A first assessment of nitrogen sources and biogeochemical processing in the speleothem record

Ambler, Scott and Wynn, Peter and Barker, Philip (2020) A first assessment of nitrogen sources and biogeochemical processing in the speleothem record. Masters thesis, Lancaster University.

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Abstract

The most common groundwater contaminant is nitrogen, which presents a threat both to human health and the wider environment. Karstic aquifers are particularly vulnerable to contamination. As up to 25% of the world’s population is reliant on karst groundwater as their source of potable drinking water, quantifying this nitrogen contamination is important for human health. Use of nitrate δ15N and δ18O to determine sources of nitrogen contamination is well established in the literature. ‘Legacy N’ – the poorly understood storage of nitrogen in the vadose zone - has been established through extensive groundwater modelling. However, there are very few reliable historic archives of nitrogen groundwater contamination that would assist in quantifying this nitrogen storage capacity. This project proposes the use of speleothem archives of nitrogen to create an historic record of nitrogen groundwater contamination. Contemporary cave monitoring of Cueva-Cubío del Llanío in the Cantabria region of Northern Spain was undertaken to determine cave dynamics has classified the basic karst hydrology, cave ventilation and cave water chemistry that influence speleothem growth and the proxies preserved within the speleothem calcite. Nitrogen concentrations and isotopes in the major components of the nitrogen biogeochemical cycle were used to quantify the cycling of nitrogen through overlying environment and the impact on cave nitrogen. This included work on speleothem material. Cueva-Cubío del Llanío was found to have a complex ventilation regime, with side chambers removed from the main density driven airflow. Drips were found to be a combination ofmatrix and fracture-fed flow, with drips in shallower chambers drying up during summer months. Seasonality was present in nitrogen concentrations within the cave waters, though was absent in isotope values, suggesting an as-yet-unidentified non-fractionating cause. A promising record of nitrogen has been preserved in the Cueva-Cubío del Llanío speleothem archive. The first successful extraction of speleothem nitrate isotopes was completed, with multiple speleothems from around the world all recording differing nitrate isotope compositions, and interpreted as reflection of nitrogen dynamics in the overlying environment. The record of nitrogen dynamics within speleothems establishes the potential for an historic record of groundwater nitrogen.