheric pressure. Here we present the results of laboratory experiments studying the interaction between liquid water flowing over a sand bed under Mars-like atmospheric pressure (c. 9 mbar). The experiments were performed in a Mars Simulation Chamber (at the Open University, UK), in which we placed a test bed of fine sand at a 25° slope. We chose to investigate the influence of two parameters: the temperature of the water and the temperature of the sand. We performed 27 experiments with nine different combinations of water and sand temperatures ranging from 278 to 297 K. Under all experimental conditions, the water was boiling. We investigated and compared the types and timing of sediment transport events, and the shapes, characteristics and volumes of the resulting morphologies. In agreement with previous laboratory studies we found that more intense boiling increased the volume of sediment transported for a given volume of water. We found four main types of sediment transport: entrainment by overland flow; grain ejection; grain avalanches; and levitation of saturated sand pellets. Our results showed that increasing sand temperature was the main driving parameter in increasing the sand transport and in modifying the dominant sediment transport mechanism. The temperature of the water played a negligible or minor role, apart from the duration of sand ejection and avalanches, which lasted longer at low water temperature. At low sand temperature the majority of the sand was transported by overland flow of the liquid water. At higher sand temperatures the transport was dominated by processes triggered by the boiling behaviour of the water. At the highest temperatures, sediment transport was dominated by the formation of levitating pellets, dry avalanches and ejection of the sand grains. This resulted in a transport volume about nine times greater at a sand temperature of 297 K compared with 278 K. Our heat transfer scaling shows that the boiling behaviour will be enhanced under Martian low gravity, resulting in more efficient transport of sediment by levitating sand pellets even at temperatures close to the triple point. Our results showed that the boiling intensity played an important role in the physics of sediment transport by liquid water. This implied that the amount of water required to produce morphological changes at the surface of Mars could be lower than previously estimated by assuming stable liquid water. Boiling is a critical process to be considered when assessing gully formation and modification mechanisms mobilized by liquid water. Our work could have similar implications for any water-formed landform on Mars, which could include recurring slope lineae, dark dune flows and slope streaks. Supplementary material: Videos of the experiments are available at https://doi.org/10.6084/m9.figshare.c.3990330